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What is the Injection Moulding Process?

28 February 2021

Injection moulding is a common manufacturing process for producing plastic products and materials in large quantities. It involves inserting melted plastic into a mould cavity which has been designed for the required shape of the end product. Although the process is short and straight forward, there are 4 stages involved in creating a finished product.   


The process begins with two halves of the mould being clamped together under pressure to accommodate the injection and cooling processes. One half of the mould is attached to the injection moulding machine and the other is allowed to slide. The clamping unit then holds the moulds together to keep them closed whilst the molten plastic is injected. The time required for closing and clamping the mould is highly dependent upon the machine being used for the process. This can be estimated using the dry cycle time of the machine.


The plastic material is then placed into the injection moulding machine and moved towards the mould by the injection unit. During the process, the molten thermoplastic material, which is in the form of pellets, is melted by heat and pressure. The molten plastic in the barrel of the machine is then injected quickly and the build-up of pressure packs and holds the material. 


The material is injected into the mould cavity using a closed mould tool; the amount of material injected is referred to as the shot. It is strenuous to accurately calculate the injection time due to its complexity, however, it can be estimated using the injection pressure, power and shot volume. Moreover, once the molten plastic is injected into the mould, hydraulic or mechanical pressure is used to exert pressure to ensure the cavities within the mould are filled.


In the next stage of the process, the plastic is left to cool as soon as it makes contact with the interior mould surface. After the material has cooled, It will solidify into the desired part. During this stage of the process, some shrinkage of the part can occur, although the packing of material during the injection stage allows additional material to flow into the mould and can reduce the amount of shrinkage. This is often the longest part of the entire injection moulding process, as the mould cannot be opened until the required cooling time has passed. The time is estimated using the thermodynamic properties of the plastic and the thickness of the part.


After the cooling time has passed, the cooled part is ejected by the use of rods, a plate or an air blast to remove the plastic component completely from the mould. Force must be applied during the ejection phase as while the part cools, it often shrinks and sticks to the mould. To do so, a mould release agent can be sprayed onto the mould cavity surface before the injection of the material. The time required for opening the mould and ejecting the part is estimated from the dry cycle time of the machine and the time taken for the part to separate from the mould. Once the part is ejected, the mould is then closed and the injection moulding process repeats itself. Although it sounds lengthy, some injection moulding processes can take just seconds to complete allowing for hundreds of products to be made every hour.

Here at Insignia Manufacturing, we can produce injection moulded products to suit your needs. Furthermore, our design team can bring your idea to life with detailed concepts and accurate prototypes of bespoke components for clients who require something more unique. If this is something you are interested in, please get in contact to discuss your project in more detail.

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