Patent GB-645356-A (GB)
Title: Process and apparatus for shaping thermoplastic sheet material to form frames
IPC: B29D12/02
Filed: 1947-07-11
Abstract: 645,356. Moulding spectacle and other frames from thermoplastic sheet material. MONCRIEFF, L. J. July 11, 1947, No. 18528. [Class 87 (ii)] Frames for spectacles, small mirrors, handlenses, &c. are simultaneously blanked and moulded from cellulose acetate sheet or other thermoplastic sheet of a thickness less than the maximum thickness of the frame by a pair of dies which when closed meet along lines which define the boundaries of the frame and also meet over marginal areas immediately outside the boundaries. The sheet material lying in these areas is thus trapped and some is squeezed sideways across the boundaries to augment the thickness of the frame. Figs. 3 and 7 are cross-sections of two pairs of dies for moulding spectacle frames. In Fig. 3 the upper die has two flat projections 15, for blanking out the eye-piece apertures, which slightly overlap recesses 10 in the lower die which receive waste material. Upstanding ridges 9 with flat tops, whose inner contour corresponds to the outer contour of the frame are provided on the lower die by mounting a plate 4 thereon, suitably machined to shape. Nose-piece projections and recesses 6 and nose-pad recesses 13 are provided in the appropriate dies. A sheet 17, which may be preheated of thickness less than that of the finished frame is positioned between the dies which are then closed, being mounted in a suitable press for this purpose. Material is trapped between the ridges 9 and the upper die and the overlapping portions of the projections 15 and recesses 10, and some of it is squeezed sideways into the mould cavity. The dies have channels 2 for the circulation of heating and cooling fluid and passages 12 allow the recesses 10 to communicate with the atmosphere to vent the recesses 10 and permit escape of waste material if the recesses are not sufficiently large. In Fig. 7 the blanking projections 24 for the eye-pieces are mounted on a plate 23 contoured to a size slightly larger than the inner boundaries of a plate 22 which conform to the outer boundaries of the frame to be moulded. Overlapping boundary portions are thus provided as before. Suitable projections and recesses for the nose bridge and pads are also provided. Heating and cooling passages are provided in the dies but not passages 12, the recesses 25 being made larger. If it is found that there is insufficient back pressure to squeeze sufficient material over the inner boundaries, small packing pieces 28 may be introduced into the recesses 25. Instead of positively cooling the dies, reliance may be placed on air cooling. Alternatively two presses may be used, one heated and the other cooled. After pressing out a frame in the hot press the dies are transferred to the cold press and kept under pressure until cool, a second set of dies being inserted into the hot press while the others cool, the dies being heated only by conduction from the press. A comprehensive list of suitable moulding materials is given, including cellulose derivatives and synthetic resins such as polymethyl-methacrylate and polyvinyl chloride and acetate. According to the Provisional Specification material from the inside only of the frame is transferred across the inner frame boundary to augment the thickness of the frame and one or other of the die members carried cutting edges for cutting the sheet at the outer boundary of the frame. Also, the bores for escape of excess material may be formed in either die member.