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Plastic injection moulding advantages centre around great precision and high repeatability, combined with speed, a low cost per part and a huge choice of available plastics. Disadvantages include a higher initial cost and lead time than some other processes.
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.
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.
Toolcraft offer plastic injection moulding, vacuum forming and plastic fabrication, including a range of finishing, printing & packing services. Our longstanding experience allows us to help you choose the best manufacturing process for your new project. We also share our knowledge on our plastic moulding advice pages. Or just:
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