D-M-E

Applications Engineering Department

Beryllium-Free Copper Alloy Core Pins

OCTOBER, 2004

Re: Utilization of Performance Core Pins With Standard Steel Ejector Sleeves (Thermal Expansion Differences and Tolerance Issues)

Many moldmakers utilize our standard steel (H-13) Ejector Sleeves (nitrided & not nitrided) in conjunction with our Performance Core Pins, which are a beryllium-free copper alloy.

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(Thermal Expansion Differences and Tolerance Issues):

Performance Core Pins move heat much more rapidly than a steel pin. This heat removal is done by conduction. Many times the core pin rests on a chill plate or is in contact with 70-80 F. water.

The most common problem is interference fit. For example, a .250 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./Degrees 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 utilized.

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 400-700 Degree 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.

Yes, many moldmakers will electroless nickel or hard dense chrome plate the pin. Even low temperature applications of Titanium Nitride coatings as well as Armoloy coatings have been utilized.

PVC does not attack beryllium-free copper alloy. The material may tarnish under certain conditions. Copper cleaners such as Braso will remove the tarnish. Do not use abrasives to clean the copper alloy.

Specific settings must be adhered to when utilizing EDM and beryllium-free copper alloy. Conventional EDM settings that are used for steel will result in high electrode wear and slow cutting. Call D-M-E for specific details.

Tig welding with Argon being the shielding gas works best when welding beryllium-free copper alloy. Brazing, gas welding and silver soldering are not recommended.

Call D-M-E for details.

If you have any questions regarding this or other topics please contact D-M-E Applications Engineering Department.

OCTOBER , 2004