Demystifying plastic injection moulding – Terminology Part 2

Demystifying plastic injection moulding – Terminology Part 2

In our second blog on demystifying the terminology of plastic injection moulding, we’re focussing on some of the key features of your finished part.

1. Snap fits
Snap fits are a lip and hook method of fixing two components together so that they hold on tight and lock in place. They are a quicker and more cost-effective method than using screws but they do create undercuts in your part design that provides challenges when ejecting your part from the mould. (see below)

There are many ways to make a snap fit. The design will depend on a number of variables specific to your project such as space available, material choice, snap and retention force and opening requirement. Ideally, with snap fits, your plastic injection moulder will create both interlocking parts to ensure that they work well together.

An example of where snap fits are commonly used is in battery enclosures for electronic devices but there are many, many more examples in plastic injection moulded products.

Snap fit

2. Undercuts
Undercuts make it difficult for your part to be ejected from the mould without damage during the injection moulding process. They occur when there is a groove, a recess or an overhanging feature in your product design. As part of the development process OGM will always work with you to try and design out undercuts as they add complexity and cost to your project.

Sometimes undercuts cannot be avoided as they are a necessary part of the design. This is particularly the case for more complex technical parts and if, for example, you need snap-fits to attach your part to other components.

During the design for injection moulding process, your injection moulder will discuss the different solutions to overcome or avoid undercuts on your design. These will include:

  • Shut offs – this is where the design is amended to remove material in the area under the undercut to create a slot or hole. This solves the problem by creating space to remove the part.
  • Moving the parting line – this is the simplest way to deal with an undercut and often works for designs that have undercuts on the exterior surface of the part.
  • Using bump-offs – this method may be used if the undercut is small and the plastic material used to mould the part is flexible enough to be “bumped-off” the undercut without damaging the part. In effect, the moulded part is physically forced over the undercut, deforming the plastic which then springs back into shape after removal.
  • Sliding cores – these are used when it is not possible to design around an undercut. Inserts are used to slide in and out of the mould as it closes and opens. Sliding cores therefore need additional space to operate effectively.
Undercut just part
Undercut - part only
Undercut part and tool
Undercut - part and mould

3. Screw threads
By including a screw thread in your part you will be able to attach it to another component whilst offering flexibility for future adjustments, removal or rapid assembly. Plastic injection moulded screw thread sizes are standard across industry so you can attach a wide range of components to each other in different materials such as plastic, metal or rubber.

There are male and female types of screw thread which require different methods of removal from the injection mould tool.

  • For the male thread, the mould tool is usually split in half down the centre of the thread for simple removal.
  • For the female thread, the piece of steel that moulds the inside of the thread must be unwound to release it from the mould. This usually involves hydraulics to rotate and eject the steel plug before the moulded part can be released from the mould tool.

Some common examples of plastic injection moulded screw threads include screw-on bottle caps for items such as soap dispensers or medical syringes with screw threads for rapid assembly. Sometimes metal inserts are screwed into the threads, one example is breathing masks where, when adjusted or tuned, the inserts have the vital function of controlling the amount of air or oxygen flowing into the mask.

Screw threads

To date, our injection moulding experts at OGM have never experienced a screw thread too small or too large; we have produced extremely small screw threads for medical syringes and large ones for automotive trailers.

We are up for a challenge, so if you think your next project will challenge our grey cells, give us a call.

Scroll to Top