Executive Summary: Overmolding Material Basics
Overmolding enhances cable assembly durability by encapsulating the connector and wire junction in a protective polymer. PVC (Polyvinyl Chloride) is the industry standard for general-purpose, cost-sensitive applications. TPU (Thermoplastic Polyurethane) is the premium choice for harsh industrial environments, offering superior abrasion and oil resistance. Santoprene (TPV/TPE) is preferred for extreme environmental conditions, offering rubber-like flexibility and excellent resistance to UV, ozone, and extreme temperatures.
Key Engineering Rules of Thumb:
- The Adhesion Rule: Material selection implies jacket selection. Always match the overmold material chemistry to the cable jacket material to ensuring a chemical bond (e.g., TPU overmold on TPU jacket). Without a chemical bond, you rely solely on a mechanical lock, which is not watertight (IP67).
- Shore Hardness: For strain reliefs, target a Shore A hardness of 70–90. Too hard (Shore D) creates stress concentration points; too soft fails to limit bend radius effectively.
- Chemical Compatibility: Use Polyether-based TPU for hydrolytic stability (water exposure); use Polyester-based TPU for oil/solvent resistance.
Technical Deep Dive: Engineered Polymers for Harsh Environments
Selecting the correct resin for injection overmolding requires balancing mechanical properties, environmental exposure, and cost. Adherence to IPC/WHMA-A-620 Class 3 standards often dictates materials that can withstand mechanical stress without compromising the wire insulation.
1. PVC (Polyvinyl Chloride): The General Purpose Standard
PVC is the most common overmolding material due to its low cost and ease of processing. It provides adequate electrical insulation and flame retardance (often meeting UL 94 V-0).
- limitations: PVC is prone to "plasticizer migration," where it becomes brittle and cracks over time, especially in cold environments or when exposed to UV light. It has poor resistance to industrial oils compared to TPU.
- Best Use Case: Office equipment, static consumer electronics, and indoor medical devices (using non-phthalate medical-grade PVC).
2. TPU (Thermoplastic Polyurethane): The Industrial Shield
TPU bridges the gap between rubber and plastic. It is renowned for its "memory" (ability to return to original shape) and extreme toughness.
- Abrasion & Oil Resistance: TPU offers significantly higher abrasion resistance than PVC and excellent resistance to greases, oils, and solvents, making it the standard for automation and robotics.
- Cold Flexibility: Unlike PVC, TPU remains flexible at very low temperatures (-40°C to -50°C), preventing jacket cracking in freezer or outdoor arctic applications.
- Best Use Case: Robotic arms, factory automation cabling, handheld diagnostic tools, and coiled cords.
3. Santoprene (TPV/TPE): The Environmental Specialist
Santoprene (a brand of Thermoplastic Vulcanizate, a subset of TPE) combines the processing ease of plastics with the performance of vulcanized rubber (EPDM).
- Environmental Sealing: Santoprene provides a superior tactile "soft-touch" feel and offers the best resistance to weathering, ozone, and UV radiation. It is often used for creating watertight (IP67/IP68) seals around connectors.
- Thermal Stability: It maintains mechanical properties across a wide thermal range (-60°C to +135°C), outperforming standard PVC and many TPUs in high-heat under-hood automotive applications.
- Best Use Case: Outdoor automotive sensors, marine electronics, and medical devices requiring repeated sterilization (autoclave compatibility varies by grade).
Comparison Data: Material Properties Matrix
|
Feature |
PVC (Standard) |
TPU (Industrial) |
Santoprene/TPV (Extreme) |
|---|---|---|---|
|
Cost |
Low |
High |
Moderate-High |
|
Abrasion Resistance |
Fair |
Excellent (Best) |
Good |
|
Flexibility (Low Temp) |
Poor (Brittle < -20°C) |
Excellent (-50°C) |
Excellent (-60°C) |
|
Oil/Chemical Resistance |
Poor to Fair |
Excellent |
Good |
|
UV/Ozone Resistance |
Poor (needs additives) |
Good (UV stabilized) |
Excellent |
|
Water/Hydrolysis |
Good |
Good (Polyether grade) |
Excellent |
|
Tactile Feel |
Hard/Slick |
Rubbery/Tough |
Soft/Grippy |
|
Bonding |
Bonds to PVC |
Bonds to TPU/PUR |
Bonds to PP/PE |
Frequently Asked Questions (FAQ)
Can I overmold TPU onto a PVC cable jacket?
Generally, no. While it is physically possible, TPU and PVC do not form a strong chemical bond during the injection process. The result relies on a mechanical lock (adhesion by geometry), which is prone to delamination and water ingress. For a true IP67 watertight seal, we recommend matching the overmold resin to the cable jacket material (TPU on TPU).
What is the difference between TPE and Santoprene?
Santoprene is a specific brand name of Thermoplastic Vulcanizate (TPV), which is a high-performance subset of the generic Thermoplastic Elastomer (TPE) family. While all Santoprenes are TPEs, not all TPEs have the high-performance EPDM rubber characteristics of Santoprene. In engineering specs, "Santoprene" often implies a requirement for superior fatigue resistance and weatherability compared to generic Styrenic TPEs.
Why is TPU preferred for coiled cords?
TPU has excellent elastic memory. When a spiral coiled cord is extended, the material must retract to its original shape without permanent deformation (creep). PVC tends to lose its "spring" over time, sagging and becoming loose, whereas TPU retains its retraction force over thousands of cycles.
How does Shore Hardness affect strain relief performance?
Strain relief (bend relief) is designed to transition the force from a rigid connector to a flexible cable. If the overmold material is too hard (e.g., Shore D scale), it acts like a rigid extension of the connector, moving the stress point further down the cable. If it is too soft (e.g., Shore A < 50), it collapses under the cable's weight. A graduated hardness or a segmented design using Shore A 70-90 material is ideal.
About the Author
Michael Wang
Senior Technical Engineer
As the technical lead at TeleWire, Michael bridges the critical gap between complex engineering requirements and precision manufacturing. With deep expertise in Design for Manufacturing (DFM) and signal integrity, he oversees the technical validation of custom interconnect solutions for mission-critical automotive, industrial, and medical applications.
