Sometimes it’s the little things that cause big problems.

I could hear a big truck coming down the snow-covered road in front of my office. The driver had become a friend and, like all of the others, likes to get my packages to me as early as possible so he can get a cup of coffee and scratch the ears of Jake, my black Labrador Retriever, before hitting the road again.

My package this day was a small one containing two parts and a cold runner being fed by a heated sprue bushing. I knew this molder well since we have served on several SPE groups together. We have even participated in many seminars, but our topics are usually different. He speaks about materials and I discuss (what else) troubleshooting techniques.

The interesting part of this project is that this single nylon part had replaced 11 metal parts that used to be bolted, welded, or bonded together. This had been a big cost savings for the molder’s customer but there were still a few bugs to work out—mostly cracks in the areas where the bearing section is attached to the mounting bracket.

These cracks would show up right after molding most of the time, but sometimes they wouldn’t appear until after assembly. The perception was that if the part was going to crack, it would do so sometime in the two-week period right after molding.
I looked at the parts and runners under my lighted magnifier and could immediately see many sharp corners and edges that had cracked.

Checking measurements
The dimensions of the runner, gates, and heated sprue cone looked pretty good. The full-round runner was fairly uniform at 0.110 inch, the trapezoidal gates were 0.040 inch deep by 0.060 inch wide, and the cone shape being fed by the heated sprue bushing measured 0.130 inch in diameter at the small end and 0.180 inch in diameter at the large end—just right for the heated sprue orifice of 0.125 inch.

The gating detail was curious. The tooling engineer had attached a block that measured 0.100 inch thick, 0.300 inch long, and 0.110 inch wide to the outside wall of the bearing section. If this was a tab gate, it was the biggest one I had seen on a 0.050-inch-thick wall. I made a note to ask the tooling engineer what the purpose was for this small block. Maybe they removed it after molding, or possibly it was used for positioning the part for assembly, but it did not really hurt the molding process, so I decided to forget about it for the time being.

I moved on to the gates and liked what I saw. As I mentioned before, the gates on each of the two parts had a trapezoidal design—basically a subgate. This design is one I wish more tooling engineers and toolmakers would incorporate into their designs, as it can be used with most materials and won’t let you down or cast doubt on your abilities. The dimensions of this subgate that fed into the block or tab and then into the part wall were more than generous enough for running unfilled nylon materials.

So far it looked like the sprue, runner, and gate designs would work well. An ejector pin attached to the main runner, just 0.050 inch away from the subgates, did give me some concerns, however. Since the subgate entered the part at a 30° angle, this close proximity of the ejector pin would not allow the subgate to pull out of the tunnel portion of the gate without being distorted during ejection. As long as the material heats were carefully watched, this might not be a serious problem. It just meant that automatic operation of the molding machine might be risky and should not be attempted.

I also made a note to remind the molder to be sure this mold was being run in a molding machine with a general-purpose nozzle feeding into the heated sprue bushing. The orifice in the GP nozzle would need to match the flow tube or bore diameter of the heated sprue bushing. This was not a mold that should be fed with a reverse-taper nylon nozzle.

Nothing to this point led me to think I had found the problem that had been causing these parts to crack.

Next, I looked closely in those sharp corners and sharp edges, finding 12 sharp edges and 10 sharp corners that were not radiused and only two sharp inside corners with any radius at all.

For my simple radiusing test, I held a medium ballpoint pen at a 45° angle and drew a line in each of the sharp areas. A double line, visible on the left part in the photo, meant insufficient radiusing and therefore impact and tensile weakness.

Finally, here was the reason for the part cracking! Fortunately, adding radius is not difficult for the toolmakers. The question was, how much radius should be added? The rule for a blending radius is to add radius equal to 25% of the thickness of the part wall being attached to, and for an inside corner the radius amount should equal 50% of the nominal walls. This means the radius amount required in this part design would be between 0.0125 and 0.025 inch. From what I saw on these sample parts, I decided to recommend all 0.025-inch radiuses, unless this created a fit and function problem.

Mysterious delay
Now that we knew where the cracking was coming from, why did it only occur within that two-week period immediately after molding?

It seemed unusual to me that this molder that I had known for so many years would forget that moisturizing nylon parts right after molding is one of the ways for those parts to become usable prior to at least 41 days after molding. Many of the nylon manufacturers have this information on their websites.

We used to just dump our nylon parts into a tub of boiling water and leave them to soak overnight, but this is not recommended anymore. The material manufacturers would prefer that anyone molding their nylon materials place the parts on a tray and heat them to a temperature of 100°F or a different temperature of their choosing for at least two weeks. Following this procedure reduces operator safety issues and eliminates any tendency for the soaked parts to become unusually flexible.

This moisturizing procedure would increase the part’s impact strength by several points, cause the part dimensions to grow ever so slightly, and cut in half the flex modulus or stiffness of the part.

If this moisturizing is not done artificially, it will occur naturally just by leaving the parts in the normal atmosphere. It takes six months or so for these changes to happen outdoors or in a warehouse (quicker in the summer and slower in the winter), but it will happen.

I had arrived at the finish line for this project. All I needed to tell the molder was to have the toolmakers add the missing radiuses and moisturize the nylon after molding. I called the molder and spent my typical half hour catching up and another half hour suggesting what he needed to have done. He wanted to know if the toolmakers could handle the moisturizing part of the corrective action that I passed on to him and I politely told him it should probably be the responsibility of the maintenance area.

Again, we have corrected the problem presented to us and, fortunately, it did not put the total workload on the toolroom. Sharing the responsibility of corrective action always puts a smile on everyone’s face.

It was getting late—I had spent 5 or 6 hours on this project—so I decided to take Jake to the dog park so he could catch up with all his friends. The deep snow we had this winter has made it difficult for us to get around easily, so this would be fun for both of us. 

Troubleshooter’s 
notebook
Part/material: Nylon.
Tool: Two-cavity cold runner.
Symptoms/problem: Parts cracked within the first two weeks after molding.
Solution: Radius all sharp corners and edges; moisturize the nylon parts.