Improper mold venting can, and frequently does, cause a multitude of processing problems, but now a patented, scientific method and device to help check for proper venting conditions is on the way.
Process & Design Technologies LLC, a consulting firm dedicated to providing solutions for plastics processing issues, has been issued a patent for "Apparatus and Method for checking mold vent condition." P&DT principal Terry L. Schwenk has been working to promote scientific venting for a number of years.
"Scientific venting principles are based on volume displacement over time versus the typical method which involves the vents being calculated as a function of flashing of the material," Schwenk says. "Air and/or gas in the cavity need to be displaced by the molten plastic. If you have poor venting, it will require more pressure and time to displace the air and/or gas in the cavity, with heat being generated as a byproduct."
Schwenk goes on to explain that when molten resin is injected very rapidly into a mold with a marginal vent, a lot of heat is created. Even though there are no burn marks on the part, the material could be much hotter than required. Burn marks are the result of the gases igniting, but Schwenk points out that the material can still be too hot without burn marks.
"It's the hidden problem. If they don't see a burn mark on the part, they think they're okay. That's where the science comes in," he says. "You have to determine what size vent you need to evacuate the air and/or gas. The faster the injection speed, the larger the vent you need. Using P&DT's patented device allows processors to calculate the proper vent size needed to allow the cycle times required for the process you have. "
Currently, Schwenk has a prototype of the device, which is used on the outside of the mold. It allows checking of the vents by making a fixture specific to the mold - one for B half and one for A half - which then measures the venting and calculates the vent size needed.
"Toolmakers always promote how precise their measurements are, but often they vary from cavity to cavity," Schwenk says. "A cavity on the low end of the tolerance may vent better than a vent on the high end of the tolerance. This device can determine which cavities have minimal vents and which cavities have larger ones. Non-uniform venting can result in non-uniform parts."
For new tool builds, Schwenk adds, "There's no excuse for going steel safe anymore. You can know exactly what size the vents need to be." This can be especially helpful during mold validation, when technicians often chase their tails hecking runners and gates. Designers rarely specify vent sizes.
Schwenk notes that proper venting analysis and checking can save plastic manufacturers thousands of dollars with improved injection mold performance, fewer rejects, and faster cycle times.
Schwenk says next step for Process Design & Technologies is commercializing both the device and the method to make them available to processors and moldmakers. —Clare Goldsberry