To save face, the molder needed to match the actual cycle time of this thick-wall acetal part to the cycle he quoted. Improperly sized waterlines were the culprit.

A cycle sped up, a myth debunked—all in a day’s work for the Troubleshooter.

I received a part from someone I have worked with many times before. He was molding a copolymer acetal part—pretty thick at .275 inch. He said his problem was cycle time; the best he could get was 90 seconds. He had quoted a faster cycle but didn’t want anyone to give him grief about missing the correct cycle time on such a straightforward part.

His note started out, “Now, I know acetal cycles more slowly than other crystalline materials, but what am I doing wrong?” I knew right then that I needed to talk to this guy, and quickly. Why? Acetal doesn’t have to cycle slower than any other crystalline material, even parts with thick walls like this one. If anything, for a given wall thickness, acetal can cycle faster than many other materials.

I called the molder and asked why he was confused about acetal. His answer didn’t surprise me. He said the guy who trained him in the first molding shop he worked in told him he would have problems with acetal and to forget trying to get fast cycles because it runs slowly. So I paid a visit.

The 411 on Acetal

When I got there we did the typical molding shop routine of coffee and donuts, and then got down to business. First, we went to the shop floor and watched the molding machine run. After checking the nozzle, runner system, gates, barrel heats, mold temperatures, and anything else we could see from a couple of feet away, we went to the office for another cup of coffee, more donuts, and a quick review.

I started with acetal basics:

Runners. The runners don’t have to be huge. For parts with a wall thickness of .060 to .125 inch, I usually start with subrunners 1.5 times the part wall thickness. The main runner diameter should be .025 inch larger than the subrunners, and so on. For thick-wall parts, keep the main runner diameter as big as the part wall or you might have trouble packing it out.

In this case we didn’t have subrunners, so I suggested that the main runner be as big in diameter as the part is thick. This would make the full-round main runner diameter .275 inch, the sprue O-diameter .325 inch, and the nozzle orifice .300 inch.

Gates. The difference between acetal and other unfilled crystalline materials is that acetal gates need to be bigger—two-thirds of the wall thickness for gate depth. The gate land should always be half the gate depth but no longer than .030 inch, and the gate width should be two or three times wider than the gate depth. The width of the gate depends on the amount of material being injected through it. The bigger the part, the wider the gate. This would make the gate for this part .185 inch deep, .370 inch wide, with a .030-inch land length.

Drying. Acetal doesn’t need to be dried unless it has carbon black in it for color or if it’s regrind that has been sitting around for a week or so in opened containers when the humidity level is high. In these situations you can dry it at 180°F in a desiccant dryer to get it ready to mold. These guys were not drying the material because it was a natural color.

Processing conditions. Fast injection speeds produce a glossy finish on the molded part and slow injection speeds create a matte finish.

A 50°F mold temperature gives you a more flexible part and 180°F makes the part stiffer. This molder was using 80°F water for the front half of the mold and 75°F on the back half.

Barrel heats of 350°F to 375°F work pretty well for a 9-melt copolymer acetal such as we had to work with here. You can run higher melt temperatures if necessary so long as the material’s residence time in the barrel doesn’t exceed 3 minutes.

A backpressure of 50 psi is about right for not having to deal with any dispersion problems, and a screw rpm in the medium to fast range is OK since acetal does not mind being mixed, even aggressively. In fact, acetal works best when you use a higher-compression-ratio screw. A 5:1 compression ratio is my choice for a screw tailored to acetal specifications. Otherwise, a general purpose screw has to do. Sometimes you need to use extra backpressure to get the cold acetal pellets to melt completely or plasticate before entering the mold cavity.

That’s Not Cool

After everything had been reviewed, we needed to figure out why this molder thought his cycle was too slow for the part’s wall thickness.

Since this was a two-plate cold runner system, I multiplied 250 by the nominal wall of .275 inch and got a 68.75-second cycle time. Then I added 3 seconds for the size of the molding machine and got a target cycle of 71 seconds. (For an explanation of this calculation, see “Mastering Cycle Time Estimation,” December 2003 IMM.) Yes, his current cycle of 90 seconds was theoretically slow for an unfilled acetal part with a nominal wall thickness of .275 inch. All we had to do now was find out why.

It seemed time to move past the barrel and mold temperatures and runner and gate sizing and take a look at the cooling water in the mold. What I saw made me smile. This 24-by-36-inch mold had just one delivery line in and one return line out for the front half of the mold, and the same for the back half. Between these in-and-out points were a lot of rubber jumper hoses and quick-disconnect water couplings.

