What is Glass Reinforced Plastic?

Glass reinforced plastic, (GRP), or ‘Fibreglass’, as it is commonly described, is typically a thermosetting resin reinforced with glass fibres.

The resin used is generally polyester resin, although other resins; for example: phenol, vinylester and epoxy are employed where a specific capability or performance requirement exists.

The fibres for reinforcement can be of glass, carbon, kevlar, polyester, cotton and even hemp; all have been utilised either as individual short strands, continuous fibres, bonded mats, woven cloths and some as three dimensional fabrics.

Each type of reinforcement is suitable for particular applications and performance requirements.

Processing GRP

The resins and reinforcing fibres used to manufacture glassfibre products can be processed in many ways; each production method is related to various factors including: Size, Construction, Product Volume, Complexity of Form, Unit Cost, Tooling investment and Product life.

Production processes include: High Pressure Injection; Hot Press Moulding; Pultrusion; Low Pressure Injection; Cold Press; Resin Transfer Moulding, (RTM); Vacuum Assisted RTM,( RTM Light); Casting; Spray Deposition and Hand Layup.

Resin Transfer Moulding (RTM)

Resin Transfer Moulding (RTM)

Resin Transfer Moulding (RTM) is a closed mould process.

The RTM process has an environmental advantage over traditional moulding processes as there is a significant reduction in styrene emissions.

The process involves, as with hand layup and spray deposition, first applying a mould release agent to the mould tool; this is to ensure the final product can be removed from the mould tool without damaging the tool or the product.

If required, a gel-coat, pre-mixed with a catalyst or curing agent, is applied either by brush or spray gun; the gel-coated surface will be the finished ‘A’ surface of the final component and can be pigmented to almost any colour. The gel-coat is typically made from polyester resin with additives to improve its performance for fire, durability and light fastness; thixotropic boosters are added to thicken the resin and aid application.

The gel-coat is allowed to cure; when cured the moulding process continues by first laying into the female half of the mould tool the required layers of glassfibre reinforcement, when completed the male half of the mould tool is fitted over and clamped to the female half.

Pre-catalysed rapid curing resin is injected into a closed mould which contains glassfibre reinforcement material such as mats, woven fabrics and veils, the resin wets through the reinforcement and the composite material is allowed to cure under a low pressure.

The advantages over this process to hand layup and spray deposition processes are smooth A and B surfaces, consistent product thickness, faster processing times enabling more products to be produced, typically four times the volume of hand layup and spray deposition components from a single mould tool.

Environmental advantages include; less emissions and a significant reduction in material wastage.

Tooling has to be robustly constructed to withstand the injection pressures and generally this can cause handling issues in closing the male and female mould tools.

Vacuum Assisted Resin Transfer Moulding, (RTM Light)

Vacuum Assisted Resin Transfer Moulding, (RTM Light)

A further development of the RTM process was the introduction of vacuum assistance.

The vacuum is utilised both to close the two halves of the mould and to evacuate the mould cavity, this development brought a number of benefits over the original injection only process.

Highly reinforced and heavy male tool halves were no longer required, atmospheric pressure was sufficient to resist deflection; labour intensive heavy clamps and fixings were reduced to light location clamping.

The male half of the mould tool can now be produced virtually transparent enabling the flow characteristics of the resin through the reinforcement to be monitored optimising material usage. The quality of the component produced is further improved by the almost total elimination of air voids.

The same advantages of the standard RTM process are gained over hand layup and spray deposition processes; smooth A and B surfaces, consistent product thickness, faster processing times enabling more products to be produced, typically four times the volume of hand layup and spray deposition components from a single mould tool, with the added benefit of eliminating the handling issues associated with the standard RTM process.

Environmental advantages include; less emissions and a significant reduction in material wastage.

RTM Light is the preferred production method at GBM Limited and significant investment has been made in providing this process.

Two dedicated RTM facilities have been created at the Tongland site and have been fitted out with the latest equipment.

The environmental advantages and consistent quality of the final product obtained by using this process ensure GBM Limited provide customers with high quality products, cost effectively while minimising the environmental impact in doing so.

Hand Lay-up

Hand Lay-up

Hand layup is the open mould laminating process most individuals are familiar with; the process is used for mould tool making, low volume production and large complex components.

The process involves first applying a mould release agent to the surface of the mould tool; this is to ensure the final product can be removed from the mould tool without damaging the mould tool or product.

Next a gel-coat, pre-mixed with a catalyst or curing agent, is applied using a brush; the gel-coated surface will be the finished ‘A’ surface of the final component and can be pigmented to almost any colour. The gel-coat is typically made from polyester resin with additives to improve its performance for fire, durability and light fastness; thixotropic boosters are added to thicken the resin and aid application.

The gel-coat is allowed to cure; when cured the moulding process continues by first wetting out the surface of the gel-coat with a resin applied with a brush, pre-mixed with a catalyst or curing agent, this is followed by a layer of reinforcement, typically chopped strand mat; additional resin is applied and further reinforcement added. A roller is then utilised to consolidate the resulting laminate to exclude any air entrapped in the process and ensure the reinforcement is completely coated in resin. The construction method continues until the correct laminate thickness is obtained.

The component is then left to cure; during the curing process heat is generated by the chemical reaction in the polyester resin; this exothermic reaction while normal needs to minimised to avoid distortion of the final product.

When cured, usually overnight, the component can be removed from the mould tool and machined to size.

Spray-up

Spray-up

Spray deposition is another open mould laminating process and is used for producing higher volume products and large complex components, where product thickness tolerances are less critical.

The process involves, as with hand layup, first applying a mould release agent to the surface of the mould tool; this is to ensure the final product can be removed from the mould tool without damaging the mould tool or product.

Next a gel-coat, pre-mixed with a catalyst or curing agent, is applied either by brush or spray gun; the gel-coated surface will be the finished ‘A’ surface of the final component and can be pigmented to almost any colour. The gel-coat is typically made from polyester resin with additives to improve its performance for fire, durability and light fastness; thixotropic boosters are added to thicken the resin and aid application.

The gel-coat is allowed to cure; when cured the moulding process continues by first wetting out the surface of the gel-coat with a resin applied by a spray gun, the resin and catalyst is typically mixed with the catalyst or curing agent within the spray gun. This is followed by spraying the catalysed resin and glassfibre rovings through the spray gun at the same time, the Glassfibre rovings are chopped within the chopper head of the spray gun into 12 to 24mm lengths. Each pass of the spray gun increases the product thickness, a roller is then utilised to consolidate the resulting laminate to exclude any air entrapped in the process and ensure the reinforcement is completely coated in resin. The spraying of the glass and resin mix continues until the correct laminate thickness is obtained.

Gauges can be used to check thicknesses during the laminating process, consistency overall, however, is dependent on the skill of the operative.

The component is then left to cure; during the curing process heat is generated by the chemical reaction in the polyester resin; this exothermic reaction while normal needs to minimised to avoid distortion of the final product.

When cured, usually overnight, the component can be removed from the mould tool and machined to size.