Composites Compression moulding

Both thermoplastics and thermosets can be formed by compression moulding (Fig. 24.5). The polymer, or mixture of resin and hardener, is heated and compressed between dies. The method is well suited to the forming of thermosets (casings for appliances, for instance) and of composites with a thermosetting matrix (car bumpers, for example). Since a thermoset can be removed while it is still hot, the cycle time is as short as 10 seconds for small components, 10 minutes for large tliick-walled mouldings. Pressures are lower than for injection mouldings, so the capital cost of the equipment is much less. 

Injection moulding - Elastomers Compression moulding - Composites Hand/spray lay-up - Composites... 

One limitation of epoxide moulding compositions is their short shelf life (typically 1-3 months), which necessitates strict stock control. The compounds may be compression, transfer or injection moulded, although compression moulding is preferred for long-fibre grades. 

Other types of compression moulding and stamp forming used for continuous fibre reinforced composites are illustrated in Fig. 4.73. 

Just as an example, the X-ray diffraction patterns of compression moulded samples of PVDF, poly(vinylfluoride), and of some VDF-VF copolymers of different compositions are shown in Fig. 17 [90]. The degrees of crystallinity of the copolymer samples (40-50%) are high and analogous to those of the homopolymer samples. This indicates a nearly perfect isomorphism between the VF and VDF monomeric units [90, 96], The diffraction patterns and the crystal structures of the copolymers are similar to those of PVF, which are in turn similar to the X-ray pattern and crystalline structure of the P form of PVDF. On the contrary, the X-ray pattern of a PVDF sample crystallized under the same conditions (Fig. 17 a) is completely different, that is typical of the non-piezoelectric a form [90]. 

A significant amount of waste composites is generated each year and the need for a recycling method is becoming a necessity. Environmental Technical Services has developed, with the support of the University of Missouri-St.Louis, a method for recovering valuable constituents from composite materials. The process converts the polymer matrix to lower chain hydrocarbons and fuel gas leaving behind fibres. Mechanical tests of BMC panels, reinforced concrete and compression moulded panels made with recovered fibres were carried out. 10 refs. USA... 

Processing of LFRT or LFT - long fibre (reinforced) thermoplastics injection, composite insert moulding, compression moulding, extrusion-compression. 

Towflex (Hexcel Composites) is based on carbon, glass or aramid fibres impregnated with polypropylene, polyamide, PPS, polyetherimide, PEEK. Product forms include flexible Towpreg, woven fabric, braided sleeving, UD tape, chopped compression moulding compound, moulded plates, thermoformable laminates. 

In marked contrast to conventional injection moulding, where orientation effects normally depress conductivity, in this investigation the injection-moulded composite material yielded not only a lower percolation threshold than compression moulded samples, but in the injection direction, also gave conductivity values two to three orders of magnitude higher than the latter. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

A compression moulding technique [94] has been used to prepare composites of polystyrene with layered perovskites C12Mn polystyrene powder (Mn = 145 000) and finely powdered C12Mn (average particle size 150 pm) are dryblended and subsequently compression moulded at 160 °C and a pressure of 50 kg/cm2. 

Transfer moulding involves the distribution of the uncured stock from one part of the mould called as pot into the actual mould cavity. This process permits the moulding of intricate shapes or the introduction of inserts like metals in many composite products. These procedures are difficult in compression moulds. Although these moulds are relatively more expensive than compression moulds, the actual process permits shorter cure times through the use of higher temperatures and better heat transfer which is obtained due to higher pressure applied to force the compound into the mould. 

Composites containing 0-50 vol% glass were prepared by milling at 170°C for 15 min on an open two-roll mill and compression moulding at 190°C into 3 mm thick plates. The cooling rate was approximately 40°C/min. The samples were stored at room temperature for several years before the measurements. 

Pieces of the compression-moulded sheets were pressed into dogbone-shaped specimens of thickness 1.7 mm and a gauge width of 15 mm for the tensile tests. Stress-strain curves were obtained at room temperature with a Zwick tensile tester at a strain rate of 0.0001 s. The samples were strained up to 3.6% strain and the strain was measured with an extensometer with a gauge length of 50 mm. The tensile tests were done only on the composites with 50 vol% glass beads, with different amounts of the silane. 

M.D. Wakeman, T.A. Cain, C.D. Rudd, R. Brooks, A.C. Long, Compression moulding of glass and polypropylene composites for optimised macro- and micro-mechanical properties II. Glass-mat-reinforced thermoplastics. 

Haque, E. Burr Leach, L. Matched metal compression moulding of polymer composites. In Handbook of Composites Matched Metal Compression Moulding of Polymer Composites Peters, S.T., Ed. Chapman Hall, 1998 378-396. 

Compositions of polypropylene with unmodified and modified chalk were obtained by means of a screw extruder at 210 C. The granulated product was used to prepare compression moulded films 0.4 mm thick which were pressed at 190 C and then quenched in iced water. From films obtained in this way oarshaped samples with the dimensions as in Fig. 2 were punched. In all cases the samples were cut out form the same places in compression moulded films in order to provide approximately the same flow-induced orientation of chalk particles relative to the sample axis. For the purpose of tensile properties and volume changes measurements the samples were marked (see Fig. 2). 

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