When you think of injection molding and plastic parts, you probably think of the material as being rather basic. Sure, there are different kinds of plastics, but all of it is pretty much the same, right? That couldn’t be further from the truth.
You’ve got your Acrylonitrile Butadiene Styrene, of course, but there’s also Polyethylene, Polypropylene, and High Impact Polystyrene. There is essentially an endless amount of material options, and deciding which plastic to use isn’t always a simple choice. That said, there is one way that you can alter the chemical and physical performance of plastics: additives.
Let’s take a look at different plastics strictly within the thermoplastic realm and see how additives can help achieve design and performance goals.
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Thermoplastics (TPEs) naturally possess properties that provide injection molded parts with desired qualities, like strength, durability, and thermal resistance. Different thermoplastics will inherently bring different benefits to the table. TPEs can absorb shocks, nylon is strong, acrylic can add transparency – the list goes on.
What you require out of a material for a given part will obviously change by design, but in many cases, you may need to enhance your base material in some way. That’s where additive fillings like glass fiber, ceramic or mineral reinforcements come into play.
An Inspection of Glass Additives
Looking first at what is likely the most common additive, glass fibers, we can start to examine this process in greater detail. Much like fiberglass, adding glass fibers to a molded plastic can improve strength and rigidity in a part. However, the stiffer you make a part, the brittler it becomes. It’s all a careful balance that can be customized based on your needs. Glass fiber reinforced plastics are best suited for supporting parts in stable environments.
Digging even deeper into the grit of the technicals, fill percentages of glass fiber additives will vary. They will usually stay between 15% and 45%. While you could theoretically use any plastic, typical base plastics are ABS, nylon, polycarbonate, PET and high-performance resins.
If we take a closer look at glass fibers specifically, we can see just why this additive would help improve part capability. Each fiber is a strand that is equal to the length of a plastic pellet, standard lengths being about 1/8 inch long. These fibers intermingle, lining up with the flow direction of the plastic or resin in a mold. As there are turns or obstructions in a mold, you end up with more strands at random angles to the other, increasing strength in these regions, right where it is needed.
In the case of glass fibers, the performance enhancements don’t come without some additional risk to molding problems. The fibers can restrict the shrink rate of the base resin, which can create nonlinear shrinkage and other internal stresses. When dealing with molds that have varying geometries and holes, this nonlinear shrink can present difficult challenges.
If you are seeking thermal resistance in addition to material strength, then ceramic filler or mineral additives present better alternatives. Low percentages of ceramic filler can increase temperature resistance and enhance strength. As always, these strengthening properties add rigidity, which in turn increases brittleness.
For ceramic additives, you’ll also need to think about the shape of the filler itself. Glass fibers are long and slender, as we discussed, but mineral fillers are usually flat flakes and ceramic powder is generally symmetrical, which reduces warp. Along with these variances in shape, ceramics will traditionally only slightly reduce the shrink rate of a part. Within the realm of ceramic additives, there are a variety of choices to fit specific designs.
Beyond the Basics
Glass fiber and ceramic additives have dominated the injection molding additive space for some time, but now, resins are making a strong play in the market. Thermally conductive resins present significant ease of fill in molds that can give parts conductivities somewhere between plastic and metals. Resins are also well suited in light-weighting and giving more freedom in design beyond the basic two additives (glass and ceramic).
All that said, resins still struggle with challenging geometry such as thin walls and small features, which can affect their applicability.
Simply put, injection molding is a rapidly advancing modern manufacturing process like any other. The use of additives and other molding techniques allows for increased versatility to the injection molded part.