Plastics Technology

JAN 2018

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link: http://pty.epubxp.com/i/918111

Contents of this Issue

Navigation

Page 53 of 91

EXPERIMENTAL DETERMINATION OF VENT SIZES A mold was constructed for a part that is center gated and has 18 tabs of varying combinations of tab thick- nesses and vent sizes (Fig.3). The center gate delivered the melt to each of the tabs at the same time. There were three different tab thicknesses: 0.125 in., (3.175 mm); 0.0625 in. (1.587 mm); and 0.0312 in. (0.792 mm). There were six vent sizes from 0.0005 in. (0.0127 mm) to 0.0030 in. (0.0762 mm) in steps of 0.0005 in. (0.0127 mm). Different materials were molded, and for each tab the minimum vent size where no flash was seen was recorded. The results show primarily that the thickness of the tab also plays an important role in determining the vent size. The thicker tab was able to accept larger vents without flashing. For example, with an ABS material, at the 0.125-in. tab the vents flashed at 0.0030 in.; whereas at the 0.0312-in. tab, the vents flashed at 0.0020 in. (Fig 4). There are two reasons for this. As the plastic is forced into thinner sections, the flow rate of the plastic in the area increased. The increased flow rate increased the shear rate and led to a reduction in the viscosity, making the plastic flow easier, resulting in flash. The reduced viscosity also allowed a reduced pressure drop, increasing the end-of-fill pressure, resulting in flash. The graph in Fig. 5 summa- rizes the results. The published vent depth for ABS is 0.002 in. (0.0508 mm), but the test results from the vent mold showed that a vent size of 0.003 in. (0.0762 mm) is acceptable if the part thickness is 0.125 in. This value was used on several molds successfully. Naturally, parts thicker than 0.125 in. can have larger vents, but there will be a plateau point and the increase will not be linear. Caution is required. Tests on nylons also showed surprising results. The published value for nylons is a vent depth of 0.0005 to 0.0007 in. (0.0127 to 0.017 mm); but for thicker sections, experiments showed a vent depth close to 0.0015 in. (0.038 mm) could be used. In all cases, it is important that proper scientific molding and scientific processing techniques be followed. Adding vents is an important step in mold design and mold building. Good venting leads to robust, repeatable, and reproducible molding processes. Although material suppliers provide recommen- dations for vent depths, the they mostly seem to be generic numbers. The method described above is a scientific way of determining vent sizes. For a given mold, the all the vents are usually machined to the same depth. Based on the data above, for maximizing the efficiency of venting, the vents could be machined to different depths based on their location and the wall thickness in the local area. ABOUT THE AUTHOR: Suhas Kulkarni is the president of FimmTech Inc., Carlsbad, Calif., a firm that provides resources for Scientific Molding and Scientific Processing. The firm provides training, software, and consulting. He has 26 years of experience in injection molding and is the author of the book, Robust Process Development and Scientific Molding, published by Hanser Publications and now in its second edition. Contact: (760) 525–9053; suhas@fimmtech.com; fimmtech.com. FIG 3 An experimental mold built to study the factors that determine the vent depths was center gated and had 18 tabs of varying combinations of tab thicknesses and vent depths. Tab Detail FIG 4 Vent-depth combinations in experiments showed that tab thick- ness plays an important role in determining the optimum vent depth. FIG 5 This graph shows the combination of tab thickness and vent depth where flash begins to occur when molding a medium-flow ABS. Tab Thickness = 0.125 in. Vent Depth = 0.0030 in. Evidence of Flash Tab Thickness = 0.125 in. Vent Depth = 0.0025 in. No Flash Tab Thickness = 0.03125 in. Vent Depth = 0.00015 in. No Flash Tab Thickness = 0.03125 in. Vent Depth = 0.0020 in. Evidence of Flash Tab (Part) Thickness Vent Size, in. 1/32 in. 1/16 in. 1/8 in. 0.0035 0.0030 0.0025 0.0020 0.0015 52 JANUARY 2018 Plastics Technology PTonline.com M O L D V E N T I N G T ips & Technique s

Articles in this issue

Links on this page

Archives of this issue

view archives of Plastics Technology - JAN 2018