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The major difficulty which beginners normally experience is the decision of where to start an the general procedure to adopt when designing a mould. This lesson is included as an introductory guide to permit the reader to follow a definite approach in order to establish a sequential technique of his own.7510
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In practice a mould drawing normally comprises three views: a plan view of the moving half, a plan view of the fixed half, and a side sectional view of both mould halves. This “general arrangement” drawing should contain sufficient information to permit a draughtsman to make detailed drawings of each inpidual mould part. For the novice, however, it is sufficient to draw only two views, and the views normally chosen are the plan view of the moving half and the side sectional view of both mould halves.
In the following drawings the evolution of a typical design is given in seven separate checking of the completed drawing. The operation number given in both the drawings and the notes indicates the procedural sequence of the operations.
Stage A: primary positioning of inserts. The object of the primary positioning stage is to determine the position of the impression with respect to the major horizontal and vertical centre-lines. During this stage, the size and position of the cavity and core inserts are established in both and sectional views.
Stage B: the ejector system. During this design stage the size and position of the ejector pins and push-back pins are established. The overall sizes of ejector plate and retaining plate are also determined. Before commencing to position the various ejector elements, however, it is prudent to consider the type of fluid circulation system that is most applicable tor controlling the temperature of the mould. For this basic design, a parallel drilled hole system is adopted, with two holes being incorporated in each half, the holes passing on either side of the mould insert. This preliminary consideration regarding the position of the fluid circulation system is essential to prevent difficulties being experienced later in the design.
Stage C: the ejector grid. During this design stage the drawing progresses to include detail of the ejector grid in both plan and sectional views. Further details, such as the ejector rod and ejector rod bush, are added to the ejector system.
Stage D: complete the top half. The fluid circulation system and the guide pillars are added to the plan view which permits the final outside shape of the mould to be determined and drawn and drawn. In the sectional view the sprue drawing is complete. In addition, various items such as the feed system, spure pin, bridge piece and register plate details are added.
Stage E: complete the plan. The lower half of the plan view can now be drawn as a mirror image of the top half. The runner and sprue pin details are added. Once the plan view is complete, the section cutting plane can be decided upon. It is essential that the cross-sectional view illustrates each component part of the mould. Thus, as the cross-section already shows such items as support block, push-back pin, ejector pin and other centre-line details which have been incorporated in previous stages, the only remaining items to be included are the guide pillars and bushes, and the position of the fluid holes. Note that there is no advantage in taking a section through two identical part. To de so is a complete waste of draughting time.
Stage F: complete the cross-sectional view. During this design stage the cross-sectional drawing is completed. Each part-section is considered independently in relations to the plan view to ensure that all relevant component parts of the mould are included and are drawn correctly. This will mean that the lower part of the cross-sectional drawing is completed in the process.
Stage G: complete the drawing. This final stage involves cleaning up the drawing, erasing unwanted lines, cross-hatching and indicating those parts of the mould that are to be secured together.