METHOD FOR THE USE OF GAS ASSISTANCE IN THE MOLDING OF PLASTIC ARTICLES TO ENHANCE SURFACE QUALITY

摘要

A method of gas-assisted injection molding in which: (1) a first amount of plastic is injected into an article-defining cavity of a mold so that it flows therein; (2) a second amount of plastic continues to be injected into the cavity simultaneous with the injection of a charge of pressurized gas wherein the pressurized gas prevents the flow of plastic in the cavity from stopping; and (3) the charge of pressurized gas continues to be injected after the second amount of plastic is injected to distribute the total amount of molten plastic resin in the cavity. The ratio for the first amount of plastic to the total amount of plastic required for the preparation of the article is in the range of 0.2 to 0.7. Also, the pressurized gas has a pressure between approximately 1000 psi and 5000 psi. These steps prevent imperfections on the exterior surface of the plastic article. The gas charge is maintained under pressure within the article until the article has set up in the article-defining cavity. Finally, the gas pressure is relieved in the article and the finished article is removed from the mold. The method is particularly useful in molding articles having a relative large cross-section.