By Bob Lebeaux, President, The Plastics Group of America
September 28, 2009
Truth be told, if the product wall thickness is larger than 0.125, voids are likely to occur. When molten plastic is extruded, the outside cools first, frozen in place. Then the inside begins to cool and crystallize, shrinking about 15% in the process. If the outer wall is sufficiently strong, it cannot sag inward, and a void forms spontaneously on the interior, with voids tending to locate at wall intersections and in the thickest sections.
Other causes for voids can be moisture in the material, or poorly incorporated fillers or pigments. Degrading the plastic can also cause voids. A vacuum-vented extruder can cut down on voids, but profile extruders rarely so equipped.
If not moisture-related, you may have to live with voids. Try inspecting parts with a very strong light. This may show the voids unless parts are black. If opaque, there is no way to detect voids (short of x-raying). Many fine parts contain internal voids, but noone is the wiser.
Ultimately, the only way to completely eliminate voids is to redesign a product to eliminate its thick sections. You could also have limited success by slowing the cooling rate of the extrusion.
« Back to the Plastics Group Blog
Facebook
Twitter
Digg
LinkedIn
Del.icio.us