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Plastic Injection Moulding Design Guide: Part I

Fresh product from injection moulding machine

Wheatley Plastics works closely with clients to make sure their plastic product designs are optimised for cost efficiency and an accelerated manufacturing cycle.

To make sure that the prototyping runs smoothly, we use a set of product design guidelines which help us get things right the first time.

We will share the first part of these design guidelines in this article. But before that, we will talk about the plastic injection moulding process and why it could be right for your manufacturing project.

What is plastic injection moulding?

Injection moulding is the most common technique for the mass-production of plastic products.

It is used to make a whole range of simplistic and complicated plastic products, including:

  • Disposable razors
  • Washing up bowls
  • Bottle lids
  • Medical products
  • Tool housings
  • Plastic buckets
  • Toys
  • Garden chairs
  • Car dashboards and bumpers

The straightforward process involves melting down thermoplastic pellets, injecting the molten pellets into a mould and cooling the molten material down until it is strong enough to stand up on its own.

Injection moulding is popular because of the low cost per unit when manufacturing in high volumes. Once the design is finalised and the mould created, injection moulding provides good repeatability and flexibility in manufacturing. It can also be used to manufacture to fine tolerances.

But it isn’t with drawbacks. Compared with alternatives like 3D printing, plastic injection moulding comes with significant upfront costs.

While 3D printing can create any plastic product from a simple computer file, plastic injection moulding requires the creation of a solid metal mould. And the cost of prototyping and creating this mould can run into the thousands.

Injection moulding also limits the complexity of plastic product designs. Despite the popularity and versatility of the process, it also means that there are some important design constraints that you need to be aware of. We will explore some of these constraints in the rest of this guide.

Use uniform wall thickness

Of all the different parts of the process of injection moulding – heating, injection and cooling – the cooling cycle is arguably the most important.

If you don’t get the cooling right, then it can create a lot of problems including warping and sinking.

When the product cools, it contracts. If the walls are of a uniform thickness, then the entire part will contract at the same rate – uniformly shrinking away from the mould so that it comes out smoothly.

But if some of the product walls are thicker than others, or have thick and thin parts, then the thicker areas will cool more slowly. Which can cause warping and sink marks. Solving these problems can be very expensive in terms of retooling costs, so it makes sense to try and avoid them in the first instance.

If your product needs to have walls of different thicknesses then you need to make sure that the transition between the thicknesses is as smooth as possible, in a way that allows the melted plastic to flow evenly inside the cavity.

Wall thickness should be between 1.2mm and 3mm to avoid some of the most pressing plastic injection moulding problems.

Generally speaking, designers will want to keep walls as thin as possible while still retaining the strength and quality of the product. This approach can save money, not only because you are using less plastic material, but also because thinner walls cool faster – shortening the mould cycle time and reducing machine costs.

Different materials have different tolerances and so come with different recommended thicknesses. Here are just some of the most common thermoplastics used in plastic injection moulding and their tolerances.  

Material Recommended
wall thickness [mm]
Polypropylene (PP) 0.8 – 3.8 mm
ABS 1.2 – 3.5 mm
Polyethylene (PE) 0.8 – 3.0 mm
Polystyrene (PS) 1.0 – 4.0 mm
Polyurethane (PUR) 2.0 – 20.0 mm
Nylon (PA 6) 0.8 – 3.0 mm
Polycarbonate (PC) 1.0 – 4.0 mm
PC/ABS 1.2 – 3.5 mm
POM (Delrin) 0.8 – 3.0 mm
PEEK 1.0 – 3.0 mm
Silicone 1.0 – 10.0 mm

Round out edges

One of the issues with injection moulding is that the shapes you can mould are restricted. Because the molten plastic needs to flow smoothly through the whole mould, it is important that you design rounded edges rather than sharp bends.

Molten plastic material doesn’t flow easily around corners and won’t fill out all of the cracks and crevices that you need to make sure your product is strong and durable. If a product does have sharp corners, then these are likely to stress and break if the product is bent around too much.

For interior edges, your plastic product should have a bending radius that is at least 0.5 X the wall thickness. For exterior edges, you need to add this interior radius to the wall thickness to make sure it is the same thickness the whole way around.

Hollow out thick sections

Is your plastic product made up of large cross-sections of plastic? Plastic injection moulding is not particularly well suited to creating these types of products because of the time it takes for these thick sections of plastic to cool down. If you don’t account for this in your designs, then the cooling cycle can be very long and it can lead to a number of problems including warping and sinking.

Fortunately, there are ways that you can solve these problems in the design. The best way to retain the strength of your product while reducing the overall plastic load is to use hollow sections with structural ribs to support internal and external walls.

These ribs should one be half as thick as the maximum thickness of the external wall.

Use a draft angle

Once a product has been moulded and cooled the final process is ejection from the machine. This is completed with ejection pins that push the product out of the mould (this is what creates the small circular marks on injection moulded products).  

When products have sharp 90-degree angles, then it is difficult to eject the products successfully because of the extra friction created between the plastic and the mould.

The best way to correct this is by introducing a draft angle – a tapered angle for moulded parts running perpendicular to the parting line. By using a draft angle of a few degrees it reduces the vertical friction on the cast walls meaning that the product should slide out nice and easily.

Most plastic injection moulded products should have a draft angle of at least one and a half or two degrees. This will rise up to a maximum of around five degrees for taller products.

For more information about Wheatley Plastics design and manufacture service, speak to a member of the team. Call: 0161 477 2800