The Troubleshooter, Part 19: Disk gates for big parts

This article continues our series of troubleshooting reportsfrom one of the leading on-the-spot problem solvers in the moldingindustry. Bob Hatch is manager of technical service and customersupport for Prime Alliance, the Des Moines-based resin distributor.Before his present assignment, Bob managed a molding operationfor 25 years.

I received a part this week that is .750 inch thick and about20 inches in diameter (see a section in Figure 1). It has a diskgate fed from a cold sprue. The molder is having trouble fillingand packing this structural foam polypropylene part without voids,and the part is warping.

I see problems like this quite often but usually the problem iscaused by the sprue O diameter being too small to handle the volumerequirements of big parts like this. Not so in this case. Thesprue O diameter is .375 inch, just right from what I can see.

So what is causing the problem? My first clue came from the setupsheet. The injection pressure of 1200 psi is much too high fora part this thick. The melt temperature of 450F is also high forpolypropylene, especially with the 3-minute cycle being used.The high barrel heats are causing the long cycle time.

Why then the high injection pressure? Were we pushing againsta lot of air trapped in the mold? No, I didn't see any evidenceof that. So what is the problem? I looked at the flow path andthere it was. The depth of the disk gate was very shallow andtotally restricting the flow of material through that area. Thetoolmaker had used a core pin under the sprue to form the centerhole of the part and by adjusting the length of the core pin thetoolmaker created the depth of the disk gate (Figures 2 and 3).Toolmakers-they're so predictable.

Because the core pin forming the center hole was too long, thedepth of the disk gate was only 15 percent or so of the wall thicknessof the part, which was easy to measure just by looking at theinside diameter of the hole in the part.

I called the molder and told him what I had discovered. His responsewas typical. He said the depth of the disk gate was the same ashad always been used for other parts he'd successfully molded.His biggest concern seemed to be how he would trim the gate ifhe made it any thicker.

I told him that first of all, the depth of the gate would havebeen acceptable if the part had been only .100 inch thick. Butthis is not okay for a part .750 inch thick. Second, I suggestedhe should worry more about getting the part molded in the firstplace, then figure out how to get the gate removed. I suggesteddrilling or punching as two fairly common methods being used forsituations like his. He wasn't very happy about the changes hewas going to have to make, but said he would make thm anyway,and let me know how it turned out.

While I had him on the phone, I told him he might try anotheridea. I suggested that, as long as he was going to cut the endof the core pin off anyway, he might want to use an old diecastdesign and leave a point or spreader on the end of the pin (Figure3). Doing so would leave a spreader point in the middle of thethick part of the sprue, which would taper down and out to theoutside diameter of the pin. I told him I wanted the depth ofthe disk gate to be .375 inch or half the wall thickness of thepart, and the spreader point would stick up into the sprue fromthere.

I mentioned that I like to use this design to "core"the sprue out in the thick section so the sprue won't have a tendencyto break away from the part as it is ejected out of the mold,which, of course, results in the sprue sticking in the sprue bushingand the molder having to pound the sprue out of the sprue bushingwith a brass rod and a hammer. He liked the idea of the spreaderand thought he would definitely use it in the redesign work he'dbe doing on the mold.

After a week or so went by, one day a package arrived with severalparts in it. One of the parts was the one I had helped out on,but I had not seen the other parts before. I called the molderto see what was going on. When I got him on the phone, I almostcouldn't get a word in edgewise. He was pretty excited about thechanges I had helped him with. In fact, he had such good resultsfrom the changes to the mold we had worked on that he had modifiedseveral of his other disk gated molds as well.

The other parts in the box were parts from the other molds hehad worked on. Fortunately, the molder had the foresight to leavethe sprues attached so I could see the flow path of the materialfrom the barrel through the nozzle, sprue, disk gate, and intothe part itself. He asked me to look at several parts and tellhim how they looked.

The polypropylene part we had worked on looked pretty darn goodbut still had a slight amount of warpage. I told the molder totry running the barrel temperatures a little bit lower, somewherearound 375F instead of the 450F he was using. I went on to tellhim to try raising the mold temperature from the current 70F to100F, still inject slowly, and keep the injection pressure around800 psi with a hold pressure of 800 to 1000 psi. I want the holdpressure higher than the inject pressure so we can fill the airlessvoids as they form with material we're pushing into the cavitywith the hold pressure, something we don't do with thinner parts.I still want the foam to do its thing, but I want to control thefill and pack with the machine.

One of the other parts was also polypropylene but the wall thicknesswas only .125 inch. On that part I told the molder to use thesame barrel and mold temperatures as before. But since this wasa thinner part, the hold pressure would be more like 400 psi.

Another part had some blush and flow lines around the gate area.For these parts I told him he would need to make the gate deptheven deeper. Some materials, such as ABS, acrylics, and polycarbonates-evenacetal for that matter-need deeper gates. The molder said thepart I was talking about was molded out of a polycarbonate/ABSblend so that seemed to fit. And he said he would make that changethe next time the mold was out of the press.

What started out to be a fix for one part evolved into changesfor many parts. In every case, the molder was eventually ableto correct not only the warpage problems but gate blush and slowcycles as well. These were easy to fix because the center holeswere all small diameters, such as the .500-inch to .750-inch sizes.These changes will also work for larger inside diameter centerholes as well. Just be sure the flow of material is not restrictedfrom the nozzle into the cavity area of the mold itself.

If you need to keep the inside diameter of the center hole perfectlysmooth, then just gate around the hole on the top surface of thepart. It will make the surface rough around the center hole afteryou trim the gate off, but you can always texture the nearby surfaceto make it blend in.

Disk gates are great when you're trying to keep a part round ina single cavity mold. But if the disk gate restricts the materialflow, causing you to raise your barrel temperatures and injectionpressures, you can end up with lots of problems. Fortunately,this time the problems were easy to correct.