When a console component for an ATV ripped, the Troubleshooter looked at material degradation, design, and processing as possible culprits. The problem lay in all three.

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 technical programs manager for resin distributor Channel Prime Alliance. Before his present assignment, Bob managed a molding operation for 25 years. You can reach him at bob.hatch@channelpa.com.

Material analysis revealed what a partial component could not.

I found a package on my desk last week that contained a section of a console that goes on an ATV. I looked the part over, measured it, burned it, and sniffed it—you know, the usual stuff. I saw a crack or rip on the side of the part that was marked with a piece of masking tape and three arrows pointing to the crack. From what I could see, this crack was the only problem with the part.

The molder had a note taped to the part asking for a review of the cracked area. He wanted to know if this rip was a problem attributable to the molding department or perhaps a mold or part design issue. Last but not least, he wanted to know if the ASA material was the right choice for this application.

This was quite a lot of items to look at, but not out of reason. First, I looked at the design and found it to be a pretty straightforward section of a dashboard. It had drink holders and recessed areas to hold small objects such as sunglasses, fish hooks, shotgun shells, and so on. The wall thickness was uniform at .090 inch and the ribs were radiused. The design looked pretty good to me. Next I looked at the part for the choice of material, which was a standard grade of ASA. I suppose ASA is a good choice here, but it doesn’t provide the chemical resistance that I think a part like this might need, especially during the cleaning of the ATV. On the plus side, ASA is a fairly rigid material with good impact and tensile properties. The real advantage of ASA over a material like ABS, for instance, is its excellent UV resistance.

ASA has been called “weatherable ABS” for a long time now. The “weatherable” designation comes from the rubber impact modifier in the ASA being an acrylate rubber, while ABS uses a butadiene rubber. The acrylate rubber stands up better to UV attack. This gives me enough confidence to say that ASA is certainly a good choice for a part like this that will spend a lot of time outdoors. If they ever decide they need better chemical resistance in this application, I would look at nylon or a copolymer polypropylene.

The last area of review the molder requested was how the molding department handled the ASA. In general the part looked pretty normal for ASA molded in a part with .090-inch wall thicknesses and a textured surface. I suspected the texture was there to cover knitlines from multiple gates or other surface cosmetic problems, possibly from not enough venting. However, I could not find any gates to review because the section of the part I was sent had been cut from a full part on a band saw (a strong case for always sending me a complete part with runner attached).

Physical Abuse

I checked the impact and tensile properties by closing the part in a vise and pushing and pulling it in a vigorous manner to see if I could break it. I then used pliers to grab, twist, and bend the part. To test the impact, I used a 20-oz ball peen hammer, and while the part was clamped in the vise I bent the part over as far as I could, then started lightly pounding on it, gradually tapping harder and harder until I was satisfied that it was or was not going to break.

Visually, I did not see any moisture or shear splay. All I could see was a slight graininess in the glossy area near the parting line. The only explanations for this graininess that I could think of were that the material this part was molded out of had too high a percentage of regrind, the barrel heats were too high, or air was trapped in this area from lack of venting.

With the design and material cleared in my mind, all I had left to work on were processing issues. To confirm my suspicions of something being wrong in the material mix, I only needed to send a piece of the part to my favorite lab—Marsh Tech in Minneapolis, MN.

What I like so much about Marsh Tech is that not only will they do a test for oxidation and degradation on my sample—which would tell me if the molder was causing the problem—but also Erlene Marsh or her daughter, Angie Macintosh, will render an opinion on what they think might be responsible for the cracking. I next-day-aired the sample chip to them.

While waiting for the lab report to come back, I kept looking for other causes of the cracking problem. I looked at the cracked or ripped area with a magnifying glass and could see the inside of the part wall. In the area of the crack, the material was rough in texture, not glossy. This told me the crack was not caused by a chemical attack. The part did not seem unusually brittle and had pretty good impact; same for the tensile properties. The surface was not a deep, rich, jet black, but I figured the texture on the part surface was causing this washed-out look. It had more of a utility black look than the jet black I had expected to see.

Usually when I send a sample piece off for analysis, the project gets put on hold until I get an answer back from the lab. In this case, I couldn’t seem to set it aside; something kept gnawing at me about the cracking on the side of this part. When I did my crude tests for tensile with my vise and pliers, I did not get the part to crack or split anywhere else without far more physical abuse than is usually necessary to check these details.

A-ha

Then it hit me like a ton of bricks: If the molder ran these parts a little bit undersized, and if the mounting brackets for this section of dashboard had bolt holes for mounting instead of bolt slots, then I could explain the cracking.

We all know that plastic grows when heated and shrinks when cooled. Using slots instead of round holes for the mounting tabs would allow for expansion and contraction of the plastic. If the part were held rigidly instead of sliding, it would break somewhere between the bolt holes. This could be the explanation for the part cracking, especially if I were to find out if this part went into the cold months unbroken and came out broken.

This was great! I had a cracked part due to some form of degradation, or maybe I stumbled onto a problem caused by these guys using holes instead of slots for the mounting tabs. It could also mean that the dashboard part was molded undersized; it would not take a lot of effort to accomplish this. Just running a hotter-than-normal barrel heat or mold temperature, or even a lower packing pressure could cause the part to shrink more. A faster cycle time would certainly cause the part to shrink more.

If the mounting tabs would not accommodate expansion and contraction forces, the part would break somewhere, either at the tabs themselves or somewhere in between. If the part was molded to size or even oversized, the expansion and contraction forces would probably not be able to stretch the part to the point of breaking, but an undersized part could certainly be stretched to the breaking point. I got my answer from Marsh Tech the next day: “This material has been compromised.” Erlene said the analysis proved something had compromised the material, but she had no way of knowing if it was too much regrind or some kind of hydrolytic or thermal degradation. She said that no one could test what percentage of regrind any sample of material had in it. (This is what I like about Erlene—she tells me the way it is.)

What had this new information added to what I already knew? We now understood that the material had been degraded somehow, and this would reduce the tensile strength and impact of the material. My crude tensile and impact tests told me otherwise, but I trusted the lab results more than my vise and pliers.

A Satisfied Customer

With the material analysis in hand and a good grasp of the other details in my mind, I placed a call to the molder and passed along my thoughts. It did not take me long to figure out I was doing all the talking. I asked him what he thought, but all he said was that he would get back to me and please do not call his customer.

Two days later the molder called back. He thanked me for the material analysis and said that my suspicious nature had changed his procedures in the molding shop; excessive regrind, deviations from the prescribed setup, and drying issues would no longer be a problem. He added that he had sent the mold to the toolroom to make the bolt holes into bolt slots, in the direction of expansion and contraction, naturally.

He went on to say he was not sure, but he thought the cycle time may have been speeded up a bit last year, at a time when one of the engineers was experimenting with running colder mold temperatures in order to achieve faster cycle times, but he couldn’t be sure whether or not this part had been involved. That would explain how they ended up with some undersized parts; I would bet that this part came from one of the test molds. It is not often that I get confirmation on all my suspicions, but this time it looks like I hit the jackpot.

TROUBLESHOOTER’S NOTEBOOK Part: ASA console section for an all-terrain vehicle (ATV). Tool: Two plate, cold runner. Symptoms: Massive crack through the part. Problem: Material was compromised; part was designed with bolt holes for mounting instead of slots, which prevented the part from adjusting between hot and cool conditions; parts possibly undersized following experiments with running a faster cycle. Solution: Change processing procedures—to improve the material, reduce excessive regrind and deviations from the prescribed setup; in the mold, change the bolt holes to slots in the direction of expansion and contraction.