Polymer Fillers and Additives

Learn how additives like glass and colourants can improve cosmetic and functional properties of moulded parts

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.

At Konlida, our injection moulding services support a wide range of filled and unfilled resins across multiple industries.

Glass Fibres

Glass fibres are the most commonly used additives in plastic injection moulding. 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.

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

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 available include: ABS, nylon, acetal, polycarbonate, liquid crystal polymer (LCP), PBT, PET, PPS and high‑performance resins like PEEK and PEI.

The addition of glass fibres also affects the moulding process. A fibre is a strand that is approximately 3.2 mm in length, depending on the extrusion. As the material flows through the cavity, the fibres align themselves in the direction of flow. As the flow approaches a core pin that creates a hole in the part, the fibres must separate and swim around the obstruction, changing the angle of the strands. 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 in the flow direction than in the transverse direction. This creates non‑linear shrink and exacerbates internal stress that increases warp risk. Unfilled resins typically have uniform shrink during cooling whereas glass fibres create different shrink factors in flow and transverse directions. 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 is 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 ensures proper part ejection without drag marks. Smoothing out the flow path by placing the gate at the long axis of the part and minimising through holes and turns will help keep the glass fibres aligned with the anticipated shrink in the mould cavity. Paying attention to these concerns helps minimise the inherent risk with fibre fillers. For a deeper look at how glass fibres affect mouldability and part performance, see our material alternatives for plastic injection moulding guide.

Ceramic and Mineral 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 is 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 minimises the effect on uniform shrink rates. So again, it helps to reduce internal stress caused by filler.

For applications where flame retardancy is a critical requirement—common in electronics and automotive sectors—our flame retardant materials and UL classifications guide provides essential guidance on material selection and compliance.

Thermally Conductive Resins

Konlida also supports some thermally conductive resins based on geometry and ease of fill. 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.

For guidance on selecting materials based on mechanical, thermal, and electrical property requirements, refer to the injection moulding material selection guide .

Colorants

Let’s switch gears now from additives that modify the mechanical properties of resins to ones that provide cosmetic modifications—colourants . Konlida offers a limited selection of colourants as well as custom colourant (based on a Pantone number) that we can add to a base resin. 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, 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. Our quoting system will automatically indicate which colourants are compatible with which materials.

The interaction between colourants and surface finish is discussed in our cosmetic defects in injection moulding guide, which covers how colour choice can affect the visibility of moulding flaws such as sink, knit lines, and swirl.

Pre‑Compounded Resin

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 pre‑compounded resin . Pre‑compounding means a resin supplier will mix all of the additives into one pellet ensuring uniform distribution and colour. All pellets are the same in pre‑compounded material rather than the salt‑and‑pepper mix that has a random dispersion of pellets.

For detailed material property data across different resin families—from commodity plastics to high‑performance engineering grades—see our materials comparison guide .

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