Common part-quality issues such as burns, flash, and warpage cause frequent headaches for molders. Fortunately, careful consideration of one key aspect of mold design—venting—can help eliminate these problems.
Venting is a small part of tool design that has a large effect on part quality, and the number of vents and their locations are therefore critical. But not all plastics designers and processors pay close enough attention to this crucial design aspect.
Inadequate venting traps air, steam from resin, and nonaqueous volatiles (NAVs)—gases from resin and/or additives. These can cause many problems:
GAS BURNS are black carbon marks caused by the plastic burning (“dieseling”) due to compressed gases. These burns can be more than a surface issue if the mold becomes damaged as the chemical action of the gases etches away the steel underneath.
PLATEOUT is a film deposit of chemical solids on the cavity. These deposits, most often seen on clear polycarbonate parts, can leave a cloudy mark on the part. Issues with plate out are often seen near the gate due to an initial influx of hot gases.
FLASH occurs especially when molding thin-wall parts. This common problem is a result of material running out of room in the mold, due to gas that should have been expelled occupying part of the cavity space.
SHORT SHOTS happen when gas compression in the cavity adds resistance to polymer flow, slowing the cavity fill rate and allowing the melt front to cool before the part is full. When this occurs, viscosity rises and more pressure is required to fill the part.
OILY BUILDUP, a gummy or sticky substance, can build up near the venting locations and can plug the vent openings. This usually occurs due to overheating.
Aside from location, there is virtually no limitation on venting options. Vents should not be placed where part function, cosmetics, or flash may be a problem. Beyond that, however, venting should be used as much as possible to avoid the aforementioned part quality issues. How you do it is the only open question. This article will look at two common venting options—using traditional machined vents and venting through porous materials—to help you decide which is right for your application.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
