Selecting the right plastic tube for an industrial application involves more than choosing a material and specifying dimensions. One property that directly determines how a tube performs under real operating conditions is its Shore hardness. This single measurement tells engineers and procurement professionals whether a tube will flex around tight bends, hold firm under pressure, or seal reliably against a fitting. Understanding the Shore hardness scale and how it maps to common tube materials removes guesswork from plastic tube selection and helps prevent costly mismatches between tubing and the application. If you are looking for tubes designed with these specifications in mind, explore the tube product range to see how hardness ratings translate into real products.
This article explains what Shore hardness measures, how the two main scales differ, and why a shift of even a few points on the durometer can change the way a tube performs in the field. Each section builds on the last, moving from measurement fundamentals to material comparisons and common specification errors worth avoiding.
How the Shore Hardness Scale Measures Flexibility
Shore hardness quantifies a material’s resistance to permanent indentation. Developed by Albert Ferdinand Shore in the 1920s, the test uses a spring-loaded instrument called a durometer to press a standardized indenter into the material surface. The reading falls on a dimensionless scale from 0 to 100: a value of 100 means zero penetration (maximum hardness), while 0 indicates maximum penetration and an extremely soft material.
The most widely referenced testing standard is ASTM D2240, which defines 12 different durometer types. For plastic tubes and flexible hoses, the two scales that matter most are Shore A (for softer, flexible materials) and Shore D (for harder, more rigid materials). International equivalents include ISO 48-4 for rubber and ISO 868 for plastics, though the underlying measurement principle remains the same.
Because Shore hardness is a surface measurement, test conditions matter. The standard specimen must be at least 6.4 mm thick; thinner samples produce artificially high readings because the indenter senses the hard surface underneath. Temperature also affects results, since polymers soften as they warm. Consistent ambient testing conditions are essential for accurate, repeatable values.
For engineers specifying industrial plastic tubes, the key takeaway is straightforward: a lower Shore number means a softer, more flexible tube, and a higher number means a stiffer, more rigid one. That relationship holds within a single scale, but comparing numbers across different scales requires understanding the differences between Shore A and Shore D.
Shore A vs. Shore D: Which Scale Applies to Your Tubing
Shore A and Shore D cover different segments of the hardness spectrum and use different indenters and spring forces. Shore A employs a blunt, truncated cone indenter with approximately 8 N of force and is designed for softer materials such as silicone, flexible PVC, and thermoplastic elastomers. Shore D uses a sharper, pointed cone indenter with roughly 44.5 N of force and applies to harder materials like rigid nylon, HDPE, and stiff polyurethane.
The two scales overlap in the middle range. A material reading 95 on the Shore A scale is approximately equivalent to 50 on the Shore D scale. As a practical rule, if Shore A readings consistently land above 90, the material should be measured on the Shore D scale instead. Conversely, materials softer than about 50 Shore D are better expressed using Shore A values. Beyond roughly 65 Shore D, materials become rigid enough that Rockwell hardness testing is more appropriate.
For tube material selection, the choice of scale depends on the tubing type:
- Shore A: Flexible tubing materials such as silicone, soft PVC, and softer polyurethane grades
- Shore D: Semi-rigid and rigid tubing materials such as nylon (PA), HDPE, and harder polyurethane grades
- Rockwell R: Very rigid plastics like polypropylene, which sit above the Shore D range
Knowing which scale a supplier uses prevents a common source of confusion. A tube listed at “80” means something very different depending on whether that value is Shore A or Shore D. Always confirm the scale when comparing datasheets from different manufacturers.
How Hardness Affects Tube Performance in Real Applications
The durometer value of a plastic tube is not an abstract number. It directly influences flexibility, fitting compatibility, pressure handling, and service life. A deviation of just ten Shore A points can transform a flexible damper into a rigid conduit, so specifying the right hardness range is essential for reliable performance.
