|
David Taylor, Scott Bader
This section briefly describes the main manufacturing techniques and where they are used. Specific references for manufacturing techniques are given in the text. For further details on manufacturing techniques see references at the end of this text.
Introduction
A wide range of different processes have developed for moulding of composites parts ranging from very simple manual processes such as hand lay to very sophisticated highly industrialised processes such as SMC moulding. Each process has its own particular benefits and limitations making it applicable for particular applications. The choice of process is important in order to achieve the required technical performance at an economic cost.
The main technical factors that govern the choice of process are the size and shape of the part, the mechanical and environmental performance and aesthetics. The main economic factor is the number of identical parts required or run length. This is because composite parts do not generally come as standard components but are custom designed for a particular application. Pultrusion and continuous sheeting are exceptions but most processes will have an initial investment or set up cost that must be amortised over the length of the project. This is a major factor in the choice of process and is one of the reasons for the proliferation in processing methods.
Open moulding - hand and spray lamination
Open moulding is by far the most common process used to fabricate composites parts accounting for over 40% of composites processed world-wide. It is a relatively simple process with low investment cost but a high degree of manual handling. Virtually all types of reinforcement can been used in open moulding which together with the use of core materials to create sandwich structures enables access to the widest range of mechanical and structural performance of any composites process. Unsaturated polyester resins dominate in this area but epoxy and vinyl ester resins are also common. Open moulding can be used for a very wide range of mouldings from caravan parts and cladding panels to boat hulls and radomes. Typical economic run lengths range from 2 or 3 individual parts up to several hundred.
Hand lamination
The process starts with the construction of a mould. The mould is most commonly constructed from composites material using a model made from wood, plaster or any other suitable modelling material. Only one, normally female, mould half is needed which defines the exterior surface of the part. Most often the first stage of moulding is to brush or spray a polymer coating or gel coat onto the mould surface. Gel coats are available in a wide range of different colours and effects and are selected to protect the part from environmental degradation or chemical attack and provide the desired aesthetics.
After the gel coat has been allowed to cure fibre reinforcement in sheet form is laid in place in the mould. Glass fibre is the most common reinforcement in the form of chopped strand mat but other fibres such as carbon or aramid may be used. A very wide range of reinforcement types are available and can be positioned and oriented in the mould giving the ability to vary the mechanical performance across the part. The design of the glass fibre pack is a key to the performance of the composite part and the ability to change it at will gives great flexibility to the hand lay process.
The next stage of the process is to pour liquid catalysed resin over the reinforcement and to work it into the reinforcement using rollers. This process is very labour intensive but extremely important as it ensures even distribution of the resin, full impregnation and wetting out of the reinforcement and removal of air. Unless the process is carried out effectively the composite part will not perform correctly. Further layers of reinforcement and resin are applied according to the requirements of the part and core materials such as rigid foam, balsa wood or honeycomb may be included to create sandwich structures.
When the lay up is complete the moulding is left to cure. This normally occurs at ambient temperature and can take anything up to 10 hours. The moulding is then released from the mould and trimmed to remove excess material from the edges of the moulding. Sometimes mouldings are cured at slightly elevated temperatures to improve speed and productivity and may also be post cured at even higher temperatures to achieve the maximum performance.
Spray Lamination
Spray lamination is very similar to hand lay-up apart from the method of placing the reinforcement and the resin into the mould. Rather than using reinforcement in the form of mat, resin and reinforcement are co-sprayed into the mould using a combined resin spray and chopper gun. This process is much easier and faster than hand lay-up and can be automated using robotised spray guns. This does eliminate much of the manual labour involved in positioning the reinforcement in the mould and will result in higher productivity although rolling is still necessary to ensure proper consolidation of the part. Spray lamination can be used in combination with hand lay where different types of fibre or constructions are required to achieve specific properties.
Vacuum Infusion
Vacuum infusion (VI) dates back to the 1950's when patents for a broadly similar process were published in the USA. The process however has come to prominence in recent years due to increasing pressure on the control of VOC emissions from the open mould process. The basic principal of VI is that reinforcing fibres are placed in a mould, which is sealed using a plastic film, or vacuum bag and resin is drawn into the mould under vacuum. Moulds for VI are fitted with a peripheral channel to enable vacuum to be applied and catalysed resin is fed in at the centre of the part and allowed to diffuse through the reinforcement to the edge of the mould.
