Injection Moulding: A Guide to Plastic Parts


Are you embarking on an injection moulding project? Before you get to the actual moulding process, we have devised some ways to make sure this incredibly useful system goes smoothly, to save you money and time in the long run.

Here are 8 things that are useful to know when you’re making plastic parts. There are lots of things to learn about the process; this is a good starting point.

1 – Choosing the Right Material

You don’t need to be an expert to choose the right material for the job, but it’s important to do your research. What precise type and quantity of materials do you need? Consider the flexibility, rigidity and elasticity you require for the application.

2 – Think About the Parting Line

When you create a plastic part with injection moulding, you have to make two parts – halves that will be joined together once set. Once the liquid plastic is poured into the mould, it needs to set and be injectable. That is, each space that the plastic fills must be shaped in a way that allows ejection, otherwise, you could risk some of it getting stuck.

3 – Beware of Warping and Shrinking

Warping and shrinking can occur during the cooling period of your part. Warping happens when there is too much pressure placed on an area of the part, which causes the shape to become distorted. Shrinking occurs when the mould has varying dimensions, causing the cooling process to work on it inconsistently. You can help prevent these by taking into account the nature of the materials, tools and machinery used for the job.

4 – Aim for Consistent Wall Thickness

When making your parts, you need to ensure that its walls are relatively even in terms of thickness. Too many inconsistencies in the wall can lead to problems and make the product tricky to eject. The optimum thickness of the walls will depend on the shape and size of the part you’re making.

5 – Be Careful About Undercuts

Some parts need undercuts to work properly, but this can make ejecting difficult. Simple ejection isn’t possible with undercuts – you’ll have to use other methods of retrieving the piece intact from the mould. This can be done with additional tools, but if you can avoid undercuts altogether, by designing a part that doesn’t require them, you’ll save yourself the effort.

6 – Keep Corners Smooth

Sharp edges and corners lead to more stress, which can cause the part to crack or distort. Using radii will help smooth out sharp edges and reduce the stress. Also, any corners you do have should aim to be no less than 25% as thick as the part’s walls.

7 – Structural Integrity

With thin walls, you can use ribs to improve their structural integrity. You can also use projection and bosses to prevent collapse, but it needs to be done properly to avoid mould problems. You can also use the draft to angle the walls, thereby preventing weakness that can arise from walls that are completely vertical.

8 – ​​​​​Insertion Options

Adding inserts means either moulding them in (which costs more time and money) or adding them afterwards in a secondary operation. Choose the best method for you and your budget.

This is simply a brief overview of what you need to think about when making plastic parts with injection moulding. We hope you’ve found it useful, and that you can use it as a checklist for future projects. For more information see Active Plastics Injection Moulding NZ.

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