Compression moulding is a high-volume, high-pressure method suitable for moulding complex, fibreglass-reinforced plastic parts on a rapid cycle time. There are several types of compression moulding including: sheet moulding compound (SMC) which are, bulk moulding compound (BMC), thick moulding compound (TMC), and wet lay-up compression moulding. Compression moulding tooling consists of heated metal moulds mounted in large presses.
Process Description – The mould set is mounted in a hydraulic or mechanical moulding press. The moulds are heated to 2500 to 4000 F. A weighed charge of moulding compound is placed in the open mould. The two halves of the mould are closed and pressure is applied. Depending on thickness, size, and shape of the part, curing cycles range from less than a minute to about five minutes. The mould is opened and the finished part is removed. Typical parts include: automobile components, appliance housings and structural components, furniture, electrical components, and business machine housings and parts.
Moulds – Tooling is usually machined steel or cast alloy moulds that can be in either single or multiple-cavity configurations. Steel moulds are hardened and sometimes chrome plated for enhanced durability. The moulds are heated using steam, hot oil, or electricity. Side cores, provisions for inserts, and other refinements are often employed. Mould materials include cast of forged steel, cast iron, and cast aluminium. Matched metal moulds can cost fifty times as much as an FRP open mould and tooling in the $50,000-$500,000 range is not uncommon.
Major Advantages – Compression moulding produces fast moulding cycles and high part uniformity. The process can be automated. Good part design flexibility and features such as inserts, ribs, bosses, and attachments can be moulded in. Good surface finishes are obtainable, contributing to lower part finishing cost. Subsequent trimming and machining operations are minimized in compression moulding. Labour costs are low.
Pultrusion is a continuous process for the manufacture of products having a constant cross section, such as rod stock, structural shapes, beams, channels, pipe, tubing, fishing rods, and golf club shafts. Pultrusion produces profiles with extremely high fibre loading, thus pultruded products have high structural properties.
Process Description – Continuous strand fibreglass roving, mat, cloth, or surfacing veil is impregnated in a resin bath, then pulled (pul-trusion) through a steel die, by a powerful tractor mechanism. The steel die consolidates the saturated reinforcement, sets the shape of the stock, and controls the fibre/resin ratio. The die is heated to rapidly cure the resin. Many creels (balls) of roving are positioned on a rack, and a complex series of tensioning devices and roving guides direct the roving into the die.
Moulds – Hardened steel dies are machined and include a preform area to do the initial shaping of the resin-saturated roving. The dies include heating which can be electric or hot oil. The latest pultrusion technology uses direct injection dies, in which the resin is introduced inside the die, rather than through an external resin bath.
Major Advantages – The process is a continuous operation that can be readily automated. It is adaptable to both simple and complex cross-sectional shapes. Very high strengths are possible due to the fibre loading and labour costs are low.
Vacuum Bag Moulding
The mechanical properties of open-mould laminates can be improved with vacuum bagging. By reducing the pressure inside the vacuum bag, external atmospheric pressure exerts force on the bag. The pressure on the laminate removes entrapped air, excess resin, and compacts the laminate. A higher percentage of fibre reinforcement is the result. Additionally, vacuum bagging reduces styrene emissions. Vacuum bagging can be used with wet-lay laminates and prepreg advanced composites. In wet lay-up bagging the reinforcement is saturated using hand lay-up, then the vacuum bag is mounted on the mould and used to compact the laminate and remove air voids.
In the case of pre-impreg advanced composites moulding, the prepreg material is laid-up on the mould, the vacuum bag is mounted and the mould is heated or the mould is placed in an autoclave that applies both heat and external pressure, adding to the force of atmospheric pressure. The prepreg-vacuum bag-autoclave method is most often used to create advanced composites used in aircraft and military products.
Process Description – In the simplest form of vacuum bagging, a flexible film (PVA, nylon, mylar, or polyethylene) is placed over the wet lay-up, the edges sealed, and a vacuum drawn. A more advanced form of vacuum bagging places a release film over the laminate, followed by a bleeder ply of fibreglass cloth, non-woven nylon, polyester cloth, or other material that absorbs excess resin from the laminate. A breather ply of a non-woven fabric is placed over the bleeder ply, and the vacuum bag is mounted over the entire assembly. Pulling a vacuum from within the bag uses atmospheric pressure to eliminate voids and force excess resin from the laminate. The addition of pressure further results in high fibre concentration and provides better adhesion between layers of sandwich construction. When laying non-contoured sheets of PVC foam or balsa into a female mould, vacuum bagging is the technique of choice to ensure proper secondary bonding of the core to the outer laminate.
