Plastics Technology

JAN 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Experiments reveal the relationship between vent depths and part thickness, allowing molders and moldmakers to more precisely determine vent dimensions. Determining Vent Depths in Injection Molding In injection molding, air inside the cavity needs to be evacuated for the plastic to fill the cavity. Vents are therefore machined in the cavity blocks in the mold. If the air is not evacuated, it will cause inadequate filling, resulting in several defects such as short shots, poorly packed-out parts, burning of the plastic, voids in the parts, dimensional variation, and so on (see Fig. 1). Over time, the mold steel can get damaged because of the excessive air pressures in the local area at the end of fill or in cor- ners where the air and plastic tend to get pressurized. Lack of vents can also create excessive pressures in the cavity, causing the mold to open sufficiently to cause flash on the parting line. The vents are machined typically on the parting lines, from where the plastic reaches the end of fill to the outside of the mold. The end of fill here must not be confused with the end of fill during filling of the part. End of fill for this discussion refers to the stopping of the polymer flow in the particular local area. The viscosity of the plastic should be high enough to prevent it from flowing out of the mold through the vent. Figure 2 shows 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. The dimensions of the primary vent are the most critical. First, the vent depth (D), should be such that it helps the evacuation of air but not let the plastic escape. Second, the vent land (L), should not be large enough to cause a substantial drop in pressure and prevent air from being pushed out nor should it be so short that the plastic easily finds its way out to the secondary vent. Typical land lengths should be around 0.060 to 0.080 in. (1.2 to 1.5 mm), assuming the vent depth is designed correctly. For larger parts such as car bumpers or doors, the land lengths need to be higher, depending on the volumetric flow rates. Third, the vent width (W) should be at least 0.25 in. (5 mm). On the high end, it can be as wide as desired, and in some cases, it can run around the entire perimeter of the part (ring vents). Vents in By Suhas Kulkarni FimmTech QUESTIONS ABOUT MOLD VENTING? Visit the Molds & Tooling Zone. Good venting leads to robust, repeatable, and reproducible processes. WHAT YOU WILL LEARN 1. WHY VENT?: To evacuate air that might otherwise cause part defects. 2. VENT SIZE: Make them large enough to develop a robust process without causing flash. 3. MATERIAL SUPPLIER INPUT: Tends to be generic. 4. PART THICKNESS: Thicker parts tend to need deeper vents. 50 JANUARY 2018 Plastics Technology PTonline.com Tips and Techniques

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