Melt stagnation

They include melt stagnation, less probability of overpacking, less chance of streak formation in the part, and reduced abrasion due to the absence of metal-to-metal contact. Lower viscosity melts may require the use of the check ring valve. 

The screw consists of a feed section, a rapid transition section, and a metering section a rounded forward end prevents stagnation. The breaker plate that converts the rotary motion of the melt into smooth, straight flow should have as many holes as possible both ends of each hole should be countersunk for streamlined flow. 

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. 

Ribe NM Smooke MD (1987) A stagnation point flow model for melt extraction from a mantle plume. J Geophys Res 92 6437-6443... 

When a stagnation flow on the melt-crystal interface is created by any mechanism and is intense enough to lead to a thin solute boundary layer, the velocity field in the boundary layer can be described by... 

Streamline the flow to avoid abrupt changes in the flow passage that may cause stagnation areas. Stagnated flow may lead to thermal degradation of the plastic melt as the melt is exposed to high heats for long periods. 

Streamline the die flow passage to avoid flow stagnation areas. Such areas facilitate degradation of the polymer melt due to prolonged exposure at elevated temperatures. 

A conventional nozzle (Fig. 6.31) with a reverse taper can be used with FEP, PFA, and ETFE. The size of the bore should be as large as possible and it should be tapered in order to prevent stagnation and abrupt changes in the melt velocity. The sprue should extend into the nozzle a sufficient length (13-25 mm) to minimize the likelihood of cold slug formation in... 

Material Flow Shorter hold-up times. Less resin stagnation in the barrel. EfScient pressure transmission to the melt. EfScient mold filling. 

Savino, J.M. et al, 1970, Experimental study of freezing and melting of flowing warm water at a stagnation point on a cold plate. 4th Int. Heat Transfer Conf 1, 2- 11. 

Choker bar (restrictor bar) n. A bendable metal bar incorporated in a sheet-extrusion die for controlling flow distribution and lateral sheet thickness, and for reducing stagnation in the melt. The shape of a flow passage between the choker bar and the lower die body is altered by turning bolts coimected to the bar. 

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