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Issues with Plastic Pellets in the Injection Moulding Process

plastic granules

Every plastic injection moulded product starts life as a humble plastic pellet. A small uniform cylinder or ball of plastic that will be melted down, pressured into a mould and cooled.

Hundreds of these small units of plastic will go into a wide range of plastic parts – including large plastic products like car bumpers and wheelie bins and smaller parts.

But these tiny pellets can cause big problems if plastic injection moulders don’t take steps to avoid them.

In extreme cases, ignoring some of these problems can have a dramatic effect on the outcome of a cycle, causing weaknesses or other inconsistencies in the final product.

Getting the right kind of plastic

Like with so many other things, you get out what you put into plastic injection moulding.

To get a product that meets your specifications, you need to start by feeding the right type of polymer into the machine.

Before you get to the factory floor, you need to have answered a few questions about your final product. You need to know what type of plastic you will use, what grade of raw material you need, how the material flows and what kind of fillers or additives you will use.

Plastic injection moulding can use a wide variety of thermoplastics. There are just a few of the most commonly used materials in plastic injection moulding:

  • Acrylonitrile-Butadiene-Styrene ABS
  • Nylon PA
  • Polycarbonate PC
  • Polypropylene PP
  • Polystyrene GPPS

These materials all have different attributes making them more suitable or less suitable for a specification.

For injection moulding, one of the most important polymer attributes is how the material flows when it is melted. Specifically, you need at the viscosity of the melted polymer and, ideally, simulate how easily it will flow into a cavity mould.

Experienced plastic injection moulding companies like Wheatley Plastics have the resources and the expertise to help guide you through this initial stage of the process.

They will help you choose the plastic polymer that’s right for your product, advise you on material grades and additives and use special software to help you simulate the flow of a molten polymer through the injection moulding machine.

Non-uniform pellets

For effective plastic injection moulding, you need the plastic pellets to be as uniform as possible. If pellets differ in size or shape then they may cause different degrees of friction in the screw chamber, which can lead to irregularities when the polymer melts.

Although plastic pellet manufacturers take great care to ensure that their pellets are equal in shape and size, you are unlikely to get 100% uniformity. In a batch, pellets may vary by between a few millimetres or a few centimetres if pellets clump or fuse together.

Pellets that are particularly large or particularly fine can cause the most problems.

Any pellets that are uncommonly large may not shear on the flights of the mechanically turning plastic injection moulding screw, which uses the friction of the flights against the barrel of the machine to generate extra heat.

This can slow the reciprocating screw down and lead to a different amount of plastic material in each product.

Finer pellets pose a different problem. These pellets will melt faster than normal sized pellets and reduce the amount of friction that’s required to melt each pellet. This can cause burns or other imperfections in your finished product like black or white specks in the part.

Although a few millimetres here and a few centimetres there is unlikely to make a significant difference on the majority of plastic injection moulding projects, it is something that’s worth troubleshooting if you experience persistent problems and imperfections with your plastic injection moulded products.

Hopper problems

Before plastic pellets start the melting process, they start in a hopper and are fed into the feed throat.

A fault in the hopper design can lead to a fault in the way that pellets are fed into the injection moulding machine. Hoppers need to be designed to have ‘mass flow’ to ensure the uniformity of flow through the feed throat, rather than ‘funnel flow’ in which a few pellets will ‘rat-hole’ down into the heating chamber.

Rat-holing can mean that pellets are not fed into the melting barrel at an even rate and can result in some pellets around the edges of the hopper never making it further into the machine.

Mass flow can be created by angling the hopper correctly, making sure the feed throat is clean and by ensuring the geometry of your pellets is such so that they won’t catch on or otherwise interrupt each other as they pass through the hopper.

Plastic pellet pollution

This final problem doesn’t necessarily affect the plastic injection moulding manufacturing process but represents a more general problem with plastic pellets.

High profile TV shows and celebrity crusades like the one mounted by Sir David Attenborough have highlighted the impact that plastic pollution has had on wildlife and humans in developing countries.

Although chemically most plastic pellets are deemed to be ‘safe’, if they are mishandled they can do significant harm to the planet.

Plastic pellets are particularly nefarious if they leak out into the environment because their small size means that they can be swallowed by animals and fish.

Images of decaying animals with small pieces of plastic are particularly evocative for the general public.

When plastic pellets are being transported from the factories to an injection moulding plant, they are liable to be spilt from trucks, trains and cargo containers. They are also very easy to spill when they get into the factory, where pellets can easily find their way outside or into drains.

Plastic injection moulding plants need to ensure that they have strict practices in place to make sure that plastic pellet loss and pollution is kept to an absolute minimum.