Design for injection moulding - 5 considerations for a quality part

Design for injection moulding – 5 considerations for a quality part

Plastic injection moulding is perfect for high volume production of plastic parts and components for a wide range of industries. This is largely due to the fact that once in production, it is an extremely efficient, cost effective and quick method of making parts. In addition, a wide range of materials can be used for injection moulding with different plastics providing particular properties, making the process extremely versatile for producing high quality parts with minimal finishing required and low waste.

To optimise cycle time and part quality, there are 5 main considerations when designing for injection moulding:

1. Wall thickness
To ensure your part isn’t at risk of warping and cracking as the plastic cools down, it’s important to maintain a continual and even flow of molten material around the mould tool. To achieve this wall thickness should be kept as uniform as possible.

By having an even wall thickness your components will be stronger and at less risk of having unsightly sink marks or air pockets (voids) that can be caused by walls being too thick.

Having consistent wall thickness also means that the material will fill the mould cavity much easier which improves cycle times. Different materials have different recommended wall thicknesses which is why it’s always best to seek the advice of your plastic injection mould company when designing for injection moulded parts and deciding on which materials to use.

Wall thickness can also have an impact on costs and productivity. A thin wall obviously uses less material, weighs less and cools down much quicker so cycle times are reduced and more parts can be produced over time. These combined factors can deliver substantial cost savings particularly for high volume production of your components.

Producing moulded parts with differing wall thicknesses is possible but it’s important that any changes in thickness are very subtle and your moulded part may need support structures such as ribs. Need advice? Talk to one of our design for injection moulding experts.

2. Material choice
Material choice doesn’t just have an impact on the appearance or strength of your finished part. Other reasons to consider material type include:

  • Flow rates
  • Drying times
  • Special features you are including in your part design
  • Whether post-production machining will be required
  • Hygiene regulations your component needs to meet
  • Cost

There are many points you need to take into account when choosing which polymer to use so it’s a discussion you should be having at an early stage in the design for injection moulding process.

3. Round edges
Designing in round edges, rather than sharp corners, ensures that your chosen material will flow smoothly around your mould to produce a part that is strong enough to withstand warping. Sharp corners will restrict material flow resulting in stress during the plastic injection mould process and weak spots in your finished parts.

Our injection mould design experts recommend a minimum interior radius of 0.5 times the wall thickness and a minimum exterior radius of 1.5 times the wall thickness to provide a high-quality part that can easily be extracted from the mould during production.

The importance of avoiding sharp, 90 degree, corners shouldn’t be underestimated as they are one of the key contributors to part failure. However, if it’s not possible to use purely round edges in your design, an experienced injection moulder like OGM will be able to advise you on the best options to produce a component that meets all of your requirements.

4. Ribs
Ribs introduce additional strength to plastic injection moulded parts, offering greater rigidity without adding thickness. They are used to support the walls or replace thicker walls to prevent issues such as warping, sink marks and air pockets also known as voids.

Rib thickness should be 40-60% of the thickness of the wall they are supporting or they can also be placed away from the walls in completely separate areas of the mould. By using short ribs it is easier to fill the mould with the molten plastic and the part can also be more easily removed from the mould once it has cooled.

If ribs are needed to maintain the integrity of your part and make the manufacturing process as simple as possible, your injection mould designer will be able to advise:

  • how many ribs you will need
  • how thick and high they should be
  • how far apart they should be positioned (if multiple ribs are needed)

5. Draft
Draft is our fifth consideration and there’s a reason this is the one we are finishing with. Draft is important at the end of the injection mould process when your part is ejected from the mould tool. As the molten plastic starts to cool and harden it shrinks into the walls of the tool. Without draft, considerable force would be needed to extract the hardening part. This could cause drag or punch marks on your components or damage the mould itself.

The amount of draft, and the angle it is applied at, depends on:

  • the size of the part
  • whether it has a textured finish
  • the size and shape of the mould
  • the material used

By calculating the correct draft required, your design for injection moulding partner will be able to improve the quality of your part as well as reduce the production cycle times.

The different variables and parameters above are reasons why design for manufacturing for the injection mould process is so important. 

By speaking with experts like our team at OGM at the beginning of your project, you will be able to select the best design and materials for your product specification.

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