This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in
the molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.
- Gate diameter for hot runner molds
- Gate diameter of 50 percent of wall thickness for easy-flow PE and PP.
- Gate diameter of 75 percent of wall thickness for ABS, SAN, and acrylics.
- Gate diameter of 90 percent of wall thickness for polycarbonate/acetal/ PVC/glass-filled materials and PPOs.
- Land length for hot runner molds
- Land should be no more than .005 inch.
- Check with your hot runner manufacturer for correct land length.
Problems with hot runners and heated sprue bushings always seem to be in one of three places. The first problem area is usually the gate diameter, second is the gate land, and third is the size of the nozzle orifice.
When troubleshooting hot runner problems, I start with the gate diameter and land length. The gate diameter should be at least half the wall thickness for polyethylene and polypropylene and bigger for the more shear sensitive amorphous materials such as ABS, acrylics, and polycarbonates. The land length of these hot tip gates should be .005 inch, usually indicated as a sharp edge on prints.
When the flow path is restricted because of a small gate diameter or a long land length, you will usually see higher heat settings being used by the molding technicians to get the parts to fill and pack out. The problem with this approach is that it usually causes warpage of the parts, cosmetic defects around the gate area, and long molding cycles.
If the flow path is sized correctly, you will be able to set the same heat settings on the front zone of the barrel, the nozzle, and the heated manifold, such as 450F across the board for ABS or 525F for easy-flow polycarbonate. If you find you have to run the manifold heats higher than the front zone of the barrel you probably have gate sizing problems. If you have to run the heats up on the nozzle, then the nozzle orifice probably hasn't been drilled out to match the flow tube diameter of the heated bushing (Figure 1). Notice that it isn't just sizing the nozzle orifice to match the hole in the heated sprue bushing. First you might have to increase the hole size of the sprue bushing to match the flow tube diameter, then drill the nozzle out to match that.
Figure 1. To optimize the performance of the hot runner, the sprue bushing orifice should be the same size as the main sprue bushing flow channel. Increasing the nozzle diameter reduces the pressure loss during both injection and decompression.
A case in point is a two-cavity hot runner mold that a good friend of mine is running in Arkansas. As often happens, he inherited this mold from another molder. The parts look like upside down banana split bowls (Figure