Plastic moulding

The reason why clay exhibits plastic behaviour can be found in the packing of clay sheets with water molecules and hydrated ions in between. At the surface of such a clay sheet the water molecules are tightly bound by means of H-bridges between the water molecules and the surface charge of the sheet. Further away from the sheet the water molecules are interconnected by means of H-bridges but these forces are weaker and consequently those water molecules can move more easily than the ones which are bound to the clay sheet. In that movement both the loose water molecules and the hydrated ions act as a lubricant between the sheets and can move parallelly in relation to each other the plastic behaviour (figure 9.9). 

Clay mostly already contains organic components which affect the plasticity in a positive way. The fine ceramics industry makes use of this fact in order to make masses more plastic, especially in the manufacture of thin-walled articles. 

Rigid masses have a remaining height of more than 25 mm and for soft ones it is less than 16 mm. In figure 9.11 you can read off the desired plasticity (remaining height) and the required water content. The desired plasticity differs for every product, table 9.3 supplies some examples. 

Very occasionally bricks are still made by means of hand moulding. A lump of clay is forcefully thrown into a mould and then the surplus clay is removed. The result is a lively and variable product. When large quantities of bricks are produced the process does not differ much from that of hand moulding. However, now the clay is pressed into a mould and a rather uniform product is the result, e.g. the common brick. 

In the extrusion process the plastic clay mass is forced through a die by a screw mechanism (figure 9.12). 

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BS 7010 1988 A System of Tolerances for the Dimensions of Plastic Mouldings. London BSI. PD 6470 1981 The Management of Design for Economic Manufacture. London BSI. 

GROVES, w. R., Plastics Moulding Plant Vol. 1, lliffe, London (1963)... 

Such lenses may be made by machining from rod. More recently processes have been developed where the monomers are cast polymerised in tiny plastics moulds whose cavity corresponds to the dimensions of the lens and using procedures very reminiscent of those described for the manufacture of acrylic sheet (see Section 15.2.2). 

Phenolic moulding powders, which before World War II dominated the plastics moulding materials market, only consumed about 10% of the total phenolic resin production by the early 1990s. 

Stmctural plastics mouldings (a) furniture of all kinds (b) car body parts (c) TV, radio and loudspeaker cabinets (d) brush handles and gun stocks... 

Acid anhydride and amine hardening agents, Adhesives, plastics, moulding resins and... 

AMD 1 Safety footwear. Part 1 Safety footwear other than all-iaibber and all-plastics moulded types (AMD 6273) dated 28 February 1990. Superseded by BS EN 345 1993 and BS EN 346 1993... 

It is becoming common practice to have the cross-section of a plastic moulding made up of several different materials. This may be done to provide a permeation barrier whilst retaining attractive economics by having a less expensive material making up the bulk of the cross-section. To perform stress analysis in such cases, it is often convenient to convert the cross-section into an equivalent section consisting of only one material. This new section will behave in exactly the same way as the multi-layer material when the loads are applied. A very common example of this type of situation is where a solid skin and a foamed core are moulded to provide a very efficient stiffness/weight ratio. This type of situation may be analysed as follows ... 

Once the foamed plastic moulding has been converted to an equivalent section of solid plasdc then the long term design procedures illustrated in the... 

If a plastic moulding fails in the performance of its normal function it is usually caused by one of two factors - excessive deformation or fracture. In the previous sections it was pointed out that, for plastics, more often than not it will be excessive creep deformation which is the limiting factor. However, fracture. 

Extrusion, by its nature, is ideally suited to the production of continuous lengths of plastic mouldings with a uniform cross-section. Therefore as well as producing the laces as described in the previous section, the simple operation of a die change can provide a wide range of profiled shapes such as pipes, sheets, rods, curtain track, edging strips, window frames, etc (see Fig. 4.18). 

A large Biot Number means that conduction controls the energy transfer to/from the plastic and large temperature gradients will exist in the plastic. A small Biot Number means that convection is the dominant factor. The above analysis was for conduction heat transfer (B, - oo). When the plastic moulding is taken out of the mould we need to check the value of B,. In this case... 

Cooling the moulds of plastic-moulding machines with chilled water... 

The pressure which must be exerted on each square unit of the surface of the product for satisfactory moulding, i.e., proper consolidation and freedom from porosity. It is usually expressed in lb/in2 or kg/cm2. Moulding pressure is not critical for rubber (700 lb/in2, 5 MPa minimum) but is very critical for efficient moulding of the various types of plastics. Moulding pressures are calculated on the projected area of the product and should include the area of the spew groove. 

Polystyrene can be moulded, cast or extruded in sheets, rods and tubes. It can be easily pigmented. It finds wide, application in making household goods, lenses, plastic moulds, toys, wood laminates, films, battery boxes and electrical components. Expanded polystyrene is used as thermal insulating material and packaging. It is also used in lifejackets and floats. 

Copper/Constantan 400-62QK tol = 0.0041 T-2731K 1073 5566 10741 food processing, stack gases, plastic moulding, lubricating oils, producer gas plants. 

In the usa plastics moulded the happy-go-lucky American dream with the fluorescently pigmented Hula Hoops, Frisbees and Big Wheels of the... 

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