Melt leakage

The mould cavity must be filled completely without any flash occurring. The simplest control for mould filling uses limit switches on the screw travel. These actuate after the correct volume of melt has been injected, reducing the injection pressure to a lower holding value. However, any melt leakage past the screw non-return valve, or at the nozzle, causes the part mass to vary, with a consequent variation in the product dimensions. A better, more direct, control method uses a melt pressure transducer in the mould, which detects the rapid pressure rise, once the mould is full the... 

Melt leakage occurs, mold wear, higher injection pressure required, poor cycle repeatability, etc. 

Blow molding accumulators take one of two forms. The separate accumulator is a heated chamber which is an integral part of the blow molding machine. The capacity of such a chamber can be very large and can, if necessary, be served by several extruders. The other type of accumulator is built into the parison head (Fig. 10.36). This takes the form of an armular ram that surrounds the mandrel and torpedo. The construction of the parison head becomes quite complicated and there is the potential for melt leakage and degradation... 

A low melting (5°C) gallium—indium—tin alloy has been the choice for small spiral-groove bearings in vacuum for x-ray tubes at speeds up to 7000 rpm (71). Surface tension 30 times that of oil avoids leakage of the gallium alloy from the ends of the bearings. 

Sediment. Sediment is most commonly used as an operational check of filter efficiency and leakage, although some customers, especially those who iatend to melt the sugar iato clear solutions, write sediment restrictions. The measurement is normally done by passiag the 50% solution used for the color determination through a half black—half white filter pad and visually counting the white and black specks. 

General description. Burnthrough as discussed here specifically refers to the melting of tube metal in the vicinity of the weld such that a cavity is formed. If burnthrough is severe, a continuous channel may be produced that can cause leakage. 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

Mixed thermal/thixotropic systems improved resistance to leakage may be realized for low or moderate melting point systems. Above its melting point, the liquid can still be immobile because of thixotropy. 

The ordinary casing will be outside the elastic one with air between and welded together to prevent water leakage. When the ice melts, the material will go back to its original shape again. 

Tablets are flat with an oval-, pear-, or bullet-shape to facilitate the insertion into the vagina but often present a hard surface and dried ingredients that may cause undesirable irritations on the other hand, pessaries are characterized by a smooth surface and by excipients dissolving or melting in physiological conditions, but they are messy to apply and undergo leakage.

So far we have neglected the effect of the flight clearance. As small as the clearance is, polymer melt is being dragged across the clearance by the barrel surface and the pressure drop may pump melt across the flight width. This creates a continuous leakage flow from downstream locations to (one turn back) upstream locations, reducing net flow rate. 

Now that the physical configuration has been clarified, we can consider in detail the mathematical model for the process. First, we find a relationship between the flow rate and the geometrical and operational variables. Neglecting leakage flow between the screws, as well as that between the screws and the barrel, this is simply given by multiplying the velocity Vi with the cross-sectional area of the melt-filled channel, Am, as shown in Fig. 6.48... 

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