IMM 2/99

I'm working for a company that provides connectors for the automotive industry. We are supposed to ship zero defect products on time and at no cost. (Aren't we all?) We use multicavity molds with a lot of core pins. The chambers of the connector housing are formed with core pins. One of the many individual core pins on the multicavity molds often breaks and causes production of faulty connector housings. We currently use 1.2343 for such core pins. Molds are four to eight cavities with six to 25 core pins each. Does anybody know of a solution, steel, heat treatment, or design recommendation to prevent the breaking of the core pins? Or does anyone know of process control measure within the injection molding process in order to identify occurrence of core pin breakage? Cycle time is 10 to 15 seconds.

I have seen many core pins break for lots of reasons. Two very common and often overlooked causes are injection pressure or unusual filling patterns. Check the location with the gate and fill pattern. Does the core pin telescope into a nest to prevent bending and breaking? If not, add a nose pin and have a built-in P/L lock on the end of the pin. Another way to add strength to a pin is to make the body diameter larger than the cored out hole and add a small radius to the pin where the body meets the pin area of the cored out hole. Just a couple of ideas to help keep the pins straight; good luck.
-R. McDonald, Model Mold & Machine Co. Inc., Indianapolis, IN, (317) 926-1505.

An imaging vision system will help detect missing or broken pins and reduce scrap. If you are insert molding, it will also detect operator error and prevent breakage. They are a wise investment in reducing scrap, downtime, and repair cost.
-D. Smith, Van Dorn Demag, Temecula, CA, (909) 694-3364.

I bet there is a lot of history to this core pin breakage problem you're not telling us. What have you tried? How often are the molds cleaned during the press run? Is the core pin closest to the vent location the one that breaks most often?

Core pins can be readily purchased in M-2 and Standard and High Hard H-13. You can dial in the toughness by drawing back the hardness in a tempering oven at your local heat treater. See your steel selector book. Also, experiment with other steels bought in blank sizes: L6, O1, S7. 

Coatings are getting popular now. I would try titanium dioxide (the gold-colored one you see on cutting tools). The lower temperature ones don't alter the above-mentioned tempers. Use these to increase the surface toughness, mold release capability, and chemical resistance.

For maximum, all-around toughness, mold release capability, and chemical resistance, use cryogenic tempering (race-proven in Nascar Winston Cup transmissions) in addition to conventional heat treat and titanium dioxide coating. We have a growing list of Winston Cup race teams and manufacturers of high volume bottle cap molds. It would cost less than the SPE golf outing for a hundred or so core pins of this size.
-D. Wheeler, Polar Bear Cryogenics Co., Kalamazoo, MI, (616) 349-8679.

Misalignment is a common cause of pin failure. There are many proprietary surface coating and conversion treatments to strengthen core pins, and there are specialist suppliers of pins. Progressive Components in Wauconda, IL is an example. The fax number is (847) 487-1027.

We have helped to cure several persistent pin breakage problems by specifying MAR 18 250, a maraging steel which offers much higher fracture toughness and endurance limits than conventional tool steels. We also have a monograph with additional information on this topic and would be happy to e-mail it to anyone who requests it.
-T. Gerson, F.T. Gerson Ltd., Toronto, ON, (416) 364-2457.