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The Strange World of PTFE and UHMWPE

PTFE, also known as polytetrafluoroethylene or more popularly, Teflon, has become well known for both its high performance, but also for the unfortunate results of sloppy manufacturing. PTFE is a popular type of material with excellent physical characteristics, best among them its extremely high chemical resistance and slippery surface. However, the chemical category of Pressure Sensitive Adhesives (PSA), of which teflon is part, has been tarnished since its birth due to sloppy manufacturing, improper chemical disposal, and terrible handling, resulting in exposure to carcinogens. There has been a dash to reduce the industry’s dependence on PTFE for these legal reasons–and in some states, it is almost outright banned. 

However, PTFE is still a crucial material that, while it does require special handling, has many important physical characteristics that can benefit many different industries. 

The Invaluable PTFE

PTFE is an invaluable material for seals, because of its “conformability” (low modulus) and viscoelasticity. While it does not recover from deformation as well as an elastomer, its recovery is significantly better than other thermoplastics and meets the needs for seals. Its slick surface makes it excellent for components that will have movement, such as a reversing valve on a heat pump. 

The Properties of Teflon

It recovers from deformation, not 100% like an elastomer, but good enough for seals. It is slippery, so a seal with a wiping motion works very well. An example is a reversing valve on heat pumps. 

PTFE is typically processed by compression molding and has a high viscosity at melt similar to Jello. It expands by 30% at that melt condition and has surprisingly poor elongation. A 600F block of PTFE with a hole in it, put over a steel pin of close size will crack during cooling. It doesn’t work well with injection molding, tending to deform significantly during the process. On the other hand, compression molding of PTFE is a lot like free sintering, where a compacted powder is placed into a sintering oven to solidify. 

A Wide Range of Applications

PTFE is best known for its beneficial ‘stick-slip’ property. While it may hamper control applications, PTFE has little to no difference in static and dynamic friction, making it excellent for pneumatic slip valves, balls, plugs, and butterfly valves, especially in demanding chemical applications.

In addition, powdered PTFE is a common additive for many applications. Here there are fine differences between powders that are made from virgin resin and cryogenically ground in ball mills expensively, and powders that have been irradiated and ground less expensively because the damaged particle is more brittle. There have also been some instances where the irradiated version has led to stress cracking in high concentrations with a few materials. PTFE powder-coated frying pans are the most recognizable application, but this type of coating can also be found across many industrial applications too. 

The Poor Man’s Teflon

UHMWPE (ultra-high molecular weight polyethylene) is an equally fascinating material often used in discussion with PTFE. UHMWPE has the appearance and slick surface of an ice skating rink, but with extremely high wear resistance and cut resistance in addition to low friction.

What does ultra-high mean?

But what molecular weight actually constitutes ‘ultra high’? This is a debate that’s gone on for decades. Some manufacturers have argued that either 2 million or 5 million is high enough to fit that description, and many fell into one camp or another depending on the products they were making. For instance, snowmobile manufacturers required the increased impact strength that comes with 5 mil molecular weight, and the twin screw extruders that processed UHMW economically at 2 million molecular weight disagreed with the ram extruders that developed 5 million molecular weight. Then there was the difficulty in measuring the molecular weight. Traditionally, manufacturers used intrinsic viscosity in a hot decalin solution, but it tended to give widely different results due to cross-linking that took place.

Processing UHMWPE

Processing UHMWPE has been mostly limited to hot compression molding, twin screw extrusion, and ram extrusion. However, a very unique method of processing was developed by Crown Plastics where they used a “Formica” continuous compression molding machine that uses a metal conveyor belt between heated platens. They were limited in thickness and width for a long time, but couldn’t be matched when it came to high molecular weight and price. 

Properties and Applications

Like PTFE, chemical resistance is remarkable but at a significantly lower temperature. Irradiated, cross-linked parts have the unique characteristic of recovering from dramatically distorted shapes and subsequent recovery. 

UHMWPE ends up in the most unique applications thanks to its high molecular weight and unique properties.

  • Pulleys for wire rope
  • Protective sheets to protect the sides of the ships from the steel wire 
  • Steel components of fishing gear
  • Protective liners for concrete chutes 
  • Wear surfaces on food processing equipment and snowmobiles  
  • Bottoms for Skis and Snowboards 
  • Jigs and fixtures in manufacturing because it is easy to machine to an extent, and readily available as stock shapes. 

While UHMWPE is somewhat easy to machine, it does require experience and practice. The problem with machining is that it has a high coefficient of thermal expansion so as the workpiece is being cut with a tool it heats up and changes dimension. High coolant streams help but are not always practical. As an example turning a 2” diameter bearing to a .002” tolerance might take 2-3 trials with an hour or so in between. The second one would be faster because you then know what you need as a hot dimension.

Final Thoughts

These two materials differ dramatically yet so often end up in similar conversations. PTFE has a high specific gravity of 2.1, where UHMW is .96. PTFE is twice to three times the price of UWMW, but with the specific gravity is even more expensive per cubic inch hence the tag of poor man’s Teflon for UHMW. PTFE has a much higher temperature and chemical resistance. Both are slippery and are used as additives in resin blends. UHMW is much more wear, and cut-resistant.

Amorphous Thermoplastics, Another Quick Study

Amorphous thermoplastics are the more popular of the two major types of injection molding thermoplastics used today. The other type, crystalline materials, typically have higher performance capabilities and are used more often in metal replacement applications. Like crystalline materials, amorphous materials cover a gamut of options and applications in the injection molding arena.

Amorphous Thermoplastics Can Be Clear

Amorphous materials are typically clear or almost clear. Starting with the least expensive Polystyrene (often called crystal styrene), to the most expensive: Polyethersulfone (Ultem) which has a brown tint but still quite clear.

