The actual materials used in the injection moulding process do not consist of just pure polymers.They frequently include numerous additives, some of which improve the processing characteristics of the material while others improve the performance of the final moulded product. Some additives modify the flow behaviour and shrinkage of the polymer. Described below are common additives and their effects on polymer processing behaviour.
Nucleating agents
Flow promoters
Fibrous reinforcement
Antioxidants
Fungicides
Particulate fillers
Flame retardents
Blowing agents
With semicrystalline materials, particularly Nylon, grades are sometimes available which contain nucleating agents. These additives promote the rapid and uniform development of the crystalline regions. This helps to reduce the cooling time needed to achieve solidification, and to give more consistent physical properties in the finished moulding.
Flow promoters are used by manufacturers in their materials to improve the flow of a material and in many cases a lubricant can be used for this purpose.
Many materials are available with fibrous reinforcement, glass fibre being the most popular, although carbon and asbestos are also used to a limited extent. This type of additive greatly improves the tensile strength, rigidity and heat distortion characteristics of the moulded product. Different grades of fibre-reinforced materials are available with varying fibre content and fibre length. The addition of fibrous reinforcement frequently enables much shorter cooling times to be used owing to the moulding having much improved strength at high
temperatures. However, glass fibres in particular are very abrasive and may cause rapid wear in the mould, barrel and screw assembly.
With all fibrous materials the final product may be subject to irregular shrinkage. This is caused by some of the fibres becoming highly aligned in certain directions owing to the flow of materials through the cavities. Where this alignment occurs, the part will display a much higher shrinkage in directiont at right angles to the fibre alignment, compared with the shrinkage parallel to the alignment. Materials that behave like this are said to be anisotropic.
Certain materials require antioxidant additives to protect them from progressive deterioration when used for components that need to survive long-term exposure to elevated temperatures. Some plastics, plasticised PVC for example, are susceptible to
attack by microorganisms when exposed to warm, humid atmospheres. Adding fungicides to the moulding compound can reduce this problem.
Particulate (or powder) fillers can also be incorporated. These improve the stiffness of the component and reduce overall shrinkage. Fillers of this type include talc, chalk, limestone
powder and clay, and other additives are used to modify the surface characteristics of the material. Most polymers are not coloured, so that when coloured components are required, dyes or pigments have to be added. Despite the fact that these additives are used
in very small amounts they sometimes have a marked effect on processing conditions. For this reason it is quite common to find that when moulding a component in one specific material, but in a range of colours, different processing conditions may be needed for
certain colours.
The majority of plastics burn far too readily. In an attempt to overcome this problem, many materials are available with flame retardant additives, which make the material more difficult to ignite, or less likely to burn so fiercely. Unfortunately, quite a number of
additives of this type in use today cause considerable problems for the moulder in that they tend to accelerate degradation of the polymer as it passes through the cylinder and in some cases corrosive by-products are also released.
Some moulders are required to produce mouldings that have a cellular or microporous structure, for example, cushioning, thermal insulation or lightweight rigid thick-section
components. There are several techniques that can be used to produce this type of component, some of which require a machine with special adaptations so that gas, under high pressure can be injected into the melt as it fills the mould. Similar results can be
obtained on standard injection moulding machines if an appropriate blowing agent is added to the moulding material. This additive is a substance that breaks down to produce bubbles of gas as the melt undergoes a rise in temperature due to rapid flow of the melt
through the restriction of the gate.
Walsh's Plastic consulting. 