Heated sprue

A type (a) mould of three-plate design with heated sprue bushing is converted using an HR manifold and nozzles with side gating. The cycle time has been reduced by 35%, and the production cost has fallen by 43%. 

Figure 4.95 Heated sprue bushing with extra temperature balancing...

Heat conduction from the manifold takes place chiefly via the spacer pads and fastening bolts, and perhaps also via the non-heated sprue bushing. A diminution in heat losses caused by conduction may be attained by ... 

The manifolds most frequently used have a very long heated sprue bushing located on the mould axis and operating on the principle illustrated in Figure 9.10. As can be seen, the melt flowing out of the sprue bushing may, when the mould is taken away from the injection machine nozzle, gets into the ejector plate space. 

The additionally needed opening width used for runner demolding can be disadvantageous. The combination with a heated sprue channel decreases in the bulkiness of the runner is therefore not rare. Three- plate mold- gates are used for applications with very small gate distances not achievable by hotrunner nozzles because of their space requirements. A typical example is day light of 7 mm for the individual gating of narrow chambers which are positioned next to each other in molded parts for medical use. 

Productivity is directly related to cycle time. There usually is considerable common knowledge about a geometry and process conditions that will provide a minimum cycle time. Practices such as using thinner wall sections, cold or hot runners for TPs or hot or cold ones for TSs narrow sprues and runners, the optimal size and location of coolant (or heat) channels, and lower melt or mold heat, will decrease the solidification time reducing the cycle time. 

When a thermoplastic polyurethane elastomer is heated above the melting point of its hard blocks, the chains can flow and the polymer can be molded to a new shape. When the polymer cools, new hard blocks form, recreating the physical crosslinks. We take advantage of these properties to mold elastomeric items that do not need to be cured like conventional rubbers. Scrap moldings, sprues, etc. can be recycled directly back to the extruder, which increases the efficiency of this process. In contrast, chemically crosslinked elastomers, which are thermosetting polymers, cannot be reprocessed after they have been cured. 

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

Where the maintenance of a clear channel between sprue and the slowest freezing part of a charge is impractical, cavitation is avoided by casting charges in layers, each of which is allowed to crust over before pouring the next TNT melts at 81°. It forms eutectics with RDX, Tetryl (68°), PETN (76°), and other impurities in the mix and makes these materials more soluble at higher temps. Thus, there is a general tendency for the solid content and, hence, the apparent viscosity of most castable mixts to decrease as the temp is increased. However, a reversal of the tendency toward the reduction in viscosity has been noted in Comp B when it heated above 100°... 

Sprue fed from injection unit under heat and pressure... 

The two most widely used methods for molding thermoset are compression and transfer molding. Thermosets, you will recall, undergo a permanent set, i.e., become essentially insoluble and infusible under the action of heat. Consequently, techniques for fabricating thermosets must take due cognizance of and make allowance for the fact that sprues, runners, and gates are not reusable and therefore constitute rejects. 

FIGURE 2.4 Molding qrcle of a plunger-type transfer mold, (a) An auxiliary ram exerts pressure on the heat-softened material in the pot and (b) forces it into the mold, (c) When the mold is opened, the cull and sprue remain with the molded piece. (After Frados, J. ed. 1976. Plastics Engineering Handbook, 4th Ed., Van Nostrand Reinhold,... 

One can see from Figure 2.10 that a three-plate mold operation necessitates removal of the ruimer and sprue system, which must be reground, and the material reused. It is possible, however, to eliminate the runner system completely by keeping the material in a fluid state. This mold is called a hot-ruimer mold. The material is kept fluid by the hot-runner manifold, which is heated with electric cartridges. 

Pressure loss develops, longer molding cycle, requires increase in heat requirement, premature freezing of sprue, etc. 

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