Mould finishing

Mould finish. For a high-gloss finish a highly polished steel mould or a chromium plated mould is required and the mould temperature and material temperature are critical. Finishes or textured surfaces offering stippled, matt, satin, etc. are also widely available. 

Near-net moulded components, resulting in reduced post-moulding finishing processes and less material scrap. 

A series of hard, acrylic urethane, in-mould coatings which can also be employed as a post-mould finish, of. excellent weatherability. 

The tools themselves heat up and quite rapid mould turn-round can be achieved on large mouldings. Finish is not as good as with hot press moulding and tool life is limited. 

A possible reduction in post-moulding finishing operations to remove large sprue gate witness marks... 

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plMtic. Those polymers which in their original condition will fiow and can be moulded by heat and pressime, but which in their finished or cured state cannot be re softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

Many of these problems can be solved by using continuous casting (Fig. 14.3). Contraction cavities do not form because the mould is continuously topped up with liquid metal. Segregation is reduced because the columnar grains grow over smaller distances. And, because the product has a small cross-section, little work is needed to roll it to a finished section. 

Polymers have some obvious advantages for wheels. The wheel can be moulded in one operation, replacing a metal wheel which must be assembled from parts. It requires no further finishing, plating or painting. And its naturally low coefficient of friction means that, when loads are low, the axle may run on the polymer itself. 

Materials which reduce the friction of mouldings and other finished products when these are rubbed against adjacent materials which may or may not be of the same composition. The most well-known examples here are graphite and molybdenum disulphide used in quantities of the order of 1-2% in nylons and other thermoplastics used in gear and bearing applications. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

In recent years rotational casting methods have made the slush moulding process virtually obsolete. In these processes an amount of material equal to the weight of the finished product is poured into a mould. The mould is then closed and rotated slowly about two axes so that the paste flows easily over the cavity walls in an oven at about 200-250°C. When the compound has gelled, the moulds are cooled and the moulding removed. Compared with the slush moulding process there is no wastage of material, little flash, and more even wall thickness. Completely enclosed hollow articles such as playballs are most conveniently made. 

The above process is limited to simple shapes whose principal dimension is not more than four times, and preferably less than twice, that of the next largest dimension. More intricate shapes must be made by machining or in some instances by a coining operation which involves stamping a sintered moulding of the same weight and approximate dimensions as the finished part at 320°C. 

Because of its high cold flow, poly(vinyl acetate) is of little value in the form of mouldings and extrusions. However, because of its good adhesion to a number of substrates, and to some extent because of its cold flow, a large quantity is produced for use in emulsion paints, adhesives and various textile finishing operations. A minor proportion of the material is also converted into poly(vinyl alcohol) and the poly(vinyl acetal)s which, are of some interest to the plastics industry. 

Plasticisers are sometimes added to the polymer, dibutyl phthalate being commonly employed in quantities of the order of 5%. Use in moulding powders will enhance the melt flow but somewhat reduce the mechanical properties of the finished product. 

The ease with which acrylic monomers may polymerise with each other and with other monomers has led to a host of compositions, frequently of undisclosed nature, being offered for use as moulding materials, casting resins, coating resins, finishing agents and in other applications. 

About 95% of the polymers are processed by injection moulding. Virtually all the remainder is extruded into sheet and rods for subsequent machining into finished parts. 

Because of the above properties, together with other features such as the ability to mould to close dimensional tolerances, low warpage, low shrinkage, low moisture absorption and good surface finish, polycarbonate-ABS alloys have become widely used in the automotive industry, for electrical applications and for housings of domestic and business equipment. 

Typical melt temperatures are in the range 300-360°C (e.g. 320°C). Mould temperatures are usually about 135°C in order to optimise the amount of crystallinity and hence give mouldings of greatest stiffness, dimensional stability, thermal stability and surface finish. It is, however, possible to use relatively cold... 

Other, more recently developed, uses include microwave oven parts, transparent pipelines, chemical plant pumps and coffee machine hot water dispensers. One exceptional use has been to produce, by an extrusion moulding process, very large rollers for textile finishing for use where cast nylons cannot meet the specification. Also of growing interest are medical equipment applications that may be repeatedly steam-sterilised at 134°C, filtration membranes and cartridges for ink-jet printers. 

In the mid-1950s cellulose propionate became commercially available (Forticel-Celanese). This material is very similar in both cost and properties to CAB. Like CAB it may take on an excellent finish, provided a suitable mould is used, it is less hygroscopic than cellulose acetate, and is easily moulded. 

Another commonly employed low-cost organic filler is coconut shell flour. This can be incorporated into the moulding composition in large quantities and this results in cheaper mixes than when woodflour is used. The mouldings also have a good finish. However, coconut shell flour-filled mouldings have poor mechanical properties and hence the filler is generally used in conjunction with woodflour. 

Preform mouldings are particularly useful in carrying containers and protective covers. Examples of their use include television receiver backs, moulded suitcases and typewriter cases. Although the finish obtained during moulding is frequently adequate in industrial applications some improvement is necessary where a good appearance is desired. Methods used include painting or... 

Whilst the injection moulding process has now been widely accepted for phenolics the transition from compression moulding has been less extensive with U-F materials. The basic reason for this is that the U-F materials are more difficult to mould. This has been associated with filler orientation during moulding, which can lead to stress peaks in the finished product which the somewhat brittle resin in less able to withstand than can a phenolic resin. 

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