Moulding techniques

Turning takes place on a potter s wheel and is only suitable for so-called rotation symmetric shapes. This means that the objects which are made, such as plates, dishes and vases, are symmetric along an axis or point of symmetry. 

As you can see in figure 11.1.1 each point X on a rotation symmetric object has a mirror image X with respect to the axis or point of symmetry. 

---

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. 

Reaction injection moulding techniques, developed primarily for polyurethanes (see Chapter 27), have also been adapted for nylon 6 in what must be considered as a variation of the polymerisation casting technique. 

The high melt strength, facilitating thermoforming and blow moulding techniques. 

In the late 1970s several developments occurred causing renewed interest in poly(ethylene terephthalate) as a plastics material. These included the development of a new mouldable grade by ICI (Melinar) and the development of a blow moulding technique to produce biaxially oriented PET bottles. In addition there appeared a glass-fibre filled, ionomer nucleated, dibenzoate plasticised material by Du Pont (Rynite) (see Chapter 26). 

For mass production purposes matched metal moulding techniques are employed. Here the preform or mat is placed in a heated mould and the resin poured on. The press is closed and light pressure ( 501bf/in ) applied. Curing schedules are usually about three minutes at 120°C. It is possible to produce laminates using less resin with pressure moulding than with hand lay-up techniques and this results in better mechanical properties. 

For some years there has been concern at the amount of styrene vapour in workshops preparing reinforced polyester laminates. More recently this has increased interest in polyester-polyurethane hybrids and in the further development of closed moulding and resin transfer moulding techniques as well as greater use of lower styrene levels. 

Since 1960, the inner core has been made from c/5-poly-butadiene by the compression moulding technique. This replaced the earlier material made from a suspension of barytes or bentonite clay in water and glycerine or the winding of rubber threads made from t /5-polyisoprene, either from latex or a dry rubber compound. A typical thread recipe is given Table 4. 

PE and EVA foams were manufactured by a two-stage heat and chill compression moulding technique, which allowed more efficient utilisation of the platen area than other compression moulding process variants and which was suited to low density foam production. The interrelationships of base polymer type, crosslinking and blowing agent concentrations and physical properties of resultant foams were investigated. 13 refs. 

A compression moulding technique [94] has been used to prepare composites of polystyrene with layered perovskites C12Mn polystyrene powder (Mn = 145 000) and finely powdered C12Mn (average particle size 150 pm) are dryblended and subsequently compression moulded at 160 °C and a pressure of 50 kg/cm2. 

A suspension is a mixture which arises when solid particles are mixed optimally in a liquid. The suspended solid particles have a diameter of appr. 200-0.5 nm and the mixture is also called a colloidal dispersion . The liquid is the medium of dispersion. A clay suspension is suitable for the production of so-called hollow, non-rotation symmetrical articles, such as sanitary ware. Until the beginning of the 20th century these products were made by beating the clay into plaster of paris moulds, the so-called dies. Gradually people discovered not only the physically and chemical properties of suspensions but also how to change them and thus the technique of clay moulding developed and complicated shapes could be made. The science of colloid chemistry has been essential here. In the field of technical ceramics the moulding technique is also applied with other raw materials than clay. 

Reaction injection moulding is associated with polyurethanes and is different entirely from moulding techniques used commonly for thermoplastics in that the starting materials usually are liquids at room temperature. As the name implies, a chemical reaction takes place in the course of moulding and complex polyurethane items can be made in one step from mixtures comprising polyol, polyisocyanate, and a suitable promoter of the reaction. 

After the introduction of processing facilities for wood-plastic composites (WPC) by extrusion and later by injection moulding techniques, WPC production and its use increased almost exponentially. 

---