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October 12, 1998

9 Min Read
The Troubleshooter, Part 17:  Cosmetics

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.

Every so often I get a package with parts and runners in it that seem pretty easy to troubleshoot. Then, after getting into it a little deeper, I see more problems than I was asked to comment on in the first place.

This time the package contained ABS parts and a runner. The parts looked like oval-shaped covers that might go on a clock radio. Maybe they were covers for the battery compartment, if I were to venture a guess. The mold is a four-cavity, cold runner with a heated sprue bushing. The parts are to be plated after they are molded.

The customer had a mold filling analysis done to establish fill pattern and knit line locations, but the parts still had cosmetic defects-haze on the curved section and blush at the gate-that messed up the plating process. And the cycle time at 40 seconds was slower than the customer thought it should be.

It's amazing that the customer spent almost $5000 on a mold analysis and still didn't have usable parts. Somebody needs to figure out a mold analysis package that can be run quickly, costs no more than a couple of thousand dollars per mold, and provides useful information. Then we can build molds that'll run like they are supposed to-not like today, where we have to run barrel heats too high, injection speeds too slow, and the molds too cold because we don't have good information to get the molds built right in the first place.

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Runner Analysis
I started my review with the runner system. The runner was definitely undersized for ABS, especially plating-grade ABS. The existing runner was trapezoidal with a connector runner between the sprue and a straight-line main runner with a subrunner off each end of the main runner. The main runner fed into the middle of each subrunner for balancing purposes. The parts were edge gated, which is good for flatness, but the gate was a wedge shape, which is generally considered a high-shear design and not recommended for ABS.

I could see sink in the runner just inside the main runner where the connector runner joins it. That spelled out pressure loss to me; sure enough, the connector runner was undersized at .160 inch deep and .170 inch wide; this was especially bad since the main runner is .195 inch deep by .210 inch wide.

The heated sprue bushing orifice was a little small at .225 inch. I pretty much knew I would end up suggesting a larger size, but first I needed to size the runners.

I could see why the runners were undersized. The spacing between the main runner and the subrunner was only .100 inch. To enlarge either the main runner or the subrunner, we would have to remove steel entirely off one side of the runner and leave the .100-inch spacing between the runners alone, which could be done.

I suggested we make the main runner .250 inch deep by .275 inch wide. The subrunner would be .200 inch deep by .225 inch wide. The connector runner would need to be .300 inch deep by .325 inch wide. Since the heated sprue bushing was only feeding one runner, the circle diameter of the connector runner and the heated sprue bushing orifice would be .312 inch so the nozzle orifice could be .290 inch.

These changes made a big difference in the flow path. The molder was now able to run lower barrel heats, and lower barrel heats mean faster cycle times.

The runner needed to be vented to get rid of excess air that was being forced through the gates and overloading the parting line vents. Runner vents needed to be .003 inch deep (I like to feel the flash on the ends of the runners), as wide as each runner's width, then go out .060 inch from the parting line and drop into a .040-inch-deep channel to atmosphere. Polishing the vent lips to a high polish, in the direction of air flow, made them self-cleaning.

I told the molder to vent the runners at every 90° turn and to not forget to vent the sprue puller. Venting is one of the most often overlooked "absolutely necessary" procedures that needs to be done when building molds, especially the runner venting.

Sizing the Gates
That took care of the runners. Next we were down to the gate sizing. The original gates were way undersize for a part with a .100-inch wall thickness and 6 inches of flow from the gate to the end of fill. The gates were .060 inch deep by .075 inch wide. For ABS, that is too small, although maybe for PE or PP, they would be OK.

For ABS, the depth of the gate should be at least 3/4 of the wall thickness, which would make it .075 inch deep, and then twice as wide as it is deep, or .150 inch wide for a part this size. This is just the starting point for sizing an ABS gate. If you still get blush on the part when you start up again, the gates need to go a little deeper.

