Thermoplastic injection moulding

Thermoplastic injection moulding is the most common way to manufacture plastic parts. Thermoplastics are polymers that can be heated to soften or melt, and cooled to solidify as a physical change, rather than a chemical change that takes place during molding of thermoset materials.

The thermoplastic injection moulding process begins by adding pelletised material to a hopper. In most cases, the material must is dried prior to moulding, and frequently requires the addition of a colour concentrate before loading. The material is gravity fed into a heated barrel and screw. Rotation of the screw results in shearing action on the raw pellets, causing them to melt.
The screw rotation also pushes the molten plastic forward in the barrel towards the mould. The material is then injected into the closed mould at high pressure, through a runner system to fill all the cavities. The mould is clamped shut under enough force to keep the mould halves together, while the molten plastic is flowing. On a cold runner system, the plastic in the runner solidifies and must be discarded or ground into pellets to be reused, which we refer to as “regrind.” If a hot runner system is used, the plastic in the runner stays molten, and no material is wasted.
When the mould cavities are filled, the part cools until rigid enough to be ejected. Part cooling within the injection mould is accomplished through water lines cut into the mould. At the completion of the cooling cycle, the mould opens and the part(s) are ejected for removal.
As a thermoplastic injection molding company, The Merriott Plastics Group uses a process called “decoupling”. In this process, the cavities are filled to approximately 95 percent of their capacity using high injection pressures. At a specific programmed position of the injection screw, the pressure is reduced and the 5 percent remaining portion of the mould cavity is filled at a lower pressure. This process eliminates over packing of the moulded plastic component and the resulting high internal stresses caused by over packing.

Thermoplastic injection moulding equipment

The Merriott Plastics Group uses the latest equipment for the thermoplastic injection moulding process. A current list of this equipment can be found at our facilities list page. We have thermoplastic injection moulding presses ranging from 25 tonne to 1100 tonne clamping pressure. Programmable controllers are used to fine tune the injection moulding process. The flexibility of having different press sizes allows us to manufacture parts here in the UK from the smallest plastic injection mouldable component to one that has a footprint of over one square metre. At The Merriott Plastics Group we always use an appropriate press size, because using a large tonnage press uses a large capacity barrel and screw. If the plastic is left inside the barrel at high temperatures for too long, the material can degrade.

In addition to the wide range of thermoplastic injection moulding presses, we also use a variety of auxiliary equipment to keep production costs down. We often use robots and sprue pickers to remove parts and runners from the moulds. The runners are dropped into a grinder where they can be chopped into regrind. We also use automatic proportional vacuum loaders, which pull the correct ratio of virgin material and regrind from containers and load it into the hopper. In many cases, we have a fully automatic thermoplastic injection moulding process, where the only labour costs are inspection, packaging, and managing the flow of material.

Merriott Plastics Group – your premier injection moulding company in the UK.

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