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May 1, 1997

6 Min Read
The Troubleshooter, Part 14: Voids in thick parts

trouble.gifThis article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.

I continue to get parts from molders who are having trouble getting rid of voids in the thick sections of their parts. The best example is a molder who is trying to get rid of all voids in a part that looks like a solid one-piece handle for a cement trowel or a large-diameter screwdriver handle.

First of all, you have to know how to determine what kind of "bubbles" or "voids" you are having trouble with. If the "bubbles" are round, they are usually moisture related. If they are elongated, then they are probably pressure related.

The first parts I received from the molder this time had both moisture and pressure voids in them. When I called it to his attention he said he had just poured the material out of the bag into the hopper. He would dry the material next time. Since the material was acrylic, I told him to be sure the temperature was set correctly for the type of acrylic he was using. In this case, it was the hard-surface acrylic, normally used for scratch resistance, which dries at 210F measured at the inlet air connection to the hopper dryer. This correct drying temperature will get rid of any moisture problems when he starts molding production parts.

The pressure voids are a different story. The key to getting rid of pressure voids is to make the flow path larger in diameter than the thick section of the part where the "airless" voids are forming.

The first sprue and runner I saw on the part in question was way undersized. The short little runner, which connects directly to the part, was .250 inch and the sprue feeding it had a .180-inch O diameter being fed by a nozzle with a .150-inch orifice in it.

We started out by increasing the size of the short runner to .500 inch in diameter with the sprue O diameter reamed out to .500 inch also. The sprue O diameter doesn't need to be bigger than the runner diameter because the sprue only has one runner coming off of it instead of the more common arrangement of two or four. With a .500-inch sprue O diameter, we were able to drill the nozzle out to a .475-inch orifice, and the parts were sampled again.

We were able to get rid of all the voids except for one little one right in the center of a thick section. Then we had a couple of choices. We could open the sprue and runner up another .100 inch and drill the nozzle out the same amount, or we could play with the heats and holding pressure.

We elected to leave the .500-inch dimensions where they were and raise the melt temperature a little bit. I also suggested that the molder raise the rear zone temperature to the same as the front zone. I have found over the years that just raising the rear zone helps get rid of that last little void in many of the parts I have molded.

We had been using lower than normal melt temperatures on this thick part in an effort to keep the cycle times as fast as possible. We found, however, that we needed to raise the barrel heats a little bit across the board to pack out that one last void.

We could have used the stiffer flow acrylics to achieve our void-free part, but that would have required an even larger sprue and runner combination than we had. The easier flow version of the hard-surface acrylic was actually doing a better job for us, and we were so close.

We raised the barrel temperatures from 400F to 410F and the void got smaller. We tried a little more holding pressure, up from 1200 psi to 1300 psi and it got even a little smaller.

We were getting close, and then the phone rang. The customer wanted to try crystal styrene instead of the acrylic. He wanted to use a less expensive material. Sure the part would scratch more easily, and it wouldn't be as resistant to chemicals, and it wouldn't look as nice, but at least he didn't go so far as to suggest we use bottle regrind.

All of our hard work was not wasted, however. Everything we changed to make the acrylic run better worked to our advantage with the crystal styrene. The thing to remember is that we opened the flow path to a large size to get the material to flow easily and allow us to use the holding pressure to keep material flowing after the fill and pack portion of the machine cycle. We just keep pushing on the material in the flow path to fill the voids as they formed in the part.

We can run the crystal polystyrene at quite low temperatures, since it is an amorphous material, and it will still flow at temperatures in the low- to mid-300F range.

We molded the parts without any voids using the large-diameter flow path, low barrel heats, and mold temperatures in the 100F to 120F range. All that was left was to cut that big gate off with a band saw and smooth it up with a sander using emery cloth. Then the gate area was polished with a polishing wheel. It was a little bit of extra work, but the finished part was worth it.

I don't usually see a sprue and runner feeding thick parts like this, but it will work. I would normally expect to see a short sprue gate in most cases, with a large nozzle orifice feeding it.

A heated sprue bushing with a large-diameter orifice could have been used since the barrel and nozzle heats are kept low enough that the nozzle won't drool or string between shots. The best of all might be a valve-gated sprue gate so the gate will be .250 inch to utilize the large-diameter flow path that again will keep the barrel heats on the low end of the manufacturer's recommendations. Plus, when the valve is closed, the gate area will be cosmetically acceptable.

The mold temperatures need to be adjusted to a middle to lower range to keep a part like this from bending over after it comes out of the mold. Keeping the part flat or hanging it vertically and letting gravity keep it straight could work also.

Don't be afraid of thick parts; just change your thinking about flow path sizes and barrel heat requirements. Voids are something you don't have to live with, but just like little kids, they need to be understood.



Thick, handle-type part.


Hard-surface acrylic.


Single-cavity mold.


Elongated and circular voids in the part and in the runner system.


Restricted flow; material not dried; unable to pack part out.


Dried material; resized sprue O diameter and runner to .500 inch, opened nozzle diameter to .475 inch. Raised barrel heats slightly to allow packing of the voids; increased holding pressure to pack out voids.


Even though material changed to polystyrene, the part packed out and the voids disappeared.

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