Secondary injection molding not only creates a smooth surface for medical devices but also enhances product functionality and added value.
Over past decade, secondary injection molding technology has revolutionized consumer product aesthetics, design concepts, and functional requirements. Medical device manufacturers have also recognized its potential advantages and are expanding its application in medical field.
Secondary injection molding is renowned for creating "smooth surfaces," but it offers many other benefits, such as ergonomic design, two-tone appearance, branding, and performance improvements. This technology can enhance product functionality (e.g., noise reduction, shock absorption, waterproofing, impact resistance) and added value.
Like co-injection molding, double injection molding, and sandwich injection molding, secondary injection molding belongs to multi-material injection molding technologies. Basic idea of multi-material injection molding is to combine two or more materials with different properties to increase product value. In this article, the first injected material is called substrate or base material, and second injected material is called cover material.

In secondary injection molding, cover material is injected above, below, around, or inside substrate to form a complete part. This process can be accomplished through multiple injection molding or insert molding. Cover material typically used is an elastic resin.
Multiple Injection Molding: If cover material's construction allows, multiple injection molding is a good method for medical device manufacturing. 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, with resin injected into mold from one or more injection points. Using same injection point is called co-molding, producing a composite part with a core resin material covered by an outer layer. Using multiple injection points is called secondary injection molding, where one material is molded on top of another, creating a multi-layered structure.
However, multiple injection molding is not suitable for all products. During secondary injection molding, slide block must be moved or mold core moved to another mold cavity; another method is to feed mold core into another injection molding machine.
Insert Molding: Insert molding is required to produce products such as fully covered injection-molded handles. To achieve complete coverage, substrate must be removed from its original mold cavity, replaced with another core and cavity for cover material to be injected.
During this process, another mold should be running simultaneously on same or a different size injection molding machine (depending on size of part). Typically, substrate is much larger than cover material and may require preheating to bring surface temperature close to melting point of cover material for optimal bond strength.

Secondary injection molding is sometimes called in-mold assembly because two materials are ultimately fully combined, rather than just creating a delamination structure. This technique can be used for both individual parts and component materials. Regardless of application, ensuring that substrate and cover material achieve required mechanical or chemical bond strength is crucial.

 

Generally, to enhance adhesion, melting temperature of cover material resin should be same as that of substrate. If melt temperature of cover material is too low, it will not melt substrate surface, resulting in insufficient bond strength. Conversely, if melt temperature is too high, substrate will soften and deform; in severe cases, cover material may penetrate substrate, leading to part failure. Therefore, selecting matching materials is crucial to ensure good adhesion.
Generally, matching materials should have similar chemical properties or contain matching composite components. When substrate and cover material are incompatible, they typically only form a mechanical interlocking effect, not a chemical bond.
Multi-material injection molding also requires attention to several issues, the most common being: insufficient chemical or mechanical bond strength between polymers, incomplete filling of individual or multiple parts, and flash burrs in individual or multiple material parts.
Injection molding machine must maintain injection consistency. Furthermore, ratio of injection volume in injection barrel to size of molded part is also a significant factor affecting injection quality. This ratio is critical for all injection molding operations, especially in secondary injection molding. A check valve acts like a sluice gate to separate cover material. Check valve operation is easier when both secondary injection materials are metal; however, it becomes more difficult with metal substrates and more elastic plastics.

