Key points
The plastic injection moulding process is capable of dealing with a great range of polymeric feedstock to create complex yet sturdy products. However, there are many instances where the material is non-virgin stock, and it is often the case that a filler or additive material will be mixed in with the polymer feedstock for specific reasons.
These break down as:
​Improving mechanical properties such as tensile strength, impact toughness and crack resistance
In this case, short-strand fibres such as glass or carbon may give the best mix of performance and rigidity but almost any powder filler will help with these properties to some extent.
To increase the bulk of the final product
This is the case, especially with more expensive feedstocks. Including cheaper fillers in with the feedstock can help make the final product cost effective and easily handled materials without significantly compromising the mould-ability or impacting its mechanical properties. Powdered materials used for this purpose can include fumed silca, calcium carbonate powder, and talc.
To simulate other materials that may come into contact with the final product
This is especially the case with added metallic powders and even some types of stone to give certain desired properties, such as decreased wear or even instilled magnetic attraction. By adding a fine powder of the appropriate material, the final properties of the product can be subtly altered while retaining the polymeric properties too. Many powdered metals can be used for this purpose.
To reduce density
Using small, hollow glass beads is the most effective means of reducing density without affecting mass.
To impact opacity and UV resistance of the product
This can include colourants and materials aimed at restricting the amount of non-visible light that can pass through a product as a protective system.
To aid safety features
The majority of retardant filler materials are typically phosphorus-based flame retardant systems. They act in the gas and condensed phase and are characterised by materials such as aluminium diethyl phosphonate in conjunction with synergistic additives like melamine polyphosphate.
Process improvement additions
Processing cycle time and mould release can be significantly reduced by the addition of lubricating materials which help reduce the tendency for the hot materials to adhere to the mould surface. Other additives may increase the structural rigidity of the moulded part so that it retains its form and can be handled at higher temperatures, allowing the mould to be closed and refilled quickly.
Softening materials
Polymeric materials that experience low flow, even at higher temperatures and the pressures associated with an injection moulding machine can be processed by the addition of different polymeric materials and even vitamin E.
There are several considerations to take into account when including a filler material in your active feedstock.
For instance, adding any inert filler will also tend to increase cure and demould times. Fillers may also contribute to a reduction in the overall heat generated and could lead to amalgamation problems and unbalanced curing in the final product, depending on its forms and sectional thicknesses, so a full and detailed analysis of the thermal issues with the material need to be carried out in a suitable stress analysis or CAD program first.
Additives to plastic injection moulding feedstock does represent some processing issues, not least of which is ensuring that the addition is distributed equally throughout the main material. Failure to ensure this can result in the final product having pockets of poorly adhered material and a resulting severe reduction in the strength of the piece.
Unfortunately, even the smallest areas of inhomogeneity can become an area of high stress and ultimate failure, so thorough mixing of both feedstock and addition is essential, both in the reservoir and the heated barrel of the machine, and on into the mould itself.
There are many common additives used in plastic injection moulding to enhance and change the final properties of the product being created.
However, regardless of what the additive is being used for, plainly it must not affect the final product, and since a specific application may call for multiple additives to be incorporated, it is essential that any potential interactions between them and the basic polymer material need to be fully investigated before use.