If you can't get the parts to fill and pack without raising the barrel heats or going over 1000 psi with your injection pressure, then the gate needs to be wider. (The gate depth controls freezeoff and the width controls the fill.) I don't like to make a gate any wider than the width of the runner, subrunner in this case, so I could go as wide as .200 inch if I needed to. I don't like fan gates in general, especially not for ABS parts. We started with the gate width at .150 inch.

The short subrunner over to the gate has a circle diameter of .200 inch, same as the rest of the subrunner. The land of the gate should be only .030 inch. A long gate land will cause a flow mark just inside the gate, so the land can't be over .030 inch for any material. All that is left to do now is get the parts vented.

Parting line vents are the second most overlooked part of tool building. I like to vent at least 20 percent of the parting line, sometimes even 30 percent works better. For ABS though, I think 20 percent will be just right. I usually put in one vent per parting line inch, and each vent is .200 inch wide. But this part is shap-ed just perfectly for perimeter venting and this is what I suggested the molder should use.

Perimeter vents for ABS should be .001 to .0015 inch deep all around the parting line of the part, starting on one side of the gate and ending up on the other side. In this case I thought perimeter vents of .001-inch-depth would be best.

Next, we need to go out from the .001-inch-deep parting line with a land of .060 inch, drop into a .040-inch-deep channel to a racetrack, then vent the racetrack to atmosphere. Then we finish up by polishing the vent lip in the direction of airflow, to a high polish, to make it self-cleaning.

After I finished troubleshooting the molder's problem areas, he made the changes and put the mold back into the molding machine. He made the changes to the barrel heats (the melt temperatures actually) and ended up running the plating-grade ABS at a melt temperature of 425F instead of the 510F he had to use before we made the changes.

With the mold properly vented, he was able to speed up his injection without getting burns on the parts. Since the parts weren't warping, the mold didn't have to run at 50F this time. He warmed the mold up to 90F, which improved flow properties of the ABS and reduced molded-in stresses.

Cycle Time
The last item on the list was the cycle time. For ABS being run in a two-plate mold, I take the wall thickness times 250 to get a target cycle. In this case, that would be .100 inch times 250, which would be a 25-second target cycle. The cycle had been 40 seconds, but after the flow path was opened and the melt temperatures were brought down, the 25-second cycle ran nice looking parts without any warpage or cosmetic defects.

The proof for all we had changed was when the parts were sent to the plater. The percentage of rejects for plating defects went down from more than 20 percent to almost none. There will always be a few rejects in the plating process, but at least the plater didn't have any molding-related problems this time. The advice I gave the molder was that if he had any future problems with plating ABS parts, such as plateout in the mold, which will cause a haze on the molded part that shows up when it is plated, then he could try vacuum venting to get rid of any more excess air in the mold.

Many commercial vacuum venting units are available today, and they all seem to work quite well. It's a little bit of the old diecast days creeping back in on us, but it works, and it is necessary when working on parts like concave mirrors, lenses, or any other parts that need very glossy surface finishes.

Plating grades of ABS are usually just very "clean" versions of regular ABS materials. Some of the additive package used for regular ABS materials is eliminated to produce molded parts that can be more easily plated.

Some of what is eliminated are the lubricants used for enhancing flow properties and helping with the ease of ejection for the molded parts. It is for this reason that the mold itself and the processing conditions must be optimized. This is a good example of how plating-grade ABS parts can be optimized, but the same techniques can be used to help regular ABS parts look and run better as well.

TROUBLESHOOTER'S NOTEBOOK

PART: Oval-shaped speaker covers.

MATERIAL:ABS parts that will be plated.

TOOL:Four-cavity, cold runner with a heated sprue bushing.

SYMPTOMS:Haze on the curved section and blush at the gate that disturbed the plating process; slow cycle time. A mold filling analysis did not indicate these problems.

PROBLEM:Undersize runners and gates, poor venting.

SOLUTION:Enlarged runners and gates, vented parting line and perimeter, decreased melt temperatures.

RESULT:Cosmetics improved, percentage of rejects from plating went to almost none.

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