Uniform Shrinkage

Their mold shrinkage is quite uniform (.006” to .008” inches per inch) so substituting one for another can be done in the same tooling. Glass filled versions have very low shrinkage (.001 in /in) so precision, stiffness, and less warp makes them candidates for structural applications. 

The structural applications benefit from high modulus, reasonable impact strength and dimensional stability in a humid environment, where nylons tend to suffer.  The exception here is Polysulfone which has issues with boiling water, but that is really extreme humidity. This humid environment aspect is a concern for most crystalline materials ranging from issues such as simple absorption and dimensional change in nylons to PEEK where the issue is quite small, finally to depolymerization in polyesters. 

Chemical Attacks Can Be An Issue

Where amorphous materials suffer is chemical attack from solvents and acids to a greater degree than crystalline materials which are not quite as susceptible to attack. A strange problem exists with ABS and Polycarbonate applications in frying oil exposure, at even room temperatures. They delaminate at the skin. Some types are more resistant to this issue, so careful consideration must be paid to the type of amorphous material selected. 

Types of Amorphous Materials

Crystal Polystyrene

Starting with the least capable, and most inexpensive: crystal polystyrene. It is clear and is inexpensive. It is brittle, not good in warm environments, and gets used in clock lenses, decorative applications or low performance components. 

High Impact Polystyrene

High impact polystyrene (HIPS) is an elastomeric modified crystal styrene that is low cost, has far better impact strength, and gets used in a great deal of applications requiring a modest mechanical strength. It is not clear.

Styrene Acrylonitrile 

Styrene acrylonitrile (SAN) is very clear and used for optical applications at a lower price than acrylics but with better physical properties than crystal styrene, and is used in cosmetic containers, toothbrushes, dishware, and disposable lighters.

Polyphenylene Oxide

Polyphenylene oxide (PPO) is a blend of PPO and styrene called Noryl . Noryl is a naturally flame-retardant material without the polycarbonate price tag when polycarbonate is purchased as a flame retardant grade. Originally developed as a competitor to Cycolac KJB ABS which was a quickly adopted material for electronic enclosures, computers, printers when flame retardant certification was required. There were times when Cycolac KJB had 12-month lead times, and GE came up with Noryl to fill that gap for their own manufacturing requirements for appliances.

Acrylonitrile/butadiene/styrene

Acrylonitrile/butadiene/styrene (ABS) is a terpolymer of these resins with better heat resistance, greater structural strength, better impact, and a better reputation than it’s lower cost cousin from which it is made. Typical applications have been automotive interior components since it can take higher temperatures. Some directly under the windshield applications have transitioned to alloys with other higher temperature resins. Non-automotive applications include kitchen appliances, tools, electronics, and non-sterilizable medical devices.

Acrylic

Acrylic, trade name Plexiglass is clear and falls into the next niche pricewise. Not often used in applications where clarity is not a requirement since ABS tends to be more popular. It does have higher gloss than ABS, and surprisingly better chemical resistance, especially to frying oils. Excellent UV resistance.

Cellulosics

Cellulosics, (Propionate, acetate, butyrate) have been forgotten to some extent. Since they are made from renewable sources(trees) I expected a resurgence due to “green” programs, but pricing has placed them in a one note band for screwdriver handles as their biggest application, and quality casino dice as another. They are tough materials. The first application I had seen for this material was in the mid 60’s for a textile machine where a spindle would hit the part rather hard. Polycarbonate hadn’t become popular yet and the initial grades of polycarbonate couldn’t compete in price and performance.

Polycarbonates

Football helmets say it all. Polycarbonates are clear, impact resistant, good temperature stability and capability. Clear lenses on the front of automobiles (Acrylics in the rear where impact strength isn’t quite as important). Boat windows. Clear globes for Mine Safety. Good UV resistance. Security devices. Medical components. Lighting applications. Less expensive than cellulosics, and much better UV resistance.

Then there is the plethora of blends, using the materials described so far. PC/ABS for those applications that need a little better performance than ABS, but not Polycarbonate properties and doesn’t need to be clear. 

Polysulfone

Polysulfone was the next tier of amorphous materials. Back in the late 1960’s polysulfone was as good as you could get in a high temp clear material. Keep it away from boiling water. It was a decent 250F material and was used in electrical, electronics, and medical applications.

Polyethersulfone and PolyEther Imide 

Polyethersulfone(clearer) and PolyEther Imide (root beer tint) now occupy the highest reaches of thermal performance for amorphous materials. 300F is the upper limit for long term exposure. Since both exhibit good sterilizability (both gamma and steam) they are used extensively in medical applications. Glass filled PEI has the distinction of having the highest thread strength of any material.

Polyvinyl chloride

Polyvinyl chloride is widely known as PVC. Why leave this material near the end? Plumbing fittings. Good UV and chemical resistance, modest temperature resistance, low material price what’s not to like? Molders don’t like it. They avoid it even more than Acetals. Fumes rust tool steel, and molding machines. Molds have to be made from stainless steel, as do barrels and screws and all parts that touch molten resin. Even then, mold platens get rusted. It is nearly as bad as running fluoropolymers such as FEP and PFA.

Polyolefins

Finally, the high-volume misfits, the semi crystalline materials, polyolefins. Often incorrectly considered amorphous because of clarity, these are the most cost-effective resins available. Impact resistant, very good chemical resistance, integral hinge with polypropylene, low specific gravity (they float), slippery, but cannot take even 150F temperatures. There are a few polypropylene compounds that can tolerate 175F. They get used everywhere: Containers. Pumps. Gas line components, milk bottles, kayaks, gas transmission lines, children’s toys, gas cans, child safety applications, the list is endless. The range of capability goes from very flexible Linear low density, low density, medium density, high density polyethylene to fairly rigid polypropylene. Along with polystyrene, these are the least expensive plastics.

