Polyurethane qualification failures often begin the same way: the technical data sheet listed a property, the molded part did not reproduce it, and the production team absorbed the blame. In many of those cases, the material was never the problem. The problem was that the TDS was read as a final promise instead of what it actually is — a set of results measured under specific, named conditions.
A TDS Is a Measurement Report, Not a Performance Warranty
A datasheet value is not a guarantee that your part will behave identically. It is a report of how the material performed under a particular test method, specimen geometry, curing profile, and conditioning environment.
Hardness is the most familiar example. A TDS may report 60 Shore A, but that number only means something when you know the durometer type, specimen thickness, conditioning time, and ambient conditions. The relevant methods are ASTM D2240 and ISO 868 [1][2]. The same formulation tested on a thin plaque and a thick block can give different readings because hardness is an indentation measurement, not an intrinsic material constant.
The same logic applies to nearly every other value on the sheet. Before comparing one TDS to another, confirm that the test methods and conditioning match. Standard laboratories usually condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity, but production environments rarely sit inside that window.
Read the Identity Block and the Processing Values First
A polyurethane TDS should be read in two passes. The first pass establishes what the material is; the second pass establishes whether your equipment can run it.
Start with the identity block
Before looking at performance values, identify the chemistry family and product form. The identity block normally covers:
- Product code and supplier
- System type: one-shot, prepolymer, two-component, or three-component
- Polyol type: polyether, polyester, or a hybrid
- Isocyanate type: MDI, TDI, HDI, or prepolymer form
- Recommended processing method: casting, low-pressure foaming, high-pressure RIM, spraying, or molding
- A/B mix ratio or index range
- Solids content and solvent content, where relevant
Two formulations with the same headline property can behave very differently in equipment. A polyester-based and a polyether-based prepolymer may both reach 90 Shore A, but their melt behavior, moisture sensitivity, and viscosity profiles are not interchangeable.
The processing values decide whether you can run it
Viscosity is the first practical gate. A rotational viscometer measurement under defined temperature, spindle, and shear conditions tells you how the liquid will feed [3][4]. A viscosity quoted at 25 °C may be misleading if your machine tanks run at 40 °C. Polyols and prepolymers thin as temperature rises, so a material that looks too thick on paper may feed well in a heated closed-loop line — or it may not.
If the viscosity is too high at the lowest practical machine setting, the metering pumps may cavitate, the mixing head may run hot, and ratio accuracy can drift. If it is too low, recirculation and pressure control become more difficult.
NCO content is another value to read before any performance property. It controls mix-ratio calculation and index setting, and it is determined by titration rather than by processing behavior [5]. A shift in NCO content means the required amount of curative or polyol changes even if the published ratio stays the same.
Pot life, cream time, gel time, and rise time are cup-test values with direct process consequences [6]. They are not final cycle times. A gel time of 35 seconds in a laboratory cup does not mean the part can be demolded in 35 seconds. It means the reacting mass reached a defined viscosity transition under those mixing and temperature conditions.
A published rise time is useful only inside the same cup-test conditions used to generate it. The Role of Rise Time in Polyurethane Foaming covers the process variables that move that value.
Hardness, Density, and Mechanical Properties Are Conditional
Performance values on a polyurethane TDS are conditional results. They are most useful when you keep the test method attached to the number.
Hardness: the scale changes the number
Shore A and Shore D are different indentation scales with different springs, indenters, and useful ranges. A value of 60 Shore A and a value of 60 Shore D describe very different materials. Soft elastomers are usually measured on Shore A; rigid materials may be reported on Shore D [1][2].
When comparing materials, record the durometer type and the test temperature. A Shore A reading taken immediately after molding may differ from one taken after 24 hours of post-cure or full environmental conditioning.
Bench comparisons only work when the durometer scale is the same and the specimen geometry matches. Shore A vs. Shore D Hardness in Polyurethane covers how gauge choice and test conditions shift the meaning of a hardness reading.
Density: free-rise and molded values are not the same figure
Density is tested by displacement or pycnometer methods such as ASTM D792 and ISO 1183 [7][8]. For solid elastomers, density is relatively straightforward. For foam systems, the distinction between free-rise density and molded density is the one that causes the most production surprises.
Free-rise density describes the foam expanding without mold restriction. Molded density is measured from a part formed under cavity pressure and pack-out. A foam with a free-rise density of 220 kg/m³ may form a molded part at 280 kg/m³ or higher once the cavity restricts expansion.
If you are turning a material specification into a cavity fill calculation, density alone will not close the loop. How to Calculate Shot Weight in Polyurethane covers how part geometry, material density, and shot size fit together.
Tensile, tear, elongation, and compression set are test results, not service loads
Tensile strength and elongation at break come from dumbbell specimens tested under ASTM D412 or ISO 37 [9][10]. The resulting tensile strength is a material property measured at a controlled strain rate. It does not mean the part can carry that load continuously in service at any temperature, under fatigue, or in aggressive environments.
Tear strength is a separate failure mode. A sharp notch, flash line, or undercut can initiate tearing well before tensile failure appears. Tear is reported according to test geometries such as trouser, angle, or crescent specimens under ISO 34-1 [11].
Compression set measures whether a deformed part returns to its original dimensions after a specified time and temperature. It is particularly relevant for seals, pads, rollers, and rail fastening components. Typical methods include ISO 815 [12]. A low compression set is not necessarily better unless the value comes from the same recovery time and temperature the part will actually see.
