Smaller Parts and Runners Have the Same Needs as Large Ones

By Bob Hatch

Related links: Small Runners | Books on Runners

Watch out for the transitions from runner to subgate and don’t undersize those runners.

This month I found an envelope, not a big box, on my desk and the small size made me wonder what I would find inside. It turned out to be the basis for another great story.

I could tell from the part’s 45° angle that it was a small corner piece for a picture frame or window. From the sprue and short runner, I could tell it was a single-cavity mold running in a smaller-sized molding machine.

With my measuring equipment, I checked the sprue and runner. From bending the sprue, I could tell the material of choice was in the crystalline family because it was pretty easy to bend—probably polypropylene or a high-density polyethylene. It doesn’t really matter when it comes to sizing sprues, runners, and gates, because both materials require about the same sizing. I couldn’t find a note in the box so this would have to do.

The sprue, runner, and gate were on the small side, which is OK for most crystalline materials, but something did not look right to me. The sprue measured .160 inch at the O-diameter and .235 inch where the short main runner was attached. With the thick section of the part sized at .180 inch, this sprue likely was freezing off prior to the part’s thick section freezing off. This could make it difficult to pack out the part’s thick section enough to eliminate any voids.

The main runner was a trapezoidal design—just .120 inch deep and .140 inch wide, which is pretty small to fill and pack a part the size of this one, but I could see another problem. The runner-to-subgate transition was poorly designed, hindering material flow through this area. Also, it looked like the runner was not extended far enough from the sprue to achieve a full runner depth flow down and into the subgate. This is definitely a restriction to flow, and along with the sprue diameter being smaller than the thick section of the part, I could see why the molder had sent me this part.

The runner-to-subgate transition depth should have been .120 inch (same as the runner depth where the subgate attached to the runner), instead of .070 inch, and the gate diameter measured .060 inch where it attached to the part. This was also a pretty good restriction to flow and would not provide the volume of material needed to fill and pack this part without a little help from the toolroom and some interesting changes in the processing conditions.

Final investigation
I decided to keep checking for other problems in case we had more work to do before this mold would be ready to run production again. At least, I hoped they weren’t running production with a mold in this condition.

I checked the part for radiuses in all of the sharp corners and found that about 75% of the sharp corners or sharp edges had been radiused, but the thick section right in the middle of the part was not radiused. I drew a line on the part with my medium ballpoint pen where the center section attached to the rest of the part, and ended up with a double line. This indicated that this middle section had a 90° sharp angle instead of a .025- or .030-inch radius.

I then checked the ends of the parts to see if any dullness or grainy conditions existed in those end-of-fill areas. That always indicates that we need more venting. In this case, though, all the end-of-fill areas looked nice and glossy like the rest of the part. At least something was done right.

The gate was halfway down one of the part’s wings and it appeared to be breaking off somewhat cleanly, although it was leaving a little too much vestige on the part and possibly pulling some material out of the gate area when the part was ejected. This could be attributed to an oversized subgate or an underpacked part, and not having a good, solid area from which the gate could shear without distortion in the gate itself. I imagined this ugly gate look would correct itself when we got the sprue bushing sized correctly and worked on the blending of the runner into the subgate.

What to change
I put together my suggestions for the molder. First, I recommended increasing the sprue diameter by .050 inch at both ends. Next, I reminded him that the trapezoidal runner needed to be larger to allow the proper material volume to flow to the subgate for a good fill-and-pack condition. The proper runner depth for crystalline materials is based on the thick section of the part, which in this case was .180 inch. In general the runner that feeds the gate should be equal to or up to 11?2 times the thickest wall section. In this case, I decided to make the runner .200 inch deep and .230 wide. I could have gone bigger, but the volume requirements were not high and with the increased sprue diameter, it should make a good transition for the material to fill and pack the part quite nicely.

With the sprue and runner both enlarged, all I had left was sizing the gate and determining the nozzle type and orifice size for this mold. At .060 inch, the subgate diameter was not equal to 50% of the thick section of the part, so it needed to be increased to .090 inch. Now, do I really think it would take the full .090 inch to properly fill and pack this part using extra barrel heat and injection pressure? No, I do not. A small part like this doesn’t require a huge gate to get the volume it needs to fill and pack the cavity before the gate freezes off, so I recommended a change to the subgate diameter of .075 inch from .060 inch. They can always go bigger later if needed.

A good reason for bigger gates is that it allows us to bring down the barrel heats a little bit and possibly speed up the cycle time. A faster cycle plus fewer rejects equals more profits for the molder. On the nozzle, the rule for crystalline materials is to use general-purpose nozzles with an orifice 10% smaller than the sprue O-diameter.

A couple of days later I got a call from a molder wondering if I’d had time to look at his part. After getting a description of the part and runner, I discovered he was the molder I was looking for to give these suggestions to. I have not heard how the changes to his mold worked out, but I am pretty confident he will be happy with the results.

April, 2007 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.