Crystalline Thermoplastics: A Quick Study

We are looking at crystalline materials, among crystalline resins, common materials considered for working components in industrial applications include, Acetals, Nylons, PBTs, and a new one POK. Non-common materials, considered specialty resins are PPS, PEEK, and PI. There are others, and even cost-effective talc filled PP gets used from time to time when demands are not onerous.

Amorphous materials such as ABS, HIPS (High Impact Polystyrene), Noryl, Polycarbonate, Polyether Sulfone, Polysulfone, and PEI(Ultem).

How do these differ?

Typically, amorphous materials are clear (but not always), they have lower shrinkage values, so they warp less during molding, have high friction/poor wear, poor chemical resistance, and a different temperature curve of property (such as flexural strength) versus heat. Amorphous materials have flex strength curves that are more linear, and lower at lower temperatures, but maintain properties until a softening point.

Crystalline materials have higher values at the lower temperatures, then a dramatic drop at the glass transition temperatures. Glass fiber filling of crystalline materials dramatically improve this performance. Amorphous materials are more often used for enclosures or parts requiring good aesthetics.

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What makes an engineer consider one over the other? 

Depends on the engineer. It should be the least expensive option that gets the job done without a lot of testing prior to production for low quantity type production applications. Experience with resin satisfies that issue, but for high volume application, assurance that the material with the lowest cost gets chosen will require life testing.

Sometimes corporate goals such as a green initiative can play a factor as well. There are answers here.

Acetals, trade named Delrin, Celcon, and Duracon for example occupy the low cost bracket. Decent friction, wear, high flexural modulus, low moisture absorption are strong points. Unless impact modifiers are used, impact strength is low. Addition of glass fibers reduces impact strength further, but stiffness is increased. This resin has very good fatigue resistance. A negative for molders is the smell of formaldehyde that is quite upsetting during purging or when the material is over heated. Applications shouldn’t exceed 150F in the long term. Specific gravity of 1.42 increases cost per cubic inch compared to nylon 6 and 6/6. There is high shrinkage, resulting in increased warpage, voids, and sinks with thicker wall sections. Surface finish is sometimes difficult to process away. They are easy to acquire as rod stock to prototype, difficult to 3D print due to shrinkage. Sometimes we see machined parts from Acetal transitioning to molded parts as volume grows. There is copolymer, and homopolymer versions with minor differences. Homopolymer have better stiffness, and higher heat resistance to a minor extent, while copolymers process more easily and have better impact. Again, additive packages for both types blur these lines. A Delrin 100T has far better impact than Celcon M90 for example.

Nylons are more commonly used and have far more variants. Nylon4, 6, 6/6, 6/7, 7 9, 10,11,12 are examples of this diverse universe, all with a place at the table when specific requirements dictate. We will see shortly how POK makes them somewhat redundant, however. Starting at Nylon 4 and going to Nylon 12 the amount of water absorption decreases, the temperature resistance reduces, vapor transmission reduces, clarity gets better, and dry as molded impact strength improves. When trying to define nylons there is a broad spectrum, especially when additives get included. Glass fiber is a big game changer, nylon really likes to bind to the sizing agents on the glass fiber so major gains in performance with small amounts (13%) of glass fiber are added, Add an impact grade of 6/6 Nylon along with the glass fiber and a very durable material with little moisture encumbrances results. What hurt glass filled nylon in electronic applications in the 1970’s was the poor performance when flammability additives were added. They were conductive. That’s when PBT resins took nylon’s place in that industry. Nylons are less slippery than Acetals, but again additives such as Carbon fiber, PTFE, UHMW, greases, and Kevlar fibers make them attractive friction/wear choices. 

High temperature Nylons: PPA (Amodel), and 4/6 nylons have slightly higher(20-40F) temperature capabilities than standard grade with higher pricing.

Nylon has been at the forefront of metal replacement initiatives. From under hood in automotive, to aluminum die cast parts. There seems to be greater acceptance to the material’s capability for strength, performance in hot applications, and chemical environments. While GF 6/6 nylons can be used in radiator applications, one side of the part has to be exposed to atmosphere, a kind of strange drawback. A second weak point is nylon’s lack of resistance to alcohols in gasoline. Many carburetors have been clogged with decomposed Nylon components. For submerged cooling, the high temp. nylons are used. A 60% long glass fiber 6/6 nylon, with UV protection, heat stabilizers, and lubrication additive package is a formidable structural option. They can and will absorb a lot of moisture, up to 3% (6 and 6/6 versions) in some environments with attendant dimensional changes. When in sub-freezing, low dew point applications, without the impact packages nylons can have very low impact values. You will always see data with “dry as molded” and “conditioned” value that are quite different. Nylons also have serious shrinkage issues and warp when thick and thin wall are present on a single part. The thick sections can shrink at .030” inches per inch, and thin walls at .010”.

Polyesters, PBT and PET. We are going to talk about thermoplastic polyesters, not thermosets which all cross link during processing. One thinks of boats when polyester is discussed, but low profile, bulk molding compounds of polyester, glass fiber, various fillers, and a little polystyrene for surface appearance have need used in automotive, and high-capacity electrical devices for over 60 years.

Thermoplastic polyesters such as Valox, Rynite, and Celanex started to be used more broadly in the 1970’s when it was found that V-0 additives were more compatible than with nylons. Weld line strengths were not as diminished, and electrical properties were maintained with PBT in comparison to nylons.PBT is more apt to be used when fillers are not required, and PET which is not as process friendly gets used when filled with Glass or carbon fiber. PET is less crystalline as evidenced in clear soda bottles, and does not process as quickly as PBT without an additive package. An early application for PBT was in water pumps for outboard motors, and it performed well until temperatures exceeded 150F when it’s well known depolymerization when moisture is present took place. During processing PBTs can very easily be compromised when not dried adequately, and because it does not show up visually can be a significant problem. An impact test with finished parts is the fastest and best method of insuring adequate drying.

