GB861326A - Molding process and apparatus - Google Patents

Abstract

861,326. Moulding plastic articles. OCEANA INTERNATIONAL Inc. Feb. 27, 1957 [March 1, 1956(2)], No. 6602/57. Class 87(2). A quantity of liquid or semi-liquid additiontype resin sufficient to fill the cavity or cavities of a heated two-part mould is disposed between two sheets which are inert towards the resin at least one being of a highly stretchable material and being arranged between the resin and that part of the mould which contains the cavity or cavities or the deeper cavity or cavities whereby the sheets are forced into and caused to line the walls of the cavity or cavities by the compressed resin as the mould is closed. In one arrangement for the moulding of ployester buttons, the resin 76 is placed between two sheets of polyvinyl alcohol 74, 75, which extend a considerable distance beyond the outer mould cavities in each direction. The resin so sandwiched is then placed on the bottom mould part 70. The mould, which is heated, is then rapidly closed by raising the lower mould part and the resin is first squeezed between the lower part of the mould and a spring-loaded floater plate 59 containing apertures 67 corresponding to the mould dies 53, 71. The resin and upper film are squeezed into these apertures and as the mould continues to close and the floater plate rise relative to the die-retainer plate 52, more spring pressure is brought to bear on plate 59 and additional resin is forced into its apertures, very little being left in the areas between cavities (where flash is normally formed). At the same time the dies 53 enter the apertures, pressing the liquid resin and overlying film to the shape of their cavities and into the cavities of dies 71, the lower film 74 being stretched and forced against the wall of the lower cavities. There is a slight excess of resin in each aperture so that as the press approaches the closed position this excess is forced out to form a flash between the floater plate and the bottom half of the mould. The thickness of the floater plate 59 should be such that in the closed position of the mould, the bottom surface of the plate is spaced from the top surface of the retainer plate 70 by a distance not substantially greater than the thickness of the two films in their compressed condition. As a result, the resin encounters greater resistance to flow in the horizontal than in the vertical direction, preventing excessive flash and possible imcomplete filling of the die cavities. If desired, a ridge 77 on the floater plate extends about the moulding area and enters a groove 74 on the bottom plate 70 as the mould is closed in order to prevent the lateral extrusion of the resin while yet allowing escape of excess resin as the pressure approaches the maximum. In a modification of the apparatus, the floater plate is dispensed with and surrounding the dies 53, is a continuous sealing ring which slides in a recess in the upper mould plate and which can rise only against a spring resistance. This ring normally projects below the surface of plate 52 to an extent that it engages the top film 75 before any considerable pressure is applied against the mass of resin and becomes effective in sealing the films before the resin begins to spread between them. The two films may be sealed at one, two, or three sides, leaving the other side(s) open for the ecsape of air until they are sealed by the clamping action of the mould. If a plastic bag is used, air is removed by inserting a hypodermic needle attached to a vacuum pump, the hole made by the needle being sealed upon removal of the air.