Mechanical Properties of Polyurethane
Urethane casting is the process of injecting polyurethane and additive resins into a soft mold usually made of silicone elastomer.
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This is Chapter 4 from a full article called Urethane Casting from the IQS Articles - Your Source For Industrial Information
Urethane Casting - Chapter 4
Compared to metals and other plastics, urethane castings have its set of properties that makes them stand out. Below are some noted properties of polyurethane resins.
Abrasion can be classified into two types: sliding and impingement abrasion. Sliding occurs when a soft and hard material slides or rubs into each other, with or without contaminants between the surfaces. Impingement abrasion, in contrast, happens when particles impact the surface causing erosion. Because of these properties, cast urethanes are excellent material for wheels and rollers.
Cast urethanes with low coefficient of friction and high tear strength offer good sliding abrasion resistance. For impingement abrasion, cast urethanes with good resilience are used. Resilient polyurethanes can easily yield elastically. Forces from the impacting particles are easily distributed on the surface.
Abrasion resistance is achieved by the right composition of the urethane resin. Among the polyol compounds for making polyurethane, polyesters exhibit better tear and abrasion resistance.
Like abrasion resistance, urethanes possess good impact resistance because of its good resilience. Urethane castings can elastically deform to absorb the impact and return to its shape while dissipating the energy throughout the body.
Hardness is the relative resistance of a material to localized surface deformation. It is usually determined by measuring the depth of indentation on the material by a standard indenter, ball, or presser foot. Materials are then graded according to its relative hardness from one another. For elastomers, hardness is characterized by Shore hardness number as measured by a durometer. The Shore hardness scale is categorized into 12 scales, each scale has its own indenter configuration, profile and force applied. The most common scale for urethane casts are Shore A and D. Shore A scale measures hardness of soft, semi-rigid polyurethanes. Shore D, on the other hand, measures hard rubbers and rigid polyurethanes. However, keep in mind that high hardness does not correspond to high rigidity or strength.
Elastomer tensile strength, like in metals, is the amount of force or stress the material can take without failing. Urethane casts can be formulated to meet strength requirements for up to 14,000 psi by using composite materials such as fiberglass and carbon fibers.
For flexible urethane casts, flexural strength is a key property which tells how much bending force is required to break the material. Urethanes have excellent flexural properties, especially in thin cross-sections. They usually have 17,000 psi flexural strength, up to 39,000psi for composite casts.
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