Spotlight: Making the right choice with injection moulding

Materials World magazine
1 May 2018

Protolabs, UK, explains how to make the step from planning to production using injection moulding. 

You’ve spent months developing a product concept, secured funding from investors, and it’s time to deliver a product to your customers. Now what?

Making the leap into production can be intimidating. Not only does it require large financial commitments, but also many manufacturers can be frustrating to work with. But, with the right prior knowledge, you can ramp up production on the right foot.

While 3D printing is a popular manufacturing solution, it’s usually not economical at production volumes – the same goes for computer numerical control (CNC) machining. At Protolabs, in addition to 3D printing and CNC machining services, we offer rapid injection moulding, which is often the most effective way to scale manufacturing operations for plastic components. 

There are many factors that go into injection moulding, so we’ve broken down the process to be more digestible.

Pick the right tool

When you hear the term injection moulding, your first thought may be high-volume manufacturing – this could not no further from the truth. 

For part quantities in the millions, moulds are typically machined from steel. It usually takes months to create a steel tool and the financial investment is substantial. The advantages of steel tooling go beyond high volumes, though. It can support more cavities –meaning more parts per shot – and handle more complex geometries and surface finishes.

3D printing has become another method for producing moulds. The moulds are low in cost and fast to create compared to those machined from metal. But, keep in mind that the rough surface finishes inherent in 3D-printed parts will require some secondary processing to smooth out. The idea of low-cost moulds is alluring, but will only save money in extremely limited runs. After 100 shots or so, the mould will degrade and need to be replaced.

Aluminium tooling is a solid middle ground between 3D-printed and steel moulds. Even though aluminium moulds cannot handle the same volumes as steel, they can be suitable for anywhere between 10,000 and 50,000 shots, and even up to 100,000 or more in some instances. And, by optimising your injection moulding design, you can improve the longevity of the aluminium tool.

Select the right material

Another key factor to injection moulding design is material selection. It’s important to determine which mechanical properties are going to be critical to your part’s function. Start by asking yourself, will my part have to support weight? Does it need to endure high temperatures? Is transparency a requirement? Answering questions like these will help narrow your options, of which there are thousands, and help guide you through the material selection process.

For our purposes, let’s touch on a few of the most common thermoplastics.

Acrylonitrile butadiene styrene, commonly referred to as ABS, will work for many plastic parts. Its cost is not too high, it’s pretty strong and tough, and it also looks nice with its glossy finishes – Lego bricks are a great example of ABS plastic components. Another benefit is that it can easily be painted. Some of its limitations include poor UV and chemical resistance. Polypropylene (PP) is an affordable plastic. It’s used in many consumer products such as automobile interiors, patio furniture, and food storage containers. PP is useful for designs that incorporate living hinges, and it also has decent stiffness, heat resistance, and clarity.

Acrylic is the go-to material for applications that require a transparent plastic part, like an enclosure for an LED. It’s also useful for outdoor products because of its UV stability – meaning its appearance won’t degrade from long-term sun exposure.

This is just a brief overview of thermoplastic materials. There are many others that you will routinely encounter like nylon, polyethylene, polystyrene, and liquid silicone rubber. 

Design for mouldability

If your design was created with the intent to be manufactured via 3D printing, you might need to make some adjustments to ensure it can be moulded. Whereas 3D printing is capable of handling complex geometries, injection moulding has a few requirements to ensure consistent, quality results.

Adjusting features to be suitable for moulding will help resin flow well into the mould, allow it to cool evenly, and reduce the chance of cosmetic defects such as sink, knit lines, flash, and warp.

Here is a quick rundown of how to handle common features in injection moulding designs:

  • Wall thickness – consistent wall thickness will reduce the potential for warped parts, prevent sink, and help the material cool uniformly so unsightly knit lines do not form or are less prominent. A good rule of thumb is to design walls that are no less thick than 40–60% of adjacent walls.
  • Rounded features – avoid sharp edges and corners to help resin flow through the mould, reduce stress, and add strength to your part. If sharp corners are critical to your part’s function, there are materials that support sharp features better than others.
  • Draft – on a similar note, you should apply draft to vertical walls. This is the angle applied to the surfaces of a part that run parallel to the motion of the mould opening. Our guidance is usually one degree of draft per inch of cavity depth. Draft will ensure that your part cleanly ejects from the mould.

Find a manufacturing partner

The type of tooling your product requires will be the primary factor in determining your ideal manufacturing partner. Just like how you might approach buying a new car, it’s good to explore many suppliers.

Selecting an injection moulding manufacturer is a substantial business decision, so there are more factors in play than just price. Some of the most important items to consider are lead-time, minimum order quantity, and the total cost of tooling and parts.

It’s also important to note the manufacturer’s level of experience, as injection moulding is a complex process. There are numerous aspects that affect manufacturing quality, such as material handling, quality controls, machining capabilities, and more. Customer service is another element that should not be overlooked. It’s invaluable to have a manufacturing partner that is responsive, actively communicates, and is quick to act on the behalf of your requests.

Use for prototyping

Injection moulding is often thought of as a process reserved for production. Many believe 3D printing or, in some instances, machining is the only way to cost-effectively prototype plastic parts. But there is a substantial business case for injection-moulded prototypes.

For example, if you need to conduct market testing, prototype volumes can often creep into the hundreds, and 3D printing or machining at these volumes can get expensive. With low-cost, rapid injection moulding that uses aluminium tooling, you can prototype with production-grade materials. Additionally, sometimes alu

minium tooling pays for itself, even for a few prototype parts, by providing certainty that your moulded design will function as intended once in production.