Weak Plastic Coat Racks - Notch Sensitivity

By Bob Hatch

Related links: HDPE | Cold Runner System

There’s strength in numbers—proper radiusing numbers, that is.

This month I received a white plastic coat rack to look at. The runner was missing but I could see what the problem was right away. I doubt if anyone will be surprised to find out that notch sensitivity issues were the problem with this otherwise nice-looking, fairly tough part.

	I don’t like it when I have to assume something, but I couldn’t catch the molder in his office and no one else at his company had a clue what his questions were or where the part came from, for that matter. I could see vestige from two large gates on the bottom side of the part, which I suspect came from a cold runner system that could have been a balanced or unbalanced design. Since I didn’t see any real problem with filling, I decided to go with unbalanced, just because I’ve seen parts similar to this one that used an unbalanced design. I didn’t have to calculate the sprue, runner, and gate sizes since I didn’t have a runner. However, the molder seemed to be fighting a design problem related to proper radiusing. I found it in two areas of the part design, but it was so subtle I could see why the tool designer and the toolmakers overlooked it. Now, the hooks that hold the coats were designed well and had enough radius to prevent breakage in the sharp corners. What was wrong was that the two upright hooks that attach the coat rack to the top of an interior door were missing the proper radiusing where they connect to the rack. It wasn’t a difficult problem to catch, but could be easily overlooked. Proper radius numbers for any part design are pretty easy to remember. A rib attached to a nominal wall should be radiused at 25% of the nominal wall thickness. For an inside corner, the radius should be 50% of the nominal wall thickness. The trick is to know what your situation happens to be. In this case, I treated this design issue as a radius for an inside corner since we had two round sections of the part design coming together with dimensions of equal thickness. The rest of the part seemed to be well radiused—no doubt two different moldmakers worked on this project. One knew how to properly radius the joining of equal-dimension components and the other moldmaker either did not know the proper techniques or simply overlooked this critical requirement for maintaining the proper strength in each of these critical areas.
That’s one sharp corner in the over-the-door hook to this HDPE coat rack. The result was breakage at the weak point. Proper radiusing (50% of the nominal wall thickness for an inside corner) will make a stronger part

Let’s go over the numbers. The over-the-door hook pieces were .290 by .310 inch, or an average dimension of .300 inch; they had a full-round hook attached to them with a diameter of .390 inch. These two dimensions were not exactly the same, but I treat any dimensions greater than .250 inch as the same for radiusing purposes—and their junction needed a radius. This inside corner radius would need to be equal to 50% of the ball dimension of .390 inch, for a radius of .195 inch.

Since the part had zero radius, the addition of a .195-inch radius would go a long way to improve the toughness in this part of the design and should eliminate any tendency for breakage at these attachment points.

Material change
With the trouble spots out of the way, I looked at the rest of the part. Even though I could see several other areas without proper radiusing, I couldn’t see any reason to correct them since I didn’t see any stress or breakage predictors. When I work on a design from the beginning, I always try to get all the critical areas properly radiused, but when I am troubleshooting, I just try to correct the areas that are breaking. It all depends on what the molder wants to do and how much time we have to work on the mold.

This was a pretty good design. If these hangers broke for the customer, it would be pretty easy to send replacement hangers and not have to replace the entire part. Maybe that was the thought, but why not just design it so it wouldn’t break from the beginning? Sometimes I fail to see the logic in the problems I am presented with.

One other thing that I noticed is that high-density polyethylene was probably not the best choice of material. I suspect that a stiffer product would be in order. As the material gets stiffer, the impact value often goes down, and we needed a combination of rigidity and toughness in an application like this coat rack.

I checked the IDES plastics database, in the free section, naturally, and could see that many grades of polypropylene had a higher flex modulus than this HDPE. I would have probably chosen one of them for this project instead.

There are also other HDPEs with a flex modulus greater than 200,000 psi that would work in a coat rack application, and these could be good candidates for future designs.

The database gave me many suggestions for alternative materials and when I called the molder with my design suggestions as well as other choices for materials, I again heard he was not available. So I did the next best thing. I sent him an e-mail outlining my suggestions and added my number for him to call me directly if he had any questions. All that was left to do was prepare a bill and send it to him.

June, 2007 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.