Tips To Help You Succeed With Hot Runners

A properly designed hot runner system can significantly reduce cycle time and resin cost. Here are several success factors, presented by Dow Plastics' Mike Martin and Ken Kerouac in Dow's TechLink newsletter, that should be considered early, in order to produce cost-effective parts when using hot runner tools.

Design

Pay special attention to detail. The design guidelines that apply to the machine nozzle also apply to the hot runner system. Plastics that are sensitive to high heat and excessive shear require more attention to design detail. Insert plugs that close off manifold bores should be smooth. Runner diameters should impart shear stress no more than 1 percent of the resin's tensile strength. The pressure drop from sprue bushing to tip should be no more than 25 percent of the maximum plastic fill pressure, with the resulting temperature increase remaining as close to the desired melt as possible. A runner volume of 25 percent of the part volume, and a shear rate of about 1000 sec-1, are good targets for most applications.

Control temperature. Ideally designed, the hot runner system eliminates heat exchange between the melt and the manifold. When carefully prepared, the system should maintain melt temperature and uniformity on its own. The watt density provided by the heater elements should be minimized--2w/cu cm of volume is a good target. Thermocouples should be placed in steel near the center of the heater zone for optimal closed loop control.

Eliminate hot or cold spots in the manifold through careful design and placement of the thermocouples and heater elements, regardless of type: cartridge heaters, coil heaters, tubular heaters, heat pipes. Hot-tip nozzles use separate control of the body and tip temperature to help eliminate excessive heat soak in the nozzle and control the gate appearance. Mold cooling of the gate area should, whenever possible, use a separate water circuit.

Choose a practical gate design. The ideal gate design should freeze off during a profitable cooling time, and be difficult to detect. At optimal injection velocities, it should not cause shear in excess of the resin's critical shear stress or have different properties at the gate and elsewhere. Consider the end function when determining the level of gate vestige.

Processing

"Soft start" the hot runner system. Many heater designs include a compacted magnesium oxide layer that promotes efficient operation but absorbs moisture when idle. When starting the system after an extended period of time, first soft start it at a lower temperature for 30 to 60 minutes for moisture to bake out. That will prevent heater shorts and promote heater life.

Keep barrel nozzle temperature below melt temperature. The barrel nozzle temperature should be set 25 deg F below the melt temperature, which then becomes the starting point for the manifold temperatures. Hot runner temperatures typically ramp downward from the runner bushing to the tip. If the runner temperatures are too cold, they can cause excessive early pressure, resulting in cold plugs.

Handle heat degradation. If heat degradation is suspected, first determine if it is occurring in the barrel or the runner system. Then determine the heat soak time required to cause the purge patty to discolor or degrade, and compare it to the time it takes to discolor molded parts. If the time is about the same for both, and less than twice the intended cycle time, all temperatures should be reduced. If the part degrades first, lower the manifold temperatures; if the purge patty degrades first, lower the barrel nozzle temperature.

Keep an eye on system controllers after molding begins. Watch the hot runner system controllers for a few cycles after the initial molding conditions are set. A zone that is energized for less time than other zones may indicate shear heating, improper cooling, improper placement or wiring of the thermocouple, or a zone that's too hot. A zone that gets too much power may mean a weak heater cartridge or overcooled zone, which is typical of a runner system nozzle tip.

Optimize injection velocity. At low speeds, cold shear or splay is evident because flow channel velocities aren't fast enough to efficiently drive resin in the manifold. At high velocities, shear heating and burning will be noticeable. Set the correct velocity, which typically is higher than can be achieved without a hot runner system. And keep in mind that every time the flow splits, the shear rate is reduced by half.