Technical: Design & Engineering

Material Selection

For the best urethane formulation consider:

1. For your application, prioritize performance requirements.
2. Although there is no perfect elastomer for all applications, there good choices worthy of prototype testing. Occasionally, it is necessary to specially blend urethane materials to meet performance requirements. Fortunately, an almost infinite variety of urethane properties are possible. It is essential to allow adequate time to your development phase if you expect to optimize the urethane characteristics.
3. Your prototype test program should simulate the operating conditions as nearly as possible. Care must be taken to not attempt to accelerate testing [cyclic rates or amplitudes] exceeding the application parameters. Excessive heat build-up in urethanes [hysteresis] will compromise your data and test results.
4. Expect to have to modify the part design or urethane compound to correct deficiencies found during testing.
5. Field trials and monitoring results are beneficial before a major product launch.

Any development of a unique urethane compound will take into account best physical properties for the intended service while lessening any interference from undesired properties. Care must be taken to understand the importance and interdependence between the urethane selected and a host of issues which include: climate cycles, ultra-violet light, resistance to weather, ozone, temperature, oxidation, humidity, stress and load factors [static / dynamic] resistance to chemicals, grease, oils, etc. UI Engineering can help you.

Specifications

Thoroughly understanding where and how the urethane part is to be used is essential to compound selection. It is a common belief that urethane hardness alone can determine optimum material choice. Not so! Each material has its own unique set of physical properties, which will yield dramatically different performance characteristics even if of the same hardness. Proper engineering of part / feature tolerances are essential to competitive bidding. Tight tolerance requirements where not really justified by end use will needlessly increase cost. Care must be taken to ensure all bidders are dealing with exactly the same specifications to avoid some party winning the bid by cheaper material selection not best suited for end results. Things for you to consider in selection of a good material include: Elongation, Hardness, Tensile Strength, Abrasion, Tear Strength and Modulus at 100% to 300%, plus any specialized environmental performance needs. Make sure your Design department incorporates comprehensive requirements and / or specifications on drawings before your Purchasing Departments issues a quotation request.

Urethane Innovators uses premium performance compounds, with no fillers or extenders unless required to fulfill a needed technical purpose or by the customer request. If the optimum material is not affordable to you, UI will help you make an intelligent compromise between material and cost. We also have compounds that meet FDA and USDA requirements.

Resistance to oils and chemicals is determined by immersing small billets totally in the fluid. Immersion test data is usually generated at room temperature. Elevated temperatures often increase a fluid's effect on the urethane. It is always wise to test specific compounds under your particular field conditions and applications as near as possible.

Load-Bearing Capacity

Urethanes have higher load bearing capacity relative to other elastomers and deflection and recovery capabilities are better than plastic or metal.

Molding Tolerances

Often customer prints are detailed to tolerances which are unrealistic for the application. Generally, tighter tolerances result in a higher cost part. For non-critical dimensions we can advise you on what is realistic for molding and optimal cost. Where very close tolerances are a must, they can be achieved by tight tolerance tooling [often requires tune-in] or by subsequent machining or grinding operations after the part is cast. Urethanes have a very high coefficient of thermal expansion [i.e. ten times greater than steel or four times greater than aluminum]. (.001" to .0015" per inch per 10°F. If a 1" feature tolerance is +/- .005"inch tolerance, as measured at 72°F, it will be out of tolerance at 20°F or 120°F. Generally achievable "As Molded" tolerances are:

Standard Tolerances for Machined Urethanes (unless otherwise noted)	Standard Tolerances for Cast Urethanes (unless otherwise noted)
Decimal Dimensions	Decimal Dimensions
3 Decimal Places         +/-.005	3 Decimal Places         +/-.015
2 Decimal Places         +/-.015	2 Decimal Places         +/-.030
Hole locations              +/-.010	Hole locations             +/-.030.
Angular Dimensions	Angular Dimensions
In Degrees                +/- 30 min.	In Degrees               +/- 60 min.
In Degrees                +/- 30 min.	In Degrees               +/- 30 min.
Fractional Dimensions   +/- 1/64	Fractional Dimensions   +/- 1/32

Urethane Bonding to Metals

During molding, urethane can be attached permanently to metals where bond strengths exceed the tear strength of the urethane. The bonding surface must be free of all contamination. Cured urethane can be bonded to most metals, composites and other materials. For instance, ball bearings with tough urethane treads are available in a wide range of sizes and uses.

