Transfer moulding problems with

A major disadvantage is that the mould temperature must be kept rela-tivety low so that reaction times are lengthy if the potymer surface in contact with the hot mould cures too quickly (before the rest of the charge is heated and therefore liquified) a poor moulding can result and the mould cavity pressure may then be inhomogenous. These problems are reduced by preheated screw feed and also in the related technique of transfer moulding. 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

Mouldings are usually designed so that the melt flows from thicker into thinner sections, to ensure that the extremities are fed properly. A two-dimensional heat flow calculation illustrates a problem that can arise during solidification. Figure 6.12a shows isotherms for the cooling of a rib, with a heat transfer coefficient of 1000Wm K at the mould interface, calculated by finite element analysis. The thinner rib solidifies first, and an isolated island of melt, cut off from the melt supply, is left at the intersection of the rib and the plate. Contraction of this melt either causes sink marks in... 

---