By integrating soft, elastic thermoplastic elastomers (TPE) with rigid substrates, TPE overmolding delivers innovative solutions across industries, from consumer goods to automotive components.
What Is Overmolding?
What is overmolding? Overmolding is a specialized manufacturing process where one material is molded over another to form a single, cohesive part. Typically, a softer material is injected over a harder substrate, resulting in a product that benefits from the strengths of both materials.
What is TPE Overmolding and TPE Characteristics
TPE overmolding involves injecting TPE onto a pre-molded rigid substrate, typically through two-shot or insert molding. In two-shot molding, the substrate and TPE are molded sequentially in the same machine, ensuring a seamless bond. Insert molding, on the other hand, involves placing a pre-formed substrate into a mold before injecting TPE. Both methods rely on precise temperature control to achieve strong chemical or mechanical bonding without adhesives.
TPE Characteristics
TPE is a versatile material that combines the flexibility and elasticity of rubber with the ease of processing like plastic. It offers adjustable hardness, good slip resistance, strong adhesion to various substrates, and meets environmental safety standards. Additionally, TPE can be melted and reprocessed multiple times, making it both functional and sustainable.
Key Considerations for Successful TPE Overmolding
Achieving high-quality results in TPE overmolding requires careful attention to following factors.
- Material Compatibility: The TPE and substrate parts must have compatible solubility parameters to ensure strong bonding.
- Melt Temperature: The TPE injection temperature should be sufficiently high to partially melt or “burn through” the substrate surface, promoting adhesion.
- Wall Thickness Balance: Maintain TPE thickness between 0.5-3 mm, with a minimum of 1 mm to promote adequate bonding and tactile sensation.
- Smooth Transitions: Avoid sharp corners by incorporating generous radii to enhance contact between TPE and the substrate, reducing stress concentrations and improving adhesion.
- Proper vent system: Incorporate air vents (0.01-0.02 mm) along the mold perimeter or at fill endpoints to avoid air traps, ensuring smooth TPE flow.
- Gate and Runner Systems: Select gate sizes based on TPE type and thickness, employ balanced runner systems or hot runners to ensure uniform flow and consistent bonding quality.
- Optimized Mold Design: Use tight mold tolerances (0.01-0.03 mm) to prevent flash, where excess TPE leaks from gaps.
- Pre-Drying: Hygroscopic substrates like nylon must be dried to below 0.1% moisture to prevent defects like bubbling or weak adhesion.
- TPE Flow Characteristics: TPE must have good fluidity to navigate long flow paths and thin-wall areas.
- Maintain proper L/T: Maintain a flow length-to-thickness (L/T) ratio between 80:1 and 150:1 for optimal mold filling and adhesion.
- Surface Design: Incorporate textured mold surfaces to prevent sticking and mask aesthetic imperfections, creating a refined finish.
- Adhesive Use (if needed): For substrates with poor chemical compatibility, select a high-quality adhesive or rely on mechanical interlocks, such as undercuts or textured surfaces, to enhance bonding strength.
- Injection Parameters: Use high injection speed and pressure to ensure proper TPE flow without deforming the substrate.
Common TPE Overmolding Types and Base Materials
TPE overmolding is the process that a soft thermoplastic elastomer molded over a rigid substrate materials. Below are the primary TPE overmolding types and their corresponding base materials:
TPE Over Rigid Plastic
A soft TPE layer is molded over a rigid plastic substrate to create a cushioned, non-slip surface that enhances grip and comfort. Typically achieved through two-shot molding, where TPE is injected sequentially after the plastic substrate in the same mold.
Base Materials:
- Polypropylene (PP)
- Acrylonitrile Butadiene Styrene (ABS)
- Polycarbonate (PC)
- Nylon (PA6/PA66)
- PC/ABS Blends
TPE Over Metal
This overmolding TPE type involves coating a metal substrate with TPE to combine structural strength with a soft, resilient surface. Often implemented via insert molding, where a cast or CNC-machined metal part is placed in a mold before TPE injection, this method relies on mechanical interlocks or adhesives due to limited chemical bonding.
Base Materials:
- Steel
- Aluminum
- Stainless Steel
TPE Over Fabric
This overmolding TPE type applies TPE to fabric substrates to enhance aesthetics, durability, and tactile properties. Often implemented through insert molding.
Base Materials:
- Polyester
- Nylon Fabrics
- Textile Composites
Common Defects in TPE Overmolding and Solutions
Despite its advantages, TPE overmolding can encounter defects that compromise part quality. Below are common issues and their solutions:
Delamination
The TPE layer separates from the substrate due to poor material compatibility or moisture.
Solution: Choose TPE with matching solubility parameters (e.g., TPE-S for PP) and pre-dry hygroscopic substrates like nylon to below 0.1% moisture. Preheating substrates to ~100°C enhances bonding.
Flash
Excess TPE leaks from mold gaps, creating unwanted edges.
Solution: Adjust pressure settings and verify mold alignment.
Warping
Uneven shrinkage between TPE and substrate causes distortion.
Solution: Control cooling rates and maintain uniform wall thickness.
Short Shots
Incomplete TPE filling occurs due to poor flow or insufficient pressure. Solution: Increase injection speed and pressure, optimize gate placement, and ensure TPE melt temperature (170-230°C) is adequate.
Curling
TPE extends beyond substrate edges, causing wrinkling. Solution: Implement shut-off designs in molds to limit TPE overflow and adjust cooling to prevent uneven shrinkage.
Surface Roughness or Bubbles
Due to moisture in materials or inadequate venting.
Solution: Improve drying processes and enhance mold venting.
Mold Sticking
TPE adheres to mold surfaces, leading to defects.
Solution: Use appropriate mold release agents and maintain mold surface finishes.
Applications of TPE Overmolding
TPE overmolding is employed across diverse industries, enhancing product functionality and user experience. Whether you need soft-touch handles or functional grips, we support the production of custom TPE parts tailored to your exact design requirements. Key applications include:
Daily Supplies: Toothbrushes, knife grips, combs, and su#itcase handles.
Tools: Screwdrivers, hammers, and electric drills handles.
Consumer Electronics: Mobile phone cases, tablet covers, and smartwatch bands.
Sports Equipment: Golf club grips, bicycle handles, and ski equipment.
Automotive Parts: Steering wheel grips, gearshift knobs, and dashboard components.
Medical Devices: Surgical tools and medical equipment handles.
Partner Zhongde with Professional TPE Overmolding Solutions
Zhongde is a trusted provider of TPE overmolding service, delivering high-quality, customized parts tailored to your needs. Our services includes material selection, advanced mold design, precise injection molding, and comprehensive defect mitigation strategies. Whether bonding TPE to plastic or metal substrates, we ensure strong adhesion, durability, and aesthetic excellence.