Moulds – Moulds are similar to those used for conventional open-mould processes.
Major Advantages – Vacuum bag processing can produce laminates with a uniform degree of consolidation, while at the same time removing entrapped air, thus reducing the finished void content. Structures fabricated with traditional hand lay-up techniques can become resin rich and vacuum bagging can eliminate the problem. Additionally, complete fibre wet-out can be accomplished if the process is done correctly. Improved core-bonding is also possible with vacuum bag processing.
Vacuum Infusion Processing
Vacuum infusion is a variation of vacuum bagging where the resin is introduced into the mould after the vacuum has pulled the bag down and compacted the laminate. The method is defined as having lower than atmospheric pressure in the mould cavity. The reinforcement and core material are laid-up dry in the mould. This is done by hand and provides the opportunity to precisely position the reinforcement. When the resin is pulled into the mould the laminate is already compacted; therefore, there is no room for excess resin. Very high resin to glass ratios are possible with vacuum infusion and the mechanical properties of the laminate are superior. Vacuum infusion is suitable to mould very large structures and is considered a low volume moulding process.
Process Description – The mould may be gel coated in the traditional fashion. After the gel coat cures, the dry reinforcement is positioned in the mould. This includes all the plies of the laminate and core material if required. A perforated release film is placed over the dry reinforcement. Next a flow media consisting of a coarse mesh or a “crinkle” ply is positioned, and perforated tubing is positioned as a manifold to distribute resin across the laminate. The vacuum bag is then positioned and sealed at the mould perimeter. A tube is connected between the vacuum bag and the resin container. A vacuum is applied to consolidate the laminate and the resin is pulled into the mould.
Moulds – Moulds are similar to those used for conventional open-mould processes.
Major Advantages – Vacuum infusion can produce laminates with a uniform degree of consolidation, producing high strength, lightweight structures. This process uses the same low cost tooling as open moulding and requires minimal equipment. Very large structures can be fabricated using this method. Vacuum infusion offers a substantial emissions reduction compared to either open moulding or wet lay-up vacuum bagging.
Resin Transfer Moulding
Resin transfer moulding is an intermediate volume moulding process for producing composites. The RTM process is to inject resin under pressure into a mould cavity. RTM can use a wide variety of tooling, ranging from low cost composite moulds to temperature controlled metal tooling. This process can be automated and is capable of producing rapid cycle times. Vacuum assist can be used to enhance resin flow in the mould cavity.
Process Description – The mould set is gel coated conventionally, if required. The reinforcement (and core material) is positioned in the mould and the mould is closed and clamped. The resin is injected under pressure, using mix/meter injection equipment, and the part is cured in the mould. The reinforcement can be either a preform or pattern cut roll stock material. Preforms are reinforcement that is pre-formed in a separate process and can be quickly positioned in the mould. RTM can be done at room temperature; however, heated moulds are required to achieve fast cycle times and product consistency. Clamping can be accomplished with perimeter clamping or press clamping.
Moulds – RTM can utilize either “hard” or “soft” tooling, depending upon the expected duration of the run. Soft tooling would be either polyester or epoxy moulds, while hard tooling may consist of cast machined aluminium, electroformed nickel shell, or machined steel moulds. RTM can take advantage of the broadest range of tooling of any composites process. Tooling can range from very low cost to very high cost, long life moulds.
Major Advantages – This closed moulding process produces parts with two finished surfaces. By laying up reinforcement material dry inside the mould, any combination of materials and orientation can be used, including 3-D reinforcements. Part thickness is determined by the tool cavity. Fast cycle times can be achieved in temperature controlled tooling and the process can range from simple to highly automated.
To further investigate the potential suitability of composites for your needs, contact Composites Australia Inc. today, tell us about your special interest, and we will put you in touch with those member companies best qualified to assist you.