Sponsored By

March 1, 2001

10 Min Read
The Troubleshooter, Part 47: From concept to reality

0301i12a.jpg

Extensive troubleshooting before the tool build helped this project stay on track. Minor blushing problems were solved by enlarging the gate, and knitlines were covered by the graphic, applied in a secondary operation.

I had a visitor several months ago who wanted to ask some questions about a project he was about to embark on. I have known David for more than 20 years and even though I was up to my neck in alligators I told him I always have time for an old friend. 

David presented me with a briefcase filled with drawings and prototypes. I have to admit I was impressed with his preparation skills. We cleared a spot on my desk and he spread out drawings, coins, and other interesting items. 

He told me he wanted to make a coin display for collectors to show off the new quarters being minted for each of the states. His concern was that he didn't want it to look cheap; it needed to be of high quality and contain enough coin pockets to display quarters for all 50 states. 

He also wanted to allow for coins from both the Denver and Philadelphia mints. That meant that 100 coins would eventually be displayed. 

Part Parameters 
David wanted to make the display double sided to save space, which I thought was a good idea. He was going to display coins from the Denver mint on one side and coins from the Philadelphia mint on the other. It also helped me with the question about how thick the walls should be. They would have to be as thick as two coins, with a thin wall between them. The outside frame would have to be a little thicker to allow for the clear window that protects the coins from scuffing and dust. Our calculations dictated a double-sided coin display wall .150 inch thick and an outer frame .500 inch thick. 

Naturally, I didn't want the outer frame to be a solid .500 inch thick, so I suggested he core it out with two or three ribs, which would cut any given wall thickness in the outer frame down to no more than .080 inch, or possibly .100 inch at the most. This satisfied David because he was thinking about using a gold foil on the outer rim of the part to give it a high-quality look. 

Next, he wanted to know the material that would work best in this application and I gave him all the insight I could muster. Did he want chemical resistance? How about impact? Heat deflection requirements? What color or colors would it be? I asked all the questions I could think of and when we were finished it turned out that ABS was the best material. I told him that ABS prices are down from past years and that the resin provides good value for the money. 

A second consideration was that the shrinkage for ABS is similar to possible replacement materials, including high-impact polystyrene, acrylic, polycarbonate/ABS, and polycarbonate. All of these are materials that he might want to evaluate in the future for whatever reason. 

It is much better to anticipate a problem and solve it ahead of time instead of having to go back to fix it later.

David felt comfortable with this choice and wanted to know what shrinkage numbers he should give the toolmaker. I told him shrinkage is a guessing game, but I would tell the toolmakers to start with .006 inch/inch. Thicker walls shrink more than thin walls and processing conditions play a big role, but this design should respond well to the .006 inch/inch number. 

He asked about molders in the area capable of handling his project and I gave him several suggestions. Then he was off to begin the process of getting the mold built. He said that when he received his first samples he would bring them back for my comments. 

The Mold 
A couple of weeks later I got a call from Rod, a toolmaker I have known for many years. He now owns his own molding shop and also builds molds. He wanted to know what size and type of gate he should use for David's product. I suggested he make it a single gate and use a heated sprue bushing to feed the gate. 

Rod is a very good toolmaker and has turned out to be a good molder as well. Whatever he does is usually very well thought out and would probably be right. I just reminded him to size the orifice of the heated sprue bushing to handle the large volume of ABS needed to fill and pack out the part without extra heat or pressure in the process. I didn't want the part to warp because of something we didn't allow for in the tooling. 

Rod said he was going to use a hot tip gate diameter of .090 inch to start and open it up later if necessary. I agreed and reminded him to be sure to drill out the nozzle to match the flow tube diameter of the heated sprue bushing, and not to forget to taper the gate land to reduce shear. A .002-inch land at the gate is all you want and need. 

First Run 
I didn't hear from David for a couple of months, but before I knew it he came back with samples that looked pretty good. It is amazing how nice everything can look with proper planning. The parts were flat, the coins fit in the pockets, and David was pleased. I thought the parts looked great for the first samples out of a new mold. 

