Hand lay-up processes

Some typical properties of polyester-glass laminates are given in Table 25.1. From these figures it will be seen that laminates can have very high tensile strengths. On the other hand some laminates made by hand lay-up processes may have mechanical properties not very different from those of thermoplastics such as the polyacetals and unplasticised PVC. 

Wet Lay-Up Process. Formulations are impregnated by hand or sprayed into fibers that are in the form of woven, knitted, stitched, or bonded fabrics. This is usually accomplished by rollers or brushes. Spray-up is a partly mechanized version of the hand lay-up process. The thermoset precursor is applied by means of a spray gun to the mold surface simultaneously with the chopped glass-fiber roving. Laminates are left to cure under standard atmospheric conditions. 

The hand lay-up process can be used for fabricating boat hulls, automobile bodies, swimming pools, chemical tanks, ducts, tubes, sheets, and housings, and for building, machinery, and autobody repairs. 

Figure 5.1 7 Automated-integrated RP vacuum hand lay-up process that usesTS polyester prepreg sheets...

Figure 6.3 Hand lay-up process. Cross-section of GRP boat hull being removed from the mould...

In the spray-up process, a mould is made, just as in the hand-lay-up process. Continuous roving is fed into a chopper gun, which as its name suggests chops the fibres and sprays them onto the mould resin is also sprayed at the same time. The fibre and resin streams meet each other on the mould surface. The laminate is then rolled to remove as much air as possible. The fibre content achieved with this process is typically between 25-35 wt%. This method is rarely used by itself for corrosion-resistant vessels but is used in combination with filament winding for larger vessels. 

Manufacturing and assembly of the individual segments will be done manually to ensure effective quahty control and to be able to accommodate late design and material changes. Instead of a negative form, a positive mall will be used. The hand lay-up process will start with the inner foam layer and work to the top of the complete composite sandwich which will be finalised by applying the inner fibre layer. 

The possibilities for making bonded structural sandwich elements in a variety of materials are very real. However, whilst there exist structural examples such as aluminium honeycomb panels (used in aircraft and transport applications) and metal skinned foam sandwich panels (used as the monocoque chassis in refrigerated transport applications), these composite constructions are normally utilised in non- or semi-structural ways. Typical skin materials are steel, aluminium, GRP and plywood, and common core materials are rigid foam polystyrene, polyurethane, polyisocyanurate, PVC, and honeycombed aluminium. In some instances the foam core is injected between the skins and adheres to them in others, adhesives are used to bond the separate components together. The nature of the manufaeturing process depends on the type of structure to be made, and the degree of investment in produetion maehinery. Both flat and eomplex eurved forms ean be made by a hand lay-up process as well as in an automated way. 

For hand lay-up processes, trial pieces should be made and tested. These may be either sacrificial parts of the unit that can be removed for testing or specially prepared samples made at the same time and by the same process. In both cases care should be taken to ensure that the sample is representative of the material in the finished unit. 

Resin transfer moulding is used to replace the hand lay-up process for better productivity and quahty. In this process, resin is injected into a closed cavity mold that is filled... 

Figure 6.6 Hand lay-up process for producing highly reinforced parts with fiber matting and polyester, epoxy, or other thermosetting liquid resin systems.

Similarly, special SMC formulations were developed for the ERF Lorry cab, and in the case of the High-Speed Train cab, the hand lay-up process is developed. In the following sections of this chapter various aspects of these processes are discussed at greater length, and some comparisons are made, especially between SMC and the hand lay-up process which may compete with one another. 

In contrast to the hand lay-up process hot press moulding is ideally suited to long production runs of relatively small components. The moulding operation is as follows ... 

Prior to ERF producing SMC-clad lorry cabs the company had acquired several years experience of moulding GRP panels for their LV series lorry cab. However, these panels were manufactured by the hand lay-up process which has certain limitations, see Chapter 15, PST 4. 

A special grade meets the demand for halogen-free materials for pultruslon, resin transfer moulding, filament winding, and hand lay-up processes, giving highly retarded composites at a comparatively low addition rate. With this additive, at approximately 50 parts per hundred resin (phr), the Epiradiateur NF 92-501 test - one of the most stringent in Europe - can be passed with a classification of Ml, where 250 phr is required with aluminium hydroxide. With epoxy resins, the same low smoke density is produced as with polyester laminates, at addition levels of 20-50 phr. The aviation standard FAR 25.8 53 can be passed with an addition of 25-35 phr. A liquid-form additive is available where no powder additive can be used. 

Most of the high-volume automotive polymers arc thermoplastics the common virtues of polypropylene, ABS (acrylonitrile butadiene styrene), PVC (polyvinyl chloride), nylon, polyethylene and polycarbonate etc., are their versatility, ability to be injection moulded or extruded into intricate shapes, and suitability for mass production. Thermosets used in significant volume include phenolics, long used as electrical components, and now finding application in non-burning creep-resistant underbonnet items, and glass-reinforced polyesters, used originally in hand lay-up processes, but nowadays widely used in SMC. 

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