Johnny Xiong
Rapid Tooling Expert
Contents
Every successful injection molded part begins with the right material choice. While factors like tensile strength, temperature resistance, and color are crucial, one of the most fundamental properties is plastic hardness. Hardness is more than just a number; it defines a product's physical performance, its tactile feel in a user's hand, and its long-term durability.
From our experience at HordRT, we know that a material’s hardness has a profound impact on the entire manufacturing process. It influences everything from the initial mold design and material flow dynamics to the required cooling times and the final quality of the part. Making the right choice upfront prevents costly re-tooling and ensures your product performs exactly as intended.
This guide will demystify the two most common scales used in our industry—Shore A and Shore D—to help you make informed decisions for your next injection molding project.
Core Hardness Scales: Shore A vs. Shore D at a Glance
When we talk about the hardness of plastics and rubbers, we are typically referring to a material's resistance to permanent indentation. This property is measured using an instrument called a Shore durometer, which applies a specific force through a standardized presser foot (the indenter). The depth of the indentation determines the material's hardness value on a scale from 0 to 100.
The two primary scales for plastics, Shore A and Shore D, are distinct because they are designed for different material families. The key differences lie in the shape of the indenter and the force of the spring used.
- Shore A Scale: Uses a durometer with a blunted, 35° conical indenter and an applied force of approximately 8 Newtons. It is designed for soft, flexible materials that can be easily indented.
- Shore D Scale: Uses a durometer with a sharper, 30° pointed indenter and a much stronger applied force of approximately 44.5 Newtons. This setup is necessary to accurately measure the hardness of rigid plastics.
Here’s a quick breakdown of their core differences:
Shore A: The Scale for Flexible & Overmolded Parts
The Shore A scale is the domain of soft and pliable plastics that feel more like rubber than traditional hard plastic. If you can easily leave a temporary mark in a material with your fingernail, it's likely measured on the Shore A scale.
Applications and Common Materials
This scale is ideal for products where flexibility, vibration dampening, and a soft, ergonomic feel are paramount. Think of seals, gaskets, soft-touch grips, and flexible protective cases.
At HordRT, we frequently work with a variety of Shore A materials, including:
- TPE (Thermoplastic Elastomer): Often used for grips on power tools or soft toothbrush handles, typically ranging from 30A to 90A.
- TPU (Thermoplastic Polyurethane): A highly versatile material popular for its excellent abrasion resistance. The TPU shore hardness can range from a very soft 60A (like a gel insole) to a semi-rigid 95A (like a skateboard wheel or a tough phone case).
- Soft PVC (Polyvinyl Chloride): Used for flexible tubing and electrical insulation, commonly in the 50A to 90A range.
Key Injection Molding Process: Overmolding
Shore A materials are essential for overmolding (also known as two-shot molding). This advanced process allows us to mold a soft, flexible material directly onto a rigid plastic substrate. The result is a single, seamlessly bonded part with both hard structural properties and soft, user-friendly touchpoints. This is how we create everything from watertight seals on electronic enclosures to comfortable, non-slip grips on handheld devices.
Shore D: The Scale for Structural & Rigid Parts
When a product requires rigidity, structural integrity, and high impact or scratch resistance, we turn to materials measured on the Shore D scale. These are the workhorse materials of the plastics industry, forming the backbone of countless consumer and industrial products.
Applications and Common Materials
The Shore D scale is the standard for any hard plastic material used for enclosures, load-bearing components, internal chassis, and gears. These parts need to maintain their shape under stress and withstand daily wear and tear.
Our precision molding processes at HordRT are optimized for these high-performance materials:
- ABS (Acrylonitrile Butadiene Styrene): Known for its toughness and impact resistance, used in LEGO bricks and electronic housings, typically around 75D.
- PC (Polycarbonate): Valued for its exceptional strength and optical clarity, used for safety glasses, protective shields, and durable enclosures, often measuring 80D to 85D.
- Nylon (PA): Excellent for applications requiring low friction and high wear resistance, such as gears, bearings, and snap-fit components, usually around 85D.
- HDPE (High-Density Polyethylene): A cost-effective and robust material used for items like construction hard hats and industrial containers, typically around 75D.
Key Injection Molding Process: Precision Molding
Working with Shore D materials demands precision. Because these parts often serve structural roles and have tight tolerance requirements, the injection molding process must be meticulously controlled. This involves precise mold temperatures, injection pressures, and cooling cycles to manage factors like shrinkage and prevent warping, ensuring every part is dimensionally stable and performs flawlessly.
Why Choose HordRT for Your Injection Molding Projects
Understanding the difference between Shore A and Shore D is the first step. Turning that knowledge into a successful, high-quality product requires a manufacturing partner with deep expertise. At HordRT, we bridge the gap between design theory and real-world production.
Our value is built on three pillars:
- Expert Material Selection: With thousands of plastic resins available, choosing the right one can be daunting. Our engineering team works collaboratively with you to analyze your product’s functional needs, environmental exposure, and cost targets. We help you select the perfect material with the right plastic hardness to ensure optimal performance and manufacturability.
- Advanced Process Capability: We have mastered the distinct processes required for both soft and hard plastics. Our expertise in overmolding allows us to create complex, multi-material parts with seamless integration, while our command of precision molding ensures that rigid, high-tolerance components are produced with perfect consistency.
- Robust Quality Assurance: Our commitment to quality is unwavering. Throughout the production process, we implement rigorous QC checks, including systematic durometer testing. This verifies that the material's hardness conforms to specifications, guaranteeing that every part we deliver is reliable, durable, and ready for its end application.
Conclusion
A deep understanding of Shore hardness is fundamental to creating successful injection molded products. It dictates how your product will look, feel, and perform in the real world. Choosing between Shore A for flexibility and Shore D for rigidity is a critical decision that impacts your entire project.
By partnering with an experienced manufacturer like HordRT, you ensure that your design intent is perfectly translated into a functional, high-quality finished part. Our team is ready to apply our expertise in materials and processing to make your next project a success.
Contact our engineering team today to discuss your project needs and find the perfect hardness solution for your product.
