Method and apparatus for making flat molded plastic articles
View Patent ↗In a method for producing flat molded plastic parts, for example panes, a mold is first filled completely with plastic material and additional plastic material is subsequently added, in such a way that the mold distends in a defined manner in opposition to the clamping force. The distension compensates for material shrinkage during a subsequent cooling operation, thus preventing sink marks and permitting the production of, for example, panes of high optical quality.
1. A method of making a molded plastic article, comprising the steps of:
closing a cavity of a positive mold by applying a clamping force;
fully filling the cavity with plastic material, while maintaining a size of the cavity constant;
adding plastic material so as to distend the positive mold in opposition to the clamping force until the cavity of the positive mold expands to reach a defined size for producing a defined article thickness;
closing the positive mold until reaching a residual distending opening and molding the plastic material into a plastic article while applying the clamping force to thereby maintain the plastic material compressed; and
removing the plastic article.
2. The method of claim 1 , wherein the adding step is controlled in dependence on a distance traveled by an advancing screw.
3. The method of claim 1 , wherein the adding step is controlled in dependence on a distending motion of the positive mold.
4. The method of claim 1 , and further comprising the steps of measuring an internal pressure in the positive mold, and applying the clamping force in dependence on a profile of the internal pressure.
5. The method of claim 1 , and further comprising the step of applying a higher clamping force upon the positive mold at a location closer to a sprue than at a location farther away from the sprue.
6. The method of claim 1 , wherein the molding step is carried out at constant clamping force.
7. The method of claim 1 , wherein the plastic article is a thick-walled, flat molded article.
8. The method of claim 1 , wherein the plastic article is a disc.