Sheet forming moulding

Table 2.17 analyses some examples of the relative processing cost versus the annual production in units. The various technologies listed are not suitable for all materials or parts machining is used with numerous materials rotational moulding uses liquid resins vacuum forming uses thermoplastic sheets blow moulding uses special grades of thermoplastics... 

PE, being a commodity polymer, is used in its different physical forms viz. fibres, sheets, membranes, moulds with different backbone chemical configurations (LPE, LLDPE, LDPE, HDPE, UHMWPE, UHSPE etc). Each of these forms of PE requires surface modification at some stage of application. The surfaces of PE fibres are often modified to make them compatible in the composites, whereas PE sheets/tapes are modified to achieve adhesion. Moulds are frequently surface-modified for probability and membranes for selective permeation. In the same way, different chemical configurations of PE, by the virtue of their properties, are used for different applications after surface modification. 

Sheet forming is carried out on a sheet, in most cases extruded, sometimes calendered. The sheet is first heated to above its softening temperature, then formed and thereafter cooled. The technique most frequently applied is vacuum forming the force required for deformation is brought about by a vacuum below the heated sheet, which sucks the sheet onto the mould. In its simplest form, this process is sketched in Figure 11.28. Heating is mostly achieved with infra-red radiation sources, cooling with compressed air or water sprays. 

Vacuum forming Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mould. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary moulds. 

Primary forms sheet, rod, and tube Casting from monomer for optical properties Extrusion from thermoplastic resins to produce sheet. Injection moulding for small complex parts... 

Prepreg—Ready to mould or cure material in sheet form which may be fibre, cloth or mat impregnated with resin and stored for use. The resin is partially cured to a B-stage and supplied to the fabricator for lay-up and cure. 

Sheet Moulding Compound (SMC)—A mixture of fibres, fillers and thermosetting resin in sheet form similar to DMC, but having longer fibres and higher fibre content. It is used in hot press moulding processes. 

Parallel to thermosetting SMCs and BMCs are thermoplastic moulding compounds in sheet form, known as glass mat thermoplastics (GMTs), and compounded into standard granules for injection moulding and extrusion. Most thermoplastics are theoretically capable of such combination with reinforcement, but the main types used commercially at present are polyamide (PA) and polypropylene (PP). 

Flat laminates can be produced using the processes described in other chapters of this handbook. For example, a double belt press or compression moulding can produce the precursor laminate. Figure 5.3a shows thermoplastic composites with unidirectional fibres in each layer consolidated into a laminate with a picture frame mould. A picture frame mould goes into a hot press or autoclave and turns composite lamina into a fused precursor. To some extent, sheet forming adds cost to a final structure because a manufacturer creates two components the flat precursor and the formed structure. 

Thermoplastic sheet forming is a thermal process and thermal conduction limits the production rate. Typical matrix polymers are poor conductors. Injection moulding and extrusion make polymer components affordable because they do not depend on conduction to provide all the heat needed to make a thermoplastic flow. Sheet forming, however, must bring a composite sheet to a uniform temperature and complete the forming process before any matrix - typically, the matrix at the surface because matrix there touches the cooler tool - becomes too rigid for forming. 

Agriculture Sheet or moulded forms for time-release fertilizers, plant clips... 

Various types of sheet are used for mouldings which are not too intricate. Examples include pol5 hene for thermosetting sheet moulding compoimds (SMC), chlorinated rubber, regenerated cellulose (Cellophane ), cellulose acetate in sheet form, and poly(ethylene terephthalate) (i.e., Melinex , Mylar etc.). 

Another old fashioned well proven sheet forming technology which is found in specialty paper production only is the cylinder mould former (Fig. 6.87). This sheet forming technology is still used for the production of cotton based banknote paper. The paper production on a cylinder mould former enables the application of three key security features ... 

Successful processing of unplasticized or rigid PVC (UPVC) requires lubricants, such as lead or calcium stearates, stearic acid or hydrocarbon waxes. UPVC is frequently extruded, and so lubricants melting at 100-120°C are most effective for extrusion at 165°C. UPVC is also widely used in sheet form, obtained by compression moulding multiple plies of calendered foil. 

These plastics are processable by injection moulding, blow moulding, extrusion and sheet forming and can be plated. Applications include electronic connectors, TV components, printed circuitry, projector lamp housings, and filter bowls. Produced in the USA, it is worthwhile monitoring the availability of polysulphones as applications are planned. 

In vacuum and blow forming, sheets produced by extrusion are shaped by vacuum or pressure forming. Heat-softened sheet is pressed into a mould by atmospheric pressure when a vacuum is created between the mould and the sheet. Fig. 24.4(a). Plastic bottles are made by blowing instead heated tube is clamped in a split mould and expanded with compressed air to take up its shape (Fig. 24.4b). Both methods are cheap and quick, and can be fairly accurate. 

Fig. 24.4. (a) Vacuum forming is good for making simple shapes out of sheet. (b) Blow moulding is used to make plastic containers. 

The methacrylic polymer remains a useful glazing material. In aircraft applications it is used extensively on aircraft which fly at speeds less than Mach 1.0. They form the familar bubble body of many helicopters. On land, acrylic sheet is useful for coach roof lights, motor cycle windscreens and in do-it yourself cabins for tractors and earth-moving equipment. Injection mouldings are frequently used for plaques on the centre of steering wheels and on some fascia panelling. 

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

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