1) OPEN MOULDING
Open moulding is a process of laminating a gel coated mould under ambient atmospheric conditions, using resin and glass reinforcement with hand lay-up or spray-up techniques.
a) Hand Lay-up
Hand lay-up is the process of applying the material, resin and fibreglass by hand. Brushes, rollers and wet-out guns can be used to apply the resin. The reinforcing materials are usually mat, cloth, woven roving, or core materials.
In spray - up moulding, chopped roving is sprayed along with catalysed resin onto the gelcoat surface and then compacted. This versatile process has proven to be a cost effective method of producing large volumes of open moulded parts.
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2) CLOSED MOULDING
a) Resin Transfer Moulding (RTM):
RTM processes are those in which a liquid resin is transferred into a closed cavity mould. The reinforcing fibre and any embedded cores and inserts are placed into the cavity before the resin is injected.
Over the years, a number of RTM variations have been developed. Examples include Conventional RTM, Vacuum assisted RTM and Vacuum infusion.
In general, the materials used in RTM, are the same as those used in ordinary open moulding, with the most significant differences in the reinforcement technology. The process production rate determines many of the requirements for materials. Higher production rates require the use of pre-made reinforcement preforms and resins with short gel and cure times.
b) Compression Moulding
Compression moulding involves moulding a pre-manufactured compound in a closed mould under pressure and often using heat. A pre-manufactured compound is a combination of some or all of the following: thermoset resin, catalyst, mould release, pigment, filler, various additives and fibre reinforcements.
Compounds can be produced in several forms including sheet moulding compound (SMC) and bulk moulding compound (BMC).
The sheet moulding compound process involves three basic steps. First a compound paste is mixed that includes all the formulation ingredients except for the reinforcement. Second the compound paste and reinforecement are combined and formed into a sheet. Thirdly, the compound is allowed to thicken or mature. The compound paste and reinforcement are combined and the compound machine-formed into a sheet using an SMC machine.
A schematic of the SMC process is shown below.
During the manufacture of bulk moulding compound all formulation compounds are combined in a mixer. Liquid components are initially added and agitated until dispersed. Dry components, except for glass fibre, are added next and mixed until thoroughly wet.The glass fibre is the last formulation component added and is mixed in until thoroughly wet. Continued mixing of the compound after glass wet out can result in unnecessary degradation of the reinforcement. BMC is ready to mould when it is discharged from the mixer.
Pultrusion is a continuous, automated closed-moulding process that is cost effective for high volume production of constant cross section parts. Pultruded custom profiles and standard shapes (channels, angles, beams, rods, bars, tubing and sheets) have penetrated virtually every market.
The process relies on reciprocating or caterpillar-type puller/clamping systems to pull the fibre and resin continuously through a heated steel die. Roving is pulled from material racks and is then wet out, typically in an open resin bath. Excess resin is squeezed out by shaped bushings positioned ahead of the die. The compacted package then enters the die, where the part cures. The cured part is pulled out of the die and finally into a saw system at the end of the machine. The saw travels downstream whilst it cuts the part to a pre set length. Puller and saw motions are synchronized, usually through computer controls. Alternative injection wet out systems have been developed. Multiple streams can be pultruded in a single die with several cavities. Heat control is also critical, and controllers are available that monitor and maintain a pre-set temperature in various zones throughout the die and mandrels.
d) Vacuum Moulding
Vacuum moulding may be used for the production of parts from medium to large dimensions. It allows for the creation of parts with both faces having a smooth appearance. This method is generally limited to the manufacture of parts with relatively simple shapes and is recommended for small to medium volumes.
Vacuum moulding works combined with low pressure injection moulding techniques. This process allows the separation of the functions of mould closure and resin flux. The mould closure is done by a peripheric circuit with a high level of vacuum. The resin flux is obtained with a low pressure injection or manually applying the resin inside the mould before its closure; when the closure follows the vacuum is created inside the mould cavity.
The more usually applied resins are low viscosity polyesters which may be combined with mineral extenders or fillers.
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3) CAST POLYMER
With the evolution of new processes and products, the name 'cultured marble' has come to be identified with gel-coated calcium carbonate filled and cast polyester matrix. With onyx (gelcoat with fillers) and densified (no gelcoat, ATH or other fillers) products coming into the market place, the market trend has been to identify all these products as 'cast polymers.'
Cast polymer is used to make vanity tops, counter tops, wall panels, window sills among other items. It can be produced in a limitless variety of colours and configurations.
Cultured marble is made by combining polyester resin with a filler. Parts are fabricated by casting the resin and filler combination in open moulds.
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4) FILAMENT WINDING
Filament winding is an automated, high volume process that is ideal for manufacturing pipe, tank, shafts and tubing, pressure vessels and other cylindrical shapes. Machine sophistication varies from basic two-axis mechanical chain-drive operation, to computer controlled multi axis and spindle systems.
The winding machine pulls dry fibre glass from supply racks through a resin bath, and winds the wet fibre around a mandrel. Both removable mandrels and in-situ mandrels, which remain with the part, can be used. For best part performance, fibre tension must be equal on all fibres. Resin is typically worked into the fibres by roll coaters or by breaker bars in dip tanks.
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a) Panel lamination
Continuous moulding between layers of film is used for production of Sheet, both coloured and translucent, in flat or profiled (such as corrugated) configuration. The main applications are in building (cladding and roofing) and agriculture
The different steps are as follows:
- continuous impregnation of rovings chopped in situ, or of chopped or continuous strand mats, by resin, on a carrier film,
- above the impregnated material, the second film is deposited, that will be used as a mould,
- shaping, normally in a progressive manner, as permitted within the curing cycle to result in the finished product on exiting the oven,
- lateral edge trimming and transverse cutting the products to the designed length. The films having served to transport the material are rewound at the exit of the oven prior to trimming. Typical machine speeds are 5 to 15 m/min., depending on the sheet width, which may be up to 3m.
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Polyester Products Application Manual; 9th addition; CCP
Cray Valley Unsaturated Polyester Application Guide
Saint-Gobain Vetrotex India Basics of Composites
Saint Gobain Vetrotex Brazil web site