Impeller Balance
IMM 08/01

We are molding an impeller about 5 inches in diameter in a single-cavity mold that appears to run with good fill. The material is a 33 percent glass-filled nylon. We're having problems achieving proper balance, resulting in scrap rates of about 15 percent. Does anyone have suggestions for ensuring good balance in a molded impeller?

Measure imbalance as soon as the impeller cools and fits properly on the balance arbor. Check imbalance amount and location and note that this imbalance data should be used as a benchmark predictor for "dry as-molded samples." To get impellers within specification after 24 hours, you may have an as-molded balance target that is out of specification. Measure imbalance on the same samples after 24 hours. If everything repeats, you should be within the balance tolerance. Good Luck. - B.Daly, PTA Corp., Oxford, CT (203) 888-0585, bdaly@ptacorp.com.

Machine capability is key. Even minute variation in shot size, fill (rates and times), and heat can lead to balance inconsistencies. I recommend a capability study that includes the following three features: part weight, imbalance, and angle of imbalance in parallel with critical machine parameters. Please note that the angel of imbalance is very important. Unless the angle is stable, balancing the part is virtually impossible. Once the three part features are in control you can permanently balance the part by adding material at the respective balance planes and monitoring the molding machine parameters. - J. Wagner, Westplex Industries Corp., Manchester, NY (716) 289-3630, westplexjimw@yahoo.com.

Here are some of the things I check when problems like this arise, but, like all things, the key is finding the right combination. First check the stock wall of the tool and then check the gating system because you may need to adjust the gates. The nylon being used will set up quickly. You should also check the barrel heats for consistent temperature. This material produces a lot of gas so maybe try using a slower fill. You can try backing down on the tonnage to allow the gas to escape the tool. Also, make sure the material is properly dried. You may want to try running the material higher than normal, profiling your barrel heats, and using a nylon tip if you can. If I can be of further help feel free to write me. - S. Clay, Ferriot Inc., Akron, OH, (330) 786-3000, sclay@ferriot.com.

The mold should have three balance adjusting pins approximately 1/4 inch in diameter that can be moved into or out of the part as much as .100 inch. Once the tool has been tuned in (with the three points specific for balance set at its midpoint), single plan balance readings are taken on the part when an equilibrium process has been reached. Then the required pin adjustments can be calculated and the whole procedure repeated until satisfactory balance has been attained. It is not a short or easy job and the tolerance on the balance must be realistic, perhaps about .006 inch for the part as described. - W. Foster, Tessy Plastics Corp., Elbridge, NY, (315) 689-2077, bfoster@tessy.com.

Run short shots to see if all the impeller parts are filling uniformly every time. If one side is filling first it will be heavier. If you are gating into the hub then you may have to add more gates to get a balanced flow. Once you get a consistent and uniform filling, all of the parts should have the same balance characteristics. Any imbalance can be corrected with judicious metal removal. - F. French, Amitima, Manchester, MO (314) 522-1252.

Gating is the key. Generally speaking, when you're dealing with something that's round and filling radiates out from the center, it is crucial to use a membrane gate. That gate is in the center of the impeller and tends to fill from the center of the part. To locate the gate on one end of a hub would actually be improper. You want to locate the gate down into the depth of the part, preferably where the melt starts to flow into any of the ribs that radiate out from the center of the hub. You want that impeller to fill evenly from the center-out to have adequate venting. I suggest a gate thickness of .040 inch, .030 inch minimum; otherwise, the gate can get plugged by lumps of glass, which interferes with the even distribution of the polymer. If it's a big impeller, you require a thicker gate. Then you do a secondary operation to shear out the gate after ejection. - E. Barno, Multiform Plastics, Sterling Heights, MI, (810) 726-2688, multiform@ameritech.net.

Equip your tool with pressure transducers and your machine with an eDart monitor from RJG Inc. Real-time process data will tell you whether the part is good or bad. We have had success balancing fill on an eight-drop hot manifold with RJG's technology. - J. Schramzki, Acra Inc., Traverse City, MI, (231) 947-8733, jims@acrainc.com.