Sizing of Moulds

The first stage in mould design is to determine the overall size a task governed by the dimensions of the press platens to which it will be fitted. In an ideal world, the mould should be able to be sited on the platen within the effective heating area. This generally requires an outer margin of three to four centimetres. Modern press platens are usually equipped with a guard zone . This allows the outermost parts of the mould to receive additional heat, which compensates for losses from the exposed outer surfaces. 

The design of the moulding tool is determined from the shape of the components that will be produced. For this reason it is of prime importance that the designer of the components works closely with the tool designer and the moulding technologist. Cooperation at this level is vital to produce a tool design that will work well in production. 

Failure to recognise these constraints will result in collapse, or delay, of the project [1]. 

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Table 1.6 Size of mould and required mass of material for proctor test...

The term completely randomized design (CRD) means that we determine the total number of experimental units needed in the experimentation, and then select experimental units randomly to be executed first or last. Consider, for instance, that in lithographic nanofabrication experimentation, an engineer would like to study the output from using two levels of a chemical applied to three nanoparticle types and deposited on four sizes of mould. Therefore, a total of 24 runs must be executed. This, in turn, implies that the experimenter would have to make 24 slurry preparations and apply each to 24 moulds. If experimenters make only six slurry preparations and then divide the slurry to four portions and then deposit on the different moulds, this procedure is not a CRD. (To overcome this situation in practice, we suggest the use of a split plot design and its variants.)... 

The typical size of moulding produced by spray is perhaps lOm but it is suitable for any size larger than about 2 ml Moulds which are much smaller than that are inefficient and difficult to control. Generally spray machines are operated a rate of about 20 kg of... 

The process is slow and laborious but produces excellent properties. The size of component which can be produced is governed by the size of autoclave or oven available. A typical size of moulding is 2 but components up to 100 m have been produced. The cycle time is in the range 1 to perhaps 20 hours but is typically 12 hours. Hence the process does not lend itself to mass production and is usually restricted to runs of less than 500 and, more typically, 50 off... 

The size of mouldings which can be compression moulded is dependent on the size of press available but 0.5 m is typical, with a maximum of about 3 m. ... 

The selection of the press is directly related to the type of product and the size of mould to be used. The scale of everything fitted to the press is controlled by these factors. The choices to be made are as follows ... 

Often warpage and distortion can be roughly predicted at the design stages but no ready method of determination is available, due to the many unknown factors such as the changes of shape and size of moulding walls. 

One of the primary limitations of PET is related to its slow rate of crystallization from the melt. A consequence of this is that relatively long cycle times are required to provide crystallinity in PET. When this is achieved, it is often accompanied by opacity and brittleness, due to the relatively large size of crystallites formed by thermal crystallization. Crystallinity itself is often desirable in moulded parts, due to the higher thermal and mechanical stability associated with it. Crystallinity is especially desirable when parts are intended to be subjected to elevated temperatures since if the PET components are amorphous they will anneal at temperatures above 80 °C. 

A twin-screw extruder is generally preferred for producing rubber-toughened, glass-filled PET compounds for injection moulding applications. The PET and impact modifier are added at the throat while the glass reinforcement is added downstream. The size of the rubber domains will depend on the amount of energy and the capability of the equipment used for dispersion. 

The part sizes are limited by the tool sizes such as moulds, dies, autoclaves or winding machines and by the power and the size of equipment such as presses, bags, pultrusion machines, etc. 

This manipulation requires that the mass to he formed be of sufficient tractability to be easily introduced into the cavities of the mould by kneading, and that, when pressed, It be of sufficient tenacity as not to bend or yield upon removal. It must, therefore, acquire consistency, and dry in the mould—a condition which Tenders it absolutely necessary that the moulds he constructed of substances of a porous nature, otherwise the pieces would adhere and lose their shape upon removal. The moulds are, therefore, generally formed of gypsum—plaster of Paris—and sometimes of burned clay, and are of every variety of size and pattern, a complete set of pattarns being required for evory new design, and for every size of the same pattern. Moulds for plates, dishes, and other shallow articles, consist of only one pioco, while for jugs, vases, and more elaborate forms, several pieces or moulds are employed. 

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