The Troubleshooter, Part 15: Flat, full parts

This article continues ourseries 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.

A customer sent me a box of parts and runners and asked for help in keeping his parts flat and eliminating short shots. The parts were round, a little larger in diameter than a silver dollar, with raised areas on the outside edge in a couple of places. They were edge gated and ran in an eight-cavity balanced runner mold with a cold sprue. The material was 33 percent glass-filled nylon 6/6. The customer also asked for help in running his parts fully automatic.

The parts each had a notch cored out on one side where it could be gated (Figure 1). The current gate was on one side of the notch, and there was a sharp corner on the other side that was trapping air because it was the last place to fill. Short shots were fairly common because the mold wasn't vented very well.

The problem as I saw it was a restricted flow path, a mold not vented sufficiently, and the wrong kind of gate for running the mold automatic. I started to correct his problems by sizing the runner system. Since I figured we would end up with either a chisel gate or a curved tunnel gate, I recommended a full round runner be used because it works best with all kinds of gates.

The Diagnosis

The diameter of the runner that feeds the gate needs to be sized for the material. In this case, a .200-inch subrunner was about right. The main runner needs to be larger in diameter, so a .250-inch main runner took care of that.

The small end of the sprue, or the sprue O diameter, needs to be bigger than the main runner diameter, so a .312-inch O was a good fit. That allowed the nozzle orifice to be drilled out to .290 inch, or we could have used a replaceable nozzle tip of that approximate size. I generally don't use a reverse taper nozzle with glass-filled nylons, so I didn't have to worry about the inside orifice diameter or draft angle of the reverse taper nozzle.

The bigger sprue diameter won't necessarily slow the cycle down if the barrel heats are kept on the low side of the material manufacturer's recommendations. With nylons, I use the correctly sized sprue orifice not only for keeping pressure on the melt in the runner system, but also to keep the nozzle from freezing off. After sizing the runner, I usually move to the gates, but since we had to change the entire gate design I decided to save the gates for later.

Next in line was venting. On round parts, I like to use continuous or perimeter venting. In this particular case, a vent depth of .001 inch was about right for the glass-filled material. That is, of course, if I kept the barrel heats on the low side of the material manufacturer's recommendation, which is now achievable, since we have removed all the flow restrictions from the nozzle, sprue, and runners.

With the vent depth at .001 inch and the entire parting line vented, all that was left to do was to make it self-cleaning. We did that by going out .040 inch from the parting line, dropping into a .040-inch-deep channel to a racetrack, then venting the racetrack to atmosphere. Polishing the entire vent lip in the direction of airflow to an A1 finish made it self-cleaning.

In addition to the parting line vents, we needed to be sure the runner was vented. We vented down the sprue puller and at the end of each runner section. The only difference between part vents and runner vents is the depth of the vent. Runner vents are deeper than part vents because I like to feel a little bit of flash on the end of each runner to prove I am getting rid of the air.

We made the runner vents .003 inch deep for this material. The width of the runner vents is equal to the diameter of the runner being vented. We vented the runner by going out from the parting line of each individual vent with a land of .040 inch to a deeper channel to atmosphere just like with the part vents. Again, we polished the vent lips to make them self cleaning.

With proper venting, the injection speed could now be increased to a fast fill to achieve good fill and pack conditions, all without having to raise the barrel and nozzle heats.

The Gate

With everything else done, it was time to figure out what kind of gate we needed to run the parts automatic and keep them flat.

Since the part didn't have enough of a side wall to put a subgate in, we decided to stay with an edge gate. We could have used a rectangular subgate if we had had a little bit of side wall to work with, but it didn't work out that way. We decided the chisel gate would be the ticket for keeping the parts flat and also provide for automatic ejection. All we would have to do is put a sprue picker on the machine. This would also reduce the operator involvement by at least 50 percent, maybe even 75 percent, which helped in quoting.

Ejection

Because we were using stiff materials, we decided to use an ejection delay system with the chisel gate (Figure 2). Basically, with a chisel gate you have an ejector pin on both sides of the gate, one close to the gate on the runner side and one close to the gate on the part itself.

The ejector pin on the runner side is cut off about 1/2 inch short, so plastic will fill in the hole and make it into an undercut to hold the runner in place during ejection. When the ejector plate starts to move, the ejector pin on the part side pushes the part away from the gate and shears it flush with the edge of the part. The gate breaks cleanly at the part due to the notch sensitivity of the sharp corners and edges of the chisel gate design. The ejector pin on the runner side of the gate will begin to move after the ejector delay space has been taken up. At this point, the runner is ejected out of the undercut and lifted up and out of the way by the sprue picker.

The keys to the success of a chisel gate are the placement of the ejector pins and the delay space below the head of the runner-side ejector pin.

Of course the sharpness of the chisel gate design utilizes the notch sensitivity of the material being used to ensure that the gate will break cleanly away from the edge of the part. For the depth of the gate, I figured 90 percent of the wall thickness, which gave us .090 inch for the wall of .100 inch. The width of the gate for a part this size is twice the depth, so it would be .180 inch wide.

With the runner, sprue, and nozzle sized correctly and the chisel gates in place, we put the mold back in the press to see how it was going to run. With the flow path opened up we could lower the barrel heats to 540F from the previous 580F setting.

We were able to increase the injection speed now that the mold was properly vented. We raised the mold temp from 150F to 180F to help bury the fiberglass and get rid of as much molded-in stress as we could.

The results were great. The parts were coming out flat, the cycle time went down from 32 to 24 seconds, and the operator,replaced by a sprue picker, was given a different job.

For quoting purposes, the molder still costs the job out with 25 percent

of an operator since someone has to pick the boxes of parts up from the conveyor belt, but the machine is really running in fully automatic mode.