Pressure compression moulding

Up to 15 3-10 500-20000 Low-temperature pressure compression moulding Car body elements... 

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. 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

The injection-compression process is an important variation in which rubber is injected into a partly closed mould under low pressure used for moulding-on of rubber soles to footwear uppers. Rubber is injected into a gap between the shoe upper and the sole plate, which is held 3-4 mm wider than it would be in its final position. After injection at the first mould station of a multistation machine the sole plate is closed in the manner of compression moulding at the second mould station and rubber is pressed along the sole and over the toe cap. The importance of the process is that it becomes possible to mould without using normal high injection pressures which tear and distort canvas, leather or synthetic uppers. 

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. 

Three types of moulding processes are available - compression, transfer and injection moulding. In compression moulding the rubber blank is placed directly into the cavity of the mould where it is heated by conduction which causes rubber flow by application of pressure. Transfer moulding uses prewarmed rubber which is heated during transfer and forced through small orifices into the mould cavities in a three-part mould. In the injection moulding process, the rubber compound is pushed under pressure from an injection head where it has been heated and plasticized into a closed heated mould where cure is completed. 

This process—injection compression moulding—offers other advantages besides lower clamping forces compression of the hot melt gives excellent surface finish and lower pressure at injection reduces orientation in the melt, so improving dimensional stability. The method is used currently to make compact discs of high quality from polycarbonate. 

FIGURE 4.6 Dependence of PTFE melting point on pressure. (From Compression Moulding, Technical Information, Publication H-59487, 05/95, E.I. Du Pont de Nemours Co, Wilmington, DE, p. 17, 1995. With permission.)... 

Figure 4.22. Variation of hardness H with tensile Yt (solid symbols) and compressive Yc yield stresses (open symbols). O,, compression-moulded samples A, A, annealed samples at atmospheric pressure , , CEPE samples. (From Flores et al, 2000.)...

Figure 4.23. Plot of Hj Yc vs log[(tan j8Ec)/ fc] for O, compression-moulded PE A, annettled PE at atmospheric pressure , CEPE <>, work-hardened mettds. The straight lines...

For further thermal and mechanical characterization of the blends, films were prepared from the pellets via injection moulding followed by compression moulding. The compression moulding was performed at a pressure of 5 MPa at 220 °C for 10 s. After moulding, the films were cooled between two metal blocks for 2 min. Films of different starting PET/PC ratio were prepared and tested by WAXS and found to be completely amorphous. 

Kau and Hagerman (1986) experimentally mapped pressure and temperature profiles for SMC compression moulding and noted the presence of two clear flow and cure regions, together with a correlation with lubrication flow. 

Thermosetting caps are compression moulded on either platen or rotary-type equipment. Pressure, dwell time and temperature are critical to the curing operation, otherwise brittle or soft caps may be produced. 

All the measured samples have been obtained by compression moulding of powders up to a pressure of... 

---