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The most common GRP moulding techniques
There are a number of GRP moulding techniques that can be used to create strong and robust GRP products.
From simple rods to enclosures, all GRP structures undergo a lengthy process to ensure they are of a high standard. Below, we look at the most common GRP moulding techniques used by manufacturers in the industry.
Pultrusion
Pultrusion is a continuous process used to manufacture composites in GRP tubes, rods and a range of other shapes that have constant cross sections. The reinforcement must first be passed through a bath that allows for resin impregnation, it is then pushed through a shaping die to achieve the desired shape. Curing takes place early on in the process before the laminate can leave the cross section.
Hand lay-up or spray lay-up moulding
This technique involves impregnating a glass fibre mat with a liquid polyester mix. A manufacturer will produce composite structures by applying composite materials completely by hand. It’s generally built up layer by layer on a mould shaped like the final product.
Spray lay-up is another variation of this process and but involves spraying the material into the mould, rather than laying it in by hand.
Resin transfer moulding
This technique requires molten resin to be transferred from a melt-pot into a mould using high pressure. Before the process begins the mould must be heated to a temperature higher than the melt-pot, as this helps to compensate for any shrinkage that may occur.
Compression moulding
Possibly the second most common technique, compression moulding is a batch technique, rather than a continuous process. It is achieved by placing a blend or compound into a mould and heating it under pressure with a steam-heated press. It’s this combination of pressure and heat that cause a reaction in the mixture, transforming it into the final product. It’s a slow process, generally used for larger sheet mouldings.
Injection moulding
This GRP moulding technique is traditionally used to make products like electrical enclosures, security housings or dish aerials. It’s a similar technique to that of thermoplastics but modified to ensure the material does not harden whilst in the machine. Heated material is injected into a mould under high pressure, to quickly create accurate mould copies. It is then left to cool before the mould pops open revealing the product.
Reaction injection moulding
Reaction injection moulding is a GRP moulding technique that produces polymers such as polyamides and unsaturated polyesters. This outcome can be achieved by using a modified injection moulding technique or alternatively, a transfer moulding technique.
Filament winding process
This GRP moulding technique is highly automated and repeatable for a very low cost. To achieve the desired outcome, a mandrel is suspended between two supports whilst a fibre application instrument, often known as a ‘head’, moves back and forth along the mandrel. As it rotates on a horizontal axis, fibre is placed onto the tool in a predetermined structure.
The fibre material is either placed through a resin bath during the process (called wet winding), or a fibre pre-impregnated with resin can also be wound. The dried fibre is then removed and used as a preform in a number of other moulding processes.
For more information on GRP products visit www.kingsleyplastics.co.uk
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