Thermoplastic resins naturally possess mechanical properties that, depending on the material, can give injection-molded parts strength, durability, impact resistance, and other benefits. For example, nylon is strong; polycarbonates have good temperature resistance; TPEs are flexible and absorb impact; and acrylic possess a high degree of transparency. Depending on a part’s geometry and the application, the base resin may work great. But when the performance of the standard plastic needs to be improved, one of the easiest ways to do that is to include additive fillings such as glass fibers, ceramics, or mineral reinforcements in the resin.
Glass fibers are one of the most commonly used additives in plastic injection molding. Depending on the percentage of fill, glass fibers significantly improve the strength and rigidity of parts, but with strength comes brittleness. If a part does not have to endure high-impact stress and deflection, for example, and instead lives in a stable environment where it is merely supporting weight, then glass-filled parts work well.
Product developers often use glass-filled materials to increase the strength of parts such as this one.
Fill percentages vary, but typically range from 13 to 45%. Glass-filled resins include ABS, nylon, acetal, polycarbonate, liquid crystal polymer (LCP), PBT, PET, and PPS, as well as high-performance resins such as PEEK and PEI.
Adding glass fibers also affects the molding process. A fiber is a strand equal in length to the resin pellet. Standard fibers are about one-eighths of an inch long, and long glass fibers can be three-eighths to one-half inch in length, depending on the extrusion. If you think of the fiber as a fish and the number of fibers as a school of fish, as the material flows through the cavity, the fish line up nose-to-tail swimming through the resin. As the school of fish nears a core pin that creates a hole in the part, the school separates and swims around the obstruction. This changes the angle of the strand. So, the more geometry or obstructions in the way of the resin flow, the more the strands are at random angles to each other.
Glass fibers also restrict the shrink rate of the base resin. That restriction is different nose-to-tail than when perpendicular. This creates nonlinear shrink and exacerbates internal stresses that increase the risk of warping. Unfilled resins typically shrink uniformly during cooling, whereas glass fibers create different shrink factors in the flow and transverse axes. Anticipating that change in shrink becomes difficult in geometries with numerous holes, changes in flow length and shape, and changes in nominal wall thickness. Adding glass not only adds performance enhancements, but also risk.
Here are some tips for working with glass additives. As with any material, glass-filled or not, adding radii to part geometry can improve flow; it’s simply easier for resin to move around curved radii and fillets than against a 90-deg. angle. Uniform wall thicknesses help resins cool at the same rate. Draft lets your molder pack the part harder in the mold to better fill the cross-section. Smoothing the flow path by placing the gate at the long axis of the part and minimizing through holes and turns in parts helps keep glass fibers aligned with the anticipated shrink in the mold cavity. Paying attention to these concerns helps your molder minimize the inherent risk with fiber fillers.
With much less regularity, low percentages of ceramic filler and mineral-reinforced additives are used to increase parts’ temperature resistance. Like glass, the fillers also strengthen parts—but, again, make them more brittle. The caveat with brittle parts is that they are susceptible to cracking or chipping upon impact.
Designers should think about the shape of the filler being used, as well. Glass fibers are long and slender; it has a direction from nose-to-tail. But mineral fillers tend to be flat flakes that are dimensionally different and have direction. Powder is symmetric; it packs well and is more evenly distributed within a part’s cross-section, so it reduces the risk of warp due to filler. Powder also typically does not change a uniform shrink rate to a linear shrink rate. It may only slightly reduce the shrink rate. Glass bead is another filler shape that is dimensionally different. Think of a ball pit: The bead is typically a ball shape; it stacks well and increases the thermal deflection of the material, but typically does not increase the structural strength like glass-fiber fillers do. The ball pit rests uniformly and minimizes the effect on uniform shrink rates. So again, it helps reduce internal stresses caused by the filler.
Thermally Conductive Resins
Protolabs is also now supporting some thermally conductive resins based on geometry and ease of fill. The CoolPoly product line of thermally conductive thermoplastics use a special proprietary filler to create their conductive properties, which land somewhere between plastic and metal. The materials work well for those looking to reduce weight in parts and increase freedom of design. Keep in mind, however, that challenging geometry like thin walls and small features may prevent the use of thermally conductive resins.
Some molders, including Protolabs, typically use a 3% salt-and-pepper colorant mix to give the final part a particular color.
Let's switch gears from additives that modify resins’ mechanical properties to ones that modify their cosmetic (e.g., colorants). Many molders offer a limited selection of colorants—Protolabs has about 45—that can be added to a base resin, usually at no additional charge. Thermoplastic base resins primarily consist of black, natural, and clear; colorants can be added to the latter two. Protolabs generally employ a 3% salt-and-pepper mix, with smaller percentages for transparent resins such as polycarbonate. The mix is not an exact color match. Even though a material can accept a particular colorant, part colors remain approximate. Some modelers will let you know or find out (using an online app) which colorants work with which materials.
When a product requires an exact color match or the use of several additives to ensure parts and resins perform in the intended environment, designers need to supply molders with a pre-compounded resin. Pre-compounding means a resin supplier (such as polyone.com or RTPcompany.com) mixes all of the additives into one pellet, ensuring uniform distribution and color. All pellets are the same in a pre-compounded material, rather than the salt-and-pepper mix that has a random dispersion of pellets.
Molders are not necessarily material experts; however, they have a well-rounded understanding of commonly used materials based on industry and geometry. They can suggest alternatives or two or three different resins that may work. Simply running a few resins for testing will assist in fine tuning the geometry and material selection.
Tony Holtz is an applications engineer at Protolabs.