The Troubleshooter, Part 29: The secrets of hot runner molds

A molder recently sent some parts from a hot runner mold with a list of complaints, including problems with brown streaks, black specks, difficulty filling out the ribs, and on and on. My usual procedure is to look at the runner system, but in this case, I could only speculate on how the hot runner mold had been put together. Did the molder or his tool shop buy a hot half from one of the high profile sources, such as Mold-Masters or Husky or D-M-E, or did they buy the entire mold?

Many times, I have found purchasing the entire mold from a single source is better, but to save time and money, most molders or tool shops will buy a hot half and fit it into their mold. The problem with doing it yourself is that quite often the hot half is correctly sized, but the half done in your own shop doesn't have the proper gate diameter and the land isn't sized correctly for the wall thickness of the part.

In this case, it again looked like the hot half might be OK, but the cold half missed the gate size and land requirements for the part. This part appeared to be a storage box, about 6 by 6 by 8 inches, with multiple hot tip gates in the bottom of the part (Figure 1). The material is a flame-retardant polycarbonate/ABS material, and the mold is a four-cavity tool.

Begin at the Beginning
The first thing I do is measure the wall thickness of both the wall where the gates are and the side walls. Here the side walls measure .125 inch, and the bottom wall, where the gates are, measures .110 inch. Not a good situation for any part, but especially bad for parts that are on the large size. I like to have a thicker bottom wall feed the side walls to get the part filled out and packed correctly.

So that will be my first suggestion: increase the thickness of the bottom wall to .150 inch, and leave the side walls at .125 inch. This will be an easy change because they will be taking metal off the core, which is much better than having to add metal.

The second thing to look at is the gate itself. I attended a Mold-Masters seminar last summer and came away with a couple of good tips. The first concerns the gate area where they recommend sizing the gate for the wall thickness the gate feeds into. They weren't specific about a polycarbonate/ABS material, but I know the gate diameter needs to be 90 percent of the wall thickness for this type of material to fill and pack the part evenly. In this case, we will have a bottom wall thickness of .150 inch to work with, and 90 percent of that means a single hot-tip gate needs to be .135 inch to provide the volume of material through the gate to fill and pack the part.

But this part has five gates on the bottom wall. When I have more than one gate filling a part, I can reduce the diameter of each gate by 5 percent or so from what I would need for a single gate. I have four extra gates, so I would reduce my single gate size by 20 percent for each of the five gates. That would give me a new gate diameter of .110 inch for each. I have found I don't need to be exact with this type of dimensioning. I just use a practical, common sense approach, and it works.

Now we know the gates will all be .110 inch in diameter. The next step is to be sure the land length of the gate is no more than .005 inch. Actually, the Mold-Masters people told me they are currently recommending a land length of no more than .002 inch. On the print, you will see this in the gate detail as a .002 inch callout, or more often as a "sharp corner."

The key to a hot-tip gate is to relieve the area under the gate at a 45° angle back up into the flow path of material that flows through the hot runner drop. Husky has had this detail in its literature for several years. Mold-Masters tells customers about it, but I haven't seen it in the literature yet.

We have the gate diameter, the land length, and now the 45° angle back into the material flow. Next we have to look to see if we have a recess area at each gate. The recess areas are often used to give any gate vestige a place to hide. Not a bad idea, but if we can get the gate tip temperature down where it belongs, we won't have any gate vestige in the first place.

Here's another tip from Mold-Masters. If you design a recess area into the gate, you must have a transition dimple or melt puddle or whatever you want to call it under the recess area to maintain the wall thickness through that area of the mold (Figure 2). If you do not have a recess area for your gate, you do not need a transition dimple under the gate. In this case, we have a recessed area at the gate, and we have the transition dimple inside the gate on the end of the core, so we should be OK.

Matching the Nozzle
The next area of concern for a hot runner or heated sprue bushing is the machine nozzle matching up to the sprue bushing in the mold. The nozzle has to be drilled out to match the flow tube diameter of the hot runner, which is at least 1/2-inch diameter when the material is ABS, SAN, acrylic, polycarbonate, or, as in this case, a polycarbonate/ABS alloy (Figure 3).

I have seen flow tube diameters as small as 1/8 inch for polyethylenes and as big as 1 inch for some of the stiffer flow materials. If the nozzle isn't sized correctly, you will often have high shear areas that can cause brown streaking on your part or pressure losses that will make it difficult to fill and pack your parts, or even trouble in changing colors. Many people have trouble changing colors in their hot runner molds, and most of the time it is because they did not drill the nozzle out to match the flow tube diameter in the mold.

I called the molder, and he conferenced his tool maker in for the call. We discussed all the important things to do when working with hot runner molds. They were not opposed to any of the changes but wanted to go through them one by one. The thicker bottom wall was not a problem as long as it would not change the shrinkage. I told him that by opening up the flow restrictions in the mold they would be able to lower the melt temperature of the material somewhat, and they should be able to fill and pack more easily and be able to keep the flow path open longer to get better control of the packing pressures, which will help them control the shrinkage better than before. That made sense, so they decided to give it a try.

I told them there is the same problem with buckets, pails, and anything shaped like that. You always want to gate into a thick section and flow the material through walls where the dimension gradually decreases. That's all we are doing here, going from thick to thin. Wherever the gate is should be the thickest section of the part.

The next question was why they had designed so many gates into the bottom of the part. They didn't know for sure, but they thought a new designer at their customer's location might have suggested it, and nobody thought to ask why. I told them I always divide the wall thickness into the flow length to see how far I can push material through a single gate.

For this type of material, I look for a resulting flow number of 200 or less to tell me if a single gate will work or not. In this case, I used the side wall thickness of .125 inch and divided it into the 12 inches of flow required to fill and pack the part and came up with a flow number of 96, certainly within our window. I suggested a single gate would have been enough to fill and pack these parts. I told them I sometimes use multiple gates on a part like this to lower the pressure drop at each gate, which will allow me to fill and pack the parts without having to use the full 4 or 5 tons/sq inch that is typical of this material.

The molder and the toolmaker agreed that the latter was no doubt the reason they went with the multiple gates because his biggest machine wasn't quite enough tonnage to get the job done at the 4 to 5 tons/sq inch requirement. I suspect they will get by with 3 tons/sq inch or less by using the multiple hot tips.

We resized the gates, drilled out the nozzle to 1/2 inch to match the flow tube diameter, and thickened up the bottom of the parts. When the mold was sampled, we saw a drop in barrel melt temperature by 50 deg F and the injection pressure dropped to below 1000 psi, which I like to see.

The molder left the cycle time where it was, around 40 seconds, because that is what he quoted and didn't want to take any chances. I suspect he could get it down to 35 seconds now that we have optimized the material flow path.

He has eliminated the rejects he was getting from the higher barrel heats and shear at the gate. That alone was more than enough for him.