This invention relates to methods and apparatus for controlling the direction and/or magnitude of warpage in molded plastic parts through strategic positioning of the non-homogeneous melt conditions across a stream of a laminar flowing fluid to a desirable circumferential position. This may be used in combination with more conventional process variables. The invention is useful in flow channels generally that flow a stream of laminar flowing material, such as thermoplastic or thermosetting plastics. The invention is particularly suitable for solidifying or non-solidifying runners, such as cold-runner or hot-runner injection molding machines that flow thermoplastic or thermosetting melt into a single or multiple cavity mold. The invention is also applicable to extrusion dies in which the melt conditions of the plastic can be strategically repositioned to achieve a desirable output condition from the flow channel to impart a desired material property to the flowing melt, such as to control a magnitude and/or direction of plastic part warpage.
Thermosets require heat to transition from a fluid to a solid state (the heat induces a chemical reaction) whereas thermoplastics must be cooled from a hot molten state to solidify. This is not a chemical reaction as found with thermosetting materials, but rather a phase change from liquid to solid. Thermosets are injected into a mold (via an injection molding machine or with use of a “transfer molding” process).
With thermoplastics the mold is cooled so that the plastic will solidify. A cold runner mold will also cool the runner after mold filling and the melt in the runner will solidify and must be removed every molding cycle. A hot runner will allow the runner material to remain molten during the entire molding cycle.
With thermosetting materials, the process is somewhat opposite to thermoplastics. A heated mold is used to allow the material to solidify. During injection molding or transfer molding, a fluid material is injected into a heated mold. The mold heats the material and initiates a chemical reaction causing the material to cross link and solidify. Normally the runner travels along the parting line similar to a cold runner thermoplastic mold. However, the runner is hot and the runner material solidifies with the molded parts and must be removed during every molding cycle. A cold runner system allows the material to remain fluid much like a hot runner used in thermoplastic molding.
Warpage of plastic parts is a result of variations in shrinkage within the part as it is being formed. Sources of such warpage of molded plastic parts have previously been poorly understood. These variations in shrinkage have generally been attributed to side to side variations in mold temperature, anisotropic shrinkage variations resulting from flow induced polymer and filler orientation, and global shrinkage variations (shrinkage variations between regions of a part) resulting from differences in wall thickness, mold temperature, melt temperature and melt pressure. Accordingly, when warpage in a particular mold design was discovered, attempts to correct the warpage typically involved modification to the melt temperature, mold temperature, fill rates, or an adjustment in pack pressure or pack time, or modifications to part geometry or gate locations.