Flexibility and Bend Radius
Softer tubes bend more easily and achieve tighter minimum bend radii before kinking. A tube at 50 Shore A will route smoothly around corners in a compact assembly, while one at 90 Shore A resists bending and works better for straight runs or high-pressure systems. The interplay between wall thickness and hardness also matters: increasing the wall thickness of a harder tube provides structural support for static bends, while softer materials handle dynamic, repeated bending more gracefully.
Tubes that are too soft for a given application, however, can kink under external pressure or collapse under vacuum. Clips, supports, or thicker walls may be needed to compensate.
Fitting Compatibility
Tube hardness dictates which fitting type can be used, and a mismatch here is one of the most frequent causes of leaks and failures in fluid systems:
- Barbed fittings: Require soft, flexible tubing (typically below 90 Shore A). Using hard tubing with barbs causes longitudinal stress cracks.
- Push-to-connect fittings: Typically require 95 Shore A or higher tubing to maintain a secure grip and seal.
- Compression fittings: Designed for hard or semi-rigid tubing. Soft tubing deforms too much to create a reliable seal.
Reduced blow-off and pull-off resistance is a direct consequence of pairing the wrong durometer tubing with a fitting not designed for it. Always verify the fitting manufacturer’s recommended hardness range.
Peristaltic Pump Applications
Peristaltic pumps are a particularly hardness-sensitive application. The recommended range for peristaltic pump tubing is generally 50 to 65 Shore A. Tubing that is too hard puts excessive load on the pump motor, shortening its life or preventing operation altogether. Tubing that is too soft may collapse under vacuum or fail to recover its shape between roller passes, leading to inaccurate dosing.
In critical dosing applications, even a small hardness deviation can mean the difference between years of precise delivery and premature failure from motor burnout or tubing occlusion.
Matching Shore Hardness to Common Tube Materials
Different polymer families occupy different zones on the hardness spectrum. The table below provides a practical reference for the most common tube materials, their typical Shore hardness ranges, and the scale used to measure them.
- Silicone: 20 to 90 Shore A (typically around 50A for standard sizes). Highly flexible, good for medical and peristaltic pump applications.
- Flexible PVC: 50 to 90 Shore A. Softer grades (50A to 65A) suit general fluid transfer; firmer grades (75A to 90A) handle higher pressures.
- Polyurethane (PUR): 70 to 98 Shore A. Excellent kink recovery and abrasion resistance. Harder grades work with push-to-connect fittings.
- Nylon (PA): Varies by grade. PA11 is softer and more flexible (approximately 70 to 80 Shore A), while PA6 is harder (80 to 90 Shore A). Harder nylon grades register on the Shore D scale (60D to 80D). Strong chemical resistance.
- HDPE: Approximately 34 to 39 Shore D. Lightweight, chemically resistant, and semi-rigid.
These ranges are approximate because formulation, plasticizer content, filler loading, and even pigmentation can shift the final hardness by several points. A flexible PVC tube with heavy filler content will read harder than one with minimal additives, even if both are nominally the same material grade.
Product Comparison: Three Tubes, Three Hardness Profiles
To illustrate how hardness differences translate into distinct application profiles, consider these three tube products:
- ToppTube™ PA11 (70 Shore D): A rigid nylon tube suited for high-pressure pneumatic lines, fuel systems, and applications requiring strong chemical resistance. Its high hardness makes it compatible with compression fittings and ideal for straight-run installations.
- ToppTube™ PA11P40 (63 Shore D): A semi-rigid nylon tube that offers more flexibility than the standard PA11 while retaining good pressure handling. This middle ground works well where moderate routing flexibility is needed without sacrificing structural integrity.
- ToppTube™ PUR C98A (98 Shore A): A firm polyurethane tube at the top of the Shore A range. It balances flexibility with enough rigidity for push-to-connect fittings and handles abrasive or rough-surface environments well. Its kink recovery makes it practical for pneumatic tool connections and automated systems.
