about 0 010 to 0 015 34 is typically in the range of about 0 5 to 2 5 seconds at a temperature in the range of 170 u00b0 to 300 u00b0 f when the substrate is sufficiently softened the cool pins 11 are pressed into the opposite surface of the substrate by further lowering of shaft 20 where the depth of the imprint of the pins is controlled by a stop on the press not shown then the anvil is released from the substrate with the pins 11 still in place thereby allowing the substrate to cool and fixing the pore size and shape this prevents heat buildup which would cause nonuniform pores the pins 11 remain in the substrate 12 dwell time on an average of about 2 seconds and then are withdrawn the substrate 12 is then moved so that pores may be imprinted on a different area if desired the pins may be used to puncture almost entirely through the substrate but it is preferred that the pores penetrate to a depth of about 5 u00d7 to 20 u00d7 the pore diameter in an alternate embodiment of the invention referring to fig2 a heated head 30 accommodating a plurality of pins 31 is utilized the head 30 and pins 31 are heated with an internal heating element 32 located within.