MOULD TEMPERATURE

Table 8.1 Heat required (enthalpy required) to raise polymers to their processing temperatures from an ambient temperature of 20°C and the heat required to be removed in cooling a polymer from the melt to mould temperature...

Polymer Melt temperature i°C) Mould temperature (X) SG Specific heat (Jkg- K ) Heat required to melt Heat removed on cooling ... 

The meli and mould temperatures and the value of the heal removed per gram on cooling are taken from the paper by Whelan and Goff. The values for the amount of heat required to raise the temperature to the melting point and the heat requirements per unit volume (both for heating and cooling) have been calculated from these data by the author. 

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. 

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),). 

Some data on the amount of heat required to reduce the temperature of a polymer from a typical melt temperature to the temperature of the mould (in terms of both per unit mass and per unit volume) are given in Table 8.1. Whilst a moulding will usually by withdrawn at some temperature above the mould temperature, the data do provide some comparison of the different heat requirements of different polymers. It will be noticed that there is a more than 7-fold difference between the top and bottom polymers in the table. 

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. 

An increase in mould temperature (which allows more time for crystallisation to occur at a reasonable rate). 

Polymer Clarity of moulding Temperature resistance Toughness... 

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. 

The manufacturers stress ease of processing as a particular feature of the material. Recommended melt temperatures are in the range 320-340°C and mould temperatures are 135-165°C. Mould shrinkage of glass-filled grades is usually of the order of 0.2-0.4% in the flow direction and up to twice this value in the transverse direction. The materials are notable for their ability to withstand vapour phase and infrared soldering processes. 

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. 

To reduce strains in mouldings, fairly high mould temperatures are recommended (65-95°C in unfilled and up to 120°C in glass-filled grades). 

Typical melt temperatures are in the range 300-360°C (e.g. 320°C). Mould temperatures are usually about 135°C in order to optimise the amount of crystallinity and hence give mouldings of greatest stiffness, dimensional stability, thermal stability and surface finish. It is, however, possible to use relatively cold... 

The polymers absorb some moisture and at the high moulding temperatures employed this can be very troublesome, leading to streaks and splash marks. 

For extrusion and blow moulding the polysulphones used are of higher molecular weight. Melt temperatures for blow moulding are of the order of 300-360°C with mould temperatures about 70-95°C. 

C. Typical mould temperatures can be as high as 165°C. At these temperatures small amounts of moisture can be serious, so that drying of granules is essential (e.g. 3h at 150°C). 

Mould temperature Injection pressure Screw back pressure Screw speed Curing time... 

In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a hardener (accelerator). This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate. A very large number of such latent acid catalysts have been described in the literature, of which some of the more prominent are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate. 

Melamine-based compositions are easily moulded in conventional compression and transfer-moulding equipment. Moulding temperatures are usually in the range 145-165°C and moulding pressures 2-4 ton/in (30-60 MPa). In transfer moulding pressures of 5-lOton/in (75-150MPa) are used. An in thick moulding required about 2 minutes cure at 150°C but shorter times are possible with preheated powder. 

The injection moulding of melamine-formaldehyde moulding powders is now carried out on a small scale. Temperatures are somewhat higher than for U-F (e.g. barrel temperatures 100-115°C mould temperatures 163-177°C). Otherwise the considerations are the same as for the urea-formaldehyde compositions. 

Crystalline, opaque, mouldings are produced by using a mould temperature of about 130°C and grades containing crystallisation accelerators . The crystalline grades retain their shape up to temperatures near to and therefore for many applications may be used above T. ... 

---