二重射出成形会社 - 複雑な製品のためのマルチマテリアルソリューション

二重射出成形会社 - 複雑な製品のためのマルチマテリアルソリューション

 

Single-material plastic components have fundamental limitations. A rigid plastic cannot provide a soft-touch grip. A transparent material cannot offer a colored opaque backing. An electrically insulating material cannot shield against electromagnetic interference. These limitations explain why product designers increasingly turn to a double injection molding company for solutions that combine two materials in a single integrated part.

で パーツマスター, we have developed deep expertise in double injection molding (also known as two-shot or multi-material injection molding). This article explains the technology, its applications, design considerations, and why choosing the right partner matters for product success.

What Is Double Injection Molding?

Double injection molding is a process that produces a single part from two different materials using one molding cycle. The process requires a specialized injection molding machine with two injection units and a mold designed with two cavities or two injection stations.

The typical sequence works as follows:

• First shot: The first material (typically a rigid substrate like ABS, PC, or nylon) is injected into the primary cavity, forming the base component.

• Mold rotation or core movement: The mold opens slightly, and the core with the first-shot part rotates or moves to a second cavity position.

• Second shot: The second material (typically a softer elastomer like TPE, silicone, or TPU) is injected over or around the first material, bonding chemically or mechanically.

• Cooling and ejection: The complete two-material part cools and is ejected.

The result is a single, seamless component with properties that cannot be achieved by assembly or overmolding alone. A qualified double injection molding company makes this complex process look simple, but the engineering behind it requires exceptional precision.

Double Injection Molding vs. Traditional Overmolding

Many people confuse double injection molding with traditional overmolding (also called insert molding). The distinction matters for both cost and capability.

A double injection molding company like パーツマスター recommends double injection for volumes above 50,000 parts per year where the automation and consistency advantages outweigh the higher tooling investment. For lower volumes or simple geometries, traditional overmolding may be more economical.

Common Applications for Double Injection Molding

The versatility of double injection molding has made it indispensable across multiple industries. パーツマスター has delivered double injection solutions for:

Consumer electronics. Soft-trip buttons on rigid housings, two-color LED light pipes, waterproof gaskets molded directly into enclosures.

Power tools. Vibration-damping grips overmolded on structural handles, two-material triggers with different surface textures.

Medical devices. Syringe plungers with soft sealing ribs, ergonomic instrument handles with antimicrobial rigid base and cleanable soft surface.

Automotive interior. Dashboard components with decorative rigid skin and energy-absorbing foam core, shift knobs with leather-like texture on rigid structure.

Household appliances. Sealed control panels with transparent windows for displays, refrigerator door handles with soft-grip inserts.

Material Compatibility in Double Injection Molding

Not all material pairs bond successfully. A specialized double injection molding company must understand polymer compatibility to achieve chemical bonding rather than merely mechanical interlocking.

Chemically compatible pairs (bond without adhesive):

• ABS + TPE (many grades specifically formulated for ABS overmolding)

• PC + TPE (for transparent rigid base with soft seal)

• Nylon (PA6/66) + TPE (high-temperature applications)

• POM + TPE (requires specialized grades; mechanical bond often needed)

Incompatible pairs (require mechanical interlocking):

• PP + most elastomers (PP has low surface energy; design with mechanical anchors)

• PE + TPE (similar to PP; requires physical undercuts)

• PBT + TPE (possible with specific grades; test required)

パーツマスター maintains a material compatibility database and works with resin suppliers (including Kraiburg, RTP, PolyOne, and Avient) to recommend proven material pairs. For custom applications, we conduct bond strength testing before full tooling commitment.

Design Guidelines for Double Injection Molding

Designing for double injection molding differs significantly from designing single-shot parts. A knowledgeable double injection molding company provides DFM feedback to prevent common failures.

Shrinkage matching. The two materials must have compatible shrinkage rates. If the second material shrinks significantly more than the first, it will pull away from the substrate or cause warpage. We typically recommend shrinkage difference below 0.5%.

Bonding area. The interface between first and second shots requires adequate surface area for bonding. For chemical bonding, the second shot must flow over and wet the first shot. Sharp edges or recessed areas can trap air and prevent contact.

Parting line location. The mold parting line for the second shot must be carefully positioned. Flash from the second shot can be difficult to remove if it occurs on cosmetic surfaces.