Design of the reinforcement and setting up of the plastic film or vacuum bag, which normally incorporates tubes or channels to help even distribution of the resin, is absolutely critical. However once optimised a major advantage of the VI process is that it can be reproduced exactly each time without the need for the use of skilled laminators. The mould is also fully enclosed during the moulding process virtually eliminating VOC emissions. A further advantage of the use of vacuum is that parts are extremely well consolidated, even at high fibre content, with very low air content giving very good structural performance.
One disadvantage of VI is that the excellent consolidation favours thin high fibre content parts, which may not have sufficient stiffness. To combat this effect core materials of various types can be used to bulk the moulding out and give the required thickness. A further complication with VI is that whilst the lamination process itself is very quick the setting up of the mould with the bag and resin distribution network is time consuming and costly as these materials are thrown away.
Typically VI is used for parts where high performance and quality are the main criteria and where the extra cost associated with the disposable elements can be supported. The process has been used particularly in the marine industry for boat hulls and masts but is still very much in its infancy.
Resin injection
Resin injection moulding accounts for some 5% of composites processing worldwide and is growing due again to increasing regulation of VOC emissions but also the drive for more automated higher productivity processes. There are a number of different versions of the Resin Injection Moulding of which the most widely known is Resin Transfer Moulding (RTM); although all are based essentially on similar principals. RTM uses two matched mould halves to create a cavity that defines the shape of the part. The mould is opened and a gel coat applied to one or both mould halves if required. Dry reinforcement is placed in the mould and resin is injected into the cavity wetting out the fibre. The part is then allowed to cure, the mould opened and the finished moulding removed.
Resin transfer moulding
The basic set up for RTM moulding is a set of matched moulds and a resin injection machine. Moulds can be constructed from composites materials but other more durable materials such as aluminium, electroformed nickel and steel have also been used. Mould accuracy and strength are important in RTM as the two mould halves need to be well matched to ensure consistent part thickness and be sufficiently rigid to withstand the pressure generated during the injection process. Moulds can also become hot during processing and may be fitted with heating elements to speed up moulding so mould materials need to be heat resistant. A range of injection machines is available for the RTM process ranging from simple pumps that inject pre-catalysed resin to sophisticated mixing and metering systems which incorporate automatic injection sealing and cleaning.
The RTM process is much faster with a far lower manual labour content than open moulding and has the advantage that parts have two smooth sides and can be produced under controlled conditions with very little VOC emission. Core materials can easily be incorporated as part of the RTM process and the development of low profile systems enable very smooth good quality finishes to be achieved.
The disadvantage of RTM is that compared to open moulding the initial investment cost is much higher requiring a more expensive mould and a resin injection machine. The higher investment cost of RTM means that the minimum economic run length is probably of the order of 500 parts ranging up to about 3000 parts/annum.
Typical parts manufactured by RTM include lorry, bus and car parts where the two finished sides, tighter tolerances and higher volume manufacture are important. Some large mouldings have been produced in RTM but generally as the moulds increase in size beyond about 6 square metres the costs of making the mould sufficiently rigid and the size of injection machine needed make the costs prohibitive. Simpler versions of RTM have been developed using lighter weight moulds and vacuum either on its own or with some positive pumping in order to limit costs and enable larger mouldings to be produced economically at lower volume. Some of these processes have been given names such as RTM Lite, VARI and Vacflo but are all based on similar concepts.
Prepregs -Vacuum Bag And Press Moulding
Prepregs are very often associated with high performance aerospace or sporting applications although they have more recently found application in architectural mouldings and infrastructure repair. Several types of prepreg are available but most commonly they are based on epoxy resin and carbon fibres although other resin fibre combinations are not uncommon. A prepreg is basically a reinforcement material that has been pre-impregnated with resin using a specially adapted impregnation machine. The material is supplied as a sheet by the prepreg manufacturer ready for use by the moulder. There are two basic methods for moulding prepregs namely Vacuum Bag or Press moulding.
Vacuum Bag Moulding
There are two main versions of the vacuum bag process for prepregs both of which start in a similar way. Both use a single sided mould onto which sheets or strips of the prepreg are laid according to a specified pattern. This pattern can be complex and is designed to meet the mechanical requirements of the finished part. Precise positioning of the prepreg is required and can either be achieved by manually or, as frequently employed in the Aerospace industry, by using a tape laying machines. A bleeder fabric, normally a felt of synthetic fibres, and a vacuum bag are fitted and sealed onto the mould. Vacuum is then applied which fully consolidates the prepreg, squeezing excess resin out into the bleeder fabric.