POK, known as Polyketone is the new comer, and it has great potential. It is made from 50% flue gas from electric generation facilities, so is considered “green”. Property wise, mechanically it falls between Acetals and 6/6nylons. It has better chemical resistance than both. It has very low water absorption like Acetals, but higher impact strength more typical of a toughened nylon. Friction is very low, better than Nylon and Acetal. It is more susceptible to warp with differential mold temperatures (it will preferentially warp towards the hot side of a mold to a controlled and greater degree than other materials. Cost is comparable to toughened nylons. A unique aspect is that while it has been around for over 20 years it never became competitive until it was acquired by a Korean company that spent an amazing amount of money fully trouble shooting manufacturing, documenting properties, and getting certifications across the globe before widespread implementation across the industry. US and Eu approvals for Dairy exposure for example were in place very early in the ramp up. A worldwide certificate for a V-0 flammability rating with good impact strength, elongation, and flexibility is a big advantage. A negative aspect (or positive from a user’s perspective) is that when left at processing temperatures for 15 minutes or more, it starts to crosslink and has poor flow. Processors need to keep on their toes. This means long term constantly improving heat tolerance.

How do these materials compare with each other in real world applications and real-world issues such as weld line strength? Nylon has better weld line strength, POK a close second, PBT next and finally Acetal. When glass fibers are introduced, these comparisons change to PBT first, POK next, then Nylon and Acetal. Chemical resistance has POK the clear winner with each of the others having specific weak points such as alcohol for Nylon, and water for PBT. Acetals have always been used for inexpensive wear items like rollers, or pulleys due to good friction, low wear, and high modulus. POK will be taking some of this marketplace with even lower friction, better wear and quieter operation.

Where does talc-filled polypropylene fit in this cast? 

Unfilled PP is considered a commodity resin, used in packaging, fishing tackle boxes for example, and never really considered an engineering material. Talc filled/glass filled PP has demonstrated a capable presence in lower temperature structural applications with very attractive pricing, chemical resistance, and satisfactory impact strength. Automotive applications got everyone’s attention at the beginning, air cleaner housings for example, then dashboard structure (versus cosmetic) applications. When the quantities are high, the loads light, and cost is very important, filled PP needs to be considered. The breakthrough for this performance has been getting superior binding systems to lock fibers to the resin.

These crystalline materials have been the favorites when looking at metal replacement applications. Glass or carbon fiber concentrations of up to 60% in Nylons (including long glass fibers, and long fiber polypropylene) are the first cost effective candidates. High stiffness, working temperatures of 200F or higher long-term, built-in color, decent impact values all contribute to the popularity of these materials.

Fine Tuning Part Design For Injection Molding

The solid model is complete, the quotes are in, the tooling order has been placed. What is there left to do? Fine tuning those minor details that won’t get addressed until tooling is being designed. Why wait until now to do this? No one wants to invest time in these technical issues until they know that another molder won’t benefit from their efforts.

Let’s start with the details that often aren’t included on the print and are never included on a solid model

Starting with the details that often get left off of a print, and never shown on a solid model:

What are the tolerances needed to fit mating parts? How flat does the part have to be? Some materials warp more than others. Is sink going to be an issue?

What are functional surfaces? Is a parting line going to interfere with function? What surface can the gate be positioned, and do the knock out pins(with eventual flash) hinder function? What are seal surfaces? Does the part need to be void free, or can some level of porosity be allowed?

  • KO Pins Flush to .005 below surface
  • No Flash
  • Tolerance ± .005
  • (X.xxx) ± .010 (X.xx)
  • Color: Grey (Pantone XXX)
  • Surface Finish: SPE #B-2

“No flash allowed” is not realistic

A common note on a print is “no flash allowed” which is an impractical wish. You might not see it, or measure it, but it will always exist at the parting line, or insert witness line. An “O Ring recess” will always leak if there is a witness line perpendicular to the diameter. A witness line following the O ring diameter might actually improve sealing.

The color is noted on the print as gray. Is it a specific pantone color, or can the molder use existing colorants? Is the color critical to match other mating parts?

Has the amount of regrind been considered? Some materials like PEI, 6/6 nylons, HIPS, and ABS can use considerable amounts of regrind. Some molders believe 100% recycled 6/6 nylon actually runs better than virgin pellets. There may be a color shift, or more black specs (which also might be defined) with increasing amounts of regrind. Other materials (such as POM)  really shouldn’t use more than 25% regrind as a general rule.

While there are a lot of questions, there are a lot of answers. The answers will vary depending upon the experience of the molder, and by experience gathered by years of overcoming diverse issues.

Gates are a requirement on an injection molded part

Gates are where molten plastic enters the cavity of the mold. When a molder starts considering the ideal location, there are many variables. The first being appearance, the second is where will the weld line (and inherent weakness) and subsequent venting, cause the least issue? How long is the flow length relative to part thickness?  Gates should be located to provide thick to thin melt flow to reduce sinks and minimize warp. Solidification of the hot plastic in the mold is thickness dependent, so you want flow to stop or slow down towards the gate, keeping pressure in the still molten areas.

Gates should be directed towards a feature in the cavity which will impede flow and prevent jetting which is an uncontrolled “squirting” of plastic across the cavity leaving a rough surface that looks like a worm. A long flow length, with thin walls will mean greater warp with some crystalline materials such as nylon, than with amorphous materials such as ABS, but warp will exist in both.

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Differential mold temperatures are one way of mitigating this warp, but not always. Machining the gate is an expensive option, but sometimes is the best solution. Such as when no weld lines can be tolerated, or with a fiber filled material such as PEEK on a bearing where roundness is critical. Subgates degate automatically and are preferred by molders. A blemish on the outside of the part is a result, just as edge gates leave a blemish. (See picture.) 

