Plastic Molding Mfg. is a Massachusetts based injection molder specializing in tight tolerance precision molding, insert molding, over molding and a variety of part joining processes. We are an engineering driven company, providing our customers a "one stop solution" for the design and production of polymer based products.
Part Joining: Overview of Assembly Methods
Identifying the appropriate part joining method requires careful consideration for material selections and end-product requirements. Common assembly processes include:
Solvent Bonding
Ultrasonic Welding
Heat Staking
Over Molding
Each of these unique methods has distinct advantages and limitations.
Solvent Bonding is a process in which the surfaces of a material, commonly an amorphous thermoplastic, are treated with a solvent for joining. The solvent causes the material to swell and soften at the area of application and under pressure applied to the joint and evaporation of the solvent, the surfaces are bonded. Due to the nature of the process, solvent bonding is effective only if the materials being joined are soluble with the same solvent or in a mixture of solvents. Joint or interface design is important for structural integrity and cosmetic appearance.
In the Ultrasonic Welding process, high-frequency acoustic vibrations are locally applied to the components being held together under pressure. The material is melted at the contact point to create a solid state weld. This process is often favorable when joining dissimilar materials. Consideration for the material thickness is necessary as thicker materials will not bond and very thin materials are susceptible to delaminating.
Heat Staking is a process in which a precise amount of heat and pressure are used to reform a post or boss molded into a plastic part to retain or attach the part to another component. The heat staking process is often chosen for complex or delicate parts, not appropriate for ultrasonic welding, that would otherwise present assembly challenges. With the heat staking method of part joining, there is an inverse relationship between the temperature and pressure. Unlike other methods of melting plastic for part joining, if this relationship is maximized, particulates and debris can be eliminated by offsetting a lower temperature with an increased force to deflect the material without damaging the plastic.
The injection molding process of over molding is a process in which one material, usually a TPE, is molded onto another material, usually a rigid plastic. The most common method of over molding is the process in which the TPE material is shot directly onto a pre-molded insert. Another method of over molding, typically used in high volume production, is a process in which a specialized molding machine having two or more barrels allows for the materials to be processed into the mold during the same molding cycle. This process is referred to as two-shot or multi-shot molding. Over molding is an excellent part joining method for applications requiring enhanced product designs to provide an improved grip or better ergonomics accomplished by the addition of the thermoplastic elastomer.
Any or all of the above methods may be suitable or desirable for a variety of assembly applications. For additional information on the advantages and limitations of these part joining methods, see our part joining chart.
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