Custom extruders running high stretch ratios during the manufacture of polymeric medical microtubes can also result in significant orientation and residual stress/strain in the finished polyimide tubing . This orientation can significantly increase tensile strength and reduce tube elongation in the machine direction. It also reduces tube working pressure vs burst pressuredue to loss of circumferential strength. high stretch ratios create residual stresses that can cause severe damage during subsequent heat treatment, sterilization or aging (natural or accelerated). These thermal processes can relieve stresses or burst pressure generated during extrusion, resulting in significant shrinkage of the tube length and an increase in diameter and wall thickness.
Polyimide tubing yield strength, tensile strength and working pressure vs burst pressure and other mechanical properties of the pipe wall thickness: theoretically, the yield strength of the pipe, tensile strength burst pressure and other mechanical properties of the pipe wall thickness is not related; but in practice, according to the weak point theory of mechanical properties of materials, the thicker the pipe, the yield strength, tensile strength, etc, Tensile strength, etc. is smaller. The basic data on the mechanical properties of materials are based on certain testing standards. This means that the mechanical properties of the same material will vary for different test conditions (such as the three-dimensional size and shape of the sample, tensile speed, ambient temperature and humidity).
For example, according to the basic principle of blowing process, the blowing temperature of FEP heat shrink base tube should be between glass transition temperature and melting temperature, and the better blowing temperature is 130℃～190℃. If the temperature is lower than 130 ℃, you need a higher pressure , working pressure vs burst pressure too high tube easy to rupture;
If the temperature is higher than 190 ℃, the tensile strength and elongation of the polyimide tubing as the temperature increases and decreases, thus blowing is also prone to rupture .