Performance Core Pins move heat much more rapidly than steel pins. This heat removal is done by conduction. Many times the core pin rests on a chill plate or is in contact with 70-80 degree Fahrenheit water.
The most common problem is interference fit. For example, a .20 inch diameter Performance Core Pin has a tolerance applied of +.001/-.000. The corresponding Ejector Sleeve internal diameter (I.D.) has a tolerance of +0005/-.0000. Thus, the sleeve I.D. could be .0005 larger than the pin O.D. (outside diameter) or the pin could be .001 larger than the sleeve I.D.! This is a situation where galling will occur. Thus, the internal diameter of the ejector sleeve must be honed to accept the Performance Core Pin.
At operating temperature the desired sliding fit between the two dissimilar materials should be .001/.0015. Beryllium-free copper alloy has a coefficient of thermal expansion of .0000097in./in./degree Fahrenheit. H-13 steel has a coefficient of thermal expansion of .0000058in./in./ degree Fahrenheit. This thermal expansion difference of the materials means the copper alloy will expand at a greater rate than the steel. The coefficient of thermal expansion must be considered when designing molds with materials that expand at different rates. Consideration to the cavity and core material expansion rates must also be considered.
The plastic material shrinkage rate is another consideration when the copper alloy core pin is used in the mold. Shrink rates are typically reduced when copper alloy pins are used. One must ensure that the bearing length between the core pin and the ejector sleeve is not excessive. A general rule of thumb is that the bearing length should be 2 to 2-1/2 times the diameter of the core pin.
NOTE: When the bearing length approaches one inch, problems may result from excessive bearing length.
In conclusion, pay special attention to the working diameter tolerance of the copper alloy pin and the internal diameter tolerance of the ejector sleeve when grinding your pins to size. Remember a copper alloy pin conducts heat out of the steel and the copper alloy pin expands. Please note that the copper alloy pin will expand at even a greater rate when utilized in an environment of the 400º - 700º Fahrenheit range. You can always warm up your copper pin to the desired temperature and measure its thermal expansion. This can be done with or without being installed in an ejector sleeve.