The moulding process

There are two general methods available for producing the cellular core of SF mouldings (see PST 2). One method relies on the chemical decomposition of a blowing agent at elevated temperatures - the chemical blow method. The other method involves the formation of gas cells due to a physical change such as from liquid to gas to the expansion of a compressed gas - the physical blow method . 

BIP use the physical blow method for producing the Geest trolley base, the process being licenced from the American company Union Carbide. 

The process is illustrated schematically in Fig. 8.3. Polypropylene granules are fed into a continuously rotating screw of special design to permit nitrogen gas to be introduced to the melt. The screw feeds melt and gas into an accumulator where they are maintained under high pressure to keep the gas in solution . When sufhcient volume of melt has been accumulated (limit switch) a valve in the mould nozzle is opened and melt is forced into the cavity where, under reduced pressure, the nitrogen gas is free to expand. The nozzle valve closes and the accumulator continues to be charged with melt from the extruder barrel. 

35 MNm 2 and 17.25 MNm 2 (1500 - 2500 psi). The melt and gas are mixed, again using a decreasing flight depth and a Maddox mixing head at the front of the screw. 

The extruder feeds one of two presses from an overhead gantry. Presses are of the hydraulic upstroking type with a clamping capacity of 150 tonnes - a compact injection-moulding press producing the same-sized mouldings would need a locking force of approximately 1500 tonnes. Press platen area is 2.15 x 1.5 m (7 x 5 ft). 

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If the material has a critical stress intensity factor of 1.8 MN m and it is known that the moulding process produces defects 40 m long, estimate the maximum repeated tensile stress which could be applied to this material for at least 10 cycles without causing fatigue failure. 

In the case of paints and printing inks, the initial preparations will be in the semi-solid state because solvents are needed both in the process of dispersing the pigment in the paint or ink medium and for application purposes. These solvents dry out after the paint or ink is applied. When making coloured plastic articles, both heating and solvents may be used to aid dispersion in the plastic medium as part of the moulding process. However, from the viewpoint of the optical properties in all of these pigment uses, what is most important is that each of these media has a refractive index close to 1.5. 

Injection and blow moulding are by far the most used of the moulding processes but compression is sometimes used for specific cases. 

Even yet experts say that the best combs are saw-cut from flat uniform pieces of bone (and such combs remain on sale in more-expensive shops). Combs can be cut in the same way from plastic sheet, achieving comparable quality, but the stock must be prepared suitably and the work as a whole is more costly than the commercial method of making plastic combs in large numbers—injection moulding using materials such as nylon or polypropylene from the moulding process, after removal of the sprues, fully finished, saleable products (teeth included) can be obtained in a few seconds. 

Mould To shape plastic parts or finished articles by heat and pressure. The cavity or matrix into which the plastic composition is placed and from which it takes its form. The assembly of all the parts that function collectively in the moulding process. 

Details are given of the design of an autoclavable medical device using flexible PVC. The device was injection moulded and exposed to steam autoclaving. Surface delamination was investigated. Experiments on residual stresses after moulding and statistical analysis of the moulding process were conducted. 8 refs. 

Plastic imitations may show marks finm the moulding process, and possibly air bubbles. 

This theory of induced fit helps to explain why enzymes can catalyse a wide range of substrates. Each substrate induces the active site into a shape which is ideal for it, and as long as the moulding process does not distort the active site too much, such that the reaction mechanism proves impossible, then reaction can proceed. 

These may be wood, paper or flour filled, but be alert to possible release of ammonia, phenol, or formaldehyde residues from the moulding process. PF can be made by reacting phenol with hexamine (made from formaldehyde+ammonia). 

A variation on the moulding process is to suck out unfrozen material from the centre of the mould shortly after filling, to leave a frozen outer shell. This can be refilled with another mix to create a two-component product, such as a split with an ice cream core and a water ice shell. This is known as fill and suck (Figure 5.7). 

The composition of the device components (plastics, springs, elastomers, etc.) The composition of the device in terms of moulded and/or assembled component parts, i.e., the moulding process may effect the properties of the components... 

Scott Blair very accurately described the complexity of the situation with this thesis. Now however an attempt will be made to describe the phenomenon of plasticity and even to influence it, so is nevertheless necessary to find more precise definitions and use them in order to be able to optimize the plasticity of bodies for the purpose of plastic shaping. The generally accepted definition of plasticity as a property of solids to exhibit flow properties under the influence of external forces, which lead to permanent deformation when the force is removed, is initially applicable to all plastically deformable bodies. However from the point of view of influencing plasticity a clear distinction must be made between the body to be deformed on the one hand and the moulding process employed on the other hand. 

The moulding processes employed differ from each other here with respect to the... 

A clear distinction between the bodies to be shaped and the moulding process employed thus forms the starting point for consideration. The requirements that can be derived from these make decisions possible concerning the application of additives and the choice of the optimum additives when the chemical and physical mechanisms of action of these are known. 

Depending on the combination of material, processing moisture and the moulding process, differing demands are made on the mode of action of the additives to be employed. 

The regeneration of sand requires additional energy and causes additional dust emissions and residual dust for disposal. However, the use of regenerated sand in the moulding process allows a reduction in the amormts of lustrous carbon used. 

Spray depositing can be used to speed up the moulding process. Glass fibre rovings (rope-like form) are fed into a chopper on a special spray gun and the resultant strands are blown into a stream of liquid resin and catalyst in the correct ratio. Moulds are pre-treated as before, but instead of hand lay-up, the mixture is sprayed onto the mould, resulting in a composite which has a random array of glass fibres. 

Any material which escapes during the moulding process results in a feather edge on the moulding known as a flash. This can be removed by the operator during the curing cycle of the next moulding. 

P(2) Components to be joined shall be correctly supported or jigged during the moulding process. 

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