The “Italian” Sprue

The runner is heated electrically, but a nonelectric means keeps the material from freezing off at the gate. Instead of utilizing a probe externally heated by means of electrical heating elements, the design of the nozzle incorporates what is in effect a torpedo. The entrapped heat of the plastic heats the torpedo, which in turn heats the plastic at the localized gate area so that it does not freeze off. The torpedo helps equalize the heat within the nozzle, both throughout the material cross-section and between front and back of the nozzle.

Cartridge heaters are normally used to supply heat to some manifold designs and the following guidelines are the result of several studies:

• Use the lowest wattage heaters possible since they offer the longest life. Best results are obtained when they are operated between 30 and 70% of their rated output.
• Select the largest heater diameter that is practical. For a given design, heater performance and longevity are generally determined by its watt density (watts per square inch of heater surface area versus its physical fit in the mold for heating. By the use of larger heaters with greater surface area, the fit is slightly less critical and the larger heaters have better characteristics. A ground O.D. to insure a tight fit is the best practice for heaters that are not cast in beryllium copper.
• Make sure that all heaters, contacts and mold contact areas are clean; the presence of foreign material between the heater and the mold component will create a hot spot and cause premature heater failure,
• Extra care must be exercised when new heaters are installed or when the mold has been in storage for a considerable period to bake out the moisture that has accumulated. A bakeout for several hours at 10 to 20% reduced power is recommended. A bakeout is always necessary before sealed terminals are applied.
• Caution must be exercised in the mold design to permit an orderly» arrangement of the wiring when such is necessary.
• Ample space must be provided for hookup. This includes a check for tie-bar press clearance and other adjacent auxiliary components.
• All heater wires must be identified for proper zone control prior to press installation. It is near impossible to identify multiple heater leads after the mold is in the press.
• Evaluate the special cast-in beryllium copper heating elements for hot runner gating since they greatly simplify wiring, minimize burnouts and put the heat where it is needed.

Several patents have been issued or are pending on hot manifold molding which merit further study.

Completely integrated control panels of the closed loop-on/off type and the steady current adjustable type using triacs are available for a variety of applications. Temperature sensing probes have been developed to insure accurate reporting of thermal changes and immediate correction. Typical of the many sophisticated control systems is a solid state expandable modular electronic power controllers shown in Fig. 8.30. It is a fully time-proportioning closed-loop feedback system capable of excellent control over a wide range of operating conditions. This control is obtained by sensing the temperature at the required point by means of a thermocouple and then applying the exact amount of power called for to

maintain a pre-set temperature. The duration time that power must be applied is automatically sensed and continuously adjusted as required to maintain very close limits in properly designed system. Included features are 士 1° control, calibrated deviation meter, automatic/manual control in the event of thermocouple failure coupled with automatic shut-down in the event of such failure. Zero crossover switchover switching for extended heater or load life is used. These and similar units are supplied by several vendors.

Another useful type of control is the electronic power controller. This unit is designed to provide current control for any system that requires a very accurate and reliable variable power source for manifold, cartridge, tips, nozzles, probes, heater casts and insulated runner heaters.