Wall thickness of second shot. Soft elastomers typically require minimum wall thickness of 0.5mm to 1.5mm. Thinner sections may not fill completely; thicker sections waste material and extend cycle time.

Gate location for second shot. The gate for the second shot must direct material flow to promote bonding and avoid washing away the first shot. Gate location is often a compromise between cosmetic appearance and functional bonding.

Case Study: Double Injection for a Medical Device Handle

A medical device manufacturer needed a surgical instrument handle combining a rigid glass-filled nylon core (for sterilization resistance) with a soft, non-slip TPE grip (for surgeon comfort). The previous solution used a two-part assembly (rigid handle plus slip-on rubber sleeve) that failed sterility testing because fluid migrated between the sleeve and handle.

パーツマスター proposed a double injection solution. Engineering steps:

• Material selection: PA66+30%GF for rigid core (withstands autoclave temperatures), medical-grade TPE (Versaflex HC series) for grip.

• Design modifications: Added mechanical interlock features (through-holes in rigid core) plus chemical bonding zone on outer surface.

• Mold design: Rotary platen double injection mold with six cavities (6+6). First shot forms core; platen rotates 180 degrees; second shot overmolds TPE.

• Process development: Optimized first-shot temperature to promote bonding without degrading TPE. Established narrow process window for consistent bond strength.

Results:

• Bond strength exceeded specification (18 N/cm pull force vs. 12 N/cm required).

• No fluid migration in sterility testing.

• Single-component assembly eliminated inventory of two separate parts.

• Cycle time: 45 seconds producing six complete handles = 480 handles per hour.

• Annual volume: 350,000 handles; customer reduced assembly labor by 3,200 hours per year.

The customer has since transferred four additional products to double injection molding with パーツマスター.

Advanced Double Injection Techniques

Beyond standard two-shot molding, a sophisticated double injection molding company offers advanced variants.

Three-shot molding. Some applications require three distinct materials. For example: rigid substrate + conductive elastomer (for ESD shielding) + decorative colored skin. パーツマスター has built three-shot molds for specialized electronics and automotive applications.

Cubic molding. A cube mold rotates on two axes, allowing four different molding stations. This enables complex sequences including insert loading, two or three material shots, and even in-mold assembly.

Transfer molding. For material pairs with very different processing temperatures (e.g., PEEK at 380°C + silicone at 150°C), the first-shot part is transferred to a separate press rather than rotating within the same machine. This prevents thermal degradation of the lower-temperature material.

Quality Challenges in Double Injection Molding

Double injection molding introduces failure modes that do not exist in single-shot molding. A competent double injection molding company implements controls for each.

Bond failure. The most serious defect. Causes include contamination of the first-shot surface, incorrect first-shot temperature, or incompatible materials. パーツマスター performs regular bond strength testing (peel or pull testing) on production samples.

First-shot displacement. The second shot injection pressure can shift or deform the first-shot part if it is not adequately supported. Our mold designs include support features and optimized gate locations to prevent movement.

Flash between shots. Molten second material can seep between the first shot and the mold steel, creating thin flash. This requires precise fit between the first-shot core and the second-shot cavity.

Cosmetic defects. Splay, flow marks, or color swirls are more common in double injection due to longer flow paths and material interaction. We use Moldflow simulation to predict and prevent these issues.

Selecting a Double Injection Molding Company

When evaluating a double injection molding company, ask specific questions:

• What material pairs have you successfully processed? Look for experience with your specific combination.

• Do you perform bond strength testing? The answer should include quantitative data (N/cm or lb/in).

• What machine sizes do you offer? Double injection requires specialized two-shot presses, not standard machines with manual transfer.

• Can you provide mold flow analysis for both shots? Simulation is essential for predicting bond line quality.

• What is your typical scrap rate for double injection? Well-controlled processes achieve under 3%.

パーツマスター meets all these criteria. Our double injection facility includes rotary platen presses from 80 to 500 tons, all equipped with closed-loop temperature control and real-time process monitoring.

Start Your Double Injection Project

If your product would benefit from combining rigid and soft materials, transparent and opaque layers, or any two-material configuration, パーツマスター has the expertise to deliver. Send your CAD file, material requirements, and annual volume estimate. Our engineering team will provide DFM feedback specific to double injection molding and a firm quotation within 3 to 5 business days.

パーツマスター
Double injection molding – two materials, one seamless part
Phone / WeChat: +86 13530838604
The smart solution for multi-material products.