Selection of resin materials for secondary injection molding involves multiple factors, including characteristics of substrate and application performance. Specifically, following points are important: Chemical resistance (meets cleaning and other operational requirements). Flame retardancy (meets environmental and environmental protection requirements). An eco-label indicates that product meets environmental and social standards. Abrasion resistance (to prevent dents or peeling). Shore hardness (meets flexibility or other requirements). Medical compliance (FDA, USP Class VI, ISO 10993, and biocompatibility requirements). Sterilization methods (steam, gamma rays, etc.). Impact resistance (meets structural requirements). Melting point (meets application temperature requirements; will not soften or deform). Adhesion method (mechanical interlocking when two materials are incompatible; chemical bonding when two materials are compatible).
Over past 5-8 years, covering materials have seen significant development, with various elastic resins being developed. For example, thermoplastic polyurethane (TPU), styrene-ethylene / butene-styrene polymer (SEBS), copolyesters, copolyamides, thermoplastic rubber (TPR), and thermoplastic vulcanizates (TPV). In practical applications, newer polypropylene resins with better adhesion to polypropylene substrates are generally chosen.
These materials vary greatly in Shore A hardness. Generally, the higher hardness of a material, the stronger its abrasion resistance. Texture of material also affects hardness. Because materials with stronger abrasion resistance experience less wear in endurance tests—for example, in spinning wheel tests, harder resins show less wear—materials with strong abrasion resistance are often chosen for applications.
SEBS resin has a very low hardness, less than Shore A 30, and TPU resin has a hardness of approximately Shore A 60, similar to softness of a human hand. In the past, hardness was generally reduced by adding plasticizers or mineral oils. However, these additives can leach out during cleaning or use (or bloom), which does not meet requirements for medical applications.
Due to development of secondary injection molding resin materials, range of substrate choices has become increasingly broad, currently including acrylonitrile butadiene styrene, polycarbonate, and nylon. This expanded material range provides more space for soft-touch designs. However, application of new materials also brings new challenges, such as material adhesion, component design, and mold operation.

In secondary injection molding process design, check valves, nozzle orifices, air vents, and mold surface texture are key elements.
Check valve between substrate and cover material is extremely crucial for adhesion; it should prevent injected cover material from gradually thinning or developing burrs. Too thin a cover material will lead to weak adhesion, delamination, and edge curling. A good check valve design should clearly separate cover material from substrate. Recessed area of substrate in Figure 1 utilizes this design.

 

Figure 1. Concave design of stop device is conducive to stable adhesion.
Nozzle orifice design is equally important for success of secondary injection molding. Ratio of runner length to wall thickness is a major factor affecting adhesion. Based on experience, this ratio should not exceed 150:1, and should be maintained at around 80:1 when developing new process designs. Figure below shows ratio of channel length to wall thickness.
To minimize flow path, nozzle orifice should be located at the point of greatest wall thickness. When using TPE resin, pay attention to nozzle orifice size. Materials such as TPU require large-diameter nozzles to accommodate higher viscosity and prevent material degradation due to excessive shear force. Materials such as SEBS require higher shear rates to achieve optimal flow rates. A better approach is to use a small-diameter nozzle initially, adjusting nozzle size after initial sampling.
Similar to nozzle orifice, air outlet is also a crucial factor affecting adhesion. Controlling air margin is a significant challenge; poor control can lead to weak adhesion and burrs. Depth of air outlet is critical for preventing burrs; depending on viscosity of covering material, outlet depth should be between 0.0005 and 0.001 inches.

 

Figure 2. Generally, ratio of flow channel length to wall thickness should be less than 150:1, and less than 80:1 when designing after developing a new process.
For certain component designs, decorative surface textures can be used to facilitate part ejection. Most TPE materials tend to adhere to mold surface due to their metallic affinity or vacuum created between material and mold surface during mold opening. Since many materials haven't yet formed a stable chemical bond after ejection, adhesion to mold surface can significantly impact bonding effect.
This means that components must be handled carefully after machining. If adhesion testing is required, wait 24 hours before conducting test to allow material to form a stable chemical bond.
Adhesion can also occur if tensile strength is insufficient when mold surface is parallel to mold opening direction. Materials like TPU require a tensile strength of 5–6 Å. Furthermore, mold surface coatings can also aid in part ejection.
Additionally, surface texture design requires careful consideration. Surface texture affects softness, feel, and thickness of overlay material. Appropriate wall thickness and surface texture design can complement each other, helping to achieve desired processing characteristics. Generally, the lower material hardness, the softer it is. Optimizing surface texture can reduce injection molding defects and improve product feel, making it feel softer to touch than its actual hardness.