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October 25, 2002

6 Min Read
The Troubleshooter: Part 57: Too much of a good thing

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.

troubleshoot_gateFreeze.gif

Figure 1. Determining when gate freeze-off occurred on this part could cut a 15-second cycle to 8 or 9 seconds.

Looking back over the past year, my corrective action suggestions have been more diverse compared to the previous 10 or so years. Runner systems and gates still top the list, but waterline issues, material dryer problems, mold polishing, and design issues have all been addressed. In some instances, I’ve had all of these problems on just one mold.

My calls are coming from a more diverse clientele, too. I used to run into the guys who didn’t seem to want or need help, but they would send me parts just to satisfy the bosses. Now, in addition to processors, I hear from design engineers, OEMs, and moldmaking shops. These toolmakers ask for suggestions on problems their customers are having or, in some cases, they ask about trying to get the mold set up properly to avoid those issues.

The number one issue I am asked to address is undersized gates. Have a conversation with your toolmaker and the reason is clear: Naturally, moldmakers prefer cutting or burning to welding.

Materials like PE, PP, unfilled nylons, and PBT can get along with small gates, but amorphous materials such as PS, ABS, ASA, SAN, acrylic, and PC need bigger gates. Since the gate depth or diameter is based on the thickness of the wall into which we are gating, and since we should always gate into the thickest wall section of the part, it isn’t hard to calculate the depth or diameter of a gate.

For PE, PP, unfilled nylons, and PBT, I make the depth/diameter of the gate equal to 50 percent of the wall into which I’m gating. For ABS, acetal, ASA, acrylic, and SAN, I use a depth/diameter equal to 75 percent of the wall thickness. For PC, I use 90 percent of the wall for my depth/diameter.

No Freeze-off
Armed with these basic rules, I attacked my pile of packages this week. I always look for something that could happen to anyone but is overlooked or not studied because it doesn’t happen very often. Fortunately, I found one that offered me just such an opportunity.

The parts were low-density PE tubes with consistent .060-inch walls (see Figure 1, above). Subgates delivered the material to the two-plate mold’s cavities. These subgates weren’t causing much of a problem; in fact, according to the molder, the parts were molding quite nicely. However, the molding machine’s computer wasn’t tracing gate freeze-off, so the processor didn’t know exactly when to start screw recovery.

The cycle time could have been as fast as 8 or 9 seconds if gate freeze-off could be determined. Since the molder couldn’t verify this, however, it had to run a 15-second cycle to ensure all parts were as consistent as possible for weight, dimensions, and shrinkage.

In this rare instance, the gate was too big, so I suggested decreasing the gate size to ensure freeze-off and adjusting processing conditions to provide the same fill and pack pressures, at least until the gate freezes off; this would help control the weight and dimensions of every part. Based on the rule mentioned previously, for LDPE with a .060-inch wall thickness, the subgate diameter should drop from .040 to .030 inch. After these modifications, if everything else was in balance, cycle time should drop to a consistent 8 or 9 seconds and the gate should freeze off.

Oversized Runner
Since I was finished with gate sizing, I thought I’d look at the sprue and runner. The main runner was a short full round fed by a cold sprue. The full-round runner is always a good choice, and absolutely necessary for edge-gate designs, but with a subgate like this one, the molder also could have used a half-round or trapezoidal design. Remember, full-round runners work with all gate designs, but half-round and trapezoidal designs only work well with subgates and three-plates.

troubleshoot_subgate.gif

Figure 2. Subgate design for amorphous materials must incorporate a larger cross section from the runner to the gate.

The subgate design on this part is perfect for materials like LDPE. It could also be used for PP, but not for filled or amorphous materials. For these other materials, it would be more appropriate to use a subgate with a larger cross section from the runner to the gate, connected to the part with a .030-inch land. This subgate design for amorphous materials ends up looking like a small wart on the end of a finger when done properly (see Figure 2).

For LDPE I try to size the runner so the diameter is 1.5 times the thick section of the part. With these .060-inch walls, this meant the runner diameter could be as small as .090 inch. A .250-inch-diameter runner fed each gate on this part; if anything, the runner diameter was too big. I wouldn’t suggest a reduction in this particular runner diameter, but I would recommend this consideration for future molds.

With a .250-inch runner, the sprue O-diameter needed to be .312 inch. Since I couldn’t tell from the drawings what size the small end of the sprue was, I could only suggest the sizing that follows the rules of thumb I use. By sizing the runner this way, they ended up with some extra regrind, and this larger diameter could ultimately cause the cycle to slow down from the 8 or 9 seconds I calculated to 10 or 12 seconds. Here is a classic case of running a slower cycle than needed to give the sprue and runner time to set up, instead of running at the speed at which the part walls are setting up.

This appeared to be a nice two-cavity, low-volume mold, or possibly a prototype tool, so I was sure speed wasn’t the ultimate goal. I suspected good parts were what the molder was after, or a chance to gain experience on these particular parts before building a higher-cavitation mold.

TROUBLESHOOTER’S NOTEBOOK

Part:  LDPE tubes.

Tool:  Two plate, cold runner.

Symptoms:  Gate freeze-off could not be determined, forcing a longer cycle; too much regrind.

Problem:  Molding machine controller unable to track freeze-off; runner too big.

Solution:  Decrease subgate diameter to ensure freeze-off, adjust processing conditions for equal fill and pack pressures; Troubleshooter opted to leave runner size as is.

 

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