Patent GB-791611-A (GB)
Title: Improvements in and relating to moulded polyethylene containers and methods of forming same
IPC: B29C69/00
Filed: 1954-06-08
Abstract: 791,611. Moulding polythene. HEISLER, J. S., and HEISLER, A. June 8, 1954 [June 8, 1953], No. 16861/54. Class 87(2). Polyethylene is moulded by pressing polyethylene powder with a wiping action on the surface of a mould heated sufficiently to cause the particles to begin fusing together, removing excess powder, reheating the mould to complete fusion together of the particles, cooling the mould and removing the moulded article. The mould is preheated to a temperature at which the particles of polyethylene powder coalesce and build up a coating on the mould surface but which is not high enough for the polyethylene to fuse and run or to cause charring or thermal decomposition of the particles, a suitable temperature being 360F. When the moulded article is a container with a cylindrical body and a flat cover the body is moulded by applying successive coatings of polyethylene to the inside surface of a cylindrical mould by a tumbling operation and the cover is formed by successive coatings of polyethylene powder applied to a flat moulding surface by pressure wiping actions towards and along the surface. In the tumbling operation for moulding the body an excess of polyethylene powder is placed in a preheated cylindrical mould, sufficient to make it a quarter or half full, the mould being upright. The mould is then gradually inclined towards the horizontal while being rotated in alternately reversed directions, the excess powder spilling out and being collected in a trough. A uniform coating of about 20 mils thickness is thus formed. Excess polyethylene must be removed to avoid the formation of bubbles and other surface irregularities during subsequent reheating and baking. Such reheating and baking brings about incipient fusing of the freshly applied layer of sintered particles before applying an additional coat, while the mould acquires additional heat for the next coating operation. The flat moulding section is heated when lying flat in an oven with the previous coating uppermost, and the cylindrical moulding section is either heated in an upright position or laid on its side and slowly or intermittently rolled through a heating chamber. After reheating, second coatings of polyethylene may be applied to the cover and body sections in the same manner as before, though usually a coating of the same thickness can be built up on the cover section in one pressure-wiping operation as is built up on the body section in a two-stage tumbling operation, the second coating in the latter case being about half the thickness of the first, e.g. 10 mils. In the second tumbling operation the excess polyethylene powder discharged during the first operation is re-used. The reheating and coating may be repeated again, though on the other hand the two-step tumbling coating operation may be avoided and the body coating formed in a single inclined tumbling operation, but this requires more care. The two mould sections carrying the polyethylene coatings are then assembled, Fig. 2, and clamped together by nuts 15. The two coatings are fused together in another reheating stage at about 360F., and a tight joint ensured by inverting the assembly in the oven so that the body is above the cover or, preferably, by placing the assembly on its side and rolling it, with a succession of similar mould assemblies, through a tunnel type of oven. Preformed polyethylene outlet flanges may be moulded in while the cover coating is being formed, or provision may be made for forming them in the same moulding operation, as shown in Fig. 2. Here, the cover mould section has threaded openings 32, 32<SP>1</SP> into which threaded annular members 33, 33<SP>1</SP> are screwed and associated with externally-threaded tubular members 34, positioned by spacing rings 35. Solid polyethylene particles are pressed with a wiping action into the threaded annular mould cavities so formed. The members 34 serve as vents during the final fusing operation to prevent pressure building up which would cause relative movement that would damage the joint between the body and the cover. When the moulding of the container has been completed the mould with the article therein has a cold air blast or water spray applied to its outside walls, and when it has been cooled to about room temperature water may be introduced to the inside of the moulded polyethylene container while still in the mould. The cooling causes the polyethylene to shrink, and the mould is opened and the container lifted out. The moulded container is inspected for homogeneity and is subjected to a water test to detect leaks. It may also have an electric spark porosity test. It is desirable to give the moulding surfaces a prior coating of a mould release agent such as a silicone compound in the form of a liquid or grease. Suitable materials for the moulds include black iron, stainless steel, aluminium, copper and metal-clad materials. Polyethylene powder having an average particle size of 80 mesh has been found to be most suitable. 19,000 is a suitable molecular weight, and the coating is more expeditious if the particles are generally spherical. Other shapes of container than cylindrical, e.g., cubical, may be moulded by the method of this invention, and by its means the outlets need not be central. The containers are mainly for chemicals and are intended to be transported in overpacks, Fig. 3 showing a polyethylene container 20 in an overpack 16 made of sheet iron or aluminium and provided with rolling hoops 17. A top cover 18 is secured by a split ring 19 and has openings for outlets 21, 22 from the container, the latter being a vent. The outlet 21 is shown with a split metal or fibre retaining washer 25 which screws down onto the cover 18, and the vent has a similar washer. Specifications 727,470 [Group III] and 791,612 [Croup XVII] are referred to.