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The World of Plastic Injection Moulding 

Written by MS
12/10/2020 12:13:32



Many of the plastic-based products we consider regularly are a result of the plastic injection moulding process. Everything from children’s toys, such as Lego, to garden furniture, such as the Monobloc, is manufactured by this method. 
 
The American inventor John Wesley Hyatt discovered the process in 1872 when he began using a plunger to inject plastic through a heated cylinder into a mould. The industry advanced over the years to produce items such as buttons, hair combs and collar stays. 
 
As plastics became more advanced, and scientists developed the material in powder forms, the process of injection moulding became more wide-spread. For instance, the Second World War led to increased demand in inexpensive, mass-produced items and the first screw injection machine was able to meet these demands and create more precise products. 
 
The ingenuity of early plastic injection moulding machines allowed for virgin materials to be mixed with recycled and coloured plastics, creating an extensive range of products that could be manufactured via this method. As a result, by 1979, plastic injection moulding overtook steel production, and it became the most used manufacturing method around the world. 
 
The plastic injection moulding has thus transformed from producing hair combs and such items to being a key method of production in a variety of industries including medical, aerospace, packaging, construction and children’s toys. 
 
In this blog post from The Plastic People, we are going to discuss the ins and outs of the plastic injection moulding process and outline some of the main benefits of this manufacturing method. Plastic injection moulding is just one of the many methods we use onsite to manage the production of bespoke products. Contact our Sales Team to find out more about how we create more components or items for your business: service@theplasticpeople.co.uk 
 
The Method:
 
In short, the plastic injection moulding process uses a ram or screw-type plunger to force molten plastic into a mould cavity -- or specific shape -- at high pressure. It is a process that can be utilised for both thermoplastics and thermosetting polymers and to a high volume. 
 
Usually, the plastic material is injected into the mould as pellets or granules -- small off-cuts that can be melted easily. This pre-dried material is fed into a hopper via a funnel where it is heated and melted. From here it is forced into a mould where the shape is formed. The mould is then left to cool to allow for the product to solidify. 
 
Typically, the most common materials used for injection moulding are:
 

  • Acrylonitrile-Butadiene-Styrene (ABS)
  • Nylon (PA)
  • Polycarbonate (PC)
  • Polypropylene (PP)
  • Polystyrene (GPPS)
 
Naturally, there are differences to process when it comes to moulding different types of plastic. For example, thermosetting plastics must be ejected from moulds sooner than thermoplastics to make sure they do not chemically adhere to mould itself. 
 
The moulds themselves are generally made from steel, due to its high-melting-point, but can be manufactured from aluminium in certain circumstances. Aluminium moulds, on the whole, are less well-suited to mass-production as they are more prone to wear and damage over time, resulting in a product of ill-quality. 
 
To avoid spikes in pressure, the injection moulding process usually takes place within a matter of seconds. This ensures a consistent temperature within the plastic material, and so it is unlikely the viscosity will change, causing complications. 
 
For some products, a two-shot injection method is used. This is primarily reversed for products that require multiple, connecting, parts or products that contain several colours. For thermosets, the two separate chemical components are injected and eventually crosslink, forming one concise product. 
 
Moreover, pre-moulded or manufactured products can also be inserted into the mould during the process so that the plastic material can be set around said shapes/objects. This process is known as Insert Moulding and is usually used for products that contain a variety of materials. 
 
Seam Lines or ‘parting lines’ are generally present on any resulting product and this is a side effect of the injection moulding process as they mark the place where the cavity -- or mould -- joins. To avoid these issues may amount in an increase in manufacturing costs. 
 
The Moulds:
 
The primary downside to the injection moulding process is the fact that moulds can be expensive. Typically, these moulds can resist high-pressure environments due to the amount of clamp force used to push molten plastic into the cavities, this results in the need for costly and durable materials. 
 
As mentioned previously, hardened-steels are used to create moulds. This is a highly resistant material that will not lose its shape after continuous use, allowing for the mass-production of identical products. These moulds are made via CNC machining. 
 
Moulds themselves are separated into two parts -- the injection mould (moulder) and ejector mould (mouldmaker). Typically, moulds are manufactured in such a way, so the plastic product is left in the ejector mould once the other side (the injection mould) is removed. More complex moulds may consist of excessive parts or allow for overhangs, but, normally, the mould itself exists in two concise pieces. 
 
To cool the moulds, and the machine itself, water is conventionally used and passed throughout the entire mechanism. A series of holes are drilled throughout the machine, to allow for the easy passing and exiting of coolants. 
 
One single mould can make over 100,000 parts or components in its life, demonstrating the durability and cost-effectiveness of this process. 
 
Polymers and characteristics:
 
All thermoplastics may be used in the injection moulding process, while only some thermosets and elastomers are suitable. Nonetheless, there are thought to be over 18,000 suitable polymers. 
 
Alloys and other blends, which have been previously used in the manufacturing process, can go on to be used in this process and it is the job of production workers to choose which materials will work best in conjunction with one another, given their properties. 
 
When considering which plastics to use in this method, it is worth thinking about the elasticity of each material; some products can be bent and altered to a greater degree than others. In addition to this, one should also think about the water absorption of any given plastic and its heat-reflecting properties. 
 
Typical Products:
 
To demonstrate the universality and ubiquity of the injection moulding process, here are some of the key products made via this method of manufacturing: 
  • Wheelie bins 
  • Syringes
  • Bottle lids
  • Car bumpers
  • DVDs
  • Disposable razors
  • Battery casing 
 

That concludes our concise guide into the injection moulding process. We’ve explored the plastic materials used, moulds, and the process itself. Plastic injection moulding is a key element to any production line that relies on the consistent reproduction of single components. 
 
If you operate a business that requires components for its production line, get in contact! We are championing the move from metal to plastic components within a range of industries, as it facilitates increased productivity, reduced downtime and plastics can be used in several extreme environments. 
 
We have worked with a range of businesses from the food industry to nuclear and medical With over 50 years’ experience in plastic fabrication, there is no one you could trust more with the manufacturing process than us! 
 
For more information head to our dedicated B2B and manufacturing website: barkstonltd.co.uk or email service@theplasticpeople.co.uk. You can also tweet us @barkstonplastic or message us on Instagram at the same address. 
 
 
 

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