The Importance of Choosing the Right Plastic
Choosing the proper plastic for your injection molded component can be challenging. There are hundreds of raw resins to consider, and you must decide if the project will require an additive (like a flame retardant or UV stabilizer) and/or a colorant. Some combinations can be mixed at the resin supplier’s facility, while others can be done using blending equipment on-site attached to the molding machine. Even when narrowing the field down to the few that meet your design requirements, the choices can still be overwhelming. This is precisely why you should include the input of your injection molding contract manufacturer early on in the design phase, particularly at the design for manufacturing (DFM) review. Their knowledge of their own equipment, coupled with the experience of actually molding parts with resins displaying the specific properties you are looking for, makes their involvement invaluable.
In this article, we’ll review some of the most important properties of plastics, and then discuss the strengths and weaknesses of a handful of popular resins used for injection molding.
Picking the Plastic with the Right Properties
The most basic specifications are usually those that relate to the plastic’s mechanical properties: toughness, strength, rigidity, and shrinkage to name a few. All of prime importance for parts that have to bear some mechanical load with a high amount of stress.
There are other important mechanical properties to consider as well. Lubricity is of paramount importance for bearings or sliding seals. Where parts are used for protection, abrasion resistance may be of more importance. If the product spends most of its lifecycle outdoors, extreme environmental & temperature factors come into play where UV resistance might be critical, or its ability to avoid water absorption and degradation if the part will be in frequent contact with water or ice.
There are optical and electrical properties to consider too. Do you require an electrically conductive plastic because the part serves some electrostatic discharge (ESD) purpose? Or are you making electrical connector housings and plugs, in which case high electrical resistance is what you need? Optical clarity and transparency are common requirements for products that serve a more decorative purpose (parts made out of a glass-like acrylic) or components that serve as a type of window in an assembly.
Why is important to get the contract manufacturer involved early on? Some properties can significantly impact production, thus effecting cost. Shrinkage is an example of one of them. Not all plastics shrink the same amount or at the same rate when they cool and solidify. Thus, some plastics are more forgiving than others when it comes to shrink marks. The melting point and the viscosity of the molten plastic both directly impact the cycle time, and ultimately the cost per part.
Let’s now review the characteristics of a few common plastics used for injection molding:
PSU (polysulfone): This family of plastics is known for maintaining its stability, rigidity, transparency and toughness at high temperatures. In fact, this heat resistance gives PSU an exceptionally high maximum service temperature. PSU can be used over a range from -100°C to 150°C. In addition to its tolerance for heat, PSU is generally very chemical resistant. In fact, it’s so resistant to hydrolysis that it’s widely used in medical applications that require steam sterilization.
PPSU (polyphenylsulfone): PPSU is very moldable although this amorphous plastic has no true melting point. Like PSU, PPSU is resistant to hydrolysis and has low water absorption and thus can be steam sterilized as well. PPSU is very dimensionally stable, has high impact strength and is resistant to high-energy radiation.
PBT (polybutylene terephthalate): Unlike PSU, pure PBT can be degraded by UV light. However, PBT is a wise choice for electrical connectors since it’s a great electrical insulator. This is a great plastic for injection molding because molten PBT flows very well, its melting point is relatively low at 223°C, and it crystallizes quickly upon cooling. Thus, cycle times of PBT parts are lower than that of other comparable plastics.
PPS (polyphenylene sulfide): PPS has great heat, chemical, UV, and abrasion resistance. The heat resistance is a double-edged sword however, since its high melting point (280°C) makes it difficult to process. PPS is also very dimensionally stable and has low water absorption.
There are many plastics properties that must be considered ahead of time to ensure that the final product is both effective and safe for the end-user. This is where your contract manufacturer can step in and offer guidance as to which plastic (or family of plastics) will meet your design requirements while at the same time saving you money and lowering the risks during manufacturing.
Your contract manufacturer can offer valuable insight when it comes to selecting the best plastic for your project. Providence is a contract manufacturer that has experience with all of the above resins (and many more).