MOULDING PRESSURE

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. 

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required. 

Moulding conditions, i.e moulding pressure, temperature and time. 

Figures 23.21 and 23.22 show how two variables, moulding pressure and resin content, affect the mechanical properties of a laminate.

Figure 23.21. Effect of moulding pressure on the tensile and shear strength of Kraft paper laminates.

Urea-formaldehyde moulding powders may be transfer moulded. Pressures of 4-10 ton/in (60-150 MPa), calculated on the area of the transfer pot, are generally recommended. 

Melamine-based compositions are easily moulded in conventional compression and transfer-moulding equipment. Moulding temperatures are usually in the range 145-165°C and moulding pressures 2-4 ton/in (30-60 MPa). In transfer moulding pressures of 5-lOton/in (75-150MPa) are used. An in thick moulding required about 2 minutes cure at 150°C but shorter times are possible with preheated powder. 

A number of different resins are available and the ultimate choice will depend on the end use and proposed method of fabrication. For example, one resin will be recommended for maximum strength and fastest cures whilst another will have the best electrical properties. Some may be suitable for low-pressure laminating whilst others will require a moulding pressure of lOOOlbf/in (7 MPa). 

Figure 2 Light permeability of polyolefins after quenching (1-4) and of nonquenched samples (l -4 ) l,l -poly-propylene (PP) 2,2 -high-pressure polyethylene (HPPE) 3,3 -low-pressure polyethylene (LPPE) 4,4 -medium-pres-sure polyethylene (MPPE). Film thickness-150 fic moulding time-10 minutes, moulding pressure HPPE-160°C LPPE, MPPE, PP-190-200X.

The pressure which must be exerted on each square unit of the surface of the product for satisfactory moulding, i.e., proper consolidation and freedom from porosity. It is usually expressed in lb/in2 or kg/cm2. Moulding pressure is not critical for rubber (700 lb/in2, 5 MPa minimum) but is very critical for efficient moulding of the various types of plastics. Moulding pressures are calculated on the projected area of the product and should include the area of the spew groove. 

Excess flash Excessive stock loading, improper mould aligning, low pressure and improper mould design. Loading blanks of correct weight, increase moulding pressure and replace loose dowel pins in the mould. 

Blooming Undercure, low moulding pressure, low temperature. Correct heating device, ram movement and improve compounding. 

In order to establish statistically usable data on the briquetting behaviour of Rhenish brown coals, the 15 brown coal lithotypes were briquetted under identical conditions (water content, grain size distribution and mould pressure) with a laboratory press. 

One of the methods for further processing of such paste pieces is pressing in plaster moulds. The mould is porous so that it releases compressed air and simultaneously absorbs some of the water from the mix. The technology is inexpensive and used in particular with small, simple articles where low moulding pressures are required. 

Figure 7.10 Thermoforming (1) female mould, vacuum, no prestretching (2) female mould, pressure forming with male or plug, prestretching (3) male mould, vacuum, with pressurised air prestretch...

By decreasing the moulding pressure, the interchain distance similarly increases and the same type of... 

The results also indicate that it is possible to raise the values of tan 6, for a given level of c", by lowering the moulding pressure of samples. 

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