Cast elastomers, compression

Phenolics Elastomers Powder, solutions Excellent thermal stability to over 300 F generally, and over 400 F in special formulations. Broad choice of resins. May be cast or compression, transfer, or injection molded. Precision potentiometers, RF connectors, heating panels. 

On the other hand, a gradual increase in tensile strength and compression set values is observed for cast elastomers containing more amorphous hard segment contents. 

The properties of thermoplastic polyurethane elastomers are quite similar to those of cast elastomers (see Table 14.5) but the absence of primary cross-links results in higher compression set and more pronounced loss of strength with increasing temperature. Also, although the thermoplastic elastomers are insoluble in most common solvents they are soluble in highly polar solvents such as dimethylace tamide. 

Urethanes are processed as mbber-like elastomers, cast systems, or thermoplastic elastomers. The elastomer form is mixed and processed on conventional mbber mills and internal mixers, and can be compression, transfer, or injection molded. The Hquid prepolymers are cast using automatic metered casting machines, and the thermoplastic peUets are processed like aU thermoplastic materials on traditional plastic equipment. The unique property of the urethanes is ultrahigh abrasion resistance in moderately high Shore A (75—95) durometers. In addition, tear, tensUe, and resistance to many oUs is very high. The main deficiencies of the urethanes are their resistance to heat over 100°C and that shear and sliding abrasion tend to make the polymers soft and gummy. 

As explained in the previous sections of this chapter, elastomers are used to fabricate many different products to meet many different requirements (Tables 2.7 and 2.8). Practical all fabricating processes are used (such as extrusion, injection molding, blow molding, compression molding, casting, and encapsulation) are used. 

Mechanical Tests. Hardness measurements taken with a Shore A durometer are reported every time a pad specimen has been removed from its water bath for compression-deflection tests. Separate flat slabs, 0.12 and 0.50 in. thick, of polyurethane elastomer, which were cast from the same batch of material and given identical exposure to water as the pads, were employed for the hardness tests. 

Like all thermoplastic elastomers, thermoplastic polyurethane (TPU) is elastic and melt-processable. Further, it can be processed on extrusion as well as on injection, blow and compression molding equipments. It can be vacuum-formed or solution-casted and is well suited for a wide variety of fabrication methodologies. TPU can even be colored through a number of processes. But more... 

An area of continued research is in development of elastomeric material models which can incorporate cyclic behavior into one model casting aside the need to switch from non-cyclic to cyclic models. Further, other areas of research involve more accurately predicting viscoelastic behavior such as stress relaxation, creep, and most importantly for cyclic applications, compression set. As these models are developed, the accuracy and abilities of Finite Element Analysis of elastomers will improve dramatically and provide a much better method to predict the complex loading conditions over time that elastomeric parts commonly see. 

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