Flexing action

The finished piece should be flexed immediately upon its ejection from the mold, while the heat from the mold is still in it. A flex angle between 90 and 180 is recommended. The flexing action can stretch the hinge area by 200 percent or more thus, the initial 0.25 to 0.38 mm thickness (0.10 to 0.015 in.) will be thinned down to less than 0.13 mm (0.(X)5 in.). This elongation aligns the plastic s molecules and increases its tensile strength from 34 x 10 to 552 x 10 Pa (5,000 to 80,000 psi). 

The flex resistance of these materials was tested in accordance with ASTM F 392-74 (Gelboflex test). The flexing action consisted of a twisting motion, following by a horizontal motion. 

Under service conditions of dynamic flexing, wax alone will be inadequate to protect the rubber against ozone attack. The flexing action will rupture the surface film of the wax and thus provide an avenue of entry for the attacking ozone. Thus dynamic flexing requires the joint use of wax plus chemical antiozonant to attain proper protection against ozone attack. 

Both mechanical and chemical action promote ink detachment from cellulose fibers during pulping. Mechanical action includes interfiber abrasion and fiber flexing and bending. Chemical action includes fiber swelling and surfactant-promoted ink particle emulsification and solubilization. 

Later on12, Koshland proposed the induced fit model of the active site action that considers that during the formation of the enzyme-substrate complex, the enzyme can change its conformation so as to wrap the substrate like it happens when a hand (substrate) fits in a globe (enzyme). This flexing puts the active site and bonds in the substrate under strain, which weakens the bonds and helps to lower the activation energy for the catalyzed reaction. 

Figure 13.2 Antagonistic muscles their role in movement. The action of antagonist muscles is exemplified by the actions involved in delivering a punch. The arm is first flexed by contraction of the biceps muscle and simultaneous relaxation of the triceps muscle. The arm is then rapidly extended by contraction of the triceps muscle and relaxation of the biceps muscle. Such movements involve considerable nervous coordination.

Flex Test for Barrier Materials. A mechanical flex test to replace the US Specification MIL-B-131B Performance Test of water vapor proof barrier material (July 1952), which is costly and time-consuming, was developed at Picatinny Arsenal. Three mechanical flexing devices, each having a basically different action, were evaluated as possible substitutes for the above spec test ... 

In practice, a thin film of oily soil can often form on the hair surface with a contact angle of zero. In this case, it may not be possible for a surfactant to roll back completely the hydrophobic soil without additional mechanical action such as rubbing and flexing. 

On cooling, plastic materials tend to contract or shrink considerably more than other materials such as metals, ceramics and glass. For example, a copper pipe will shrink by 0.01% if the temperature is reduced by 10°C. Under the same conditions, a high-density polyethylene pipe would shrink by 0.07%, and polypropylene and hard PVC pipes by 0.04%. In addition, surfaces of plastic materials cool before their cores. Such a situation leads to the initial contraction of plastic materials at surfaces, before significant change in dimension occurs in the bulk. The skins of moulded plastics tend to be stiffer than the bulk, so are more prone to degradation by mechanical action, e.g. flexing. 

Flexing in its basic form is taken to be simple bending in one direction. Composite modes of flexing deformation are used for fabrics in an attempt to simulate what happens in service. This is usually a mixture of twisting and compression, which results in a sort of crumpling action. With such tests there is no way of determining exactly what stre.sses arc developed. 

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