Resin Transfer Moulding
Composite Processes
There are numerous ways to process your composites to achieve your final product. In this article we look at the basics of Hand lay, Spray lay and RTM. Later in this series of articles we will also take an in-depth look at the more advanced methods, autoclaves and fully automated systems and how this can affect your final part.
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Hand Lay Up
Hand Lay Up is a process expression where a simple set of roller tools are used to produce a GRP part. Firstly, the operator would prepare the mould.
To prepare the mould the operator cleans with acetone to ensure there is no contamination. Once this process is complete the operator would use a wax to ensure the finished part releases after manufacture.
There are several different types of waxes that can be used but usually a simple honey wax or semi-permanent is applied. Once the wax manufacturer’s instructions have been followed the operator is ready to apply the gelcoat this process is done using a standard brush in most cases however larger producers of GRP parts may spray the gelcoat. Gelcoat application must be in line with manufacturers recommended thickness, this can be measured using what is known as a depth gauge.
Once completing the gelcoat process and the gelcoat has hardened to the state where it is now ever so slightly tacky the operator cuts in the chopped strand matting (CSM) ensuring even distribution and full coverage of the gelcoat.
Depending on the strength required for the part you are manufacturing will determine the amount of CSM required. Some simple parts such as flower planters may only require 1 layer of CSM where as something more commercial such as a car part will require several layers and in addition perhaps some technical fabrics*.
When the CSM is in place the operator will move on to the wet-out stage. This stage is where the resin is applied by dipping the resin roller into the bucket and applying to the CSM. How much is determined by how the glass wets out meaning is you still see white fibres then more will need to be applied. Once the chopped strand matting is finally wet out the resin will harden and the part is complete and ready to remove from the mould.
*Technical Fabrics that can be used in hand lay up are; woven glass fibre and stitched glass fibre. More exotic fabrics can be used to reduce weight and increase strength such as carbon fibre and Kevlar cloths.
Benefits of Hand Lay Up
Hand lay up has been used for many years and is a relatively simple procedure, making it easy to teach. Usually room-temperature cure resins are used, making tooling relatively low cost. There are also a huge selection of resins and reinforcements that are suitable for hand lay up.
Hand Lay Up Downsides
To be workable by hand, resin should be low in viscosity, which can compromise their mechanical properties, due to the need for high diluent or styrene levels. Low viscosity can cause health and safety issues, these resins have an increased tendency to penetrate materials close by, such as clothing and furniture. The low molecular weights of hand lay-up generally means they are potentially more harmful than their high-molecular counterparts. The downsides of hand lay up are usually based on human error. Resin mixing, quantity and quality are dependent on the skills of the laminator.
What is Spray Lay Up
Spray is the method of creating fibreglass objects by spraying short strands of glass out of spray gun.
As with Hand Lay, the operator will clean the mould, using acetone or other suitable cleaning product to remove any tackiness or residue from previous moulds. A release agent is then applied – either wax or semi-permanent and polished in to allow easy-release once the mould is complete.
The next step is to apply a gelcoat to the mould surface and is cured to a tacky state. Often a second coat of gel is applied, though is not always required.
Once tacky, a chopper, is placed on top of the spray gun which chops fibreglass roving, to a set length (usually 10mm to 40mm) The spray gun forces a mixture of resin (often a DCDP or isophthalic polyester resin, though vinyl ester resins are sometimes used) and catalyst. This combines with the chopped fibreglass. The operator moves in a pre-determined pattern to ensure even coverage.
The mixture is then compacted with roller to remove air to leave a smooth and even surface this removes trapped air and to ensure all the fibres has been wet out. Where necessary the operator may use foam or honeycomb cores are embedded into the laminate to create a sandwich structure, at this point all corners and radiuses would be checked for even coverage. For improved mechanical properties, a combination of fabric and chopped fibre layers can be used.
The part is then cured at room temperature as with hand lay up (though some can be can be cured in an oven or autoclave).
The part is removed from the mould, which is now ready to be cleaned, waxed and polished for the next manufacturing cycle. The part is finished by trimming excess fibreglass and drilling holes as needed.
Spray Lay Up Benefits
Spray Lay Up is relatively low cost and can offer a quick way of getting resin and fibre into a mould. Spray lay is considered a quicker and less difficult solution to complex moulds that just may not be possible with a traditional hand-lay method. The speed allows more moulds to be covered which can often mean more parts being produced when compared to hand lay up.
Spray Lay Up Downsides
The downsides to spray lay, moulds can be resin-rich, often meaning they are very heavy, in comparison to their more advanced counterparts. Again, with hand lay up, spray lay up resins need to be low in viscosity in order to be sprayable, potentially being more harmful than their high viscosity counterparts.
What is RTM
Resin Transfer Moulding is a closed mould process for manufacturing performance composite components in larger volumes. It is primarily used for components with large surface areas, complex shapes and smooth finishes.
When creating a RTM mould, after apply mould release, the operator covers the interior mould surface with a gel coat to offer a more durable, high quality finish.
The operator then places reinforcement mat into the mould, being careful in and around corners and on edging, to make sure that full coverage is gained. The mould is then closed and clamped. The operator must take care to ensure that no air can escape from the mould ensuring it is tightly shut. At this point, catalysed low viscosity resin is pumped under pressure, whilst displacing the air and venting it at the edges, until the mould is filled, and all the fibres have been wet-out.
The curing process often uses heat. Once the mould has fully cured, the mould can be opened, and the part removed. As with spray lay up and hand lay up a mould release must be applied before every new process, unless using a semi-permanent mould release, which will need replacing as directed.
Benefits of RTM
RTM offers an automated process, which results in less human error, higher production rates and less wastage. It also offers the ability to mould complex structural and hollow shapes, which may not be possible with hand or spray lay. RTM offers a uniform of thickness and fibre loading, resulting in uniform and predictable shrinkage. RTM also offers a good surface finish on both sides due to be a closed moulding process.
Downsides of RTM
Matched tooling for RTM is considered expensive, and whilst RTM is generally limited for smaller components, benefits of RTM are often only seen with large mould quantities, often in the hundreds and thousands [of objects]. Problems can also occur with unimpregnated areas – which can result in very expensive scrap parts.
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