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Hot Runners 26 MoldMaking Technology —— AUGUST 2016 Image and figures courtesy of Ermanno Balzi S.r.l. FIGURE 1A,B size are determined by the material to be injected into the mold. Viscosity is also a function of melt temperature, and the viscosity of material flowing into a small vent changes according to injection pressure. Gases produced during mold- ing also impact venting. Materials such as polyamide (PA), polyphenylene sulfide (PPS), polyetherether ketone (PEEK), polycarbonate (PC) with glass fiber, polyethylene terephthalate (PET), polyoxymethylene (POM) and materials with flame- retardant properties produce gases during molding. This gas production generates oily deposits that contaminate venting grooves, channels and the cavity surface, causing part defects. Venting gases produced by the melt can reduce mold cleaning frequency and mold cavity contamination. Nonstandard Venting Solutions Although there are some guidelines for designing and machin- ing vents to optimize venting capacity without removing excessive parting-line bearing surface, standard venting sys- tems offer limited venting capacity and performance. There are a few nonstandard venting solutions, however, that can be added to cavity venting to improve mold performance: Porous sintered materials can be inserted into the cavity to act as a vent. This reduces injection pressure, and therefore scrap and reject rates. Keep in mind that the thicker the porous mate- rial, the smaller the venting capacity, so proper venting channels must be machined to collect the gases. Frequent cleaning of vents also is necessary, as gas deposits can build up and partially or completely block the removal of air and gas from the cavity. Vacuum technology can remove the air from the cavity before the injection process. Although this addresses the problem upstream, it also causes limitations in application and per- formance. For example, to draw the air from the cavity and generate a vacuum (no air inside the cavity), the cavity must be completely sealed. This is costly and sometimes not possible in molds with sliding elements. And although vacuum technol- ogy can solve air problems, it is not effective in dealing with gas produced during cavity fill. The intake valve closes before filling, so gases can't find a way out of the cavity. An overflow system (an area connected to the cavity via a channel) can be created by machining an exit gate that con- nects the cavity's last filling point with a cold runner that ends in a venting area with a vent measuring about 0.1 mm. A groove then is machined around the vent to collect the gases and allow them to exit from the mold. During the filling pro- cess, air can then flow through the gate, cold runner, vent and gas-venting groove. Plastic freezes in the vent and is removed after every shot. Rheology (filling analyses) or trial and error can be used to determine proper vent size in this type of system. The designer can start with a vent height of 0.1 mm and length of 10 mm, and adjust as needed. The main benefit of an overflow system is the ability to overcome vent limits. Flash in the overflow area should not be a problem as long as it will be cut from the cavity after every shot. Limitations of an overflow system include extra material being wasted after every shot, overflow cutting operation and additional space requirements. Also, the position of the vent- ing gate needs to be at a mold parting line, and this does not solve air-trap issues. Alternative Venting Solutions Dynamic (as opposed to static) venting valves are an alternative venting solution that can be either external (connected via a channel or on the cold runner, if the mold has one) or internal The body of an external mold-venting valve houses a guide for a slide and contains wide venting channels connected to the venting area between the body and slide.