Manufacturing Mould Assembly

Manufacturing Mould Assembly: 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 tool room to tool room and quite often between individual tool makers working in the same tool room; 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 illustrated in the following figures.

Stage 1. Finishing the impression: When the mould plates are received from the machine tool section, the impression form (on both plates) is in the rough machined state. Cutter marks, burrs, etc., are very apparent on the surface.

The bench fitter’s first job is to produce a cavity and core free of machine marks and to the shape and dimensions specified on the mould details drawing. Basic hand tools, such as files, scrapers and chisels are used for this operations, various sizes, grades and shapes being used as and when applicable.  In addition, wherever possible, power driven flexible shaft equipments used to speed up this operation. This equipment incorporates various heads which accommodate special needle files or scrapers. The heads can have a rotary or a reciprocating motion. Once the cavity (or core) is free of machine marks the next stage is to remove the marks left by the file and the scraper. This is achieved by one or more of several techniques depending on the shape of the cavity (or core).  These techniques are honing, lapping (including diamond lapping (including diamond lapping) and emery-cloth finishing.  The last is the most common method used for simple forms.  A medium emery cloth is used initially with a suitable backing tool to remove the deep scratches left by the file and scraper. A slightly finer grade is then used in a different direction of motion to remove the scratches left by the preceding emery cloth. This procedure is continued, using progressively finer grades of emery cloths and emery papers until, finally, a scratch-free surface is obtained. At a somewhat later stage, after all other work on the cavity or core is complete, the impression form is polished.  This is accomplished with polishing cloths, mops or bobs, in conjunction with a polishing compound such as polishing soap or rouge.  Polishing, which is a lengthy, time-consuming operation must generally continue until the impression form has a mirror-like finish. Whereas a considerable amount of the bench fitter’s time is spent on finishing the impression when supplied with a mould plate or insert which has been produced by the machine tool technique, far less time is required if certain of the other mould making techniques are used, such as electro-deposition or hobbing, etc. If the cavity (or core) plate is in the form of a built-up assembly then the insert and bolster must be drilled. Tapped and counter bored (where applicable) to permit socket-headed screws to be fitted so that the assembly of parts, in fact, functions as a single mould plate. 

Stage 2. Aligning cavity and core: Once the cavity and core have been semi-finished, the next operation is to align the two parts with respect to each other so that the moulding 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 brought together and checked to ensure that the core is in alignment with the cavity. A dummy moulding is often made at this stage, using wax, so that the wall section of the product can be checked. Any slight inaccuracies need to be corrected, of course. 

Stage 3. Bedding down: The next stage is to bed down the two mould halves. 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 impression. 

Basically the process of bedding – down is simple. 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 sports 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.  This procedure is repeated until an even film of blue is transferred from one plate to the other. 

Stage 4. Water cooling circuit: The holes drilled for the water circulation in the mould plates are tapped and plugs, baffles, or connectors fitted as appropriate. 

The circuit is checked to ensure that the flow is unidirectional and that no leakage occurs.  

Stage 5. Fitting ejector system: The holes to accommodate the ejector pins and push-back pins are marked out on the mould plate and subsequently bored and reamed.  

The retaining plate is nominally clamped in position below the mould plate. The ejector holes, etc., are spotted through to this plate. 

The retaining plate is drilled and counter bored to accommodate the ejector pins and push-back pins. The ejector plate and retaining plate are marked out, drilled, counter bored, and tapped where appropriate to permit the two plates to be held rigidly together by socket-headed screws. Assemble the ejector plate assembly. 

The ejection half of the mould consisting of the mould plate, support blocks and back plate are marked out according to the mould detail drawing, drilled, counter bored 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 bush 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 parts which require 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 machine to produce a sample moulding. This is checked and, if necessary, adjustments are made. 

The mould is now ready for production.