The Troubleshooter, Part 21: Fill, Pack, and Sticking Problems

I recently got a call from a molder I have worked with on many occasions. He told me he had sent a box of polypropylene wheels to me for review and I was to call him when they arrived. He said he was having trouble molding the wheels without getting terrible flow lines on the surface of the parts and voids in the thick sections. I told him to wait until I had a chance to see the parts and I would call him right away.

The parts arrived the next day and I quickly opened the box to see what he was having trouble with. They didn't look that bad. I could see the surface defects he mentioned and I saw the voids in a part he had sectioned through the thick sections, but this was a wheel for a utility cart used in a factory, not a wheel for a BMW. Who is going to see it?

Actually I'm just kidding about who is going to see it. I like to see all parts well-molded. If not for practical reasons alone, such as dimensions being correct and maintaining the physical strength of the part, then for the satisfaction of producing a nice looking, well-molded part that other injection molders will look at and say, "Now, there's a well-molded part."

A Closer Look
I looked at the wheel a little more closely and could see three hot tip gates evenly spaced on a collar around the center hole of the wheel. The surface around each of the gates was slightly brown in color and had the look of having been burned. The gates also appeared to be too small to adequately feed the volume of material into the cavity to properly fill and pack the parts. I measured the gates and each of them was .075 inch in diameter.

My rule of thumb for gate diameter on a polypropylene part is that the gate diameter should be one half the wall thickness. But once the wall thickness goes above .180 inch, I stop the calculation. I just treat the thicker sections the same as I would the .180-inch dimension. In this case, it means I would figure the gate diameter to be one half of the .180-inch dimension or .090 inch. Then for thick parts like this one I use multiple gates, as close to the center hole as I can get, each gate being .090-inch diameter.

First Recommendations
I called the molder and told him to open up all the .075-inch gates to .090 inch and his troubles would be over. He listened quietly, said he would have the work done and let me know how everything turned out. A couple of days went by without my hearing anything from him, so I figured we had solved yet another molding problem just by removing a restriction from the flow path.

But it was not to be. The next day I got the call. The molder said he was having all kinds of confusion about opening up the gates. He said the hot runner system drops that went down to the three gates on each wheel had three channels drilled through the tip of each drop of .090 inch each. Each .090-inch channel fed one of the current .075-inch hot tip gates. Not a bad design for molding parts with .090- or .100-inch wall thicknesses, but not one for really thick parts like these.

I couldn't believe I had forgotten to ask the molder what kind of hot runner system he was using. Maybe he told me and I just wasn't listening. Whatever the case, we now needed to rethink the whole problem again. I knew I needed more volume through each gate and each gate had its own channel feeding material to it through the probe.

The current design of the .090-inch channel feeding the .075-inch gate is perfect for most molded parts and especially good for polypropylene. But when we need to pack out voids of a thick part, we have to increase the size of the flow path to get enough material into the part to get each cavity filled and packed. We have to keep pushing with the hold pressure to fill the voids as they form, then let the gate area freeze off, and recover the screw - all of which we want to do without using higher than normal melt temperatures for the material if we can help it.

Sad Looking Parts
The molder had already tried the higher barrel and manifold heats and that had only produced the sad looking parts with the burned material at the gates that I saw in the original package. I told the molder to call the hot runner manufacturer for his system and ask if he could open the flow channels in the probes up to .150 inch, which would allow us to open the gates to something larger than the current .075 inch. Maybe we could at least get the .090 inch we determined we needed or even as much as .125 inch in case we needed it later on.

A couple of hours later I heard from the molder again. He had spoken with the hot runner people and was told he couldn't exceed .125 inch with the flow channels and not over .100 inch with the gates. This was good news. We could go bigger with the flow path and not hurt the function of the hot runner system. I reminded the molder to be sure the sprue bushing side of the hot runner was drilled out to match the 1Ú2-inch flow tube diameter, which also means the nozzle on the molding machine has to be drilled out to a .5-inch orifice so it will match up with the sprue bushing. He said he would have it all done and let me know how it worked.

More Problems
It was another three or four days before I heard back again and what I heard was that the molder was having more problems. With the larger flow-channel diameter and gate size he could fill and pack the parts without any problems and the parts looked good. He could keep the melt temperatures in the barrel and the hot runner manifold at 350F but now the wheels were sticking in the front half of the mold. He could get the parts to come out of the front half of the mold if he raised the manifold heats, but when he raised the heats, the appearance of the parts started to suffer again.

We finally decided we weren't getting enough heat to the gate area itself. We were using 350F in the manifold, which seemed okay, but we weren't getting the heat to carry over to the gates. We didn't have a separate gate temperature control so we were dependent on the heat to carry over from the manifold itself. When the heats were higher we were getting enough carryover, but we were not when we dropped the heats to stop the material from degrading.

We called the hot runner people again and asked what they would suggest for getting more heat to the gate area. They said they have many requests for exactly the same thing and have made replaceable inserts available for the gates in their hot runner molds. They said the normal gate insert is made out of tool steel and works great most of the time. If the molder needs more heat in the gate area they send him beryllium copper inserts. If he needs less heat, because of stringing or drooling, they send titanium inserts.

Another Solution
The new tips arrived the next day and the toolmakers jumped right on them. Well, they didn't really jump. They had to finish their break first, wipe off their tools, and rearrange their toolbox drawers, but before it was time for them to go home, they had the mold ready to try again.

This time the parts looked much better and the gates weren't holding the parts in the front half of the mold, but for some reason the parts still weren't releasing out of the cavity as they should. We could tell the gates were releasing the wheels because we could rotate the wheels in the cavity, something we couldn't do before. They rotated just enough to tell me the gates weren't freezing off and locking the parts in place. But something was still keeping the parts trapped in the front half of the mold.

We used an air hose and blew air in around the outside of the molded part and most of the time we could get the part to come out, but not always. It looked like we might be having a vacuum problem and it was causing the middle ring of the part to stick in the mold until we broke the vacuum with a blast of air.

I could see why it was happening. The wheel design calls for very little draft on the side wall of that middle section, and since we opened up the flow path, it was now easy to overpack the parts with the inject and hold pressures. I could also see mold-spray residue in the cavities that the molder had used previously in an effort to get the parts to come out of the front half of the mold. It was this residue that I thought was causing the vacuum problem in the mold.

It felt like we were chasing our tails. We cleaned up the mold, adjusted the inject and hold pressures, then worked on speeding up the cycle so the parts didn't have a chance to shrink onto the cores in the front half of the mold. Lo and behold, everything ran just as it was supposed to.

What a wild ride! Sometimes it seems so easy to tell someone to simply open the gate, but it is almost always just the tip of the iceberg. In this case we were fortunate. I had a molder who trusts me and a toolmaker who would try most anything we asked him to do. It took longer to get the problems solved than I might have thought, but the wheels look great and the cycle is running at 75 seconds instead of the 150 seconds it used to run.

The molder will make a little money after he makes up for all the lost time and parts he has accumulated so far. The toolmaker will get paid for his efforts and I get the satisfaction of helping solve another problem. It doesn't get any better than this!