PLASTIC INJECTION MOULDING PART CHARACTERISTICS
At OGM we offer a precision plastic injection moulding service, with full design support to help you optimise the characteristics of each part, while maintaining the highest standards of quality at competitive cost.
Successful part and injection mould tool design can be a complex process, which depends on a range of factors. The most common of these are described below.
Factors that affect the quality of injection moulded parts
Wall thickness and sink marks
Ideally, the wall thickness of each injection moulded part should be consistent across the part, to ensure optimum strength. In practice, however, the design or shape of the part may mean that the wall thickness needs to vary.
It is therefore important to consider the cooling characteristics of the molten plastic injected into the mould tool. As the plastic cools it will shrink. Cooling takes place from the outside, which causes the walls to be drawn inwards; this effect can create sink marks on the surface of the part, plus voids and stresses within the wall structure.
This problem can be overcome in a number of ways: for example, by modifying the design to minimise thickness variations, incorporating ribs or bosses to conceal the sink area, or by adjusting the moulding conditions such as injection pressure and holding time.
Where sections of the part with different wall thicknesses meet it is important to design a transition between the two; essentially, this is a slope that connects the two sections, to minimise the differential cooling rates and reduce stress.
Incorrect wall thickness and variations in shrinkage can also cause warping to occur. This effect can also be influenced by the choice of material; Acrylic and Polypropylene, for example, can generally be used safely for thinner walls than ABS.
To ensure that each part is released effectively when the mould opens, the side walls of the mould are generally tapered in the direction of opening – a feature known as draft. Failure to design the tool correctly can cause parts to stick to the mould, resulting in cosmetic surface flaws or drag marks.
It’s therefore essential to calculate the correct draft angle, taking into account factors such as the rate of cooling, the surface finish required – textured finishes normally require greater draft angles – and the complexity of the part (all surfaces require a draft angle calculation).
Most injection moulded parts will have rounded corners. These help to improve the flow of molten plastic, allowing the cavity to be filled at lower injection pressures, and reducing the stresses within the part. Rounded corners also make the overall part stronger, by distributing stresses during moulding and cooling over a larger area.
Ideally, the corners on internal features should also be radiused to improve material flow and overall strength. In some instances, however, radiused corners can add considerably to the mould tool cost, without adding additional benefit; taking time to discuss this with OGM will help to ensure that you receive the best solution for each project.