Injection Moulding of plastic parts
Injection moulding is a recognised production technology usually associated with high production volumes and long leadtimes. However, since early 2000 there has been an increasing supply of 'rapid tools', some produced using additive technologies like DMLS others machined from aluminium and steel. Whilst some of these tools have the potential to meet production quantities, their primary focus is high volume prototyping or prototyping where the production material is a necessity.
The growth of Far Eastern tooling has also provided a source of 'lower cost' tools. Thus from a prototyping perspective, injection moulding costs can come down to challenge some high quantity vacuum casting applications and delivery can come down to 2 weeks or less. These two issues cannot always be achieved simultaneously, however injection moulding must now be considered as a prototyping solution with the potential to also address early production requirements.
The greatest upside to using this process is that it delivers the production intent material and, once the tooling is in place, parts are available quickly and at low cost.
In considering injection moulding as a prototyping solution, there are many factors that have to be addressed and many alternatives that must be considered. The most important issue to consider is whether the production intent material is really a requirement, if it is, then understanding the available time and attention to the CAD model are both paramount.
Tolerances vary depending on material, filler, tooling strategy, geometry etc. It is always recommended to identify critical dimensions and make these available at the quotation phase.
Minimum Feature Size
With the advent of micro moulding any comments on minimum feature sizes became redundant! This is linked to a number of features in the geometry as well as the tooling strategy selected.
Not applicable to injection moulding unless using an additive process like DMLS. Even in these situations it is almost always necessary to dress out any layering to achieve good mould release.
The most common sized moulding machine is around 80 tonnes, and access to presses up to 250 tonnes is straightforward. From 250 to 1000 tonnes access becomes more difficult and more expensive, but remains feasible.
Negligible post processing is usually required, unless quantities are sufficiently low that some features are to be post machined to minimise tooling costs. Resins are pigmented and any texturing is included in the tooling.
Clear Parts using Injection moulding
Clear parts can be injection moulded. For full details view the injection moulding of clear parts page.
Flexible Parts using Injection moulding
Flexible parts can be injection moulded. For full details view the injection moulding of flexible parts page.
This process has two distinct elements, the tool manufacture and the subsequent plastic injection. The latter, whilst being subject to many variables, varies little in principle. The objective being to ensure that sufficient molten plastic is injected into the cavity to ensure a complete fill, and in a way that ensures a high quality moulding results.
The tool however can be made in many different ways from a variety of materials. By far the most common is to use machining (and EDM) to cut aluminium or steel. Other techniques involve the use of additive processes like DMLS to sinter a core and cavity, or even Stereolithography to produce a resin tool. Each approach has accuracy and tool life implications and needs to be carefully chosen to avoid disappointment.
Some critical issues to clarify before searching for tooling are
- The realistic leadtime that you seek. Speed will cost a premium, so an accurate appreciation of the available leadtime after making allowances for the time to raise a purchase order, is a necessity.
- The number of parts required both initially and over the life of the tool. This is important as if the initial batch is small enough it may be more economic to meet each requirement from a different solution.
- Critical dimensions and their associated tolerances as these may preclude some approaches immediately.
- Textures, are they required and if so where and to what specification.
- Changes will frequently occur and most suppliers acknowledge this. However, if there are recognised areas of high risk within the tool, these should be highlighted to the supplier. This may result in a different approach and save time later on, if changes are indeed required.
- Materials, evaluating different materials (or fillers) may be a possibility so long as there is an appreciation of the effect this can have on shrinkage and hence dimensional accuracy.
If you are new to injection moulding it is best to involve a supplier from an early stage as this will ensure that the design matures as efficiently as possible and avoids any last minute delays.
To review your project requirements please contact Plunkett Associates