Production of thermosets

11.2.3 Production of thermosets

Development of methods for shaping thermosetting materials is restricted by the need to accommodate a curing reaction and the absence of a stable viscoelas- tic state. Until fairly recently, these restrictions tended to limit the size of thermoset products. Compres- sion moulding of a thermosetting P-F resin (Bakelite) within a simple cylindrical steel mould is a well-known laboratory method for mounting metallurgical samples. Resin granules, sometimes mixed with hardening or electrically-conducting additives, are loaded into the mould, then heated and compressed until crosslinking reactions are complete. In transfer moulding, which can produce more intricate shapes, resin is melted in

a primary chamber and then transferred to a vented Figure 11.6 (a) Injection-moulding machine, (b) production

moulding chamber for final curing. In the car indus- of plastic pipe by extrusion and (c) thermoforming of plastic

try, body panels with good bending stiffness are pro- sheet (from Mills, 1986; by permission of Edward Arnold) .

duced from thermosetting sheet-moulding compounds (SMC). A composite sheet is prepared by laying down layers of randomly-oriented, chopped glass fibres, cal-

from this device. On leaving the die, the continuously- cium carbonate powder and polyester resin. The sheet formed extrudate enters cooling and haul-off sections.

is placed in a moulding press and subjected to heat and Frequently, the extrudate provides the preform for a

pressure. Energy requirements are attractively low. second operation. For example, in a continuous melt-

Greater exploitation of thermosets for large car parts inflation technique, tubular sheet of LDPE or HDPE

has been made possible by reaction injection-moulding

358 Modern Physical Metallurgy and Materials Engineering (RIM). In this process, polymerization takes place dur-

Figure 11.7 shows the typical fall in apparent shear ing forming. Two or more streams of very fluid chemi-

viscosity which occurs as the shear stress is increased. cal reactants are pumped at high velocity into a mixing

If Newtonian flow prevailed, the plotted line would chamber. The mixture bottom-feeds a closed chamber

be horizontal. This type of diagram is plotted for where polymerization is completed and a solid forms.

fixed values of temperature and hydrostatic pressure. A Mouldings intended for high-temperature service are

change in either of these two conditions will displace stabilized, or post-cured, by heating at a temperature

the flow curve significantly. Thus, either raising the of 100 °

C for about 30 min. The reactive system in RIM temperature or decreasing the hydrostatic (bulk) pres- can be polyurethyane-, nylon- or polyurea-forming.

sure will lower the apparent shear viscosity. The latter The basic chemical criterion is that polymerization in

increases with average molecular mass. For instance, the mould should be virtually complete after about

fluidity at a low stress, as determined by the standard

30 s. Foaming agents can be used to form compo- melt flow index (MFI) test, 1 is inversely proportional nents with a dense skin and a cellular core. When glass

to molecular mass. At low stress and for a given fibres are added to one of the reactants, the process is

molecular mass, a polymer with a broad distribution of called reinforced reaction injection-moulding (RRIM).

molecular mass tends to become more pseudo-plastic RIM now competes with the injection-moulding of

than one with a narrow distribution. However, at high thermoplastics. Capital costs, energy requirements and

stress, a reverse tendency is possible and the version moulding pressures are lower and, unlike injection-

with a broader distribution may be less pseudo-plastic. mouldings, thick sections are not subject to shrinkage

Figure 11.8 provides a comparison of the flow problems (‘sinks’ and voids). Cycle times for RIM-

behaviour of five different thermoplastics and is useful thermosets are becoming comparable with those for

for comparing the suitability of different processes. It injection-moulded thermoplastics and mouldings of

indicates that acrylics are relatively difficult to extrude SMC. Stringent control is necessary during the RIM process. Temperature, composition and viscosity are rapidly changing in the fluid stream and there is a chal- lenging need to develop appropriate dynamic models of mass transport and reaction kinetics.