Freeze-off

The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

A recent development of the insulated runner principle is the distribution tube system. This overcomes the possibility of freezing-off by insertion of heated tubes into the runners. However, this system still relies on a thick layer of polymer forming an insulation layer on the wall of the runner and so this system is not suitable for heat sensitive materials. 

Note that both the insulated runner and the distribution tube systems rely on a cartridge heater in the gate area to prevent premature freezing off at the gate (see Fig. 4.40). 

These calculations can give a useful first approximation of the dimensions of a thermoformed part. However, they will not be strictly accurate because in a real situation, when the plastic sheet is being stretched down into the cold mould it will freeze off at whatever thickness it has reached when it touches the mould. 

This equation may also be used to calculate the wall thickness distribution in deep truncated cone shapes but note that its derivation is only valid up to the point when the spherical bubble touches the centre of the base. Thereafter the analysis involves a volume balance with freezing-off on the base and sides of the cone. 

Some typical moulding data for a range of plastics are given in Table 5.1. Note that the de-moulding temperature will be generally about 30°C below the Freeze-off temperature in order to ensure that the moulding is sufficiently solid for handling. 

The reader may wish to check the predictions of freeze-off time for the data in the above Example. 

This equation cannot be integrated as simply as before because h is now a function of time. Fig. 5.26 shows how the depth of the cavity changes with time as the melt flows. Barrie has investigated this situation and concluded that the freezing-off could be described by a relation of the form... 

From (5.84) using the boundary condition that t = tf (freeze-off time) at... 

For the non-isothermal cases the freeze-off time, t/, may be estimated by the method described in Example 5.11. 

It is now necessary to derive an expression for the pressure loss in the cavity. Since the mould fills very quickly it may be assumed that effects due to freezing-off of the melt may be ignored. In Section 3.4(b) it was shown that for the flow of a power law fluid between parallel plates... 

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. 

Repeat the previous question for the situation in which the mould temperature is 60°C and the freeze-off temperature is 128°C. What difference would it make if it had been assumed that the material was Newtonian with a viscosity of 1.2 x 10 Ns/m. ... 

With plastics that decompose, there may be hazards such as personal bums or wounds and air contamination. Faulty controllers and/or freeze-off can cause the overheating situation from a burned out heater. Safety devices should be used that alert the plant when problems develop people have to be aware of these possible situations. Recognize that personnel injury in plants due to machinery represents 10% of all accidents (Fig. 8-77). 

One of the common problems associated with underwater pelletizers is the tendency of the die holes to freeze off. This results in nonuniform polymer melt flow, increased pressure drop, and irregular extrudate shape. A detailed engineering analysis of pelletizers is performed which accounts for the complex interaction between the fluid mechanics and heat transfer processes in a single die hole. The pelletizer model is solved numerically to obtain velocity, temperature, and pressure profiles. Effect of operating conditions, and polymer rheology on die performance is evaluated and discussed. 

By varying the "plunger forward time" Holmes et al. were able to determine the time necessary to just obtain the maximum value of L this time was called freeze-off time (tf). The following approximate relation was found ... 

Fractionation of chain molecules according to their chain length, 727 Fracture mechanics, 472 Free enthalpy of formation, 753, 754 Freely jointed chain model, 247 Free-rotation model, 246 Free surface energy, 229 Free volume fraction, 537 Freeze-off time, 806 Freezing-in process, 151 Frequency doubling, 349 factor, 751... 

In addition to improved cold impact resistance, heat sag and resilience, this polymer freezes off rapidly on injection molding and does not shrink excessively. 

Design ofthe vent (Fig. 6.51) is determined somewhat experimentally. The width of the opening and the taper angle must be chosen such that there is rapid freeze-off, minimal polymer escape, and burning is... 

---