Plastic injection moulding is perfect for very intricate parts. Compared to other techniques, moulding allows you to incorporate more features at very small tolerances. Have a look at the image to the right. You can hold this moulding in the palm of your hand and it has bosses, ribs, metal inserts, side cores and holes made with a sliding shut off feature in the tool. That's an awful lot of features on a small part! It would be impractical to make using plastic fabrication and impossible to make using the vacuum forming process.
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Once your mould tool is made, identical products can be made over and over again. And again. A decently made mould tool has a very long mould tool life, as long as it's treated well by the moulding machine setters !
Whilst there is an initial high investment for the plastic injection moulding tool, after that the cost per part is very low. Other plastic processing techniques may require multiple operations, like polishing, whilst injection moulding can do it all at once. If you chose to CNC machine the part above, it would cost hundreds of pounds per part. If you're looking to go into full production, injection moulding is the way to go.
Cycle times can be as low as 10 seconds. Combine that with a multi-impression injection moulding tool and you get a LOT of products very quickly. That part above takes a bit longer as it's a specialist material and has a lot of features to be moulded correctly, but at about 50 seconds you'd still get 70 parts per cavity per hour. CNC machining a one-off would take half a day - 3D printing it even longer!
There's a vast amount of materials available for plastic injection moulding. A range of more common materials, but also things like antistatic plastic, thermoplastic rubber, chemical resistant plastics, infrared, biocompostable...and with colour compounding or masterbatch colouring you have an endless choice of colours as well. The moulding above is boring black, but it's made out of PPO - poly(phenylene oxide) - which is an extremely rigid and flame-retardant material.
In addition to a range of colours, the injection moulding tool can be made with a special finish which will show on the moulding. Just about any finish you like, for example leather look, soft touch, sparked, high shine, you name it. You can also have logos or other text engraved in the tool. Finally, you can have your mouldings printed, as a range of inks are available that will print well on plastic.
Part repeatability is very high for injection moulding. Even the sprues and runners (the leftover bits of plastic created by the 'tunnels' through which the plastic material reaches the actual mould) can be reground and the material reused. You can explore this in more detail on our environmental impact of injection moulding page.
Sounds amazing doesn't it ? Of course there are also some disadvantages:
Often, several rounds of designing and modelling are needed before the go-ahead or production is given. Then, the injection moulding tool needed to make the mouldings will need designing & manufacturing. The mould tool is an intricate piece of work which costs manpower, material and many machining hours to make and represents the largest cost in getting injection mouldings. Of course, once it's all done, part cost is very low and repeatability very high for hundreds of thousands of mouldings.
From product conception to final part can take months of design, testing and tool manufacturing. That said, if you know what you want, you can have the finished mouldings within 6 weeks. (Toolcraft's Chinese Mould Tools Flowchart shows how that could work). And as mentioned under advantages, once the tool exists, it takes very little time to run the mouldings, especially when you have a multi-impression mould tool. (Here's a list of Mould Tool Types).
Huge machines are needed to make plastic injection mouldings. Very large parts need an enormous mould tool and become very expensive to make, in which case a process like Plastic Fabrication may be a better choice, depending on the tye of product needed.
Plastic mouldings need very careful design to avoid tooling issues like undercuts (which will send up tooling cost significantly), locked-in features and not enough draft (What Draft Angles for Plastic Mouldings ?). The material and temperature will need to be taken into account in wall design, otherwise the mould may not fill fully. The placement of ejectors and cooling lines will need to be considered to ensure the product is aesthetically pleasing.
Well, it depends on so many factors: design considerations, material, size, quantities needed, your budget. An expert will have to assess each product idea to decide the most effective and economically viable way of manufacturing it.
Once you have developed the molds, the process is extremely fast with cycle times as short as 10 seconds. It is excellent for medium and high-volume production runs for anything from 10,000 parts to well over 100,000 depending on what molds you use.
If you want to learn more, please visit our website precision injection molding.
Sometimes you can increase production by using a multi cavity or family mold, where several parts are produced from one press to further increase the manufacturing rate.
If you are outsourcing your molding then it’s also important to consider an efficient front-end design and order process – take a look at our on demand manufacturing.
There is a huge range of plastic materials that you can select from depending on what properties you need from your final part. And you are not limited to plastic, Protolabs also offers liquid silicone rubber molding.
You can even use fillers in the molding material, which adds greater strength to the completed part, and you have a huge range of colors to choose from as well. Talk to us about what you need your part to achieve – there are generally a number of different options.
If you are committing to steel mold tools for high production volumes (100,000+) then it can take a great deal of time and machining to produce and this can be a significant capital cost. If you need lower volumes of parts then this will affect your cost per part price and it may be worth exploring other options such as using aluminum molds instead.
It can take up to 12 weeks to produce steel tooling. If you need to get production running before this then take a look at our on-demand production; by using aluminum molds and digitizing our front-end process we can ship anything from 25 to 10,000+ parts in 15 working days or less from the point that you first upload your CAD. Sometimes parts shipped in as little as one day.
For very low production runs there are other technologies such as 3D printing or CNC machining which could provide a more cost-effective answer more quickly.
You will need to consider certain design elements, such as:
You also need to consider where to place your gates, ejectors and cooling lines if aesthetics are important. And remember it’s also hard to change the design of a mold – you can remove part of the mold, or add plastic to your final part, but not the other way round.
For smaller production runs, 3D printing allows you to design virtually any shape or geometry that you require. As a rapidly developing technology there is an increasing number of plastic and other materials, even metal, that we can produce.
Most people think that they should only turn to injection molding for orders of 100,000+ parts; but there are other options when you can use this process for smaller production runs.
Using more affordable aluminium molds and quick turnaround times (as fast as a day) our on-demand service helps keep your per part cost down. It is ideal for anything between 10,000 and 25,000 parts, but can help control costs for production runs even smaller than this.
For low production runs measured in the hundreds or less then it is also worth exploring other manufacturing technologies such as 3D printing and CNC machining. With these technologies the cost per part will be lower and you will not face the initial set up costs or lead times involved in producing molds.
For more information, please visit custom plastic molding.