Thermoplastic elastomer (TPE) sealing interfaces can be
Injection molded separately and assembled with the interfacing component as a second process
Directly injection molded onto the bonding component. This is called “overmolding.”
Overmolding reduces production costs. In addition, it eliminates a secondary assembly process, especially for high volume products.
How Overmolding Works
Overmolding TPE seals onto a rigid thermoplastic is a delicate task. Higher precision is required than in straight-shot injection molding (a one-time injection molding) to ensure the TPE interface and the component engage each other accurately.
With straight-shot injection molding, the raw material (melted plastic) is completely enclosed in hard steel to “mold” its shape. In overmolding, a portion of the TPE material fills the steel mold cavity. Part of the material is also injected into and around the plastic bonding component. Plastics each have different shrinkage ratios while they solidify and are not as hard as steel. Part of the TPE interface being molded against another plastic part requires refinements to make a precise overmolded product. Various modification options include fine-tuning the injection molding process or adjusting the mold cavity and cores of the overmolding tool. Identifying the best modification option that is most reliable, cost effective, and achievable in a timely manner takes expertise in design, engineering and manufacturing. PolyOne’s Overmolding Guide provides details on required expertise for overmolding TPEs.
In this example, the bonding component (shown in green) is manufactured by straight-shot injection molding. The TPE interface (shown in dark grey) is manufactured by overmolding onto the bonding component.
Design for Manufacturability (DFM)
“Design for Manufacturability” (DFM) is a collaborative process between developers and customers to enhance their original part designs. There are many dimensions to DFM. The most basic goals of DFM are to ensure that:
The final part design is physically possible to manufacture.
The manufacturing process is streamlined to be simple, efficient, and robust.
Products can easily become costly if basic DFM is not properly conducted. Tools that have delicate components (like extremely thin metal pins) may be fractured easily. Some plastic materials emit gases during the injection molding process, which need to be released regularly with internal ventilation. Tools for such materials need to incorporate gas vents to reduce accumulation and promote a predictable maintenance schedule. If not designed properly, removing the tool from the molding machine and maintaining it at a higher frequency can become an impediment to full-scale production, not to mention added cost. Unnecessarily frequent maintenance can also wear out the tool steel prior to projected life of the mold.
In other words, proper DFM can increase tool life, preventing production interruptions and premature tool maintenance, thereby enabling more production before renewal investment is necessary.
While many suppliers practice basic DFM, Enplas Life Tech goes beyond to provide more comprehensive DFM services. We believe that effective DFM involves more than just the manufacturability and proper tool building described above. The benefits of Enplas Life Tech’s comprehensive DFM practices include:
Enhancing the intended functions of the part. For TPE sealing interfaces, this includes suggesting features to enhance bonding for optimal sealing. (See design for details.)
Further improving cost efficiency. Recommendations may include slightly altered dimensions or consolidation of multiple parts, which can contribute to fewer assembly steps or possible reduction of complicated procedures. Suggestions may be made in cases where additional cavitation (i.e., number of the same parts per tool to make during the injection molding process) is possible with some small additional investment. These contribute to a reduction of the overall production cost, in particular showing a larger impact with high-volume product families.
It is crucial to select a reliable manufacturing partner who prioritizes these issues during the design phase. For over 20 years, Enplas Life Tech has worked with industry leading clients to solve challenging TPE manufacturing problems, resulting in robust prototypes and industrial-scale manufacturing of the final product. With dedication to meticulous design review and engineering, tool building, injection molding, and assembly, Enplas Life Tech will help you speed up product development and accurately manufacture your fluid-sealing interface.