The only problem I could see were prominent knitlines around each of the cores that made the coin pockets, and some blush and flow marks around the gate. I told David he could get rid of the knitlines by using blind vent pockets on the end of each core and heating up the material and the mold steel; however, he wasn't concerned about the knitlines because he had designed a graphic piece that would lay around the coin pockets and cover the imperfections. The graphic sheet had all the state names and some interesting designs incorporated that really finished off the look of the part. 

I suggested that he open up the gate from the .090 inch diameter to something bigger to reduce the blush and flow lines at the gate. David thought the bigger gate would be a good idea since he saw some material starting to string or drool at the gate. I told him the drooling was a result of the molder using too much heat at the gate in an effort to keep it from freezing off between shots. If we opened up the gate we could cool the tip and the stringing and drooling would stop. 

Armed with these new ideas, David was off to visit Rod to get the changes made. A couple of hours later I got a call from David. He wanted to know if I could visit with him in the morning to discuss the changes with Rod. 

The next day at Rod's facility I explained that when a hot runner or heated sprue bushing system is set up correctly, the effect is that the front zone, nozzle, and manifold temperatures are the same, and the tip temperature is just 10 deg F cooler than the manifold. If the gate is restricted, as this one was, the tip temperature will be 50 deg F or more higher than the manifold, nozzle, and front zone. Rod checked his processing sheet and concurred that the tip temperature was indeed higher than the manifold and agreed to make the change. 

Decoration 
David called me a couple of weeks later and wanted to know how to hot stamp a gold foil completely around the outside of the part. I told him to find someone who has a roller-style hot stamp machine, similar to those used for hot stamping wood grain foil on the front of window air conditioners. 

Two days later David called and said he finally found out who makes the roller hot stamp machines. His next question was about who was going to pay for the machine. I told him that most often the molder pays for the equipment, especially if it will be used for other projects. But if the molder wasn't going to use it for anything else then I told David he would probably have to buy it and keep it for future projects. 

Another couple of weeks went by and pretty soon David showed up at my office with several boxes of parts for me to look at. The molded parts were hot stamped with the gold foil and the graphic sheets had been installed on the center flat section. Two molded feet held the display piece up and the clear panels were attached with brass screws. It was a very attractive collector's display. 

Troubleshooter's Notebook

Part: ABS coin display.

Tool: Hot sprue, family mold.

Symptoms: Blush at the gate, stringing during processing, knitlines visible around the cores.

Problem: Gate was restricted, causing the tip temperature to be too high.

Solution: Enlarge gate to allow tip to run cooler.

Result: Blush and stringing eliminated; knitlines deemed acceptable since they were to be covered up by a graphic applied in a secondary operation.

Variations
David said he wasn't done yet. He now wanted to know how to make smaller versions of this big piece that would display one, four, or six coins. Basically, the material would stay the same. The smaller pieces would have thinner walls since the coins would only be seen from one side, the gate wouldn't have to be as big due to the smaller volume, and the cycle time would be faster because of the thinner walls. David took notes and he was off again. 

Soon after, I got a call from a different molder who wanted to know how to get rid of the blush at the gate on the smaller coin display pieces that he was running for David. I told this molder that blush at the gate is a result of the gate not being big enough. This was especially true with this second mold because he was using an edge gate. An edge gate needs to be deep enough, based on wall thickness, to allow the material to flow, fill, and pack the parts. It also needs to have a land length that is only half the depth, not to exceed .030 inch, to avoid any flow lines going straight out from the gate. In this case an edge gate that was .075 inch deep, .150 inch wide, with a .030-inch land would be just about perfect. 

The molder made the changes and the parts came out looking great, according to David. He thanked me again for being his champion and wanted to know how he could repay me. I told him just to be sure I got a couple of free samples when everything was said and done. He agreed and he was off again. 

Just before Christmas David showed up with a box in his hand. He said these finished parts were his thank you to me for helping out with his project. He went on to tell me that the coin displays would soon be available on TV and through the U.S. Mint. 

I can't help but think how easy it was to keep this project on track. Every little detail was covered and David did it all by just asking questions. Many of the troubleshooting requests I get are because the right questions were never asked. I would much rather anticipate problems and solve them ahead of time instead of having to go back to fix something that was not considered in the beginning of development. I'm not complaining, though, because that is the job of a troubleshooter. 

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like