Transfer moulding equipment

Urea-formaldehyde moulding powders may be moulded without difficulty on conventional compression and transfer moulding equipment. The powders, however, have limited storage life. They should thus be stored in a cool place and, where possible, used within a few months of manufacture. 

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. 

Kg. 5.2. Transfer moulding presses are available in many sizes and capacities. Illustrated is a 75 ton model sold by MTI Equipment, Ivyland, PA. 

The runner system of a transfer mould refers to the series of channels that allow the distribution of the moulding compound to the separate chases and eventually the individual cavities. The runners emanating from the plunger/cull area which feed each individual chase of the mould are known as the primary runners and those that distribute the material from the primary runners to each individual cavity are known as secondary runners. A typical primary runner would be hemispherical or trapezoidal in shape with a width and a depth of 3-5 mm. The secondary runners are generally around 25-35% smaller in area than the primary runners. Runners are usually machined into the ejection side of the mould and are often equipped with their own ejector pins. Whenever possible, the runners are kept short to enhance the material flow and reduce the transfer time required to fill the cavities. 

The TMS rheometer has been used to study mould release in rubbers. It contains a biconical rotor (representing the mould surface). The polymer is placed in the transfer chamber, injected around the rotor and cures in situ. Fig. 1. The shear stress required to free the rotor is taken as the mould-sticking index . The rheometer has the advantage that small experimental mixes of rubber can be evaluated. Further, the rotors are easily changed, so as to evaluate changes in mould surface, and the parted surfaces are amenable to examination by XPS and other methods of surface analysis. The obvious disadvantage is that access to special equipment (the TMS rheometer) is required. The same basic concept could be adapted for the study of mould adhesion of non-elastomeric polymers. 

It is enlightening to be able to handle a sample of the soUd polymer. A useful method for forming a sheet of polymer is to cast the Uquid into a silicone rab-ber mould, say 75 x 75 x 1mm deep. A silicone mould can only be used with materials that do not seriously swell the rubber, e.g. dispersions, epoxies and wax. Preparing a hand sample for a liquid that reacts with the mould material, e.g. solutions in organic solvents, is more difficult. One can allow a puddle on polyethene to dry or cast in a well made by waxed microscope slides (Bradley and Wilthew, 1984), but it is difficult to achieve uniformity between samples in this way. Equipment is commercially available for the purpose from polymer and paint test instrument manufacturers, and is usually more cost-effective and the results more transferable than trying to make one s own equipment. 

Production of mouldings by the transfer process requires the following equipment ... 

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