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.