These three products demonstrate that even within a single manufacturer’s range, hardness selection determines whether a tube bends easily, holds pressure, or connects securely to a specific fitting type. Choosing among them depends entirely on the application’s mechanical, chemical, and installation requirements.
Common Mistakes When Specifying Tube Hardness
Shore hardness is a useful specification, but it is frequently misunderstood or misapplied. Avoiding these common errors saves time, prevents field failures, and reduces the cost of respecifying components after a mismatch is discovered.
Confusing Hardness with Other Properties
Durometer measures resistance to indentation, not flexibility, wear resistance, or tensile strength. Two tubes with identical Shore A values but different polymer chemistries can behave very differently under bending, abrasion, or load. A 70 Shore A PVC tube and a 70 Shore A silicone tube will not flex, wear, or recover the same way. Similarly, durometer is often confused with compression force deflection, but two materials at the same hardness can have very different deflection characteristics.
A higher Shore hardness does not automatically mean better performance. In peristaltic pump tubing, for example, a harder tube is worse, not better, because it strains the motor and reduces dosing accuracy.
Comparing Hardness Across Different Materials
A 50 Shore A silicone tube and a 50 Shore A PVC tube share the same hardness number, but their flexibility, chemical resistance, and temperature tolerance are completely different. Hardness comparisons are meaningful within the same material family. Across different polymers, they provide only a rough indication of relative stiffness.
Ignoring Formulation Variables
Additives, fillers, and pigments all affect hardness. Adding high loads of pigment to achieve a specific color can increase hardness by one to three points. Plasticizer content in PVC directly controls where the material falls on the Shore A scale. When substituting materials or switching suppliers, always verify the actual hardness of the new batch rather than assuming it matches the datasheet of the previous one.
Mismatching Tube Hardness and Fitting Type
This is one of the most consequential specification errors. Using a barbed fitting on tubing that is too hard causes stress cracking. Using a compression fitting on tubing that is too soft results in poor sealing. The fitting manufacturer’s recommended durometer range exists for a reason, and ignoring it leads to leaks, blow-offs, and premature system failure.
Overlooking Temperature Effects
A tube specified at 80 Shore A at room temperature may behave quite differently at elevated operating temperatures, where the polymer softens, or at low temperatures, where it stiffens. Always consider the operating temperature range when selecting a tube hardness, not just the ambient conditions at the time of installation.
How Toppi Uses Shore Hardness Specifications to Match Tubes to Industrial Needs
Toppi Oy is a Finnish manufacturer, family-owned since 1953, with over 70 years of extrusion expertise, producing plastic tubes, hoses, profiles, and cables at its Espoo facility. As a full-service partner, Toppi handles every stage from CAD design and 3D-printed prototyping through in-house toolmaking and production. This vertical integration means that when a customer needs a tube at a specific Shore hardness, the design team can select the right polymer, adjust formulation parameters, and validate the result before production begins.
Toppi’s approach to tube material selection addresses the challenges discussed throughout this article:
- Material expertise: Toppi works with PVC, polyurethane, nylon (PA11, PA12), polyethylene, and other polymers, recommending the right material and hardness for each application’s mechanical and chemical requirements.
- Custom-tailored products: When standard hardness ranges do not fit an application, Toppi manufactures custom tubes to specific durometer values using co-extrusion techniques that can combine different materials and properties in a single product.
- Fitting compatibility guidance: With decades of experience supplying tubes to industrial customers, Toppi helps match tube hardness to fitting types, avoiding the mismatch problems that cause field failures.
- Environmental responsibility: Production runs on 100% fossil-free electricity, and the company holds ISO 14001 certification.
The ToppTube™ range, including the PA11, PA11P40, and PUR C98A products compared earlier, demonstrates how precise hardness specifications translate into purpose-built tubes for pneumatic, hydraulic, and fluid transfer applications. Browse the full tube range to find the right hardness profile for your application, or contact Toppi’s design team to discuss a custom tube engineered to your exact specifications.
