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Fabrication Processes

Fabrication processes have undergone substantial evolution in recent years. Although the traditional hand lay-up process remains the process of choice for some applications, new developments in pultrusion, resin transfer moulding, vacuum infusion, sheet moulding compound, low temperature curing pre-pregs and low pressure moulding compounds are taking the industry to new heights of sophistication, and are now being exploited in high technology areas such as the aerospace industry.

There are two general divisions of composites manufacturing processes: open moulding and closed moulding.

With open moulding, the gel coat and laminate are exposed to the atmosphere during the fabrication process. In closed moulding, the composite is processed in a two-sided mould set, or within a vacuum bag. There are a variety of processing methods within the open and closed moulding categories:

Open Moulding

  • Hand Lay-Up
  • Spray-up
  • Filament Winding

Closed Moulding

  • Compression moulding
  • Pultrusion
  • Reinforced Reaction Injection Moulding (RRIM)
  • Resin Transfer Moulding (RTM)
  • Vacuum Bag Moulding
  • Vacuum Infusion Processing
  • Centrifugal Casting
  • Continuous Lamination

Selecting which manufacturing process to select depends on a number of factors including cost, materials, size, and most important volume. Below is a summary of which processes might be used depending on production amounts:

Low Volume Production

• Hand Lay-up
• Vacuum Bag Moulding
• Vacuum Infusion Processing

Medium Volume Production

• Filament Winding
• Wet Lay-Up Compression Moulding
• Resin Transfer Moulding
• Centrifugal Casting

High Volume Production

  • Compression Moulding
  • Pultrusion
  • Reinforced Reaction Injection Moulding
  • Continuous Lamination

OPEN MOULDING

The heart of the open moulding process is saturating fibre reinforcement with resin, then using manual roll-out techniques to consolidate the laminate and remove entrapped air. A major factor in this operation is the transfer of resin from a drum or storage tank to the mould. The means used to transport the resin, in many cases, characterizes the specific process method.


Hand Lay-Up

Hand lay-up is an open moulding method suitable for making a wide variety of composites products including: boats, tanks, bathware, housings, RV/truck/auto components, architectural products, and many other products ranging from very small to very large. Production volume per mould is low; however, it is feasible to produce substantial production quantities using multiple moulds.

Process Description – Gel coat is first applied to the mould using a spray gun for a high-quality surface. When the gel coat has cured sufficiently, roll stock fibreglass reinforcement is manually placed on the mould. The laminating resin is applied by pouring, brushing, spraying, or using a paint roller. FRP rollers, paint rollers, or squeegees are used to consolidate the laminate, thoroughly wetting the reinforcement, and removing entrapped air. Subsequent layers of fibreglass reinforcement are added to build laminate thickness. Low density core materials, such as end-grain balsa, foam, and honeycomb, are commonly used to stiffen the laminate to produce sandwich construction.

Moulds – Simple, single-cavity moulds of fibreglass composites construction are generally used. Moulds can range from very small to very large and are low cost in the spectrum of composites moulds.

Major Advantages – Simplest method offering low-cost tooling, simple processing, and a wide range of part sizes. Design changes are readily made. There is a minimum investment in equipment. With skilled operators, good production rates and consistent quality are obtainable.


Spray-Up (Chopping)

Spray-up or chopping is an open mould method similar to hand lay-up in its suitability for making boats, tanks, transportation components and tub/shower units in a large variety of shapes and sizes. A chopped laminate has good conformability and is sometimes faster than hand lay-up in moulding complex shapes. In the spray-up process the operator controls thickness and consistency, therefore the process is more operator dependent than hand lay-up. Although production volume per mould is low, it is feasible to produce substantial production quantities using multiple moulds.

Process Description – As with hand lay-up, gel coat is first applied to the mould prior to spray-up of the substrate laminate. Continuous strand glass roving and catalysed resin are fed through a chopper gun, which deposits the resin-saturated “chop” on the mould. The laminate is then rolled to thoroughly saturate the glass strands and compact the chop. Additional layers of chop laminate are added as required for thickness. Roll stock reinforcements, such as woven roving or knitted fabrics, can be used in conjunction with the chopped laminates. Core materials of the same variety as used in hand lay-up are easily incorporated.

Moulds – These are the same moulds as in hand lay-up simple, single-cavity, moulds of fibreglass composites construction. Moulds can range from very small to very large and are low cost in the spectrum of composites moulds.

Major Advantages – Simple, low-cost tooling, simple processing; portable equipment permits on-site fabrication; virtually no part size limitations. The process may be automated.


Filament Winding

Filament winding is an automated open moulding process that uses a rotating mandrel as the mould. The male mould configuration produces a finished inner surface and a laminate surface on the outside diameter of the product. Filament winding results in a high degree of fibre loading, which provides high tensile strengths in the manufacture of hollow, generally cylindrical products such as chemical and fuel storage tanks, pipes, stacks, pressure vessels, and rocket motor cases.

Process Description – Continuous strand roving is fed through a resin bath and wound onto a rotating mandrel. The roving feed runs on a trolley that traverses the length of the mandrel. The filament is laid down in a predetermined geometric pattern to provide maximum strength in the directions required. When sufficient layers have been applied, the laminate is cured on the mandrel. The moulded part is then stripped from the mandrel. Equipment is available for filament winding on a continuous basis and two axis winding for pressure cylinders. Filament winding can be combined with the chopping process and is known as the hoop chop process.

Moulds – Mandrels of suitable size and shape, made of steel or aluminium form the inner surface of the hollow part. Some mandrels are collapsible to facilitate part removal.

Major Advantages – The process makes the high strength-to-weight ratio laminates and provides a high degree of control over uniformity and fibre orientation. The filament winding process can be used to make structures which are highly engineered and meet strict tolerances. Because filament winding is automated, the labour factor for filament winding is lower than other open moulding processes.


CLOSED MOULDING


Compression Moulding

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

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.

We thank the American Composites Manufacturers Association for much of the technical information reprinted from their website with permission here.