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.

Ireceived a package this week from an engineer at a company I haveworked with for many years. The part in the box was a cover witha runner attached that had a couple of problems. The first problemwas a line of splay oriented in line with the gate. The secondwas blush at the gate. In addition, the enclosed letter indicatedthe company's molder was also experiencing screw slippage on themolding machine, which lengthened the cycle.

I looked at the runner first and could see the sprue O diameterwas almost big enough to be effective, but I just knew I wouldsuggest they open it up a little bit. The only reason I like biggersprues is so the nozzle orifice can be opened up to prevent shearand pressure loss, and to allow the reduction of barrel melt temperaturesdown to the material manufacturer's recommendation.

The sprue O diameter was .285 inch, but was feeding a .365-inch-deeprunner. The molder should open the sprue O diameter to at least.375 inch so the nozzle can be drilled out to .350 inch. Thiswill provide an opportunity to lower the barrel heats, which Isuspect is what is causing the screw to slip during recovery.

Second, I looked at the runner design and saw another opportunitystaring me right in the face. The part, which was edge-gated,also had a trapezoidal runner with the gate coming off its siderather than its center. This arrangement almost always causesblush at the gate and often contributes to anomalies in the partsuch as flow marks and differences in gloss levels.

For this problem, the runner should be full round with a diameterof .350 inch, and the gate should be moved to the center of therunner. This change in runner size, in conjunction with openingup the sprue and nozzle, would most likely help get rid of thesplay.

Next, I looked at the gate and it actually looked pretty closeto what it needed to be for this part. The gate depth was .100inch on a .120-inch wall-perfect for this part and this material.This material, by the way, was one of those polycarbonate alloysthat is blended with polyester. My experience told me that itwas shear sensitive and, since I could see blush at the gate andwhat looked like shear splay on the top of the part (about sixinches out from the gate), I suspected shear at the gate was theproblem.

I could have recommended slowing the injection speed down to getrid of the blush and the splay, but I preferred going back tothe basics and getting the shear out of the gate instead. Thegate depth was OK so I checked the width of the gate and foundit to be .350 inch-312 times wider than it was deep. That wasalso OK. Since the width of the gate controls the filling or volumerequirements of the part, I would have to say that this was certainlywide enough.

I then looked at the land of the gate, which is where the plasticjumps across from the runner to the part, and I could see theland length was much too long for this material. In general, Irecommend that the gate land should be one-half the gate depthbut should never exceed .030 inch for any material. The land lengthof this gate was .100 inch. A long gate land like this is whatcauses many cosmetic defects on parts of this type and in materialssuch as this polycarbonate alloy.

I looked the part over for other indications of problems but didn'tsee any. Actually, I was looking for venting problems by checkingthe corners to see if they were trapping air. This is easy tospot because as trapped air is compressed it heats up and burnsthe material. I usually see dullness in the corners first, andthen a grainy surface; in the worst case I see the black burnsmost people are likely used to. Most materials burn with a blackmark, but some materials, such as acetal, actually develop a whiteburn. Fortunately, in this case I didn't see any venting problems.We had enough problems in the nozzle orifice, sprue bushing, runner,and gates to work on.

Running Clean Parts

I wrote up my review of the part and runner and faxed it tothe engineer who requested my help. An hour later I got a callfrom him telling me he had already sent the suggestions on tothe molder and the changes were being made in the next coupleof days. He said he would keep me up to date on their progress.

The next day I got a call from the toolmaker, and his first questionwas whether or not making the sprue bushing bigger would slowdown the cycle. He said he had always been told that making thesprue bushing as big as I had recommended would be a mistake.I told him that if barrel heats weren't reduced, as I had suggested,then he could be right. I told him that the manufacturer of thismaterial recommends melt temperatures from 475 to 540F; and ifthey kept the barrel heats at the low end of this recommendation,which they could now do since they opened the flow path, thenthe cycle would not be slowed.

The second question he asked had to do with the .350-inch nozzleorifice size I had suggested and whether it could cause stringingor drooling. Again I told him that if the processing people keptthe nozzle temperature down, this would not be a problem. Apparentlythis satisfied him, although I thought these were strange questionsfor a moldmaker. I would guess he'd been around molding shopsfor a long time and had heard the processing people talk aboutthese issues, so he was just trying to keep them out of trouble.

A week later I receivedanother box of these same parts and was pleased to see the problemshad been resolved - not exactly as I thought they would, but theparts were OK to use again. The difference was that I thoughtthe splay on the top of the part was shear splay due to the longgate land. The molder found that the splay disappeared when thebarrel heats were lowered. I suspect the shorter land still helpedto eliminate splay; otherwise, the longer land would have causedshear splay when the material heats were lowered and the viscosityof the material increased-same effect, just different explanations.It doesn't really matter since the splay went away and the partslooked very good. Compared to the first parts I received, thesenew parts were a 100 percent better.

The big improvement for the molder was that with the lower heatshe wasn't getting any screw slippage during recovery. That spedthe cycle back up to where it had been before barrel melt temperatureswere raised in an effort to eliminate the blush.

Yes, you can get rid of gate blush by raising the materialtemperatures, and even by heating the mold up a little bit, butoften the downside is cosmetic defects. When melt temperaturesare high, you have to lengthen the cycle to drag the extra heatout of the mold steel.

This is exactly why I always suggest first optimizing the machinenozzle, sprue size, runner sizes, and gate sizes, as well as ensuringadequate venting for the runner and parts.