Sponsored By

July 18, 1999

8 Min Read
The Troubleshooter, Part 31:A lesson in material substitution

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.

The UPS person walked through the door with a pretty small box that didn’t weigh very much. In it was the part that I knew I would get sooner or later.

I took the part out of the box and looked it over very thoroughly. I even put the part under a magnifying glass to see what I could find. The note in the box said the part was the neck piece for a medical device and the material was an impact acrylic.

The part had an edge gate and where the edge gate had been trimmed I could see a whitish mark, not just at the gate but around it also. It looked a little like a three-leaf-clover pattern around the gate except that it was more jagged than rounded. I’m not used to seeing this whitish look at the gate of an acrylic part; it is usually more clear than anything else. The part also had an overall hazy look to it, not the clarity with depth that I usually see. You know that clarity; it’s like looking into the paint job of one of those custom cars at a car show.

I could see cracks in the part at various places, but the note went on to say that the cracks in the part were a result of impact testing the part by throwing the part at the floor. Not very scientific, just throw it and see if it breaks. I recognized the name of the company from the label on the box as belonging to two brothers I have worked with many times in recent years.

True Believers

These guys have tried everything I have ever suggested to them and fortunately they have always been successful. They even cut my “Troubleshooter” articles out of each month’s issue of this magazine, put them in frames, and hang them on the walls of their conference room. After more than two years’ worth of articles, they have used up almost all of their conference room wall space.

I only mention this about them so everyone will understand why I am puzzled about the problems associated with this acrylic part. They have been true believers in my usual suggestions.

I have no doubts that they have sized the sprue and runner correctly for the acrylic material. I just know the nozzle in the molding machine is a full taper design and the nozzle orifice is just 10 percent smaller than the OD of the sprue bushing. I know that the gate depth is 75 percent of the wall thickness and they gated into the thick section of the part.

Last but not least I know the mold is vented, as well as the runner—not just a few vents either, but they will have cut their vents in at one vent per parting line inch or they will have used a perimeter vent around the entire parting line. I convinced them long ago that runner vents should be .003-inch deep for easy flow materials and .005-inch deep for stiff flow materials and the parting line vents will be .0015-inch deep for acrylic materials.

So what is causing their problems? Is it a material problem? Possibly, but I doubt it because these guys only use prime virgin material from reputable manufacturers and they use their regrind up as quickly as they generate it at the press. Is it a dryer problem? Could be, but their auxiliary equipment is all fairly new and well maintained. They also check dryer functions with dew point monitors and check the material with a moisture analyzer as a quality check. One of the first things they learned from me is that you can’t run a molding shop without good equipment and a top maintenance program.

I’m scratching my head on this one. The moment of truth is at hand. I pick up the phone and make the call to them so I can ask the questions necessary to confirm the details I have just related. We chatted a bit, but nothing clicked. In addition to all the things I had already figured out, they said this was a part they have been running for several years without any problems until now.

That Elusive Change

So what changed? I went through each of the possible problems with them. They said they were using the same machine, same dryer, same processing conditions, same material, and they both agreed nothing had changed. Then one of the brothers said, “Wait a minute, we did have a material change recently.”

He went on to say that the impact acrylic they had been using was replaced by an improved version of the same material, from the same supplier. Needless to say, I picked up on this new information right away. I asked them what else they knew, but that was about it. I asked them what the old material number was and what the new material was and told them I would check it out with the supplier’s tech center and call them right back with what I hoped would be the answer to their problems.

I didn’t tell them then, but I knew the material manufacturer had changed the material numbers for this product line. But I had been assured by the manufacturer that nothing changed except the numbers. They told me these products would continue to have all the normal properties we had come to expect from this line of acrylic materials, plus they would have even better clarity than before and the materials would probably process even more easily.

I made the call to the technical center and got lucky enough to talk to one of the senior technical engineers who has been around long enough to be able to tell it like it is. I told him my story in a few words and he responded by telling me that he had been in several molding trials recently where customers had been doing drop-in replacements of this new product for the old product, pretty much like our customer.

He said they got great parts when they raised the barrel melt temperatures and mold temperatures a little bit over the old setup conditions. He suggested raising the barrel melt temperatures by 30 to 40 deg F and the mold temperatures by 20 deg F over the current settings. Raising the heats had worked so well for them in these recent molding trials that they had already changed the processing information on the data sheets to reflect these higher heats.

I thanked him for the insight and grumped a little to him about not getting this information prior to having our own material replacement problems with our customers. He said he was sure the corrected data sheets had been sent out, but he would fax me one of the new data sheets for this material just to be sure I had one. I received his fax a few minutes later that did show the higher heats he mentioned.

I must have missed these changes when I received the new data sheets on the enhanced product line. It would have been nice if they had pointed them out, not just hope I would see the differences on my own. But I should have seen the changes; I just didn’t look.

Now you can see what I meant when I said I knew this was going to happen some day. I have always tried to get molders to open the flow path of the material through their molds so they could bring the barrel heats down, which in turn often allows them to speed up cycle times and eliminate warped parts plus other heat-related defects.

I must have told thousands of molders to run their material melt temperatures at the lowest temperature recommended by the manufacturer of the material, which is what these brothers have been doing ever since I got their attention. What I didn’t count on was having a material manufacturer upgrade a material to get better properties without realizing our customers would now have to run higher melt and mold temperatures than were required previously.

Well, I certainly learned my lesson. From now on when I see a data sheet for any new or improved material I will look it over for any changes that might be there. I will also ask the manufacturers if these new materials reflect any processing condition changes over the older versions.

The good news is the brothers tried the higher heats and reported that everything went back to normal and the parts did look better than ever. The clarity was indeed better and even though they did raise the heats, they didn’t see any need to slow the cycle down since the parts were not distorting or warping and none of the dimensions had changed from the previous parts. Most molders are running their barrel heats too high anyway, so a problem like this would not affect them. It is only those molders who have learned to optimize their heats, speeds, and pressures that will get caught with this type of material change.

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like