The quality of plastic injection moulded parts depends on a variety of factors; these typically include:
- The design, construction and finish of the mould tool and mould tool inserts
- The correct choice of plastic resin materials
- Control of the injection moulding process, in terms of criteria such as temperature, pressure, injection speeds and flow rates.
Getting some or all of these factors wrong will cause a range of defects in injection moulded parts, ranging from sink marks and vacuum voids to weld lines and jetting.
Some of the most common defects found with injection moulded parts are caused either by incorrect cooling during the injection moulding operation, or by pockets of air or gas trapped in each moulding as it is formed.
1. Cooling defects
Incorrect cooling typically causes defects such as warping, especially in injection moulded parts with thin or narrow profiles. These can make it difficult to design mould tools with efficient cooling channels, which follow the exact tool profile; the result is that different areas of the tool surface, and thus the moulded part, will cool at different rates.
2. Gas defects
Another common defect with injection moulded parts is caused by gas pockets that become trapped in the molten plastic before they can be expelled through the split lines of the injection mould tool. The results include scorching, burning, pin holes and poor quality part finish.
Although there are various methods of eliminating the problem of gas traps, perhaps the best option is to engineer injection mould tool inserts that incorporate gas venting.
You can read more about our vented injection mould tool inserts here, or watch our video below.
3. Flow or wave lines defects
These are generally caused by changes in the flow rate of the molten plastic as it moves through the injection mould tool. Flow lines are often found where the plastic has to follow curved or shaped contours or pass through areas with varying wall thicknesses. Flow lines can also be caused by injection mould pressures that are too low.
4. Sink marks
These typically appear if the molten plastic had not been given sufficient time to cool while in the injection mould tool. They can also form if the injection moulding pressure is too low or if there are inconsistent mould temperatures.
Jetting is often the result of poor temperature control, where the melt temperature falls too low during the injection process, causing the molten plastic to become increasingly viscous as it flows through the mould tool. As this occurs, the viscous material begins to stick to the mould surfaces, while fresh material continues to be injected into the mould tool; this pushes the semi-cooled material through the mould, creating drag marks on its surface.