Overmolding technology: Over past decade, overmolding technology has revolutionized aesthetic standards, design ideas and functional requirements of consumer products. Medical device manufacturers have also recognized potential advantages of this technology and are constantly expanding its application in medical field. Overmolding technology is known for creating "soft surfaces", but it also has many other functions, such as ergonomic design, two-color appearance, brand logo and feature improvement. Using this technology, you can increase function (for example: noise reduction, shock absorption, waterproof, anti-collision) and added value of product. Overmolding, like co-injection molding, double injection molding and sandwich injection molding, is a multi-material injection molding technology. Basic idea of multi-material injection molding is to combine 2 or more materials with different properties to increase value of product. In this article, the first injection material is called substrate or base material, and second injection material is called covering material. Various overmolding technologies.

 

During overmolding, covering material is injected over, under, around or into substrate to form a complete part. This process can be done by multi-shot or insert molding. Covering material used is usually an elastomeric resin.
Multi-shot: Multi-shot is a good method for medical device processing if construction of covering material allows it. This technique requires a special injection molding machine equipped with multiple barrels to inject different resins into a single injection mold. Barrels should be placed side by side or in an L-shape, and resin is injected into mold from one or more injection points. When same injection point is used, it is called co-molding, and composite part produced is a core resin material covered by an outer layer. When multiple injection points are used, it is called overmolding, and one material is molded on top of another material to produce a multi-layer structure. However, multi-shot is not suitable for all products. For overmolding, slide must be moved or core must be moved to another mold cavity. Another option is to feed core into another injection molding machine.
Insert molding: To produce products such as fully covered injection molded handles, insert molding is required. To achieve full coverage, substrate must be removed from original mold cavity, placed in another core and cavity to inject cover material. During this process, another mold should be running simultaneously on same or another injection molding machine of different size (depending on size of injection molded part). Substrate is usually much larger than cover material and may need to be preheated to bring surface temperature close to melting point of cover material to achieve the best bond strength.
In-mold assembly. Overmolding is sometimes called in-mold assembly because two materials are completely combined at the end, rather than just producing a layered structure. This technology can be used for both individual parts and component materials. Regardless of application, it is critical to ensure that substrate and cover material achieve required mechanical or chemical bond strength.
Precautions for multi-material injection molding, Generally speaking, to enhance bond, melting temperature of cover material resin should be same as that of substrate. If melting temperature of cover material is too low, it cannot melt surface of substrate, bond between the two is not strong enough. But if melting temperature is too high, substrate will soften and deform. In severe cases, cover material will penetrate substrate and cause part processing to fail. Therefore, choosing matching materials can ensure good bonding. In general, matching materials should have similar chemical properties or contain matching composite components. When substrate and cover materials are not matched, they usually only form a mechanical interlocking effect, not a chemical bond. There are also some issues to pay attention to in multi-material injection molding. The most common ones include: insufficient chemical or mechanical bond strength between polymers, incomplete filling of single or multiple parts, flashing of single or multiple parts.
Injection molding machine must maintain injection consistency. In addition, ratio of injection molding machine barrel shot volume to injection molded part size is also an important factor affecting injection molding quality. This ratio is very critical for all injection molding operations, but it is particularly important in secondary injection molding. Check valve can separate covering material like a sluice gate. Check valve is easier to operate when secondary injection molding materials are all metal. If metal substrate and more flexible plastic are used, check valve operation is more difficult. Material selection: Selection of secondary injection molding resin material has many factors, one of which depends on characteristics of substrate and the other on application performance. Specifically, there are following points:

 

· Chemical corrosion resistance (meeting cleaning and other operation requirements).
· Flame retardancy (meeting ecological and environmental requirements). Ecological and environmental label is a sign that product meets environmental and social standards.
· Abrasion resistance (to avoid denting or falling off).
· Shore hardness (meeting softness or other requirements).
· Medical specifications (FDA, USP Class VI, ISO10993 and biocompatibility requirements).
· Sterilization type (steam, gamma rays, etc.).
· Melting point (meeting application temperature requirements, no softening or deformation).
· Bonding method (mechanical interlocking effect when the two materials do not match, chemical bonding when the two materials match).
In the past 10 years, covering materials have developed greatly, and a variety of elastic resins have been developed. For example, thermoplastic polyurethane (TPU), styrene-ethylene / butylene-styrene polymer (SEBS), copolyester, copolyamide, thermoplastic rubber (TPR) and thermoplastic vulcanizate (TPV). In practical applications, new polypropylene resins with good adhesion to polypropylene substrates are generally selected.
Shore hardness of these materials varies widely. Generally speaking, the harder material, the more abrasion resistant it is. Texture of material also affects hardness. Since materials with higher abrasion resistance lose less in tolerance tests, for example, when a spinning wheel is tested, the harder resin wears less, so materials with higher abrasion resistance are often selected for applications. SEBS resin has a very low hardness of less than Shore A 30, and TPU resin has a hardness of about Shore A 60, which is about softness of a human hand. In the past, hardness was generally reduced by adding plasticizers or mineral oils. However, these additives will precipitate (or bloom) during cleaning or use, which is not suitable for medical applications.
Due to development of secondary injection molding resin materials, range of substrates has become increasingly broad, and now includes: acrylonitrile butadiene styrene, polycarbonate and nylon. Expansion of material range provides more space for soft-feel design. However, application of new materials also brings new problems, such as material bonding, part design and mold operation.
Process design considerations:
In process design of secondary injection molding, backflow device, nozzle hole, air outlet and mold surface texture are key elements.
Backflow device between substrate and cover material is extremely critical for bonding effect. Ejected cover material should not be gradually thinned or burred. Too thin cover material will lead to weak bonding, debonding and curling. A good backflow device design should clearly separate cover material from substrate.
Nozzle hole design is also important for success of secondary injection molding. Ratio of flow channel length to wall thickness is main factor affecting bonding effect. According to experience, this ratio should not exceed 150:1, and ratio should be kept at about 80:1 when developing new process designs. Figure 2 shows proportional relationship between flow channel length and wall thickness. In order to shorten process as much as possible, nozzle hole should be set at position with the largest wall thickness. When using TPE resin, pay attention to nozzle hole size. Materials such as TPU require large-caliber nozzles to accommodate higher viscosities and prevent material degradation due to excessive shear forces. Materials such as SEBS require high shear rates to achieve optimal flow rates. A better approach is to use a small-caliber nozzle in initial stage and adjust nozzle size after initial sampling. Like nozzle hole, air outlet is also an important factor affecting bonding effect. How to control air margin is a big problem. If it is not well controlled, it may cause bonding to be weak and fill burrs. Depth of air outlet is extremely critical to prevent burrs. Depending on viscosity of covering material, depth of air outlet should be between 0.0005-0.001 inches.