Bench of mould
irrespective of the machine tool or technique used to manufacture the various parts of the mould, the final responsibility for the finishing of the individual parts and for fitting them together lies with the bench fitter. The mould finishing and assembly procedure adopted by the bench fitter varies from toolroom to toolroom; it is therefore impossible to set down a standard pattern for the work. In consequence, we intend only to indicate the general approach to this problem without going into details. we will do this by considering the various stages in the bench fitting involved in the manufacture of a simple mould. The various stages are list as follows.
Stage 1. Finishing the impression
when the mould plates are received from the machine tool section, the impression from (on both plates) is in the rough machined state. Cutter marks, burrs, etc., are very apparent on the surface. the bench fitter’s first jobis to produce a cavity and core free of machine marks and to the shape and dimensions specified on the mould detail drawing.
Stage 2. Aligning cavity and core
the operation is to align the two parts with respect to each other so that the molding produced will have the correct wall section.this is achieved by using packing pieces between the cavity and core.
the two mould plates are clamped together and returned to the milling or jig boring machine to have guide holes bored through both plates. when this operation is complete, the clamps are removed, the mould plates separated and the guide pillars and guide bushes fitted. the two mould plates are again throught together and checked to ensure that the core is in alignment with the cavity.
Stage 3. Bedding down
this is the process of marrying the two opposing mould halves together to prevent the plastic material escaping between the two surfaces when the material is injected into the impression.
one surface (the core plate in our example) is given a very fine coating of tool maker’s blue, the two plates are then momentarily brought together and where there are high spots on the second mould plate (the cavity plate in our example), blue will be picked up. these high spots are removed by scraping and filing.
Stage 4. water cooling circuit
The holes drilled forthe water circulation in the mould plates are tapped and plugs, baffles, or connectors fitted as appropriate.
Stage 5. Fitting ejector system
(i) holes to accommodate the ejector pins and push-back pins are marked out on the mould plate and subsequently bored and reamed.
(ii) the retaining plate is nominally clamped in position below the mould plate. the ejector holes, etc. are spotted through to this plate.
(iii) the retaining plate is drilled and counterbored to accommodate the ejector pins and push-back pins.
(iv) the ejector half of the mould consisting of the mould plate, support blocks and back plate are marked out according to the mould detail drawing, drilled, counterbored and tapped where specified. the entire moving half of the mould is assembled.
Stage 6. Fitting sprue bush and register ring
Turning to the fixed mould half, the sprue busy and the register ring are located and fitted. Clamping holes are marked out (with respect to the register ring), drilled and tapped.
Stage 7. Polishing, hardening and try-out
the mould is disassembled and the cavity and core form polished. All part which required heat treatment are sent for hardening. when this operation is complete the mould is reassembled and the cavity and core form given a final polish. The mould is then sent for try-out on an injection molding machine to produce a sample molding. This is checked and if necessary, adjustments are made.
