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.
By Bob Lebeaux, President, The Plastics Group of America
September 22, 2009
The primary shortcoming of polystyrene is a low softening temperature – it will sag and distort at about 200 degrees F, lower than the boiling point of water. Since many foods are water-based, heating to boiling or steaming will destroy the container. Worse yet, the polystyrene will partially dissolve in fatty foods, contaminating the food with plastic.
Here are two material types that can handle most microwave and heated food applications: Polythylene terephthalate (PET) and Polypropylene (PP).
PET has useful properties up to 400 degrees F, unfortunately, it is also much more expensive than polystyrene. PP doesn’t soften until about 300 degrees F, which is plenty high enough to withstand boiling water and steam. PP is both lighter in weight and lower in cost than polystyrene. And, talc can be used as a reinforcing agent to increase the stiffness of PP – when supplied as a fully formulated product, talc-filled PP is competitive in cost with polystyrene. Just some food for thought.
By Bob Lebeaux, President, The Plastics Group of America
September 16, 2009
If you need to know if a resin is flexible or rigid, clear or opaque, bondable, scratch-resistant, or reactive with other chemicals, then we have a terrific resource for you – the Resin Kit™.
Available exclusively through The Plastics Group of America, the Resin Kit provides samples and complete technical information on 50 of the most commonly used thermoplastic resins. It is used by molders, extruders, educators and other processors to determine unknown resins. Whether you’re an OEM or designer who needs to use it as a specific guide for new applications or a recycler who needs to identify scrap, it’s an invaluable resource that is easy-to-use, reliable, and costs just a fraction of laboratory analysis.
By Bob Lebeaux, President, The Plastics Group of America
September 8, 2009
When companies utilize nylon resins to make products by injection molding, they need to be aware of one element that can impact the strength of their finished product – the low moisture content of air in the winter. To address this challenge, some processors simply subject the parts to moisture after molding, either through submersion of the parts in a water bath, or by aging the parts in an area with artificially increased humidity (this can achieved by a humidifier or simply by having some open water sources in the storage space).
Another possible source of molded-in stresses could be the lower temperature of your cooling water in the winter. Carefully controlling the temperature of the molds by making sure that the cooling water is always at the same temperature, regardless of the season, will eliminate this variable and positively impact quality control.
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