An existing molded part can have an additional layer of resin added to it through a process called overmolding, which is a type of injection molding. This allows for a combination of properties that would not be possible with just one type of material. Adding a layer of a soft, functional, and hands-on rubber-like material, typically thermoplastic elastomer, also known as TPE, to a hard substrate is one of the most common applications. Overmolding a part with materials of a variety of colors and finishes is another method that can be used to alter or improve the "cosmetic" appearance of a component. Brushes, medical equipment, hand tools, and even toothbrushes can all contain overmolding materials, not to mention the gaskets and seals that are contained within the components themselves.
Overmolding is a manufacturing process that eliminates the need for manual assembly of materials, in addition to providing excellent adhesion between materials that are not similar to one another. It is possible to automate the process to custom 3D printing varying degrees, which contributes to its relatively low cost. And because it simplifies the assembly process, it can help cut costs and shorten the amount of time it takes to bring new products and devices to market. However, most importantly, it has the potential to significantly expand the variety of material properties that can be utilized by product developers.
Overmolding can be accomplished in a variety of ways, the most common of which are pick-and-place molding, two-shot injection molding, and overmolding. The first method utilizes a single production mold, while the second method employs two molds. The selection of materials for overmolding can be a difficult process. Although substrates and overmolding resins can be complementary to one another, in order for them to be effective, they must be compatible with one another. Not only does the application of the overmolded part influence the choices available, but the method that is used to produce the overmolded part can also have an impact. When selecting materials to use for overmolding, it is helpful to seek the advice of a resin expert. This is because both the process and the results of overmolding are more complex than those of single-shot molding.
Transfer overmolding is a type of robotic process that involves mechanically removing a substrate part from one mold and placing it into another mold that is larger than the first. In most cases, the next substrate is being produced in the first mold while the overmolding material is being injected into the second mold to fill the space that was previously empty. Another robotic process is known as rotary overmolding. In this method, the mold itself is moved from one injection station to another in order to facilitate the injection of the base material as well as the overmolding material.
Core back overmolding can only be utilized for very particular linear geometries. The mold consists of a sliding segment that is pulled back after the first material is injected and then set in order to make room for the second material that is injected. All three methods of overmolding involve layering the material to be overmolded onto a warm substrate, which makes it easier for chemical bonds to form. Due to the high degree of automation, they are cost-effective in high-volume production, often exceeding 100,000 pieces, although all three require specialized equipment and expensive tooling.
Pick-and-place molding makes use of two molds that are entirely distinct from one another. In a mold, a number of substrate part components are fabricated and then cooled. After that, they are placed by hand into a second, larger mold that holds the substrate part and leaves room for the overmolding material to inject the overmolding material onto the substrate. This mold is then heated to a temperature that allows the overmolding material to melt. In comparison to the secondary injection method, this process makes use of less complicated equipment, molds that are less complicated, and simplifies and quickens the installation process.
Although the manual placement of substrates into secondary molds is a slower process than robotic processes, the CNC Machining Brass Parts manual process can often produce low- to mid-volume parts more quickly and at a significantly lower cost. Pick-and-place molding presents a number of challenges, the most significant of which is the weakening of the chemical bonds that exist between the overmolding material and the cooling substrate. When working with substrates, extreme caution is required to avoid contaminating the surface, as this would reduce the amount of adhesion that can be achieved. In applications involving the pick-up position, having a good bonding can also be helped along by selecting the appropriate materials.
The bond that exists between the different layers of resin is what helps to keep the layers from falling apart. It is possible that the bond will be subject to multiple forces that will pull the layers apart depending on the geometry of the part. When one of the materials is an elastomer, which can flex and pull away from the substrate, bond strength is especially important because it prevents the material from coming apart. This is true for thermoset materials as well as thermoplastic elastomers. The actual chemical bonding that occurs at the interface of two resin layers is one of the primary methods of layer bonding. The other primary method, mechanical bonding, is dependent on the physical geometry of the interface. There are two primary methods of layer bonding. A combination of the part design, the selection of the material to be used, the design of the mold, and the molding process are required to achieve acceptable bonding.
If the overmolding is supposed to improve grip, the material's coefficient of friction should give some indication of how it feels to the touch. By way of illustration, thermoplastic elastomers, also known as TPE, typically have a high coefficient of friction. When it comes to determining how well a material grips, a durometer is not a reliable measurement to use in the case of cushions. Due to the fact that many resins, including thermoplastics and thermosets, have a variety of properties, it is helpful to consult an expert in order to expedite the process of selecting the appropriate resin grade for a specific application.
Insert molding, much like overmolding, involves injecting resin onto another material. However, the other material, which is typically metal, and the injected plastic material, which is typically rigid plastic rather than a plastic substrate, distinguish insert molding from overmolding. In this manner, metal electrical components or custom machined metal parts are frequently embedded in plastic. Similarly, threaded inserts can be molded into plastic parts for the purpose of producing a stronger and more durable assembly of plastic parts such as equipment housings. Insert molding is an alternative to inserting metal parts by heat riveting or ultrasonic welding. This method involves locally melting the molded plastic part to make room for the metal part to be inserted. Molded inserts eliminate the need for a secondary insert installation process, which results in significant time and cost savings. Molded inserts also allow for better control over the packaging process and allow for better packaging than other methods.