Vacuum moulding

In rubber processing this means the same as vacuum moulding. In plastics manufacture it refers to the use of a controlled vacuum to support and form a sheet of heat-softened material into a desired contour. 

Japan Synthetic Paper has developed a PP foaming process. The non-crosslinked PP is continuously extruded to produce foamed sheeting and the sheet is further vacuum moulded by the conventional process. The foamed PP is recommended for thermal insulation of hot water pipes, air conditioning plumbing and wire coverings. 

Vinyl ester resins are similar to unsaturated polyester resins in that they are cured by a free radical initiated polymerisation. However, they differ from the polyesters in that the unsaturation is at the ends of the molecule rather than along the polymer chain. Unlike polyesters, vinyl esters show a greater resistance to hydrolysis as well as lower peak exotherm temperatures and less shrinkage upon cure. Cured vinyl ester resins exhibit excellent resistance to acids, bases and solvents. They also show improved strain to failure, toughness and glass transition temperatures over polyesters. They can be used in filament winding, pultrusion, resin injection, vacuum moulding and conventional hand lay-up. 

Alcan LV63 or Martinal ON904 + 2 wt% BYK W996 Maximum dispersion viscosity for resin transfer moulding is accepted as around 1000 cps. So, under ambient conditions, these 2 dispersions are only suitable for wet press and vacuum moulding. Source of data Ashland Specialty Chemicals ... 

The distinguishiag features are (i) the special stopcock is constructed to turn easily under a high vacuum (ii) a moulded arrow on the stopcock indicates the open position, euid a quarter turn from this position closes the desiccator and (iii) a special liquid container of the non-spill type. 

In vacuum and blow forming, sheets produced by extrusion are shaped by vacuum or pressure forming. Heat-softened sheet is pressed into a mould by atmospheric pressure when a vacuum is created between the mould and the sheet. Fig. 24.4(a). Plastic bottles are made by blowing instead heated tube is clamped in a split mould and expanded with compressed air to take up its shape (Fig. 24.4b). Both methods are cheap and quick, and can be fairly accurate. 

Fig. 24.4. (a) Vacuum forming is good for making simple shapes out of sheet. (b) Blow moulding is used to make plastic containers. 

The main reason for extruding polystyrene is to prepare high-impact polystyrene sheet. Such sheet can be formed without difficulty by vacuum forming techniques. In principle the process consists of clamping the sheet above the mould, heating it so that it softens and becomes rubbery and then applying a vacuum to draw out the air between the mould and the sheet so that the sheet takes up the contours of the mould. 

The largest outlet for polystyrene is in packaging applications. Specific uses include bottle caps, small jars and other injection moulded containers, blown containers (a somewhat recent development but which has found rapid acceptance for talcum powder), vacuum formed toughened polystyrene as liners for boxed goods and oriented polystyrene film for foodstuffs such as creamed... 

At the present time, doped ICPs are not normally capable of being processed like normal thermoplastics. Processes usually Involve high-pressure moulding of finely powdered polymers under vacuum or an inert gas. However, modification of some ICPs with, for example, alkyl or alkoxy side groups may produce soluble, and thus more tractable, polymers. 

In this section a selection procedure will be developed for injection moulding, since this process is used for the widest range of materials. The choice available for other processes such as, for example, compression moulding, filament winding and vacuum forming, is much more restricted. The approach described will be less mechanistic than the systems described in the two previous sections, requiring the prospective user to be aware of the properties of the various materials available. Because the approach is somewhat different, it would be instructive to run it parallel to the above processes and compare the results. 

A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.53. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

Variations on this basic process are (i) vacuum bag moulding and (ii) pressure bag moulding. In the former process a flexible bag (frequently rubber) is clamped over the lay-up in the mould and a vacuum is applied between the moulding and the bag. This sucks the bag on to the moulding to consolidate the layers of reinforcement and resin. It also squeezes out trapped air and excess resin. The latter process is similar in principle except that pressure is applied above the bag instead of a vacuum below it. The techniques are illustrated in Fig. 4.67(b) and (c). 

It will be seen that a polymer of molecular weight A/, may be processed either as a liquid (by injection moulding, extruding, etc.) or as a rubber (by vacuum forming, sheet blowing, warm forging). In the case of the polymer... 

The diglycidyl ether (DGEBA) and the polyglycidyl compound (PGCBA) were mixed and heated to about 120 °C. The stoichiometric amount of bisphenol A was dissolved in the resin and, after cooling to about 60 °C, 0,1% by weight of 2-ethyl-4-methyl-imidazol was added to the mixture. The well stirred composition was degassed in a vacuum oven in order to remove trapped air. Afterwards, the reactive mixture was poured into preheated moulds and cured for two hours at 140 °C followed by two hours at 180 °C. 

A method of manufacturing latex foam. Compounded latex is partially frothed and fully expanded by application of vacuum to the mould the gel is frozen, set by carbon dioxide and vulcanised by raising the temperature of the mould. See Dunlop Process. 

A method of making products such as car floor mats where the mould or contoured former is drilled with a series of small holes which lead into a backing chamber connected to a vacuum line. The vacuum holds the sheet of rubber firmly in contact with the former during curing. 

---