Mould shrinkage resins

In some moulding compositions other special purpose ingredients may be incorporated. For example, naphthalene, furfural and dibutyl phthalate are occasionally used as plasticisers or more strictly as flow promoters. They are particularly useful where powders with a low moulding shrinkage are required. In such formulations a highly condensed resin is used so that there will be less reaction, and hence less shrinkage, during cure. The plasticiser is incorporated to... 

Distitron 270 S An unaccelerated orthophthalic SMC/BMC moulding compounds. resin specifically for the manufacture of low shrinkage, low profile... 

Shrinkage is important in processing as one normally moulds direct to final shape. Apart from using a low shrinkage resin (Table 2.4), the other option is to use a filled resin. Adding filler tends to increase the viscosity, so there is a trade-off between these two processing parameters. 

As manufactured, these polymers usually contain various types of fillers and this has obvious food contact implications. The fillers are used to reduce cost, improve shock resistance and to reduce mould shrinkage. Phenol-formaldehyde resins are resistant to common solvents, acids, aqueous salt solutions and hot oils, although water and alcohols cause slight swelling. As far as food contact applications are concerned the main use of these resins is in the manufacture of closures for containers such as jars and bottles. Direct contact is less likely in these applications because of the interposition of a liner between the closure and the container. The resins are also used for moulding the handles of kitchen implements. 

Woodflour, a fine sawdust preferably obtained from softwoods such as pine, spruce and poplar, is the most commonly used filler. Somewhat fibrous in nature, it is not only an effective diluent for the resin to reduce exotheim and shrinkage, but it is also cheap and improves the impact strength of the mouldings. There is a good adhesion between phenol-formaldehyde resin and the woodflour and it is possible that some chemical bonding may occur. 

Dimension tolerances are important in compression moulding. The dimensions of the finished article are smaller than those of the mould cavity, in particular because not only thermal shrinkage but also shrinkage during the reaction counts. When using resins which develop volatiles upon curing, extra shrinkage occurs due to the escape of these components. 

---