In one version of this process the sealed mould is placed in a heated, pressurised chamber (Autoclave) where the part is fully consolidated and cured. In the other version of this process an autoclave is not used and the part is cured by employing a heated mould. The advantage of an autoclave is that relatively simple tooling can be used to produce high performance parts reliably which, if the run length is short, is an advantage. This does not of course take account of the autoclave itself which is expensive but will normally be used for many different parts. The advantage of the other process is that it is faster and simpler and whilst moulds may be more expensive and there is no need for an expensive autoclave.
Both processes are well adapted to producing high performance parts and because tooling is relatively simple they can be used effectively for even very large parts. The cost of materials, disposables and labour is however high so that parts based on vacuum bagged prepregs can be expensive.
Press Moulding
Where larger numbers of high performance parts are required such as in the sports market press moulding of prepregs has been developed. The basic prepreg material remains broadly similar but in this version of the process heated matched metal moulds and hydraulic presses are used to stamp out parts. The process consists of cutting and preparing a prepreg pack and then placing this pack in a heated matched metal mould fitted to a hydraulic press. Again the pack will be designed to meet the requirements of the part and may include several different types of prepreg as well as other materials. The press is then closed onto the prepreg forming into the shape of the mould and pressurising it in the mould cavity.
The moulds are heated to a temperature of between 150 an 180°C which is sufficient to cure the part in from 10 to 30 minutes depending on the type of prepreg used. The press moulded process is much quicker and less labour intensive than the vacuum bag process but the need for metal moulds and hydraulic presses means that only small to medium sized parts are viable and run lengths need to be of the order of at least 3 - 4000. Typical applications for this type of process include skis and leaf springs for cars and trucks.
Compression Moulding Of SMC
Compression moulding of Sheet Moulding Compound (SMC) and the closely related Bulk Moulding Compound (BMC) represent the second most widely used type of composites process used world-wide, accounting for more than 25% of all composites use in Europe and the USA. These processes are highly automated and designed to meet the requirements of industrial users where run lengths can exceed 50,000 parts.
SMC has some similarities to prepregs in that it comes as a sheet containing both resin and fibre; however in this case the resin is predominantly unsaturated polyester and the fibre almost exclusively chopped glass fibre (Usually 25mm ). SMC normally contains high levels of mineral filler and is available from the compound supplier in a very wide range of different formulations including different fibre contents, different surface qualities and different colours.
Similarly to prepregs SMC is moulded in heated matched metal moulds mounted in a hydraulic press but the SMC pack is prepared at the correct weight to fill the mould cavity and cut so that it covers only 50 - 70% of the mould surface. The pack is placed in the mould which is heated to 140 - 160°C and the press closed. The combination of heat and pressure as the press closes causes the SMC to flow and completely fill the mould cavity. Curing is very rapid and parts can be demoulded in as little as one-minute although 3-4 minutes is more typical.
Moulds for SMC are designed with a small gap or shear edge at the periphery of the mould that allows air to escape as the compound flows. However at 0.3 - 0.6 mm this is so small that the compound cures as it enters the gap effectively sealing the mould. This means that SMC parts require very little finishing and trimming after demoulding making for a very rapid automated process. A further advantage of SMC is that because it is a flow moulding process complex features such as ribs, bosses and fixings can be moulded in rather than needing to be bonded on later.
The disadvantage of SMC is that the initial investment cost is very high which means that normally a minimum of 10,000 mouldings is required to make the process viable. Also as with press moulded prepregs there is a limit on the size of mouldings due to the high cost and difficulty in manufacture of large metal moulds and hydraulic presses. In recent years the development of SMC type compounds which mould at lower pressures (Low Pressure Moulding Compound or LPMC) has allowed larger smaller series parts to be produced economically but SMC is essentially a medium to high volume process.
SMC is used in a wide variety of different applications including car and truck parts, electrical cabinets and switchgear, sectional water storage panels and modular buildings. SMC's offer increased stiffness, and higher dimensional stability compared to many other composites materials but generally have more variable mechanical properties.
BMC is similar to SMC except that it contains less glass fibre cut to a shorter fibre length than SMC and is delivered in the form of a dough rather than a sheet. BMC is more often used where mechanical performance is less critical and for smaller more complex mouldings.