Many years ago, when an operator used to be at every machine, they would cut the edge gates by hand with a pair of cutters. At Recto we had a lot of edge gates which we converted over time to subgates, getting the operators away from machines to do more meaningful work. But subgates stick in the mold sometimes and it wasn’t unusual to be running 14 cavities of a 16 cavity mold after a few days. Subgates wear faster with glass filled materials, and are more fragile. With the proliferation of 6-axis robots at Recto, edge gates with their inherent advantages have become more viable.

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Methods for ejecting from the mold

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Knockout pins, sleeve knockouts, blades, lifters, stripper plates all are used to eject the hot molded part from the mold. All knockout methods share the same rules: they need to push against the stiffest most constrained feature on the part and need to eject a soft(hot) plastic part evenly and quickly from the mold. More square inches of ejector pin always helps in this regard.

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They also do a lot of venting, so at least one of them needs to be at the weld line. Standard knockout pins are the most common, and more rather than fewer is the rule. They are inexpensive, and easy to replace. The surrounding mold cools them. Using bosses on the part allows larger diameter pins to be used. Sleeve knockouts are probably the second most common, they are around round core pins.

Here there is a trap. Because of the way molds are built, the core pin gets retained at the furthest side of the mold, (making them very long ) and if small in diameter are very hard to cool. Adding water to larger (bigger than ,250”) core pins works very well, but is now an area for potential water leaks if O Rings are used to seal the pin. Guided ejection is required to reduce long term wear since sleeves are especially susceptible to non-linear motions.

Blade knockouts are used as a last resort and used when they need to be very thin. They bend easily and getting the four corners of the blade to fit well into a rectangular opening in the mold is difficult making flash a concern. Lifters get used when there is an undercut on the part, and when they are beneficial in ejecting and preventing trapped walls. A concern here is to have a design where the part is constrained by a feature during ejection so the part doesn’t want to stick to a lifter. My favorite ejection system is a stripper plate. It is too bad they can’t be used more often. Easy to cool, can eject thin walls, venting is consistent, when designed with draft angles on cores have the best long term wear resistance and the core pin is easily cooled.

Surface finish

Is the surface of the part visual or functional? Usually finish is defined on the print, but sometimes it gets over looked. When the cavity building is done with CNC machining the surfaces need to be polished to get rid of cutter marks unless specified. In the past a quick way of telling if a mold was built offshore would be the cutter paths on the non-functional surfaces. Domestic toolmakers didn’t see that as an option. It became a criticism, and the off-shore toolmakers started polishing as well. A “paper” , meaning sand paper finish, is usually a default finish. When EDM is used to produce a feature in the mold, the oxide finish normally gets polished off, and a slight textured finish is the result. More polishing, results in better surface finishes. Some  customers like the appearance of an EDM, low gloss surface. It hides molding defects. Parts that need a good surface finish get polished at a significant cost(time).

Intentional mismatch. Regarding witness lines and parting lines, when it is beneficial to intentionally make an area smaller than a mating surface for function, inspection, or long term flash control, the best time to do this is during mold design. When the stationary side(A side) of the mold is .002” / .003” smaller per side than the moving side of the mold(B side) it accommodates any parting line mismatch caused by incidental machining differences or wear on guide pins/bushings or side locks. It is a normal detail understood by experienced mold builders but should be planned for in advance. This same concept should be considered where there are features in the mold that are separate components such as cores, or core pins. Any surface that is non-functional should be recessed by .002/.003”.

The Four Types of Robotics That Are Commonly Used in Injection Molding

The very first experience I had with robotics was back in the mid-1980s when I purchased a used Sadamel vertical press from Switzerland. It was a high-precision press designed for watchmakers. Its pneumatic picker removed parts and runners from a vertical clamp at high speed. It made vertical presses relevant again for high-speed molding. Molded-in inserts wouldn’t fall out when the clamp closed.

As with most choices in life, there are reasons you take one path when there are many to consider–and the same applies to the integration of robotics into a plastic molding company. Robotics have many applications in the industry, and the type that you choose depends upon what you want to do, how much you can spend, and how much flexibility you want. The four main types that are applicable to injection molding are described in this article, with pricing based on the use of 250-ton presses.

Sprue Pickers

Sprue pickers are the least expensive, ranging from $5,000 to $10,000, and the simplest of the four types. They are most cost-effective for basic needs, such as taking a runner and putting it into a grinder, or putting edge gated parts in a tall drum to be gated the next day. You can take this motion and remove runners from a three-plate mold or knock the runner on that mold out from between the plates. Beyond these activities, the sprue picker is limited by its three-axis capability and isn’t as flexible as other types of robotics. 

Sprue pickers are excellent for molders just getting into robotics, as well as molders running two-plate, sub-gated tools. The machines travel down into the mold, grab the sprue/part, pull away from the parting line, retract to the start point, rotate away from the molding area, extend to drop the runner/part into a grinder or receptacle, return to the home position, then rotate to the start position. A flex option on the grippers adds a fourth axis to help get a large runner into a grinder. 

This type is mounted to the stationary platen on the molding machine. Sprue pickers are typically pneumatic devices, but more expensive servo versions can be purchased for greater flexibility.

Spruce Picker

Traversing Robots

Traversing robots are mounted to the stationary platen and have a traversing motion instead of a rotation. Typically, these are servo-driven, although there have been versions with pneumatic operation on the extract axis. Because they are often servo-motor driven, there are some limited secondary operations that they can perform, such as delegating, stacking, and filling boxes. When you add fixturing, traversing robots can load inserts and labels as well. Safety cages are necessary for traversing robots, but are much smaller than those required for six-axis robots since most of the work occurs high in the air.

Traversing robots can be used to remove large parts from a press and put them on a chute or conveyor belt. They also can perform special handling of parts that require secondary operations such as drilling, or assembly onto another hot part, where you can shrink-fit onto a cold part like a ball bearing. If the hot plastic part were to be put into the inner race of the ball bearing, it would shrink away and become loose.

