Melt temperature injection moulding

Low-melt-temperature injection-moulding grade High-melt-temperature injection-moulding grade... 

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

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. 

PP-structural foam mouldings were produced on an injection moulding machine in a pre-pressurised mould cavity by the classical low-pressure process and an alternative low-pressure process. Melt temperature, injection direction and sprae diameter were varied. Cross-sections cut from the middle of the small cylinder in longitudenal orientation were investigated by site-resolved X-ray scattering. Morphological properties were investigated. 4 refs. 

Scaling up, however, a single spiral mould test is not possible. Therefore, Fritch recommends that test should be carried out at three melt temperatures and three injection rates for a total of nine tests, in such a way that they cover melt temperature and mould flow rate in commercial processing. 

The projected area of a mould includes cavities and feed systems. The injection pressure is a function of part thickness, melt temperature, and mould temperature. It can vary within the mould. The three-zone reciprocating screw should achieve good plasticisation of the material. The estimation of the clamping force needed to keep the mould shut is a complex function of the projected area and the injection pressure. A rule of thumb for the clamping force required for PE is 0.2-0.5 tons/cm of projected area. A large safety factor of 25-50% is advisable. Computer simulation of flow will provide a more accurate estimation of the injection pressure and mould opening force. 

Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

The difference between the temperature of the melt on injection into the mould (Tj) and the mould temperature (7),). 

The time available for disorientation as the melt cools from Tp to T. This will depend on the value of Tp-T where is the temperature of the environment (the mould temperature in injection moulding) since this will with the specific heat determine the rate of cooling. The time will also depend on Tp-T since this will determine the extent of cooling. 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

The polymer melts at 216°C and above this temperature shows better cohesion of the melt than PTFE. It may be processed by conventional thermoplastics processing methods at temperatures in the range 230-290°C. Because of the high melt viscosity high injection moulding pressures are required. 

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

The block copolymers are easy to process but in order to obtain maximum clarity and toughness attention has to be paid to melt and mould temperatures during injection moulding. 

The polymer is not easy to process and in injection moulding melt temperatures of 300°C are employed. In order to prevent excess embrittlement by shock cooling of the melt, mould temperatures as high as 150°C may be used. The polymer may also be compression moulded at temperatures of 250-260°C. 

These polymers may be extruded and injection moulded on standard equipment used for thermoplastics. Typical melt temperatures range from about 230°C for the harder grades down to about 200°C for the softer polymers. Mould temperatures are about 25-30°C. 

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