The Troubleshooter, Part 46: The elusive cure for surface defects

Every two or three months I am invited to join a roundtable discussion with the management group of an excellent molder here in Iowa. Mike is the toolmaker, Ron runs the molding operation, Rich heads up purchasing, and Ken is the customer, pretty much accepted by everyone as the boss. Our salesman is Dave and he sets everything up for these high-level meetings.

     It was time to meet again and this month we sat down to eat pizza and work on the problems this molder had encountered since my last visit. We discussed what diameter sprues and runners should be and what the gates should look like on molds under construction. Napkins were filling up with notes and sketches. Mike is a great toolmaker and usually all I have to do is suggest something and he takes it and runs with it.

     But today Mike and I were both challenged by the others to explain why surface splay was occurring on one of the molder's thicker parts. It's about 3 inches in diameter, 2 inches thick, and each part has three cored-out holes. The material is a PC/PET polyester blend colored black and the parts are edge-gated directly off the subrunner (Figure 1). The problem actually had already been solved, but Ron, Rich, and Ken wanted to see if Mike and I could find the solution and eliminate the splay (Figure 2).

Testing the Troubleshooter
The challenge came at the end of our lunch meeting, so we adjourned to the molding shop. We assembled in the supervisor's office where Mike and I viewed good and bad parts. Both sets of parts were still attached to the runner, and from what I could see both looked the same as far as sprue, runner, and gate sizes were concerned.

     Mike, being a toolmaker, picked up both runners and looked them over carefully. He asked about the processing conditions, which is just like a toolmaker (always sure the problem can't be in the mold). Ron, being the molding person, asked if the material was the same as before, and Rich joined in since he orders the material. (Also, just like a molder and a purchasing person, it must be the material.)

     I had heard enough. I knew that Ron and Rich knew the answer and were just being devil's advocates in an attempt to confuse us. Ken was getting a big laugh out of our efforts. He smiled and waited to see if Mike and I could determine what was done to fix the problem.

     I excused myself and went out to the machine where the mold in question was running, and producing good parts. I asked the operator what was done to correct the molding problem and he responded by saying, "What problem?" Usually going right to the source is a good way to solve problems, but not this time.

      I went back to the office and Mike was all smiles. It appeared he had found the answer, or he got pretty close. My concern grew because I didn t want to be the only one in the dark. I asked Mike what he found and he started to spill the beans. Ken and Rich said I had to figure it out on my own. I was beginning to wonder if a free lunch was worth all this aggravation when Ken bailed me out. He said Mike figured out that it had something to do with the sprue.

     I had been looking at the sprue and runner and honestly didn t see anything that would have caused the splay on the parts. The main runner and subrunners had .380-inch diameters, which is not good from a pressure loss standpoint. The main runner should be a larger diameter than the subrunners. For this material the main runner could easily be .300 inch and the subrunners could be .250 inch. This would enhance flow and eliminate pressure loss in the runners. But pressure loss was not the issue. 

I was looking for something that would cause splay on the part itself. Something like a drying problem or a restricted gate would do it, but the dryer was working fine. The subrunner feeds directly into the part so that left the gate out of the equation. I looked for splay in the runner and could see a little bit, but not nearly as much as I could see on the parts.

The Solution
We were getting down to crunch time when Mike pulled out some verniers and measured the sprue O diameter and nozzle orifice vestige on both the good and bad sets of parts. Both were easily within acceptable limits for nozzle orifice to sprue O diameter sizing, but the nozzle orifice vestige for the good parts more closely matched the sprue O diameter than it did for the bad parts.

     The answer was in the nozzle orifice and how it matched up to the sprue bushing. I guessed that the viscosity of the material, and the injection speed and pressure required to fill and pack out the sinks on these thick parts, were pushing the envelope on the processing conditions and caused the surface splay on the bad parts.

     I could have looked at material drying as the culprit, or metal in the nozzle, or any number of things before I came to the nozzle orifice as the problem. In theory the nozzle orifice should be 10 to 20 percent smaller than the sprue O diameter, and we were certainly within that window. I guess this is why I like working with molders and toolmakers. We learn something new every day. This is definitely a problem I won t forget and when I do my second book it will have to be included.

     With the solution to the problem, I reviewed all the basics with everyone assembled. The runners were oversized, but too big is usually not a problem unless the injection unit is too small for the shot size (that was not the case here). With runners it s undersizing that hurts most of the time. Oversizing works better in most cases based on the length of flow and volume requirements, along with material characteristics, such as viscosity and filler content.

     The subrunner feeding directly into the part is not a problem either as long the process allows for extra hold time. This lengthens the cycle but the thick-walled part had a fairly long quoted cycle time anyhow.

     The smaller sprue O diameter feeding the larger-diameter main runner was the only real problem. This condition creates a flow restriction and makes the nozzle matchup critical for the orifice size and design of the nozzle. I reminded everyone that amorphous materials should have a full taper nozzle feeding material into the sprue bushing, not the general purpose nozzle currently in use. I told everyone that it was my opinion that the nozzle design was causing more of the problem than the undersized nozzle orifice.

     The problem was fixed by opening the nozzle orifice up from .260 inch to .335 inch, but changing from a general purpose nozzle to a full taper nozzle should do just as good a job and would provide a more permanent fix.

     Ron, Rich, Ken, and Dave suggested that I was trying to cover up the fact that it took me longer to solve the problem than it took Mike. Perhaps, but I hate to put a Band Aid on a problem when a little more effort will fix it forever.

     I haven't been back to take part in another discussion, but when I do I want to see if any of my suggestions were followed to correct the sprue and runner sizing and install a full taper nozzle. If Mike has any say, I'll bet the changes will be made and that the parts will run not only defect free, but with a faster cycle.