Turn the TDS Into a Usable Process Window
The most practical use of a TDS is not to rank materials. It is to build a process window for your existing or planned equipment.
For each batch, a production team should check the relevant TDS values against machine capability:
| TDS value | Process question |
|---|---|
| Mix ratio and NCO content | Is the meter capable of holding the ratio within tolerance across temperature changes? |
| Viscosity | Do the tanks, pumps, hoses, and mixing head deliver the material at the recommended processing temperature? |
| Pot life and gel time | Is the dispensing and mold-closing sequence faster than the working window? |
| Cream time and rise time | Does the mold fill before the rise becomes restrictive? |
| Free-rise vs. molded density | Is the shot weight calculated from the correct density for the tool geometry? |
| Demold and post-cure schedule | Does the planned cycle match the cure state assumed by the published properties? |
A narrow ratio window is easier to hold on equipment with closed-loop metering than on an open-loop manual system. If the TDS assumes a mixing accuracy the machine cannot provide, no subsequent process adjustment will recover the missing consistency.
Common Mistakes That Turn a TDS Into a Wrong Decision
Several reading errors repeat across production and purchasing reviews:
- Comparing hardness across different Shore scales or specimen thicknesses.
- Using free-rise density where molded density is required.
- Quoting viscosity without noting the test temperature and spindle.
- Treating tensile strength as a direct safe-load rating for a finished part.
- Assuming gel time equals demold time.
- Reading a ratio or index from a TDS without recalculating for the batch coordinates actually in the tank.
- Accepting a single value without a tolerance, batch variation range, or test method note.
- Swapping suppliers on headline numbers alone while ignoring whether both sheets used the same standard.
If two TDS values disagree, the first question should not be “which material is better?” It should be “which methods and conditions produced each number?”
Request a TDS-Guided Equipment Match
If you have a TDS or a draft formulation and need to check the processing window before machine selection or line upgrade, send the relevant values to Haifeng Polyurethane Machinery at info@chinahaifeng.com or WhatsApp +86 13566296633.
Include the system type, NCO content, viscosity at the intended processing temperature, mix ratio, gel or rise time, free-rise and molded density, target output, and part geometry. The more complete the values, the faster the equipment recommendation can be checked against metering, mixing-head, mold-carrier, and temperature-control requirements.
FAQ
What is the difference between a TDS and an SDS?
A TDS describes how a material processes and performs. An SDS describes hazards, handling, storage, and regulatory information. They answer different questions and should not be used interchangeably.
Which hardness scale should I trust for a cast elastomer?
Trust the scale that matches the hardness range and the way the part will be inspected. Soft elastomers are usually reported as Shore A; harder materials are reported as Shore D. Always keep the durometer type and specimen geometry attached to the number.
Is a higher tensile strength automatically better?
No. Tensile strength is only one failure mode. A material with high tensile strength may still be vulnerable to tearing, compression set, hydrolysis, or fatigue. The application determines which values matter most.
Why doesn’t the molded part reach the datasheet density?
Because the datasheet may report free-rise density while the molded part is formed under cavity restriction. Packing pressure, venting, mold temperature, and shot weight all move the final molded density away from the free-rise value.
How should I compare two polyurethane TDS from different suppliers?
First, compare the test methods and conditioning conditions. Then compare hardness on the same scale, density on the same basis, viscosity at the same temperature, and mechanical values under the same standard. If the methods differ, the comparison is not apples to apples.
Does reading a TDS require laboratory equipment?
Not for the first pass. You need laboratory data only when you want to verify a suspect value or establish your own incoming-material control range. The more important skill is knowing which conditions are attached to each number and how those conditions relate to your process.
References
[1] ASTM D2240-15(2021), Standard Test Method for Rubber Property—Durometer Hardness. ASTM International.
[2] ISO 868:2003, Plastics and ebonite — Determination of indentation hardness by means of a durometer (Shore hardness). International Organization for Standardization.
[3] ASTM D4878-15, Standard Test Methods for Polyurethane Raw Materials: Determination of Viscosity of Polyols. ASTM International.
[4] ISO 3219:1993, Plastics — Polymers/resins in the liquid state or as emulsions or dispersions — Determination of viscosity using a rotational viscometer with defined shear rate. International Organization for Standardization.
[5] ASTM D2572-19, Standard Test Method for Isocyanate Groups in Urethane Materials or Prepolymers. ASTM International.
[6] ASTM D7487-18, Standard Practice for Polyurethane Raw Materials: Polyurethane Foam Cup Test. ASTM International.
[7] ASTM D792-20, Standard Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement. ASTM International.
[8] ISO 1183-1:2019, Plastics — Methods for determining the density of non-cellular plastics — Part 1: Immersion method, liquid pycnometer method and titration method. International Organization for Standardization.
[9] ASTM D412-16(2021), Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension. ASTM International.
[10] ISO 37:2017, Rubber, vulcanized or thermoplastic — Determination of tensile stress-strain properties. International Organization for Standardization.
[11] ISO 34-1:2015, Rubber, vulcanized or thermoplastic — Determination of tear strength — Part 1: Trouser, angle and crescent test pieces. International Organization for Standardization.
[12] ISO 815-1:2014, Rubber, vulcanized or thermoplastic — Determination of compression set — Part 1: At ambient or elevated temperatures. International Organization for Standardization.
If you’re interested, check out these related articles:
What Is Semi Rigid Polyurethane Foam
How Polyurethane Rollers Are Made From Casting To Cure
What Is Spray Polyurethane Foam Composition And Application
Shore A Vs Shore D Hardness In Polyurethane
