Patent Application
20100028704
The present invention generally relates to plastic molding technology and, more particularly, to methods of reducing mechanical stresses associated with the molding process.
It is often desirable to produce components that include a hard insert molded within a conventional plastic mold material. This process is generally referred to as “over-molding,” and is widely used in the electronics industry in cases were a relatively hard material such (such as a metal or ceramic) is used as a stiffener or other structural member within a generally plastic component.
Conventional over-molding processes are undesirable in a number of respects, however. For example, the plastic molding material typically used for over-molding shrinks by a factor of between about 0.5 and 2.0% as it cools, cures, and hardens. When a hard insert is trapped within the plastic molding material as it shrinks, significant mechanical stresses can result, both on the hard insert and the surrounding molded structure. This can lead to reliability problems, including fractures, fatigue, and subsequent chemical attack.
Accordingly, it is desirable to provide improved over-molding techniques, particular for hard inserts subject to thermo-mechanical stresses during the mold process. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures.
The present invention generally relates to a method of reducing mold-induced stresses in over-molded parts using an elastic pre-mold. In this regard, the following detailed description is merely illustrative in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any express or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. For the sake of brevity, conventional techniques related to molding technology, polymers, and mechanical stress/strain characteristics will not be described in detail herein.
As mentioned previously, conventional over-molding processes are undesirable in that the plastic molding material typically used for over-molding shrinks by a factor of between about 0.5 and 2.0% during processing, particular after the curing and cooling processes. This condition is illustrated in
In accordance with the present invention, an elastic pre-mold (or simply “pre-mold”) is provided on one or more surfaces of the hard insert prior to the over-molding process. The pre-mold layer absorbs or otherwise accommodates shrinkage of the over-mold material, thereby reducing localized mechanical stresses and reducing or eliminating the effects of stress risers. Furthermore, the pre-mold layer may reduce shock loading of the type experienced by the component during sudden impacts.
More particularly,
Next, in step 504, elastic pre-mold layer 302 is applied to insert 102. In this regard, elastic pre-mold layer 302 may comprise any suitable elastic material, including, for example, a range of conventional polymeric compounds. In one embodiment, pre-mold layer 302 comprises a thermoset elastomer, such as a silicone rubber material. Such elastomers have the added advantage of maintaining their shape in temperatures experienced during the subsequent over-molding process. Pre-mold layer 302 may be applied using a variety of conventional methods of applying elastic materials such as silicone.
In another embodiment, pre-mold layer 302 comprise a thermoplastic elastomer (for example, thermoplastic polyurethane or polyester elastomer). These materials are particularly advantageous as their increased bonding to the over-molded plastic 105 can improve retention of the insert 102.
When the insert 102 is made from an extruded shape (e.g., a rod or bar), the pre-mold layer may be an extruded outer layer. For mass production of parts with varying cross-section the pre-mold layer may be over-molded using injection molding or compression molding. For smaller production runs, the elastic nature of the pre-mold would allow it to be made as a separate part which could then be stretched over the insert.
The thickness of pre-mold layer 302 may vary or may be constant over the surface of insert 102. The nominal thickness of pre-mold layer 302 may be selected in accordance with a number of factors, including the shape of insert 102, the shape and type of material of over-mold 105, the expected stress and strain experienced by certain regions of over-mold 105 and pre-mold 302, etc. For applications where the pre-mold layer is intended to reduce over-molding stress alone, the thickness of layer 302 is likely to be less than one millimeter, but for applications where the pre-mold layer is also intended to provide some shock isolation, it may be substantially more than one millimeter thick.
In step 506, over-mold structure (or layer) 105 is formed. This may be accomplished in accordance with standard plastic molding techniques. Over-mold 105 may comprise, for example, various thermoset or thermoplastic plastics with or without filler materials. After over-mold 105 is formed, appropriate cooling and/or curing of over-mold layer 105 takes place (step 508).
The finished component incorporating the over-mold layer 105, insert 102, and pre-mold layer 302 may be any type of component, for example, plastic-encapsulated semiconductor devices, computer components, fasteners, glass windows and knob assemblies.
It should be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. For example, these methods may be used in connection with standard barcode readers and the like. In general, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.