Pultrusion
Pultrusion is a process whereby continuous fibres in the form of roving, tape or fabric are impregnated with resin, pulled through a shaped die and cured to create a continuous profile. The process is the equivalent to extrusion of thermoplastic polymers or metals. Typical sections include I beams, T sections circular and square tubing as well as more complex geometries.
Unlike most other composites processes pultrusions are available in standard sections as well as custom made profiles and so it is relatively easy to specify a particular standard profile from a maker's catalogue that will have a defined set of properties.
Pultrusions have excellent mechanical properties and are often used in aggressive or corrosive environments that would present problems for certain metals. Typical applications include access ladders and walkways on oilrigs, concrete reinforcement bars, roof trusses and space frames. Like all composites they have excellent strength to weight ratios and can be designed to meet a variety of different loading conditions by utilising a combination of on axis, off axis fibres and speciality fabrics.
For many applications standard pultrusions are available but it is relatively easy to create custom profiles for large projects as the pultrusion dies are not overly expensive. Pultrusion is a highly automated process but it is still relatively slow compared to extrusion and the raw material cost is quite high. Pultrusions find application where their lightweight and corrosion resistance are key. Most recently pultrusions have found use in bridge structures and infrastructure repair particularly in the USA. Pultrusion accounts for about 5% of the composites market worldwide but its use is growing rapidly particularly in building applications.
Filament Winding
Filament winding consists of impregnating continuous roving tape or fabric with resin and applying it to a rotating mandrel. In order to achieve the required mechanical properties fibres are oriented at different angles resulting in complex winding patterns that are normally computer controlled. The process is not entirely limited to cylindrical shapes and both conical tanks and box beams can be produced. When finished the resin is allowed to cure and the moulding removed from the mandrel. Filament winding can produce very large tanks and as the process can be very precisely controlled the fibre content and orientation is very uniform giving very reliable structural performance.
More recently in an adaptation of this technology bridge piers have been strengthened by wrapping with composite material using a modified portable version of a filament winder.
Centrifugal Casting
Centrifugal casting is primarily used for pipes and tanks and consists of co-spraying resin and chopped fibre onto the inner face of a rapidly rotating circular mould. The centrifugal force created by the rapid rotation of the mould causes the resin to impregnate though the glass fibre forming the moulding and also results in a relatively smooth inner surface on the pipe or tank. The process can also used to produce slightly tapered parts such as masts or poles. There are some very large companies in the Middle East producing large quantities of pipe by this method.
Continuous Sheeting
Continuous sheeting is produced in a factory on a dedicated machine in either flat or corrugated profile and can be coloured or translucent. The process consists of depositing a layer of resin onto a moving plastic film followed by chopped glass fibre. The fibre is then impregnated with resin by rolling and the sheet passed through an oven where it is cured, usually by heating, although in some cases UV curing is employed. In the case of corrugated sheet the corrugations are introduced using rolls or formers during the curing stage. The finished sheet is then either rolled or cut into individual lengths for packaging and transportation Typical applications include cladding, roofing and refrigerated truck bodies
Procurement
There are certain composite parts such as pultrusions, pipes and continuous sheeting that are available in standard sizes so that once manufacturers have been located specification and procurement is relatively straightforward. The vast majority of composite parts are however custom designed and moulded to meet the needs of a particular application. As described in this paper there is a very wide range of different composites processes and very often companies will specialise in one or two processes. It is however rare to find a company that proposes every process and material combination and with more than 2000 composites moulders in the UK alone procurement can be a challenge. A useful source of information and advice can be the raw material suppliers particularly the resin and reinforcement companies who will often be willing to advise on the choice of process and sometimes recommend suitable moulders.
References
John Murphy, Reinforced Plastics Handbook, Elsevier Advanced Technology
Scott Bader, Crystic Polyester Handbook, Scott Bader Co Ltd, Wollaston, NN29 7RL
J F Monk , Thermosetting Plastics - Practical Moulding Technology, George Goldwin
Hamid Kia, Sheet Moulding Compound - Science and Technology, Hanser
CIRRIA,The Use of Epoxy, Polyester and Similar Reactive Polymers in Construction, Vol 1,2 and 3
CIRRIA, Composite in Construction
Scott Bader, Processes for Composites Manufacture, Scott Bader Co. Ltd |