Plastic injection moulding is part science and part art. If things go wrong, and there are many things that could, it will result in a massive waste of money, time and materials.
While most manufacturers will ensure the best quality product and take note of any defects beforehand, we will highlight a few issues you should also be on the lookout for when inspecting a prototype of your design.
- Flow lines
This is similar to the erosion water leaves on surfaces such as shallow grooves that flow in a certain direction or is a slightly different colour to the rest of the parts. This can be solved by increasing the heat of the plastic and the pressure inside the mould.
Sometimes a prototype will arrive with sides that are not quite straight, no matter how many times you try to smoothen it. Warping happens most often because parts of the plastic cool faster than others. This creates tensions in the product, a phenomenon called uneven stress.
When released from the mould, the plastic will warp to release some of that tension. Several things can fix warping, including longer cooling times, more uniform moulds, and using plastic resins that are less prone to shrinkage. The pressure and fill rate can also cause warping.
- Sink marks
Sink marks are slight indentations in the prototype which should not be there and occur most often around the thicker parts of plastic. This is often unsightly and gives of a negative impression of the quality of the product.
Sink marks are usually caused by a lack of sufficient cooling. Since the thicker parts of a plastic product will take longer to cool, this is where they are most likely to appear. A lack of pressure can also cause sink marks. Lower moulding temperatures and higher pressure inside the mould are ways of avoiding sink marks.