GB751068A - Improvements in a process for blow molding thermoplastic hollow articles and in a device for heating the thermoplastic material - Google Patents

Abstract

Before a sheet of, e.g. cellulose or vinyl acetate, or polyethylene, is blow-moulded it is held in contact with the surface of a heated plate which surface has heat retarding means arranged in a pattern such that some parts of the sheet are heated more than others. Insofar as the shape of the mould is non-spherical some parts of the sheet, when expanded, tend to stretch more than others, but if the parts which will be more stretched during moulding are preheated to lower temperatures, and those less stretched to higher, a more uniform final thickness results and thinner sheet material can be used. One way of arranging the heat retarding pattern is to stick patches of heating insulating material, e.g. of paper, of appropriate shape, on the surface of the plate. Another way is to cut recesses in the surface of appropriately varying depth and fill them with suitable cement, e.g. a mixture of ground mica and epoxy resin. A third way is to cut thin grooves in the surface of a depth inversely proportional to the desired rate of heat transfer from the plate. The required pattern of the heat retarding means is obtained by making a sample moulding, with the plate uniformly heated. If the plastic material is one with an elastic memory the thin parts are marked and the material is shrunk back, by heating, to its original form when it forms a template which can be followed in forming the heat retarding pattern. If the material does not have an elastic memory a grid pattern is marked on the sample sheet before moulding, enabling each portion of the moulded article to be properly related to its original location on the flat blank and a suitable heat retarding pattern to be designed.