Testing and Processing # 1982 York
Residual stress in extruded thermoplastics pipe is set up during the cooling process. The stress is compressive at the cooled boundary and tensile at the adiabatic midplane. Stress level increases with the itensity of cooling. A compromise may be sought between process time and stress level.
Measurements made on internally cooled extruded pipe show an almost symmetrical stress profile which agrees fairly well with the profile calculated using an elastic model with a temperature dependent modulus. Internally cooled pipe may be expected to have better service properties than conventional pipe. Long term pressure tests are being conducted to check this hypothesis.
In the production of extruded thermoplastics pipe, the extrudate must be cooled and calibrated. It is usual to calibrate and therefore cool the outside wall in order to control the outer diameter (0.D.) of the pipe to within the tolerances specified by standards. Upon reaching the drawing bench, the pipe must have been cooled to the extend that it is sufficiently stiff to withstand the forces from the caterpillar tracks without undue deformation. The rate of cooling and the resulting transient temperature profiles at any time t, depend on the heat transfer coefficient H (W/m? K) which describes the removal of heat from the outside wall of the pipe to the cooling medium, on the coefficient of heat conduction of the pipe material \ (W/mK), on the thickness of the pipe W and on the heat content of the pipe material.