Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/918111
localized sections that do not run around the perimeter of the part are called spot vents. The secondary vents are also called vent reliefs. The dimen- sions of the secondary vents are larger than the primary vent and therefore help in easy transport of air inside the mold out to atmosphere. They are mainly used as a dump area for the air as it exits the mold. Secondary vents should be at least 0.010 in. (0.25 mm). In all cases, the vents must be well polished to avoid any buildup of residue from the gases. The vents must also be draw polished in the direction of air flow. An article authored by Randy Kerkstra, Venting: Where and How Deep? (see Tooling Know How, May '15) discussed this topic. The processor needs to have the biggest vents possible so that the plastic can flow into the mold with the least restriction and help with developing a robust process. However, if the vents are even slightly oversized, the result is flash in the part. The moldmaker will then have to weld the area and rework it to get it back to where it was. This is a very difficult and time-consuming process, so moldmakers tend to stay conservative with the vent depths. The dimensions of the primary vents depend on a number of factors. Molders and moldmakers rely on the material manu- facturers to provide info on the vent depths. Moldmakers typi- cally follow these recommendations when building an injection mold and tend to stay on the lower end of the recommendation for fear of flashing the mold. Since the recommended vent sizes always come with a disclaimer, FimmTech Inc., with the help of Distinctive Plastics, a custom molder in Vista, Calif., built a special mold to study the factors that determine the vent depths. (The mold is available to material suppliers for determining the vent sizes for their materials.) For larger parts such as car bumpers or doors, the land lengths need to be higher, depending on the volumetric flow rates. Before Venting After Venting If air is not vented from the mold, it will cause inadequate filling, resulting defects that include short shots, poorly packed-out parts, burning of the plastic, voids in the parts, and dimensional variations. After Venting Before Venting FIG 1 FIG 2 Shown here is the cross section of the mold, indicating the vent area and types of vents. The relieved section that is closest to the cavity steel (and part) is the primary vent. Its dimensions are the most critical. Tab Detail Vent Width (W) Vent Land (L) Secondary Vent Primary Vent Part A A Secondary Vent Depth Primary Vent Depth (D) Section A-A @plastechmag 51 Plastics Technology MOLD VENTING