Mould temperature measurement

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 dimensionless parameters I and x in Eq. (6.6) are the measure of runner resistance and filling time for the initial ( semi-radial ) filling stage, respectively. Injection pressure P0, mould temperature T (and the respective viscosity r (T),... 

The influence of the mould temperature on the H measured at the outer surface of the PET bar is shown in Fig. 7.6. The stepwise increase of H with Tp can be explained because below and near Tg, H values between 120-130 MPa have been ascribed to the incipient sphemlite stmcture of the polymer (see Section 4.2.2,... 

Figure 7.6. Microhardness of injection moulded PET, measured at the outer surface, as a function of mould temperature. The injection was performed using conventional samples as received (A) dried samples ( ). (From Balta Calleja et al., 1993.)...

Wherever it is possible moulds should be provided with pockets for temperature measuring and recording instrumentation. This is essential if the moulding conditions are to be properly optimised and also provides a thermal finger-print for audit of the process during the production cycle. Surface temperature probes are better than nothing, but give results which may well differ between operators and are affected by the condition of the mould surface [5]. 

The whole HR system is divided into temperature control zones temperature measurement and control are carried out with the aid of automatic regulators separated from the mould. 

Naturally, a prerequisite for evaluating the mould temperature data is the measurement of the mould temperature. Unfortunately, even today this is not often done automatically. 

A measurement point position like this is not suitable for regulating the mould temperature, because of the temperature variations referred to. The temperature gauge should therefore be sufficiently far away from the mould wall, so that at the measuring point the temperature variations have already been sufficiently dampened. But it should also be an adequate distance away from the temperature control channels, so as to exclude reverse effects from this side. A middle position between the mould wall and the temperature control channels is recommended. Because the temperature falls towards the clamping surface, the measuring point should naturally not he too close to this. It is in any case advisable to insulate the mould halves from the clamping surface, so as to reduce the interference factors. For high mould temperatures, economic considerations also require this. 

If no fixed temperature measuring point is incorporated in the mould, it is possible to use probe thermometers as an aid. To carry out any measurements in the cavity, involves interrupting the cycle. The negative effects, which then arise on the accuracy of measurement and the production cycle must not be left out of consideration. But in no case should the measurement of the mould temperature - at least at one specific comparison point or reference point - be neglected. 

Measuring the Mould Temperature - Checking the Uniformity of the Mould Temperature Control... 

At yet higher temperatures (>1.4T ) the secondary bonds melt completely and even the entanglement points slip. This is the regime in which thermoplastics are moulded linear polymers become viscous liquids. The viscosity is always defined (and usually measured) in shear if a shear stress o produces a rate of shear 7 then the viscosity (Chapter 19) is... 

Unlike other water-soluble resins the poly(ethylene oxide)s may be injection moulded, extruded and calendered without difficulty. The viscosity is highly dependent on shear rate and to a lesser extent on temperature. Processing temperatures in the range 90-130°C may be used for polymers with an intrinsic viscosity of about 2.5. (The intrinsic viscosity is used as a measure of molecular weight.)... 

There is no entirely satisfactory way of measuring flow. In the BS 2782 flow cup test an amount of moulding powder is added to the mould to provide between 2 and 2.5 g of flash. The press is closed at a fixed initial rate and at a fixed temperature and pressure. The time between the onset of recorded pressure and the cessation of flash (i.e. the time at which the mould has closed) is noted. This time is thus the time required to move a given mass of material a fixed distance and is thus a measure of viscosity. It is not a measure of the time available for flow. This property, or rather the more important length of flow or extent of flow, must be measured by some other device such as the flow disc or by the Rossi-Peakes flow test, neither of which are entirely satisfactory. Cup flow times are normally of the order of 10-25 seconds if measured by the BS specification. Moulding powders are frequently classified as being of stiff flow if the cup flow time exceeds 20 seconds, medium flow for times of 13-19 seconds and soft flow or free flow if under 12 seconds. 

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