Some PIM molders, like FloMet LLC (DeLand, FL), can’t live without them. But other knowledgeable industry experts, like Arburg’s Uwe Haupt, say that hot runner systems may not be such a hot idea.
Haupt admits that the lure of runnerless systems, which are popular in plastics molding, can be seductive, especially when you consider the cost of feedstocks and the loss of materials in sprues and runners. But he argues that PIM feedstocks often have a much narrower processing window than plastics.
“Two of the most critical parameters are the mold temperature and the nozzle temperature,” he explains. “If they change from shot to shot, or even within a shot, the result will be inconsistent moldfilling behavior, as the viscosity of the feedstock strongly depends on the temperature.”
In most cases, hot runners have a low thermal capacity even though they have enormous heating power, he adds. Temperatures in most hot runner systems are controlled via heating sensors located near the heating element. Haupt believes hot runner sensors can be fooled by the temperature of the heating element.
“By monitoring the temperature of the hot runner, it is possible to find a control parameter set where the temperature near the sensor is almost constant, but the temperatures at other points of the runner system can still vary.” His experience has proven to him that a temperature uniformity of ±3 to 5 deg C within the entire runner system is an absolute must for PIM feedstocks because they are so sensitive to degradation. He believes hot runners are incapable of holding temperatures to such a uniform degree.
The direct contact of the warm outlet of the runner and the mold cavity is another disadvantage in Haupt’s opinion. “If there is no good thermal insulation, the runner will heat up the cavity and demolding problems will be the result,” he says.
In spite of all of this, Haupt is not dead set against them. He says that he usually recommends customers first try molding their green parts without hot runners, and then maybe use a hot sprue. “It is not impossible to use them successfully,” he says. “But molders should be aware of the problems hot runners can cause, and should question whether or not they are willing to introduce more problems into the process.”
Arburg GmbH & Co.
Phone: +49 (7446) 33-3887
Fax: +49 (7446) 33-3389
E-mail: [email protected]