IMM 12/97

I have a core pin that is approximately 2 inches long and has a .120-inch diameter with a 1/2º draft. The mold is a hot runner mold with the nozzle at the top of the core pin. My problem is that the core pin deflects and shuts off against the caity wall. Does anyone have any suggestions on a core pin design that may help improve deflections?

If the design allows, you might try putting a bulllnose radius on the free end of the pin, or even sharpen it to a point. It's a fairly inexpensive trial, and you never know, the reduced turbulence around the pin might make the difference.
-E. Foy, Strategic Tooling and Manufacturing, San Luis Obispo, CA, (805) 542-8514.

If you have a programmed injection system on the molding machine, you can try reducing the injection velocity at the start of the filling cycle. This will allow plastic to surround the core rod, maintaining its position. Then, increase the velocity to the normal setting.

If the press doesn't have a programmed injection, you can reduce the injection velocity for the entire filling cycle. Reduce the peak cavity pressure by adjusting the transfer point (position) to transfer earlier in the cycle. Then, reduce packing and/or holding pressure.
-J. Baldwin, Schmalbach-Lubeca Plastic Containers USA, Lenexa, KS, (913) 894-2211, Ext. 202.

I've seen similar problems resolved by using a larger core pin and stepping down to the required diameter. This way, you can keep the smaller diameter section at a minimum length.
-R. Newsome, Analytical Design Assoc. Inc., Kalamazoo, MI, (616) 324-9767.

If the application allows, fluted core pins are much stronger than straight ones.
-A. Hall, Precision Services, Austin, TX, (512) 219-7718.

On the processing side of your problem, you already have a mold built and you have to work with the tool that was built. The bottom line is you have to reduce the cavity pressure within the mold itself (for example, melt and mold temperature and slower fill-rate pressure). Let me guess, polyurethane? Hot runner? I once worked with urethane on a hot runner vial mold for Abbot Labs and the vial was 6 inches long and allowed no draft. Needless to say, every time I got the mold filled, we bound up the mold.
-R. Gracey, Modern Plastics Technics, West Berlin, NJ, (609) 768-3232.


RJG Technologies is pioneering the instrumentation of core pins to measure deflection with a patented technology. To make a long story short, the data we have been collecting on preform molds show that the largest deviation from the center of a core pin occurs during the cooling phase of the process. Even though there is some deflection while filling and packing the part, the core generally returns to retained deflection and wall in concentricities of the part. Packing the plastic has a strong effect on the cooling repeatability, and it is proven that controlling the process with cavity pressure by instrumenting the core improves the cooling repeatability. Lastly, deflection is very sensitive to how much plastic is held in the cavity after packing. The more discharge allowed, the less the retained deflection. If anyone is interested in conducting these experiments themselves, feel free to contact a technical sales rep at RJG, as we can provide the necessary instrumentation.
-B. Watkins, RJG Technologies Inc., Traverse City, MI, (616) 947-3111.