The cost of an average traversing robot is $30,000-$45,000.

Traversing Robot

Side Entry Robotics 

These high-speed robots are typically used for installing labels into a cavity or on a core, removing the part from the press, and stacking or loading parts outside of the media in bags or boxes. With total cycle times of 2-3 seconds, they are amazing to watch with parts staking up quickly–but they are the least flexible type of robot and tend to be the most expensive. They require space around the press and are the most challenging to program and operate in order to achieve the greatest output.

Side Entry Robot

Six-Axis Robots 

Six-axis robots are the most widely seen on TV and the internet, welding or moving automotive body parts. Though more expensive than other types, six-axis robots are the most flexible and precise option, and they can be rebuilt after hundreds of hours of operation and start life anew without compromise, even if the robots are decades old. 

A new Fanuc six-axis robot typically costs $40,000 to $50,000, but a rebuilt unit costs just $25,000 to $35,000. They tend to take up more room than traversing robots, because six-axis robots are typically mounted on a pedestal outside of the molding area, while smaller versions can go on the stationary platen and take up far less space. 

Because they have so much flexibility, programming is a more critical factor for six-axis robots. From removing runners to loading metal inserts, delegating parts, loading parts onto a conveyor, and putting parts into cartons, providing a step in an automation process where the orientation of the molded part can become very costly in a separate cell, the flexibility is dramatic–but programming and robotics experience is vital.

Six-Axis Robot
Six-Axis Robot

The benefits of integrating robotics into an injection molding shop

The use of robotics has many advantages in the injection molding industry. Their use results in fewer rejects and better efficiency. Grinders are less likely to get clogged and the robotics equipment can perform its tasks reliably and consistently. With skilled and experienced employees, robotics are excellent for large parts or long-runs of production, starting with production runs as small as 1,000 parts. 

About Recto Molded Products

Recto Molded Products has well over a century of experience in injection molding, and is well known for its extensive use of robotics and automation throughout our plastic molding company. Check out our website to learn more about us and to request a quote.

Getting the lowest price for injection molded parts, without sacrificing quality.

Think like a molder 

All customers want the lowest price possible, so how do you get it? You think like a molder and address the concerns that affect pricing.

What is the price the customer is seeking? Target pricing is a goal/challenge that is a good first step. The molder then considers what is it going to take to make that happen, rather than what do we guess the customer wants. A goal if you will.

Am I getting all of the information I need? How well does the customer communicate their specific needs?  Are cosmetic requirements, reasonable tolerances, and other specifications appropriate for end use, and in what environment?

Consider the redesign

How much redesign will it take to achieve the goals? Will I have access to engineering to answer questions regarding quality issues, material selection, and cost reduction programs? Are there budgets available for testing alternative materials? This aspect of arriving at pricing goals is generally overlooked. When an engineer specifies a material it is often because of a few parameters. First no one wants failure during the life of the product, so the inclination is to over specify from a viewpoint of what has been working on similar applications. 

Choosing certain materials can lower the price of your parts

Thick walls add cost!

When the quantities are modest, long-term testing probably is less cost effective than jumping up from, say, a polypropylene to a nylon with a little bit of cost increase. Long term testing might show polypropylene would work just fine, but testing cost would be prohibitive, and lead times increase. When the molder uses larger quantities of nylon, and would need to pay a premium for low quantities of polypropylene, the answer gets muddier.

Is the customer receptive to looking at materials currently being used on a daily basis to keep prices and deliveries controllable?

Are there expensive brass inserts in the design? Can they be replaced with inexpensive square nuts?

Wall thickness is a factor

The common method of looking at a part as a machining operation is just the reverse from designing as a molded part. The first method is removing material from a solid block, the other is defining the working surfaces and supporting them with structural features. When wall thickness is kept identical, the differential shrinkage due to different walls  and accompanying warpage is minimized. When walls are too thick, higher pressures are needed to keep the part void free, resulting in greater stress. When walls on a single part are both thin and thick all kinds of problems can surface. This is why molders always ask for coring: thicker walls always add cost. Both material cost and processing cost (longer cycles for cooling). Sometimes the reason for a thicker wall is stiffness, which is true but can be also achieved by using stiffening ribs or walls, or with glass fiber in the resin. 

Sometimes the thicker walls are perceived for impact strength, which isn’t always the case. Consider a thin wall soda bottle, versus a thick wall bottle of the same design. The thin wall will flex rather than break; clearly elongation is the important consideration. I once heard from the manufacturer of Ultem that a wall of greater than 3/16” with 30% glass filled Ultem would not be beneficial in any way to part design. It would be more expensive, it would not have better impact strength, or if properly designed, be any stronger. I believe this is a good general statement for any molded part. Requiring that thickness causes voids to develop, stresses get higher, and cost climbs with material cost and processing times.

How to add value to the initial part design being quoted 

Thin-walled part with ribs to achieve impact strength with stiffness.

When the design seems complete, can we integrate other attached components into the part? How about other “free” features such as part number, or instructions for example ”uses 10-32 bolt”. Can assembly be made easier by longer lead ins on the holes for screws? Have we included radii to all inside corners to improve impact strength, and reduce stress risers?

What will the working relationship be with the QC department? Do they react appropriately to the issues? When a shipment of 100,000 parts uncovers one “short shot”, will the molder have to spend hours with a returned part, root cause analysis, paperwork that is often inadequate in the eyes of the QC representative, and a fine as punishment? A symbiotic relationship works much better.

Don’t forget the packaging

Then there is packaging: Can we use returnable or disposable packaging materials? The packaging cost for 10 large parts in a $2.50 cardboard carton  is much more expensive than one hundred in a disposable gaylord.

Can quarterly purchase orders with monthly releases work?

