1. Material Selection and Compatibility
Substrate (Rigid Part):
Common engineering plastics: ABS, PC, PA (nylon), PP, etc., which must exhibit good rigidity, weather resistance, and adhesion to soft materials.
For lightweight requirements, glass‑fiber‑reinforced materials can be used (note: surface treatment is necessary to enhance adhesion).
Overmolding Material (Soft/Sealing Part):
- TPU (Thermoplastic Polyurethane): High wear resistance and flexibility, making it the preferred choice for outdoor waterproof components.
- TPE/TPV (Thermoplastic Elastomer/Thermoplastic Vulcanizate): Suitable for areas requiring elastic sealing (e.g., handles, interfaces).
- Silicone: Stable performance under extreme temperatures, but bonding is challenging and requires special treatment.
Key Points:
- Ensure chemical compatibility between hard and soft materials (achievable through molecular polarity matching or adding adhesion promoters).
- Materials must possess UV resistance, high‑ and low‑temperature tolerance (-40°C to 80°C), and hydrolysis resistance to withstand outdoor environments.
2. Key Considerations in Mold Design
Runner and Gate Design:
- Use multi‑point gates to ensure uniform coverage of the sealing area by the overmolding material.
- Avoid weld lines in critical waterproof areas (optimizable through mold‑flow analysis).
Sealing Structure Design:
- Design grooves, undercuts, or textures at the overmolding interface to enhance mechanical interlocking of the soft material.
- Reserve compression space so that the soft material is compressed during mold closing to form a tight seal.
Mold Surface Treatment:
- Apply EDM (Electrical Discharge Machining) or etching to sealing areas to improve material flow and penetration.
3. Molding Process Control
Injection Parameters:
- Temperature: Precisely control melt temperature (TPU typically requires 190–230°C) and mold temperature (40–80°C) to prevent material degradation or poor adhesion.
- Pressure and Speed: Use high pressure and low speed injection to ensure the soft material fully fills fine structures.
- Holding Time: Extend holding time to reduce sealing failures caused by shrinkage.
Two‑Step Molding vs. Two‑Shot Molding:
- Two‑Step Molding: The rigid part is molded first, cooled, and then placed into a second mold for overmolding. Strict control of surface cleanliness and temperature of the rigid part is required.
- Two‑Shot Molding: Completed in a single mold, offering higher efficiency and more reliable bonding, but with higher mold costs.
Surface Treatment:
- The rigid part can be treated with plasma or flame treatment before overmolding to increase surface energy and enhance chemical adhesion.
4. Waterproof Structure Design
Multi‑Layer Sealing Mechanism:
- Primary Seal: Formed by completely encapsulating edges with the overmolding material as the first waterproof barrier.
- Secondary Seal: Add compressed gaskets or waterproof breathable membranes (e.g., ePTFE) to balance internal and external pressure.
Drainage Design:
- Incorporate drain holes or channels in non‑critical areas to prevent water accumulation and seepage.
5. Quality Control and Testing
Bond Strength Testing:
- Perform tensile/shear tests (e.g., ASTM D638/D429) to ensure interlayer adhesion strength exceeds the material’s intrinsic strength.
Waterproof Performance Testing:
- IPX7/IPX8 Waterproof Test: Submerge the component in 1 meter of water for 30 minutes and check for internal leakage.
- Pressure Testing: Simulate rain, spray, and other outdoor conditions (e.g., IPX5/IPX6).
Environmental Durability Testing:
- Thermal Cycling (-40°C to 85°C), UV Aging (QUV accelerated aging), Salt Spray Testing (for coastal environments).
6. Typical Application Examples
Outdoor Electronics (e.g., action cameras, GPS devices):
- Rigid PC housing + TPU overmolded buttons/interfaces, with edge seals enhanced by ultrasonic welding.
Trekking Pole/Tool Handles:
- Aluminum or glass‑fiber‑reinforced nylon core + TPE overmolding, designed with anti‑slip textures and injected silicone damping layers.
Waterproof Luggage Buckles:
- PP substrate + fully overmolded TPU, utilizing undercut structures for locking and compression sealing through interference design.
7. Common Issues and Countermeasures
Adhesion Failure:
- Causes: Poor material compatibility, insufficient mold temperature, surface contamination.
- Solutions: Add a bonding layer (e.g., primer coating), increase mold temperature, use copolymer materials.
Discontinuous Sealing:
- Causes: Improper weld line placement, poor venting.
- Solutions: Optimize gate location, add vent slots or implement vacuum‑assisted venting.
Water Seepage After Long‑Term Use:
- Causes: Material hydrolysis or UV degradation.
- Solutions: Select weather‑resistant material formulations (e.g., carbon‑black‑added UV‑resistant grades), schedule periodic replacement of sealing components.