Mould internal pressure

The mould plates carrying the moulding nest can be thoroughly bent in an area of the central bores and, above all, in the area of the ejector system, rmder the effect of the internal mould pressure. Thus, the mould plates must have sufficiently high flexural strength, if the formation of scratches is to be avoided. This danger is especially great for the mould plate on the ejector side. So, if possible, additional support columns, with an oversize of 0.03-0.05 nun, by comparison with the external supports, should be incorporated. 

It was discussed in Section 8.3 that the pressure curve in the mould (as well as in the compoimd temperature and the mould wall temperature) is decisive for the quality of the component. Therefore, the connection between injection pressure and internal mould pressure will now be indicated. With this, comes the possibility of exerting a better influence on the pressure cycle in the mould during injection and thus on the component s quality. 

As can be seen from Figure 8.4, the internal mould pressme follows the injection pressure, with a time delay. At A, the cavity is volumetrically filled, and between A and B the compoimd is packed in the mould. The maximum internal mould pressure, at B, is not reached until some time after the maximum injection pressure is obtained. 

Even if the injection pressure stays the same, the internal mould pressure drops slightly, as a result of shrinkage of the eompound, down to the euring point, C. From here the pressure drops rather faster, because now no more compound can be pushed back, right to the residual pressure, Pw remainder, when the mould is opened, at E. 

This is as much a matter of the level of mould temperatures as of their imiformity and repeatability. Economic quality improvement in injection moulding is not possible without good and imiform temperatures in the mould. Even with a more expensive injection process control or adjustment system, the negative influence of unsatisfactory mould temperatures can not usually be balanced out. If it is a question of narrowing the tolerances of the components, the first step is to check the mould temperature data, and, if necessary, these must be improved before, for example, doing any research into injection pressure control on the basis of the internal mould pressure. 

Internal mould pressures too high (shaping pressure at the gap is so high that the melt is pushed even into very small gaps)... 

Viscosity of moulding compoimd too low (high internal mould pressures and low flow resistance favour flash formation). 

ISO 1872-1 1993 Plastics - Polyethylene (PE) moulding and extrusion materials - Part 1 Designation system and basis for specifications ISO 1872-2 1997 Plastics - Polyethylene (PE) moulding and extrusion materials - Part 2 Preparation of test specimens and determination of properties ISO 1969 2004 Fibre ropes - Polyethylene - 3- and 4-strand ropes ISO 3458 1976 Assembled joints between fittings and polyethylene (PE) pressure pipes -Test of leakproofness under internal pressure ISO 3459 1976 Polyethylene (PE) pressure pipes - Joints assembled with mechanical fittings - Internal under-pressure test method and requirement ISO 3501 1976 Assembled joints between fittings and polyethylene (PE) pressure pipes -Test of resistance to pull out... 

A method of fabrication in which a parison (hollow tube) is forced into the shape of the mould cavity by internal air pressure. 

A twin-walled corrugated pipe is shown in Fig. 1.14. The external wall is formed by applying an internal air pressure to the molten tubular extrudate (Fig. 13.2), which expands against pairs of mould sections—these move with the cooling pipe for some distance, then return on a caterpillar track to the die. 

EN 804 Injection-moulded socket fittings for solvent cemented joints for pressure piping -test method for resistance to a short-term internal hydrostatic pressure. 

Drive pressure (e.g., in the hydraulic stroke drive of the screw) and injection pressiue (e.g., in the screw forward cavity) are practically proportional to one another (independent of the injection conditions) and display the same cycle (if we disregard the friction losses as the screw moves forward) - see Figure 8.2. The internal mould pressiue, on the other hand, is lower than the injection pressure, conditioned by the flow losses during injection, which depend on the viscosity of the melt, the injection speed, and the geometry of the flow path. 

Figures 8.3 and 8.4 show the injection pressure and the internal cavity pressure curves. The internal cavity pressure can be measured by sensors within the mould, and can be indicated, or visually displayed, using an oscilloscope or a pen recorder. The pressure cycle in the vicinity of the gate is the most informative factor here.

A blow-moulded container, cylindrical in shape but with one spherical end, is prepared from the polysulphone whose creep curves at 20°C are illustrated in Figure 9.9. The cylindrical part of the container has an outside diameter of 200 mm and is required to withstand a constant internal pressure of 7 MPa at 20°C. It is estimated that the required service lifetime of the part will be one year and the maximum allowable strain is 2%. What will be the minimum wall thickness for satisfactory operation ... 

Fig. 4.61 illustrates that the mould temperature is quite different from the set oven temperature (330°C) or indeed the actual oven temperature, throughout the moulding cycle. An even more important observation is that in order to control the rotational moulding process it is desirable to monitor the temperature of the air inside the mould. This is possible because there is normally a vent tube through the mould wall in order to ensure equal pressures inside and outside the mould. This vent tube provides an easy access for a thermocouple to measure the internal air temperature. 

The solid metal cylinder or plunger which is moved by liquid pressure, air pressure or a combination of both, in such machinery as moulding presses, bale splitters, internal mixer floating rams and bottom doors, etc. 

ISO 7246 1984 Pipes and fittings of acrylonitrile/styrene/acrylester (ASA) - General specification for moulding and extrusion materials ISO 12092 2000 Fittings, valves and other piping system components made of unplasticized poly(vinyl chloride) (PVC-U), chlorinated poly(vinyl chloride) (PVC-C), acrylonitrile-butadiene-styrene (ABS) and acrylonitrile-styrene-acrylester (ASA) for pipes under pressure - Resistance to internal pressure - Test method... 

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