How to design for injection moulding – Top 5 Tips

The injection moulding process can often be an afterthought when it comes to designing parts. To ensure you don’t unnecessarily delay your next project, here are our top five tips for designing parts that are fully compatible with the injection moulding process from the start.

1. Types of plastic for injection moulding
Although the choice of materials will be determined by the nature of the plastic part you’re designing, there are other considerations that may require a compromise between technical characteristics, such as temperate or chemical resistance, mouldability and cost. In many instances it’s possible to use plastics that combine outstanding mechanical or chemical properties with excellent mouldability. Bear in mind that there are 45 groups of polymers, with over 80,000 different thermoplastic and thermosets, so there will almost always be a number of potential materials from which to choose for each application.

We’ve considerable experience of working with many types of plastics for injection moulding, ranging from standard polycarbonate and polypropylene, to exotic plastics such as PEEK (Polyetheretherketone), PPSU (polyphenylsulfone) and biodegradable materials. You can read more about these specialised materials for plastic injection moulding here.

2. Injection moulding gate location
Normally the location of gates in the injection mould tool are determined by the tool designer, using the latest CAD and flow simulation software. Gate location is critical for successful injection moulding. It affects factors such as part aesthetics and surface finish, warpage, tolerances, wall thickness, moulded-in stresses and the physical properties of the finished part.

Careful consideration should be given to the location and type (fan, edge or sprue) of gates when designing parts for injection moulding. What may appear possible at the part design stage may subsequently prove problematic when it comes to the moulding process.

Again, early-stage collaboration with our technical team will enable the style and positioning of gates to be optimised, without impacting the design of the injection moulded part, or the injection moulding process itself.

3. Critical tolerances
In common with all manufacturing processes, injection moulding offers exceptionally high levels of precision. There are, however, several factors that can influence the dimensional tolerances of finished injection moulded parts. These include:

  • The choice of materials – different plastics shrink at different rates as they cool in the mould and after ejection
  • The design and CNC machining of the injection mould tool
  • The quality of the injection moulding machines, especially the positional accuracy of moving parts and automation equipment, and the ability of process control systems to maintain consistent levels of temperature and pressure
  • The skills and experience of machine operators.

It is important to consider these factors at the outset when designing for injection moulding. At this stage it will be cost-effective to adapt the part design, construction of the mould tool, choice of materials or injection moulding process to ensure that the desired level of part tolerances are achieved. Ignoring these considerations until the last minute will inevitably result in production delays and additional costs.

4. Sink marks and other injection moulding defects
Whenever a polymer melts, flows under pressure and subsequently cools there is always the risk of possible defects. This is especially true where plastic injection moulded parts with complex geometries are involved.

Common defects include:

  • Sink marks – normally appearing on the surface where underlying ribs or reinforcements are used
  • Flow lines – caused by inconsistent cooling rates across a plastic surface
  • Burn marks – typically caused by the resin, or air trapped in the mould tool, overheating
  • Warping – arising from too rapid cooling
  • Jetting – caused by excessive injection moulding pressure.

To learn more about other common defects that occur with plastic injection moulding read our blog post: 5 plastic injection moulding defects 

5. Partnering with your plastic injection moulder early in the design process
There are of course many other considerations when it comes to designing parts for injection moulding: shut-off angles, draft angle orientation, surface texturing, and the need for secondary finishing operations, such as assembly, testing or printing, to name just a few.

The common – and most important – factor throughout is to work closely with our technical team from the outset. This will ensure that your injection moulding project is completed successfully, on time, within budget and to the exact specification required by your product designers.

Design support
We offer a comprehensive range of technical support services to help our customers design parts that meet their exact product specification, as well as the specific demands of the injection moulding process.

For best results, we prefer to work with you from the early stages of each project. That way, we can offer advice and support during the design phase, to help you improve the performance and characteristics of your injection mouldings, while optimising the subsequent production process and cost for each part. Contact us to see how we can help you.

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