Top ten design tips for moulded plastic components

Written by: Tom Austin-Morgan | Published:

With the advent of rapid prototyping services being able to create components within a couple of days, a lot of the risk, delays and up-front costs incurred during the critical stages of a product's development can be mitigated – easing the transition from a prototype into a full production run.

The following article highlights key considerations for engineers when designing plastic components that are intended for this type of injection moulding.

1. Material decisions

Selecting the correct thermoplastic resin is crucially important to the form, fit and functionality of a moulded component. There will always be a trade-off with the many different pros and cons for each material's properties – such as shrinkage, impact and chemical resistance, mechanical strength, flexibility, temperature resistance, optical properties and so on. But with the list of available materials running into the tens of thousands, there's plenty of choice available.

2. Looking good

[pic – turntable]

The moulding process can bring a host of cosmetic flaws, but all's not lost. With careful adherence to some basic rules, they can be virtually eliminated. Many of these factors are discussed in this article and when discussing a product's requirements with a moulding specialist, it's important to raise any potential problems before orders are finalised.

This could be something as simple as changing the thickness of a rib, moving the position of the gate and ejector pins or adjusting the material selection - but a few small changes early in the process can make all the difference to the final product.

3. When in doubt, draft It

Ensuring that a component has a draft (tapered sides) helps prevent the component being damaged as it comes away from the mould and so avoids drag marks. Use a CAD package's draft analysis tools to provide a component design with sufficient draft to aid clean ejection. The surface finish of the component will also have a bearing on the requirements, with a surface containing a light texture needing three degrees of draft, compared to at least five degrees of draft being required on an item with a heavily textured surface. Proto Labs has a rule of thumb for part design: "When in doubt, draft it".

4. That shrinking feeling

Although the rate of shrinkage in a moulded component will vary with the material selected, there will always be some shrinkage as the resin cools. The cavity in the mould has to be oversized to take this into account; however, other factors can cause shrinkage issues.

Shrinkage is proportional to resin depth, so thicker walls are more susceptible to sink marks due to the parts cooling from the outside in. Also air and gas bubbles can be formed in thicker areas, potentially causing structural weaknesses and warp is also more likely due to

uneven shrinkage. When adding ribs into a product, it's important to make them no thicker than 60 percent of the wall thickness they adjoin to prevent sink.

Designing out excessive thickness to provide a more consistent wall thickness not only reduces the likelihood of these problems, but will also reduce the amount of resin required and cut down on the weight of the component.

5. Stress reduction

Wherever possible, corners need to be generously 'radiused' to prevent the concentration of stresses. This improves the components ability to withstand load and helps prevent warping in its geometry. Sharp corners also adversely affect the flow of resin during the moulding process, giving rise to the potential for incomplete filling of the mould.

It's important to bear in mind that the outside corners of a finished component are produced by the inside corners of a mould. Moulds are machined by a vertical milling cutter, which simply cannot cut sharp inside corners. The radius of the mould's inside corner cannot be smaller than the radius of the cutter – which will vary according to the depth of the cut.

Although rapid prototyping specialists can machine sharp external corners when creating the mould, the resulting sharp inside corners on the component can cause excessive stress, so are best avoided.

6. Word up

When designing text, symbols or logos onto a moulded component, it's better to have it raised above in the final component – as it's milled into the mould's surface. The choice of style is also important, avoiding the use of serif fonts (those with 'feet') and keeping the size to at least 20 points or more and raised no more than 0.51mm in the finished component.

The bold versions of Century Gothic Bold (default in SolidWorks), Arial and Verdana are all good choices.

7. Holding it together

Being able to incorporate an integral living hinge into a plastic moulded component is a wonderful thing, but only if it can withstand the degree and frequency of bending required to fulfil its intended use. Polyethylene and Polypropylene are the best resins for living such applications.

Getting the thickness of the hinge is critical; too thick and the stress on the outer surface is too large, too thin and it may tear. Hinges can present challenges when filling the mould, so gate placement is an important consideration that Proto Labs' team can advise on.

8. More than a feeling

The surface finish of a component can have a big effect on its functionality. From providing grip to hiding finger prints, there are many levels of polish finish available and two textures. The textures are created by bead blasting the finished mould, with T1 being light bead blast and T2 being a medium finish.

The resin type used will again have a bearing on the finish, so care must be taken when making the material selection. The issue of sink rears its head again as textured surfaces can highlight sink, creating serious cosmetic problems and draft may need to be increased to avoid sticking in the mould.

9. Cams at the core

Being able to add in undercuts into a moulded part is just one of the many uses that cams have during injection moulding process. They can also provide areas of texture, flat faces for mating surfaces a part number or even a logo.Tall thin parts with a requirement for a core are the perfect application for the use of cams – particularly as in some cases, sufficient draft would not be possible. By rotating the component and inserting the core with a side-action cam, the need for a draft can be all-but eliminated.

10. The flexible approach

Injection moulding using Liquid Silicone Rubber (LSR), can produce highly-flexible components that are strong, elastic and have excellent thermal, chemical and electrical resistance. Although the design considerations for moulding with LSR are similar to thermoplastics, some adjustments need to be made. Although it's perfect for wall sections as thin as 0.25mm, rib thickness should be 0.5 to 1.0 times the adjoining wall thickness and sink is almost non-existent in LSR.

Shrink rate is high and it tends to flash very easily during moulding, something that manufacturers such as Proto Labs can help reduce by incorporating additional features into the mould design – such as avoiding the use of ejector pins by manual retrieval of the component from the mould by the operator.

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