In a blow moulding machine having a blow moulding nozzle and a closing and opening frame for mould parts, each activated by a hydraulic cylinder, the cylinders being hydraulically connected in parallel and a centering device being provided for the moulded parts. The centering device is capable of indicating the position of at least one of the mould parts relative to its intended position in its closed state. Also, the centering device is capable of displacing each of the mould parts in the same direction as a function of such indicating position.
This invention relates to a moulding machine, for example blow moulding machines, with a hydraulically-actuated frame for opening and closing a multi-part mould. In a typical machine, there is one hydraulic cylinder associated with each of the frame parts which carry a part of the mould, these hydraulic cylinders being hydraulically connected in parallel and being coupled to one another so as to maintain synchronous operation. When the mould is open, a tubular blank of heat-deformable plastics material is intorduced. When the mould is closed, compressed air is blown into the blank by means of a blowing nozzle communicating with the orifice of the tube.
In known blow moulding machines of the aforementioned type, difficulties arise during actuation of the closing frame by means of the hydraulic cylinders to center the mould relative to the blowing nozzle orifice when the mould is closed. This problem is met in conventional machines by mechanically coupling the movable parts of the frame in such manner that the coupling members -- for example, a chain -- compensate for any force differences between the individual parts of the frame. When the machine is new, this system operates quite reliably even though the centering which is achieved may not be altogether accurate. After continued use, however, it has been found that the mould tends to become off-centered with respect to the fixed blowing nozzle due to the wear of the coupling members such that a cumulative effect of the force differences will manifest itself. Also, during closure it has been found that one or more portions of the mould may strike the contact surfaces of the blowing nozzle under force, such that both the blowing nozzle and such portions of the mould may become deformed. Furthermore, gaps may develop through which the moulding material may seep during shaping, so that the mould and possibly also the blowing nozzle would need to be repaired. Still further, the mechanical coupling members tend to complicate the construction so that the mould and the frame are not readily accessible. The mechanical coupling members also impose some limitations on the design of the machine.
