IMM 11/00

We are molding a suitcase out of virgin P in a two-cavity shell, with two different shot sizes (710g, top, and 1052g, bottom). The flow system is balanced, but we are getting flash at the gate. Also, sometimes the top shell shorts and the bottom flashes. Our troubleshooting has involved checking machine pressure, material slippage, the manifold busing (clean), and position of flow channel plugs; raising injection speed, pressure, and temperature; and controlling the waterlines. Can someone help us solve this problem?

Install a separate beryllium copper alloy gate insert with a separate gate-cooling waterline in order to control the temperature on the gate and eliminate flash and short shots
-M. DeMarchi, Norseman Plastics, Rexdale, ON, (416) 745-6980, qa@norsemanplastics.com

IMM 10/00

The information given suggests edge gating. If the parts were gated from a sprue bushing, hot or insulated runner mold, they could not flash. When edge gates flash (assuming the mold surfaces are sealed), it means that the pressure at that spot is high enough to deform the steel. If the mold has been designed using some common computer programs, the pressure in the mold is controlled by the size of the runner system. This is incorrect. The purpose of the runner system is to bring the plastic to the gate with minimum loss of temperature and pressure. This is part of the reason why hot and insulated runner molds run better and faster than cold runner molds.

In this case, opening the runner system to at least ? inch round is required. Make sure the runner is vented. It will be necessary to enlarge the gates and balance the flow using the gates rather than the runners. Doing this will dramatically reduce the injection pressure and eliminate flashing at the gate.

The second problem, parts not filling and flashing, will be cured by enlarging the plastic material feed system as suggested. Be sure the O dimension in the sprue bushing is large enough. What is happening is that one part has too large a gate relative to the other so that it fills first. The second part starts to fill, slowing down the flow into the first part, and causing the gate to freeze. The second part fills and is overpacked. The first part cannot be packed because the gate has frozen. To correct this, balance the flow in both cavities so that they both end their fill at the same time.
-I. Rubin, consultant, Brooklyn, NY, (718) 258-7363. 

In addition to the process troubleshooting you have done, you many want to check the faces of both the movable and stationary platens for flatness, and look for worn or low areas that would allow the mold to flex under plastic pressure and flash as you have stated. Also, check the mold itself for a clean, flat parting line face on both halves, and check that the support columns are placed to support the area of greatest pressure (i.e., nearest the sprue), and under all cavity areas, and that the supports are at least .001 to .002 mm thicker than the height of the rails in the mold.
-S. Hatfield, Accent Plastics Inc., Corona, CA, (909) 273-7777.

Is the flow system / runner really balanced? About 59.7 percent of flow must go to the bottom and 40.3 percent must go to the top. This will make the cavities fill more evenly.
-B. Dermont, American Allsafe Co., Tonowanada, NY (716) 807-3430.