The problem that most often causes long cycle times was staring me in the face. When you run water through quick disconnects, the effective size of the mold cooling system is only as big as the smallest diameter of any hose, mold water channel, jumper, or quick disconnect in the mold. I measured the inside diameter of the quick disconnects and could see the effective size of this mold cooling system was only 1/8 inch in diameter—not the ½-inch diameter of the rubber hoses or the 7/16-inch diameter of the mold’s waterlines.

The result was very little heat removal from the mold steel. The water flow was no doubt laminar instead of turbulent, like it should have been. The cavities and cores were running hotter than the water controller setpoint.

My approach to waterline troubleshooting is pretty simple. I pick a water circuit and put one hand on the delivery line to feel how hot it is. Then I put my other hand on the return line for that same circuit and determine if they are pretty much the same temperature. If so, then the water flowing through the mold circuit is not pulling out very much mold heat. There shouldn’t be more than a 5 deg F increase in temperature anyway, but you should be able to tell a 5 deg F difference just by holding both lines. If the temperature difference is more than 5 deg F, then the length of that water circuit might be too long (no more than 54 inches), the line could be plugged up, or, in this case, the effective size of the cooling system at 1/8 inch could be the culprit.

Gauges that measure gallons per minute (gpm) of water flow are a good way to check for water cooling problems or restrictions to flow. So, as a double-check, we installed a dual gpm flow gauge and digital thermometer in one of the water circuits. Then we checked all the water circuits. We found they were all down to less than 1 gpm flow, and the water temperature increase from inlet to outlet was much more than 5 deg F; it was more in the 50 deg F range. No wonder he was having cycle time issues!

Gone in 60 Seconds

The mold was immediately pulled out of the machine and hauled over to the toolroom. This caused a commotion and soon almost every toolmaker had gathered around to see what was going on. The toolroom supervisor became the spokesman for this group and suggested he could operate on the quick disconnects and open the flow path diameter from 1/8 inch to ¼ inch. Doubling the diameter would quadruple the water flow, which would be nice, but still not enough to get us where we needed to be.

Here’s what I recommended:

• Throw the quick disconnects in a drawer and leave them there.
• Install hose barb connectors to the mold water channels to provide a ½-inch inside diameter (ID) to deliver water to the mold.

• Hook up the ½-inch-ID rubber hoses to these ½-inch-ID hose barb connectors to work with the 7/16-inch waterlines in the mold. The trick here was to deliver as much water to the mold water channels as they could handle.

• Use flexible, clear PVC hoses that slip right on the hose barb connectors to show the water flowing through the lines. These clear PVC waterlines are great for troubleshooting water problems, but replace them later with normal ½-inch-ID rubber hoses.

• Inject red dye into the clear waterlines with a syringe to watch the red dye go into the mold and then check how long it takes for the red dye to come out the return line. From this procedure we can tell if the water flow is turbulent or laminar. Turbulent water flow extracts heat from the mold steel four times faster than laminar flow.

We removed the rubber hose loops, eliminated the quick disconnects, and put the mold back in the machine just in time for the night shift to start it up again. They started up the molding machine as if we had not changed anything. We started getting good parts right away, but the cycle time was still 90 seconds.

We started speeding up the cycle by reducing the cooling time until we started to see a little end-to-end warpage. At this point we added a couple of seconds back to the cooling time and parts became flat within the tolerance range given by the customer.

Where did the cycle time end up? We left the shop that day running the part in 60 seconds. This is another case of realizing that a target of 250 times the wall thickness is just a starting point for optimization efforts. After someone gets used to optimizing the process, it is not unusual to beat this target by as much as 20%.

We could have gotten a few more seconds off the cycle time if we had wanted to tweak the material flow path or hook up an auxiliary water pump to the chilled water to increase the pumping pressure of the water flowing through this mold. (Sometimes it’s necessary to install auxiliary pumps to the water control unit or the chiller to get turbulent flow. This extra pumping pressure can be important when increasing the diameter or effective size of the water circuits.) They can do that later. These extra seconds can sometimes be pretty hard to come by. We got the easy seconds off the cycle and helped the molder maintain his reputation with the people he works with, which was the most important part of this optimization procedure.

TROUBLESHOOTER’S NOTEBOOK Part: Thick-wall copolymer acetal part. Tool: Two plate, cold runner. Symptoms: Cycle was slower than quoted to customer. Problem: Mold cooling water was severely restricted. Solution: Replace quick disconnects with ½-inch-ID hose barb fittings.