October 2014 Design Tip

A Dash of Ingredients In the Thermoplastic Recipe: Glass Filling, Colourants and Other Additives

Thermoplastic resins naturally possess mechanical properties that, depending on the material, provide injection-moulded parts with strength, durability, impact resistance and other beneficial attributes. Nylon is strong. Polycarbonates have good temperature resistance. TPEs are flexible and absorb impact. Acrylic brings a high degree of transparency. Depending on a part's geometry and its application, the base resin may work great. But when the performance of the standard plastic needs to be enhanced, additive fillings such as glass fibres, ceramic or mineral reinforcements can be compounded in to materials.

Glass fibres are the most commonly used additives in plastic injection moulding at Proto Labs. Depending on the percentage of fill, glass fibres can significantly improve the strength and rigidity of parts versus corresponding unfilled resins, 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.

Fill percentages vary, but typically range from 13 to 45 percent, in both the plastics we stock and the customer-supplied resins we accept. Glass-filled resins at Proto Labs include: ABS, nylon, acetal, polycarbonate, liquid crystal polymer (LCP), PBT, PET, PPS and high-performance resins like PEEK and PEI.

Product developers often use glass-filled materials to increase the strength of their parts.

Product developers often use glass-filled materials to increase the strength of their parts.

The addition of glass fibres also affects the moulding process. A fibre is a strand that is equal to the length of the pellet. Standard fibres are about 3mm long, and long glass fibres can be around 9mm to 12mm in length, depending on the extrusion. If we think of the fibre as a fish and the number of fibres as a school of fish, as the material flows through the cavity, the fish are lined 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 must separate and swim around the obstruction. This changes the angle of the strand. So the more geometry that is in the way of the flow of resin, the more the strands are at random angles to each other. Glass fibres restrict the shrink rate of the base resin. That restriction is different nose to tail than when perpendicular nose to tail. This creates nonlinear shrink and also exacerbates internal stress that increases warp risk. Unfilled resins typically have uniform shrink during cooling whereas glass fibres create different shrink factors, flow and transverse. Anticipating that change in shrink becomes very 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.

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-degree angle. Uniform wall thickness helps the resin cool at the same rate. Draft allows your moulder to pack the part harder in the mould to fill the cross section better. Smoothing out the flow path by placing the gate at the long axis of the part and minimising through holes and turns in your part will help keep the glass fibres aligned with the anticipated shrink in the mould cavity. Paying attention to these concerns helps your moulder minimize the inherent risk with fibre fillers.

With much less regularity, low percentages of ceramic filler and mineral reinforced additives are used to provide parts with increased temperature resistance. Like glass, the fillers also bring strength to parts, but again cause them to become more brittle. The caveat with these parts is that they are susceptible to cracking or chipping upon impact.

Think about the shape of your filler as well. Glass fibre is long and slender; it has a direction from nose to tail. Mineral fillers tend to be flat flakes that are dimensionally different and have direction. Powder is symmetric - it packs well and it's more evenly distributed within the cross section of the part, so it reduces risk of warp due to filler. It 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 it increases the thermal deflection of the material, but typically does not increase the structural strength like glass-fibre fillers do. The ball pit rests uniformly and minimizes the effect on uniform shrink rates. So again, it helps to reduce internal stress caused by filler.

Proto Labs is also now supporting some thermally conductive resins based on geometry and ease of fill. The CoolPolyT 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.

Let's switch gears now from additives that modify the mechanical properties of resins to ones that provide cosmetic modifications - colourants. Proto Labs offers a selection of colourants that we can add to a base resin at no additional charge. Thermoplastic base resins primarily consist of black, natural and clear, and colourants can be added to the latter two. We generally employ a 3-percent salt-and-pepper mix (see video), with smaller percentages for transparent resins like polycarbonate. Note that the mix is not an exact colour match; even though a particular material can accept a particular colourant, part colours are approximate. If you upload a 3D CAD model, our ProtoQuote system will automatically indicate which colourants are compatible with which materials. Proto Labs may be able to source specific colourants for your needs, your Customer Service Engineer will be able to advise you.

Proto Labs typically uses a 3-percent salt-and-pepper colourant mix.

Proto Labs typically uses a 3-percent salt-and-pepper colourant mix.

When your product requires exact colour matches or the use of multiple additives to ensure your parts and resin selection perform in their intended environment, you will need to supply your moulder with a precompounded resin. Precompounding means a resin supplier like Plastribution or Albis will mix all of the additives into one pellet ensuring uniform distribution and colour. All pellets are the same in precompounded material rather than the salt-and-pepper mix that has a random dispersion of pellets.

Our team of Customer Service Engineers are not material experts; however, they have a well-rounded understanding of commonly used materials based on industry and geometry. They can help provide potential alternatives or suggest two or three different resins that may work. Simply running a few resins for testing will assist you in fine tuning your geometry that performs as needed in your product. For a more comprehensive look at thermoplastics and their properties, download our free Materials Matter technical brief.