Low Pressure Processing
Low pressure injection molding consists of injecting a molten resin as a 'short shot' into a mold cavity. The low pressure referred to is the cavity pressure. Low pressure processes typically result in parts with lower molded in stress, better tolerance control and require less clamp tonnage.
Nitrogen gas is used to produce hollow sections in the molded part. The gas is introduced to the short shot through a special machine nozzle or via gas pins in the mold. Special auxiliary equipment is required to manage the gas to and from the mold. Part and tool design as well as gas pin placement is critical for successful results.
Chemical blowing agents or an inert gas such as nitrogen or CO2 is mixed with molten resin to create a rigid foamed part. Foaming agents decompose in the barrel with time and temperature and create N2 or CO2. The gas and resin mixture is under pressure while in the barrel. When the mixture is injected into a vented mold, foaming occurs. The part will have a skin core skin cross section. Weight reduction is achieved by only filling the mold partially. A mold filled only 90% will produce a part with a 10% weight reduction. The surface finish of a foamed part will have significant splay. Endo-thermic blowing agents can reduce cycle times. Part and tool design with particular attention to gate size and location, vent depth and location and process parameters are essential.
A modification to standard injection, gas assist or structural foam molding, counterpressure supplies a known resistance to the resin flow front during resin injection. Dry shop air or nitrogen is introduced to the mold prior to resin injection. Molds require a seal around the parting line, ejector pins and other areas of the mold that might leak the gas. Parts molded with counterpressure enjoy uniform packing, swirl free surfaces, thicker skins and enables resins such as ABS and STYRENE to be molded without drying. A control module is required to manage the gas into and out of the mold. Sealing techniques and process parameters require special attention to ensure quality parts.
Want to use Low Pressure Processing in your manufacturing process? Take a look at the Counterpressure Control Module.