High-pressure moulding

Thermoplastics can be moulded into articles by injection moulding or blow moulding. In injection moulding high pressure is used to inject molten thermoplastic into a mould where it solidifies. Blow moulding is the most common process for making hollow articles such as bottles [75]. In this type of moulding, a molten tube of resin is extruded, a mould is closed around the tube, and air is fed into the tube to expand it into the mould. 

Moulding systems are usually divided into low-pressure and high-pressure systems. 

The sheets may be formed to some extent by first softening in hot water or steam and then pressing in moulds at pressures of 200-500 Ibf/in (1.5-3.5 MPa). Machining, using high-speed tools, may be carried out on conventional metal-working machinery. 

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. 

Clamping Systems In order to keep the mould halves tightly closed when the melt is being injected under high pressures it is necessary to have a clamping system. This may be either (a) hydraulic or (b) mechanical (toggle) - or some combination of Ae two. 

However, in the non-isothermal case the pressure is also high at low injection rates. This is because slow injection gives time for significant solidification of the melt and this leads to high pressures. It is clear therefore that in the non-isothermal case there is an optimum injection rate to give minimum pressure. In Fig. 5.28 this is seen to be about 3.0 x 10 m /s for the situation considered here. This will of course change with melt temperature and mould temperature since these affect the freeze-off time, //, in the above equations. 

Figure 2 Light permeability of polyolefins after quenching (1-4) and of nonquenched samples (l -4 ) l,l -poly-propylene (PP) 2,2 -high-pressure polyethylene (HPPE) 3,3 -low-pressure polyethylene (LPPE) 4,4 -medium-pres-sure polyethylene (MPPE). Film thickness-150 fic moulding time-10 minutes, moulding pressure HPPE-160°C LPPE, MPPE, PP-190-200X.

These include cold drawn, high pressure oriented chain-extended, solid slate extruded, die-drawn, and injection moulded polymers. Correlation of hardness to macroscopic properties is also examined. In summary, microhardness is shown to be a useful complementary technique of polymer characterization providing information on microscopic mechanical properties. 

Tearing or distortion of a moulded rubber product at the line of separation of the two mould halves (the spew line) due to the sudden release, on opening the mould, of the high pressures developed by the thermal expansion of the heated rubber other names are suck back, flash back and retracted spew. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

Figure 5.2. Principle of high-pressure injection moulding...

A ram system allowing introduction under high pressure of the dosed material into the mould. On some types of injection machine, the screw also acts as the ram. 

The required quantity of thermoplastic is compressed at high pressure into the hot cavity space between the two parts of the heated mould. 

More distinct differences (as compared to high-pressure moulding) are encountered in the analysis of the first, least studied area of the entrance ( gate ) into a mould. The authors of 89 93) subdivide the entrance area into the stages of jet and regular flow especially pronounced in moulding bulky articles. The jet flow has been visually observed by us in experiments with transparent moulds. The various stages of the jet... 

In some cases the raw material is already in a fluid state with thermosets as a resin for casting or impregnating, with thermoplasts, as an exception, sometimes as a monomer which is able to polymerise in the mould. In these cases the forming processes are relatively simple, since they can be carried out on low-viscous fluids which do not require high pressures to be transported. 

Notwithstanding this fact, high pressures are required to transport the fluid mass through the transport channels or into the mould of a processing machine with an acceptable speed. Pressure levels of 50 to 150 MPa (500 to 1500 bar) are normal, so that the machines have to be very robust. 

When the material is sufficiently heated, it is transported by a moving plunger under high pressure (sometimes up to 1500 bar) through a narrow channel into the mould cavity (Figure 11.12a and 11.12b). In the cooled mould the product cools down under pressure. After complete solidification the mould is opened and the product is ejected from it (Figure 11.12c). 

So as to fill the mould at the speed required, high pressure is applied to the melt to compensate for shrinkage, it usually is necessary to force in extra material in a second stage and to prevent over-filling at that time a lower second-stage or hold pressure may be used. 

Dispersed in an infrared transparent powder. Pellets are made with I to 2% by weight of the sample, usually in dry potassium bromide. A pellet of approximately 300 mg is prepared in a mould called a matrix using a laboratory press. This method produces pellets 1 to 2 mm thick for a diameter of approximately 13 mm. This technique can be used for quantitative studies but the spectra may sometimes differ ftom those obtained via dispersion in a liquid. This is because, under the effect of the high pressure to which the powder is subjected, the solid may undergo certain modifications. 

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