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Working miracles with acrylic, a molder produces a great part, 
with just one problem.I woke up this morning to a chill in the air but still no snow on the ground. I only mention the snow since slick streets make my morning package delivery service run behind time and mess with my schedule.

Bob Hatch

January 26, 2011

7 Min Read
The Troubleshooter: What is thick, flat, and still runs a 
normal cycle time?

Working miracles with acrylic, a molder produces a great part, 
with just one problem.

I woke up this morning to a chill in the air but still no snow on the ground. I only mention the snow since slick streets make my morning package delivery service run behind time and mess with my schedule.

Sure enough, I heard the sound of the big brown truck stopping at my door for my 10 a.m. delivery. The door opened and in walked my usual driver, with a festive Santa hat perched on her head. I signed for the packages, bid her a fond “so long,” and settled in. I picked one box to open first because I recognized the return address as one of my oldest friends in the business.
Inside the package I found a very well-molded part about the size of a sheet of paper. It was transparent and as thick as a pound of bacon.

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This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges. You can reach him at [email protected]

I guessed this to be a divider in a bigger subassembly or a shelf of some kind—maybe something on which to display collectibles.
The shelf was almost 0.400 inch thick, 12 inches long, and 8 inches wide—pretty hefty for a molded part and no doubt pretty expensive to produce and market. The cold runner was still attached to the part, and thank goodness for that, because a part this thick takes a massive gate to get the material injected into the cavity.

When I tried to figure out what material I was dealing with, I looked for a note, but as you have probably already guessed, there was none. The material was clear and rigid, which would make it amorphous. With polystyrene, SAN, acrylic, polycarbonate, and a few other commonly molded materials to draw from, I picked acrylic.

Why acrylic? Most plastic shelves are fabricated from acrylic, and guys like this usually stick with what they know.

Of course, it could just as easily be one of the other materials I thought of, since the shrinkage, viewing quality, moisture problems, and molding difficulty would be somewhat similar in each of them. The only variables I could think of would be the cost per cubic inch and cycle time differences.

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One of the two attachment tabs (top) was broken off. The other tab (center) shows that the breakage stems from nonradiused sharp corners in the design. The sprue and fan gate it fed into (bottom) both needed englargement to provide proper material flow.

Hot bushing is better
Per my usual practice, I started checking dimensions. The runner was unusual—notably, it wasn’t centered on the part. I didn’t feel the offset gate was that much of a problem, so I moved on to the fan gate thickness; that turned out to be only 0.250 inch, certainly not thick enough to feed a 0.400-inch-thick part. It needed to be as thick as the part itself, or 0.400 inch.

The cold sprue was also undersized—0.175 inch at the O-diameter and 0.375 inch at the other end, where it attached to the fan gate. A heated sprue bushing is best when feeding such a thick section, in this case with a sprue orifice diameter of 0.375 inch and the molding machine nozzle orifice opened up to 0.350 inch to match the flow tube diameter.

If they chose to keep the cold sprue, it needed to be opened up to 0.400 inch at the O-diameter and 0.450 inch where it attached to the fan gate. The molding machine nozzle—a full taper for amorphous materials—would then have an orifice diameter of 0.375 inch (90% of the sprue O-diameter).

The gate was 7 inches long, 0.200 inch deep, with a land length of 0.150 inch—again, all bogus. The gate was only half as deep as the part; it should have been a minimum of 3/4 the thickness, or 0.300. The 7-inch length was much bigger than it should have been—somewhere between 0.250 and 0.400 inch is my estimate of the correct length. The land of 0.150 inch was the real killer, since the gate land should never be more than 0.030 inch long.

The part was extremely well molded, so these ridiculous dimensions told me that rules are sometimes meant to be broken. So how could the part look so good with all of these errors?

Acrylic comes in many flow types—easy, medium, and stiff (for thick parts). I guessed this molder lucked out by using an easy-flow acrylic and probably kept the melt temperature on the low end of the manufacturer’s recommendations to pull it off (similar to the story in my book, On the Road with Bob Hatch, that explains how I was able to mold a 1-inch-thick part out of high-density polyethylene without the cycle exceeding 1 minute). Sure, it can be done, but sometimes the question is whether it’s worth it. I knew time would tell as I worked my way to the end of this review.

Now, the molder could continue to use the easy-flow acrylic or switch to a tougher, stiffer grade and raise the barrel melt temperatures by 50 deg F or so.

Radius, radius, radius
Opposite the extremely long gate, I could see a couple of attachment sections molded into the part. One was broken off and the other was broken about halfway into the length of the design. This appeared to be the reason I received the part.

This was interesting: The part was easy to mold, but the attachment areas were breaking off. Why?

I looked at the partially broken attachment point and could see the toolmaker had applied some blending tapers, but I could also see a couple of sharp edges that were not radiused. This is probably a good time to remind all of my readers that putting radiuses on some sharp corners is not enough—all sharp corners or edges need to be radiused to eliminate cracking and breakage problems, especially when molding amorphous materials.

Now I knew that the problem was in the part’s design or at least toolmaker followup, not in the molding itself. I am warning the toolmaking community right now: I’m out to teach you everything I know. Beware!

I called the molder on his cell phone (does anyone have a phone on their desk anymore?). His wife answered and said she would have him return my call as soon as he got off the golf course. Sure enough, a couple of hours later I got my call back. I talked and he listened for a good 10 minutes. Then I listened for 10 minutes. Eventually we got caught up on our families’ activities, and then we got down to business (kind of like sending cards to each other for the holidays).

He was glad the molding department got a clean bill of health and he appreciated my suggestion for adding a radius to all sharp corners. I also mentioned that acrylic materials come in medium- and high-impact grades, and he might want to give them a try to see if the part strength could be improved in addition to the design changes.

I felt so good about solving this problem that I decided to celebrate the holidays by calling a friend and going to lunch. The other packages would just have to wait until later.

Troubleshooter’s 
notebook
Part/material: Thick shelf made of an amorphous material, possibly acrylic.
Tool: Single-cavity cold runner.
Symptoms/problem: Attachment tabs were breaking off.
Solution: Radius all 90° angles. And for better molding, increase the fan gate thickness, sprue diameter (both ends), and gate depth; decrease the gate length and land length.

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