How Does Injection Moulding Work
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Introduction
This guide is intended for people who are looking to source plastic mouldings. It gives a much needed insight into all that is involved with creating plastic parts, from the mould tool required to the moulding process itself. It also explores what to look out for when obtaining quotes and comparing them. If you want to explore further, the guide covers types of mould tools, as well as special finishing processes such as colours & plating. Words that are underlined can be found in the glossary in the appendix...
Part I : Moulding: The Basics
The Advantages of Injection Moulding
Plastic injection moulding is a very precise process that offers several advantages over other plastic processing methods. Here are just 5 benefits:1. Precision
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 mould tool. That's an awful lot of features on a small part!
2. Material choice
There's a vast amount of materials available for plastic injection moulding. A range of standard 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 just black, but it's made out of PPO which is an extremely rigid and flame-retardant material.....
The Injection Moulding Process
The injection moulding process involves heating & injecting plastic material under pressure into a closed metal mould tool. The molten plastic cools & hardens into the shape inside the mould tool, which then opens to allow the mouldings to be ejected or removed for inspection, delivery or secondary operations.
Stage 1
Material granules from the hopper feed into the heated barrel & rotating screw.
Material melted by heat, friction & shear force is forced through a check valve to the front by the rotating screw....
A Look Inside a Mould Tool
The core & cavity design of the plastic injection mould tool is what gives the final product its shape, but there are several other functions of the tool that are crucial for the correct formation of the end product...
Part III - Beyond the Basics - Tooling Options
Manufacturing in China can save you a lot of cost, but it can come with problems such as delivery delays, miscommunications, inferior quality and the paperwork surrounding importation. Using a UK moulder with existing ties to China can remove the risk and still result in cost-saving....
Part IV - Beyond the Basics - Mouldings
Appendix
Boss - On a moulded part, an upright column which can take a metal insert or a screw for example.
Cavity - The part of an injection mould tool that gives the plastic product its shape, that does the actual moulding of the plastic. Also see mould tool chapter on pages 5 and 6 for all terms associated with the mould tool
Cycle time - The time it takes for a mould cycle to be completed, i.e. from material feed & melting; material injection; cooling time and ejection to the re-closing of the mould tool ready for the next cycle.
Draft angles - The walls of a moulded part should be slightly tapered in the direction in which the part is ejected from the mould tool, to allow the part to be ejected easily. This angle at which they are tapered is called the draft angle.
Ejector stroke - The pushing out of ejector pins to eject the moulded part from the mould tool. Ejector stroke speed, length and timing needs to be carefully controlled to prevent damage to the ejectors and mould tool, but at the same time make the moulding cycle as short as possible.
High polish - A special finish on the cavity of the mould tool which ensures the plastic part is super smooth
Locked-in features - Features on a plastic part design that would make the plastic part impossible to remove from the mould tool, or that would cause the mould tool to need expensive mechanisms to be able to remove the part.
Overlocking - When a mould tool has been set into a moulding machine incorrectly, causing the tool to shut too hard and so damaging the mould tool
Part repeatability - The ability to create identical plastic parts time after time
Radii - Perfectly straight corners are impossible to eject from the mould tool. A slight radius should be added to any straight corners. Ribs - When a plastic part has thin walls, ribs are added to the design to make the thin walls stronger
Side cores - Side action which produces a feature on a moulded part, at an opposing angle to the normal opening direction of the mould tool. The side core needs to be able to retract as the plastic part cannot be ejected otherwise.
Sparked finish - A special finish on the cavity of the mould tool which ensures the plastic part has a slightly gritty texture – think about some car dashboards, keyboards, computer frames for example
Tolerance - The margin by which a moulded part is allowed to deviate from the sizes specified on the drawing
Tool bolster - A near complete standard mould tool which can take a core and cavity insert.
Walls - The sides of a moulded part
The text on this page is a sample from our full White Paper ‘Injection Moulding for Buyers’
