As we know, injection parts have encountered many molding problems, such as short shot, weld line, residual stress, warpage and so on. Some products are thin and huge, for example, automobile parts. Therefore, the runner system plays a very important role in the molding process. If the cold runner system is used, short shot problem might occur due to insufficient pressure. Too much pressure is wasted or consumed by cold runners. Another issue, such as the welding line is very obvious with cold runner system. There is poor cosmetics on product surface. Hot runner technology has been the solutions to these issues on many plastic products.
Hot runner systems were first developed and came into sporadic use in the early 60s with generally negative results. They gained popularity in the 80s and 90s as technological advantages allowed improved reliability and the escalation of plastic materials prices made hot runner systems more desirable and cost effective.
Runner System
The runner system consists of three sections, the sprue, runner, and the gate. The primary task of a runner is to deliver and distribute melt from the screw chamber into the mold cavity through the gates. The filling pattern, material characteristics, and the cycle time are all related to the construction/design of the runner system. More importantly, it affects the quality of final products.
As we know, we have cold or hot runner systems. The purpose is to distribute the melt form the injection machine nozzle to the part gate without imparting any characteristics to influence part quality.
Appropriate temperature control to maintain temperature for melt in the channel and to avoid cooling down with melt in the cavity.
Hot Runner System
Hot runner systems are also referred to as hot runner-manifold systems or runnerless molding. The hot runner is an extension of the machine nozzle that transports the resin to the cavities while maintaining the processing condition of the resin.
A hot runner system is mainly composed of hot runner gate, hot nozzle, manifold and heating coil for nozzle, heater for manifold, piston cylinder to control the valve and so on.
•The hot runner system is comprised of a sprue bushing, manifold, nozzle assemblies, heaters, and steel plates that enclose the manifold.
•The sprue bushing acts as a carrier for plastic from the machine nozzle to the manifold.
•The manifold distributes the resin to reach the specific drop locations.
•Heaters are embedded into the manifold and maintain the resin at the required processing temperature.
•From the manifold, the resin enters the nozzle housings.
•The nozzle housings are heated by a band heater which ensures that the resin is maintained at its optimum processing condition.
•From the nozzle housings, the resin enters the nozzle tips which direct the resin flow into the gate area.
The hot runner design should be balanced for even flow throughout the runner system. Externally heated manifolds work the best for RTP TPE materials. Internally heated systems, which use cartridge heaters, are not recommended as they generally have hot spots which could lead to material degradation. All flow channels should be highly polished with radius corners to minimize the possibility of material stagnation. To maintain high shear, low residence times, and provide for self-clearing, the flow channels should have be 0.250 to 0.375 in (6.35 to 9.52 mm) diameter. Balance volume per shot size.
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