Shape Factor

The compression [deflection] relationship of a urethane part is affected by the shape of that part. Simply put, Shape Factor is the ratio of the platen area of a urethane mass subjected to a compressive load divided by the sum of the areas which are free to bulge [or the ratio of one loaded surface area to the total area free to bulge] . As the shape factor increases, the strain produced by given stress decreases. This is a critical consideration in avoiding heat build-up in dynamic applications [Hysteresis]. It is also important in static load bearing applications such as structural bearing pads where compressive stress relaxation versus time is to be avoided. Shape has only a minor effect in tension and shear.

Consider two blocks of urethane: one a 3" diameter cylinder and the other a square block of the same thickness and cross-sectional area [7.07 in2] and both blocks are made from the same compound with the top and bottom surfaces bonded to metal plates perpendicular to the top and bottom surfaces. If the same weight is placed on each block [interestingly, neither block loses volume during deflection] the deflection will be greater for the rectangular block than for the cylindrical block. Why? Because the area free to bulge at the sides is 12.8% greater for the square block [10.63 in2] than the area free to bulge for the cylinder [9.42 in2]. By increasing the area free to bulge makes bulging easier and permits greater vertical displacement.

What would happen if the design factor where changed to a rectangular pad 1" x 7.07" long and the same thickness as the above pads? The area free to bulge would be 16.14 in2 so the deflection will be greater. The influence of shape is substantial and can be expressed numerically as shape factor (SF). Designers are wise to take this into consideration.
Parts of the same compound and shape factor behave almost identically in compression, regardless of the actual shape, provided the pieces have parallel loading faces, whose thickness is not more than two times the smallest linear dimension (to prevent buckling) and top and bottom surfaces are not free to move laterally.

Shape Factor In Design

Shape Factor is critical in urethane selection. Assume the above urethane pad 7.07in2 and 1" thick, made of a 93A durometer compound. How much will the pad deflect under a load of 14,140 pounds?

The shape factor of the pad is:

14140/7.07 = 2,000psi
For a 93A durometer material the curve for a shape factor of .67, 2,000 psi produces 8% deflection. Therefore, the pad will deflect about 0.080" under this load.

Similarly, the amount of impact that a cushion could take repeatedly for a specific design criteria [% deflection] can be calculated. For instance, a 70D urethane pad with deflection of 5% and a shape factor of 0.8 will take 4,500 PSI compressive stress. Calculations show it can absorb an impact force of 120,000 lbs at a frequency of 1 impact / minute indefinitely. Of course, this will vary for different materials, but UI engineering can assist you with your specific needs.

Urethane Superior To Over Metals, Plastics and Rubbers:

Urethane Limitations

• At 200°F urethane properties are reduced to less than half that measured at 75°F. Accordingly, it is advisable to not have dynamic applications above 200°F. Urethane physical properties typically do not suffer from heat aging at high temperatures up to 250°F even for weeks and are almost completely reversed when tested again at 75°F. Dynamic applications are best between temperatures -40°F to 160°F. At 160°F the urethane properties begin to decline and bonding to metal weakens significantly above 160°F.
• Urethanes do not dissipate heat build up [hysteresis] from dynamic action readily. By controlling the amplitude of the deflection, applications can usually be successfully designed. A trick for large deflections is to use multiple urethane elements in series.
• Extended exposure to hot and humid environments should be avoided; however, some urethanes are more tolerant of such environments and we will help you select the best choice.
• Certain chemicals such as concentrated acids and polar solvents attack urethanes should not be put into continuous service in these environments. Reference our Chemical Resistance Table.