Views: 0 Author: Site Editor Publish Time: 2025-11-17 Origin: Site
Before we talk about softness, we must address the most fundamental principle: The two materials must bond together securely.
This bond can be:
Chemical: Molecular adhesion between the materials. This requires compatible polymer chemistries.
Mechanical: Physical interlocking through undercuts, holes, and textures that the soft material flows into, creating an "anchor."
The selection process always starts with the hard substrate. Once you've chosen that, you select a soft material known to bond with it.
These rigid plastics form the structural skeleton of your product.
ABS (Acrylonitrile Butadiene Styrene): Easy to process, excellent surface for bonding, and very common. A great all-rounder.
PC (Polycarbonate) & PC/ABS Blends: Offer high strength, impact resistance, and heat resistance. PC/ABS is often preferred for its balanced properties.
PP (Polypropylene): Low-cost and chemical resistant, but has a low surface energy, making it notoriously difficult to bond to. It requires specific soft material partners.
PA (Nylon / Polyamide): Known for its strength, toughness, and resistance to wear and oils.
These elastomers bring the function and feel to your product.
| Material | Abbr. | Key Characteristics & Bonding |
|---|---|---|
| Thermoplastic Elastomers | TPE/TPR | The most versatile family. Offers a wide range of softness and is compatible with ABS, PC, PC/ABS, and PA. Special grades are needed for PP. |
| Thermoplastic Polyurethane | TPU | Excellent abrasion resistance and toughness. Bonds well to ABS, PC, and PA. It feels more "robust" and less "squishy" than TPE. |
| Thermoplastic Vulcanizates | TPV | Superior heat and weather resistance. The go-to choice for overmolding onto PP, with which it forms an excellent chemical bond. |
| Liquid Silicone Rubber | LSR | Extremely heat resistant, biocompatible, and inert. It does not chemically bond to most engineering plastics and requires mechanical interlocking designs. Perfect for medical and food-grade applications. |
Hardness, measured on the Shore A scale, is the most critical factor for touch and feel. Here’s how to translate the numbers:
| Shore A Hardness | "Feels Like..." | Recommended Applications |
|---|---|---|
| 10A - 30A | Gel, Soft Putty | Extreme cushioning, high-grade seals, very soft grips (e.g., premium razors). |
| 40A - 60A | Pencil Eraser, Car Tire | The classic "soft-touch" zone. Perfect for tool handles, toothbrush grips, and general consumer goods. |
| 70A - 90A | Shoe Sole, Shopping Cart Wheel | Durable, resilient feel. Ideal for phone cases, car steering wheels, and anything needing high abrasion resistance. |
| 95A+ (Shore D) | Hard Plastic, Skate Wheel | Used for flexible structural parts that need to resist impact and wear without feeling "soft." |
Here are some proven pairings from the real world:
Power Tool Handle:
Hard Substrate: PC/ABS or Nylon (PA)
Soft Material: TPU or TPE
Hardness: TPU: 75A - 85A (for a tough, durable grip) / TPE: 55A - 65A (for a softer, more comfortable grip)
Smartphone Case:
Hard Substrate: PC (or the phone itself)
Soft Material: TPU
Hardness: 75A - 85A (The perfect balance of shock absorption, scratch resistance, and "slip-in-the-pocket" feel).
PP Tool Handle (e.g., Screwdriver):
Hard Substrate: PP
Soft Material: TPV
Hardness: 50A - 70A (TPV is the champion for bonding to PP, creating a secure, non-slip handle).
Electric Toothbrush Handle:
Hard Substrate: ABS
Soft Material: TPE
Hardness: 50A - 60A (Provides a comfortable, secure grip that is easy to hold, even when wet).
Start with the Hard Substrate: Your choice here dictates your options for the soft material.
Hardness is Function: Softer ( lower Shore A) isn't always better. Match the hardness to the product's physical demands.
Prototype, Prototype, Prototype! Never finalize your material choice without real-world testing. Get samples in different hardness levels, feel them, bend them, and test the bond. Your hands are the ultimate judge.
By mastering the interplay between material compatibility and hardness, you unlock the full potential of overmolding to create products that are not only functional and durable but also a genuine pleasure to use.