Has this existing customer, or will this new customer provide adequate lead times, or will every order be an emergency? How much inventory will I need to maintain to meet lead times?

How does the customer pay their invoices? 10 day payment terms always reduce costs.

Bottom line is it all comes down to understanding both parties’ needs and expectations, and how the cultures complement each other.

.50 cardboard carton  is much more expensive than one hundred in a disposable gaylord.

Can quarterly purchase orders with monthly releases work?

Has this existing customer, or will this new customer provide adequate lead times, or will every order be an emergency? How much inventory will I need to maintain to meet lead times?

How does the customer pay their invoices? 10 day payment terms always reduce costs.

Bottom line is it all comes down to understanding both parties’ needs and expectations, and how the cultures complement each other.

A Detailed Look at the Quoting Process for Plastic Injection Molding

Firsthand Advice from a Injection Molding Expert

How do molders go about determining the appropriate price for an application? 

When a molder receives a print or a solid model and production quantity, the conservative first pass or ballpark pricing will always reflect the unknowns that can–and often do–surface later down the line. The more upfront data provided to the molder, the more accurate and typically lower the upfront price. This data can include a wide range of factors such as yearly consumption, specific color requirements, special packaging, payment terms, and quality specifics. And these are just a few of the aspects that the molder will ask about. 

A case study in why having complete information is important

Perhaps the most difficult request that plastic injection molders can receive is a request for a part price on something that is currently being made through a different supplier. This could be for a simple ‘price check’, an attempt to find a new supplier, or ‘reshoring’ parts previously made offshore. 

Regardless of the reason, these types of quote requests often come with a history that the customer may not be aware of, such as:

  • The material used and how it might be colored
  • The number of cavities in the mold
  • The required cooling time
  • The type of mold steel used
  • The condition of the tooling
  • The use of edge gates or sub-gates
  • Is extended cooling time required due to inadequate ejection
  • The potential for dimensional or warpage issues

And the list goes on. 

Bringing as much information as possible is important for an accurate initial price. However, the molder will always have questions, and answering them as thoroughly as possible will give them the information that they need. Bringing sample parts can be a large benefit, as well as any information on the mold itself and its design, such as the knockout pins, parting lines, gates, cavities, and more. 

What happens with a preliminary design?

The other type of quote request that molders typically get is for a preliminary design for a component, often including a 3D model of the part as well. Other information should be included as well, such as the function of the part, the environment that it will need to withstand, assembly concerns, and any cosmetics required. The more the molder knows about the entire product, the more successful the project will be.

With a new design to quote, design questions regarding draft angles, cosmetic surfaces, loads, temperatures, colors, assembly techniques, location and quantity of ejectors, gates, parting lines, areas where warpage may occur (flatness), the use of threads versus self-tapping screws, molded in place metal inserts, thick walls, and sink are all considered. Features such as undercuts, which can be designed out of the part, reduce tooling costs.

Who chooses the materials?

Material decisions are typically made in collaboration between the molder and the customer, but they depend heavily on the experience of the injection molder. Molders prefer materials that they can source in large quantities at reasonable costs, with many inventory turns per year–and their customers benefit from the molder’s expertise, getting lower prices and faster deliveries. 

As an example, the short-run part that needs 50 pounds of a specific material a year and can only be purchased from the requested source in 2,000-pound minimum quantities is not a good fit with pricing and long-term inventory issues. 

Having a molder with extensive material application knowledge is invaluable in all cases.

Who chooses the tooling?

Experienced molders will typically source the tooling for a project, based on the parts that they make and their own personal preferences. For instance, Aluminum tooling is less expensive but doesn’t last as long as fully hardened tool steel. At Recto, we avoid aluminum molds and typically use hardened steel or a similar material. Some molders use them exclusively. Aluminum molds running 30% glass-filled polymers don’t last long, while the same molds running polyethylene can function very long with little to no damage to the cavity. 

When a customer starts the conversation with the statement, “We will only need 100 parts, ” aluminum must be considered, as it’s a cheap material that will last long enough for the run. However, aluminum isn’t a feasible choice for large production runs of thousands to tens of thousands of components. In such cases, hardened steel molds are the more cost efficient choice, despite being more expensive upfront. 

A case study in material selection and experience

The more experienced the quoter has become, the more questions are asked, and experience is a great educator. Many mistakes/missteps have made the quoter wary of missing information, which has a significant effect later. 

A good example was an application for a coffee vending machine, a solenoid dispensing valve. Requirements were for boiling water, chemical resistivity for coffee, cleaning agents, and suitable long-term seal surfaces. CPVC was initially chosen, but before the ink was dry on the tooling purchase order, a change to Polysulfone was requested. 

A material supplier had convinced the customer that the CPVC was not the correct choice, but their material was better. The molder, who was inexperienced, agreed to the change on the conditions that the material supplier had to have technical responsibility from tool design to initial samples and testing. 

Six months later, and two production shipments later, the customer reported that the parts were cracking during assembly. When the molder went to investigate the issue, they discovered that the assembly operation was taking place next to the floor’s solvent welding operation. Polysulfone, the material requested by the customer, is susceptible to solvent attack. The solvent operation was moved, but the customer never used Polysulfone again. 

The moral of the story is this–not all material selections should be made from data sheets. Tensile strength is not an indicator of impact strength as much as good elongation values, and high modulus with low elongation is only sometimes a stronger part. Not until the material is brought into real world conditions does it truly prove its worth.

Conclusion

The quoting process of any plastic injection molding project often has an extensive impact on the quality of the overall relationship between the company and the customer. Not until the customer says that the molder has done their job and done it well, has the molder earned their customers’ trust.

Choosing the Right Molding Company

One size does not fit all. A customer in a highly regulated environment will need a similar injection molding vendor familiar with, and able to comply with the various certifications. There are developing requirements in some industries to “go green” which is a challenge in general for some plastics processors, but not impossible. After these initial requirements are satisfied it becomes easier to source.

