The Troubleshooter: Venting

This article continues The Troubleshooter’s Rules of Thumb series. 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.

We have covered nozzles, sprues, runners, and gates, and now we are ready to discuss venting issues. It amazes me that one of the most important areas of molding is so often ignored by mold designers and toolmakers. It must be because they do not have any immediate feedback from the molding shop floor to the toolroom and engineering departments.

The runner and each cavity must be vented extremely well if you want to get rid of the air in the runner channels and in the part cavities. What happens if we do not get rid of air quickly enough? First, we get burns in the part’s air traps, such as corners and between approaching flow fronts. Second, to get rid of the burns, the molding technicians slow the injection speed to give that trapped air time to get out.

After we slow the injection speed, we notice we are not packing the sinks out in the areas opposite the gates. We also see more prominent knitlines at the end-of-fill areas. Why do we allow this situation to occur? Usually it is because the more experienced molders have been promoted to positions in the front office and most of their expertise is no longer available to the newer people on the molding floor. They also are not available to the toolmakers for suggestions on how to correct problems with the tooling. It’s always the new guy doing all the new stuff that gets us in trouble.

So we do the best we can and that usually means to do the best you can with what you have to work with. I saw this same thing happening in my own molding shop and the way I tried to keep from losing expertise to the front office was by optimizing the molds as soon as I could—during the initial build, if possible. If I could not get it done in the initial build, then I would take care of it as soon as I received it in my shop.

Last but not least, we would correct the mold after the first sample, but by then it is pretty late. I could accomplish the optimizing at any one of these steps along the way, but it must be done.

When we do not optimize prior to running production parts, we typically see parts that are out of dimension, warpage that makes the parts bow or bend, and cycle times that are much slower than what we quoted to the customer. The out-of-dimension and warped parts result from running the barrel heats higher than the material or part configuration requires just to get the part to fill and pack without getting burns in the corners. We also run a slow cycle for the same reason. Too much heat going in requires more time to get it out.

Deep and Wide
We previously discussed the early stages of optimizing a mold and now we are ready to vent the mold. Most toolmakers are not even slightly aware of how much venting it takes to get a mold ready for sampling, let alone for production requirements. I say “most toolmakers” because I do know a few who have figured it out. I used to say I could count the aware toolmakers on one hand, but now I am pleased to say that many more than that are starting to get the picture.

When I start my venting review I like to begin with runner venting. Runner vents are deeper than part vents. Runner vents are .003 inch deep, as wide as the runner channel being vented, and use a vent land of .060 inch. Then drop from the vent land to a .040-inch-deep channel to atmosphere. Draw polish the vent lips to an A1 finish to make them self-cleaning.

I also like to see the sprue puller vented, but the vent depth on the sprue puller is treated like a part vent depth when it comes to venting. Sprue puller vent depths can be anywhere from .0005 inch deep per side (for easier-flow materials) to .002 inch deep per side (for stiffer-flow materials). Draw polish the end of the sprue puller to make it self-cleaning.

Part vents are not as deep as runner vents. Part vent depths, like runner vents, are based on the type of material being run. Nylon part vent depths are only .0005 inch deep; for ABS you can use .0015 inch up to .002 inch for a maximum vent depth (see Table 1). I have seen deeper vents used without getting any parting line flash at the vents, but only when the barrel heats and injection pressures are under control, which means when the mold is optimized.

Quantity and Location
Now that we know how deep the vents should be, the next step is to know where they go and how many will be needed to vent the mold properly.

Runners are vented at the sprue puller and at the end of each runner. Parts are vented much more than the runner is—you can vent 100 percent of the parting line if you want to. Part vents can be individual or perimeter types (see Figure 1, p. 88). Remember, it is the vent depth, not the width, that flashes, so the best part vent is the perimeter vent because it gets rid of the air more quickly than individual vents. I usually recommend perimeter vents when using a gate on the inside of the part. This inside gate could be a sprue gate, a valve gate, a curved tunnel gate, subgates into a pin, and so forth.

Perimeter vents are .0005 to .002 inch deep, depending on the type of material. You will want to go out .060 inch from the parting line and drop into a .040-inch-deep channel to a racetrack; you can then vent the racetrack to atmosphere using vent depths and lands similar to runner venting. Don’t forget to draw polish the entire vent lip, in the direction of air flow, to make it self-cleaning.

Individual vents should be installed on the parting line—I like to use one vent per parting line inch. I usually use individual vents when I have an edge gate on the parting line. When I put in one vent per parting line inch I make each vent .200 inch wide. Sometimes on more complex parting lines, mostly where the design of the part jumps around a lot on the parting line, I put in two vents per parting line inch. When I use two vents per parting line inch I make the width just .150 inch. In either case, with the individual vents, I only vent 20 or 30 percent of the parting line by using the one or two vents per parting line inch rule of thumb.

After you have the correct depth and width of the individual vents, extend the vent land length .040 inch from the parting line and drop into a .040-inch-deep channel to atmosphere. Draw polish the vent lips to make them self-cleaning, as before. All of these individual vents make it look like you have rays of sunshine streaming out of the mold cavities. The main difference between the individual vents and the perimeter vents is that the perimeter vent land is .060 inch and the individual vent land is only .040 inch.

Another area I like to cover under venting is blind pocket vents (Figure 2) It is easy to cut a small relief area in the steel next to an air trap and vent out the air through a vent channel to the relief area. Design the vent channel for this blind pocket just like a parting line vent. The depth is always based on the type of material; the width should be either .150 or .200 inch wide, or if the area to be vented is really small, make the vent as wide as you can, up to .150 or .200 inch.

Don’t Feel the Burn
The key to venting is that you should be able to keep increasing the injection speed to see if you get any burns on the surface or in the corners of your molded parts. If you do get burn marks, then you haven’t vented enough or your vents are not designed properly. Check my sketches (Figure 3) to see what you have done wrong. Don’t forget that acetal burns white and most other materials burn with black marks.

The advantage of good venting is avoiding these burns and a smoky look in the knitlines where the material comes together and compresses the area between flow fronts. The heat resulting from this compressed air burns the part surface.

When everything is finally balanced, you will see parts with better cosmetics, no plateout in the mold, and faster cycle times than you might have thought possible.

Put this article in your three-ring binder following the nozzle, sprue, runner, and gate information. Remember to use a sequential approach to optimizing. Start by sizing the subrunner that feeds the gate, and then move upward through the runners and sprue until you get the proper style and orifice size for the nozzle. Then size the gates, install the proper runner, and vent the runner and parts.

We’re getting pretty close to completing the optimizing program. All we have left to do in this series is to review a couple of hot runner and heated sprue bushing situations, a little bit on material dryers, and how to estimate cycle times. See you in a couple of months.