In the general molding categories, all will state that price, delivery, and quality are paramount requirements. But even in these simple aspects, there are other considerations.

Price. Fundamental until other aspects are considered. Can quality or delivery be compromised? Is price a function of equipment, or people? The simplest vendor would be one person, responsible for all aspects, with no chance for communication errors. When the size of the molder increases, so do the complexity and possibilities of error. That is why standards of documentation such as ISO are important to the largest molders. But even ISO doesn’t protect all shareholders from the weakest link, it just reduces the opportunity for error in theory. A company heavy with documentation, and redundant communication channels will naturally have higher costs, and therefore higher prices. That may be important in some industries that justify the additional costs.

Service. Molders running at 80-100% capacity will not have the flexibility of rapid schedule changes. If the customer industry is a “job shop” in nature a 2-3 day turnaround to start getting parts may be critical. Some customers will value short runs as more important. Service can also mean responding to pricing requests quickly, and certainly problem-solving. It can also mean providing the technology necessary to manufacture the product that isn’t available at the customer’s disposal. Material selection, part design, and quality suggestions are all important.

Quality. Is the molder satisfied with maintaining quality that is acceptable, or do they aspire to improve as an integral part of their culture? Quality while measured in PPM, or dimensional problems is the initial consideration, however, the response to issues that develop is just as important. How quickly and thoroughly the concerns are addressed is equally important. A molder with QC people with high turnover, rather than fewer with a long job history (and memory) has to be considered. How well does management support the QC department, one of the places where fault is generally cast? Are the drawings kept current, and accurate? Does the company culture provide inspection devices that make measurements accurate and easy for off-shifts to use? Does the molder have knowledge and understanding of the component’s function to identify potential issues during production, and the life of the product?

Then there is general culture. Profitable companies typically have better (precision, speed) molding machines, dryers, robots, tool room equipment, and general hand tools. Getting better tools in the hands of employees makes them more effective, and less frustrated. While servo-electric machines are faster, more accurate, and use less energy than servo valve machines, they might not be required to handle the molding project that a less expensive machine would be capable of running. And neither machine can perform adequately if there are wear issues taking place such as a worn screw/barrel. An expensive new all-electric molding machine with wear on its screw/barrel won’t be as good as an older servo valve machine with no wear. However, when given an option, buying the more expensive, more capable all-electric machine is usually the best choice. A six-axis robot has far more capability than a three-axis traversing robot. The culture of “doing things right on a long-term basis” is a formula that will provide the required price/delivery/quality aspects, and should be foremost in selecting an injection molding partner.

Plastic News

Since 2005 Recto has initiated a practical “green” culture. Not to be confused with the current push for green at any cost, our perspective is that green can have many tints, and the one that seems the most practical is one that is simple and has appropriate payback. Not to say that we might take on a project that looks good as well as filling a need. Trees for example. The City of Cincinnati Parks Department has planted trees around our building for appearance, while the shade keeps cars cool under the shade. This cooled air also makes it into the building. We have obsoleted old molding machines for many reasons, but oil and electric consumption were the primary reasons. Along for the ride came lower maintenance, better performance and a cooler building. We got rid of air cooled chillers (160 tons), and went with water cooled chillers after replacing our old, leaky cooling tower(with twice the capacity). With the new cooling tower came a water cooled chiller, and heat pumps to cool and heat high population areas of the building. As the old leaky roof was replaced, white TPO was chosen to cool the building, increase usable life, and ease of repair. When we traded an old molding machine for a 2500 gallon water tank for rain water, we found free water, and also a cleaner cooling tower. There are fewer chemicals used to keep the buffering chemicals in suspension. We added 10,000 more gallons this year. When an addition was added, four 1” PEX tubing lines were added to the perimeter of the foundation. We use our 90 degree plant cooling water to keep the addition warmer in the winter, and reduce usage the of the cooling tower.

We use smaller compressors, located strategically located to add heat in the winter, and are located where the heat can be sent outside quickly in summer. Smaller compressors means lower startup demand and lower operating costs when variable air usage conditions exist. When air operated devices are not heavily used, only 20 horse power is running. When a lot of air is being used, 60 HP motors are used.

Like most molders that work a 5 day, 24 hour schedule, our peek electric demand soars on Monday morning startups. We have a device that monitors our energy consumption with a set of traffic lights. Red lights indicates employees to start a smaller press instead. This device also monitors power factor, showing when someone has failed to switch on a capacitor bank. Our peek demands still were a factor from 10am until 4pm as every startup and sample has begun. We added 196 solar panels to absorb that peek demand, 44kw of electricity that only provides $4,000 worth of electricity but helps cut our peek demand charges by about $7,000 per year. The solar installation was finished late 2009, and because of all the incentives (that are no longer available), paid for itself in 2 years.

In 2003 Recto had 45 molding machines. Today we have 27, all but two either servo electric, or servo pump. Servo technology hasn’t completely licked the problems with long pack times, so we keep two servo valve machines in good condition to solve that problem.

In 2003 Recto reversed it’s air quality fans so that instead of inside air being extracted in the molding area, outside air was brought into the warm molding area in the winter. Extraction fans at the opposite side of the building draws the warm air to all other parts of the building with “helper” fans in some walls to further mix the air. In the summer the fans all extract air from the building with make up air coming from the tree shaded zone around the building. Our inside air temperature is cooler than outside in the heat of the summer. This saved considerably on our heating bill. 

During this 20 year span Recto has reduced it’s electric usage in half, and it’s gas usage by 90%. That’s really green no matter how you measure it. Our building roof reflects sunlight, or trees add cooling and look good, and we collect stormwater to reduce the load slightly, at the city water treatment plant. Our employees benefit with better working conditions.