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The Troubleshooter, Part 50: Vent before you mold

August 1, 2001

9 Min Read
The Troubleshooter, Part 50: Vent before you mold

This 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. 

When I opened my next-day-air packages today I found a runner system that was just begging to be talked about. The part was clear with pretty thick walls, and it looked like a handle or knob for something. The runner system was three-plate and was being fed by a heated sprue bushing. 

I looked over the runner (Figure 1) and part and came up with a hit list of problems that needed to be addressed. The feed point to the runner was too small to properly pressurize the runner system and I suspected the molding machine nozzle had not been drilled out to match the flow tube diameter of the heated sprue bushing. 

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Figure 1. Though the main runner of this three-plate system was acceptable, other adjustments were necessary to eliminate unwanted cosmetic defects.


The subrunners were trapezoidal, which is OK for a three-plate, but they weren't deep enough. The sucker pins were located directly over the drops and were probably restricting the flow of material as it transitioned from the subrunners to the drops. Finally, the gates were slightly tapered, but not enough to do a lot of good, and the gate land was definitely too long. 

The reason I had been asked to look at the parts was because they had a small cosmetic defect that looked like melt fracture or shear splay radiating out from the gate (Figure 2). 

I felt pretty confident that the gate was causing the problem since the defect was in a direct line with it. I didn't really care what the defect was because I should have been able to eliminate whatever it was by correcting the basic problems in the mold. 

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Figure 2. In this clear, thick-wall knob, shear splay or a melt fracture produced a cosmetic defect near the gate (see area of part highlighted by mark).

Fine-tuning the Runners 
I got out my trusty verniers and started to check out the runner system. I really don't try to rebuild the entire runner system when I get these parts in the mail; I typically look for a starting point that is pretty close to what I want, and then go from there. This usually works well because in most cases the runners are undersized, if anything, and moldmakers don't like to rebuild the runner plates if they don't have to. They don't mind cutting steel but don't ask them to weld anything. 

In this case the main runner was a trapezoidal shape .340 inch deep by .390 inch wide. This was actually bigger than it needed to be and would no doubt work. 

The feed point to the main runner was .150 inch and the heated sprue bushing orifice was .125 inch. The feed point needed to be opened up to something as big as the main runner depth or pretty close to it. I didn't want to get too big here to avoid stringing and drooling. I started with a .275-inch opening to the runner and a .250-inch-diameter orifice in the heated sprue bushing. 

Stringing and drooling are controlled by the amount of heat being used in the orifice area of the heated sprue bushing. If we don't use unusually high heats, which we wouldn't in this case with the runner problems corrected, then we won't see any stringing or drooling. 

I planned to recommend to the molder that he verify that the nozzle was drilled to match the heated sprue bushing flow tube diameter. For amorphous materials the flow tubes in these heated sprue bushings or hot runner manifolds are almost always .500 inch in diameter, so the nozzle needed to be drilled out to .500 inch also. 

The subrunners were .190 inch deep by .245 inch wide—smaller than they should be for an amorphous material. I recommended that the subrunners be opened up to .250 inch deep by .300 inch wide. This is a little deeper than I usually go for a subrunner, but we had three of them coming off each end of the main runner, so a little larger size here would help with the extra volume requirements. 

The drops were also on the big side. Where they attached to the subrunner, the diameter was .345 inch, most likely a result of the moldmaker's attempt to eliminate the restriction to material flow caused by the sucker pin interference. 

I didn't see this as a huge problem; it was just going to create a little more regrind and possibly add a little bit to the cycle time to get the thick section of the drop to set up enough before the mold opened. 

Defect Source: The Gate 
The gate was .075 inch in diameter with a .090-inch land. These three-plate gate lands should always be tapered right down to the part. The actual land measurement is typically only .002 inch when done correctly, not .090 inch like we had here. 

Not only that, but the current gate diameter of .075 inch was actually too small for the .200-inch wall thicknesses we were feeding into. For any amorphous material I usually recommend the gate depth of an edge gate or the diameter of a hot tip gate to be 65 to 90 percent of the wall, depending on material. Polycarbonate gates are on the bigger side of this rule, acrylic in the middle, and polystyrene and SAN are on the smaller end. 

I just knew this material was either polystyrene or SAN so I suggested the molder open the gate diameter to .125 inch. The small end of the drop needed to be tapered down to the surface of the part and the gate diameter needed to be .125 inch where it met the part. When properly tapered, the small end of the drop would look like the taper on the writing end of a retractable ballpoint pen, with the point retracted. 

I knew the gate land was the culprit because I could see where the gate had snapped off from the end of some of the drops, at the point of the step where the existing taper began. These accidental built-in sharp corners create areas of notch sensitivity that often result in unwanted breakage. 

I like to control where my runners break. When the gates break off like these did, you lose production; you have to shut down to get the plastic out of the gate. 

Figure 3. Venting is one of the most important steps in designing a mold, yet it is commonly omitted. Shown here are two methods for venting the cavity: (A) Individual part vents can be placed around the cavity; or (B) a perimeter vent can run around the entire parting line, which then vents to atmosphere.

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Necessary Venting 
At this point I was just about finished with my review and all I could see that was left to do was to work on the venting requirements for this mold. After a quick call to the molder, I found out that the mold had no vents. 

Runners need to be vented to eliminate as much air as possible before the material pushes it through the runner channels and the gates. I know it's hard to believe, but as I travel this great country of ours, I still see molds that have runners that are not vented at all—even in shops that I consider the best of the best. It's just not something that's done day in and day out—and it should be. 

Troubleshooter's Notebook 

Part: Thick-wall, clear, PS or SAN knob.

Tool: Three-plate, heated sprue.

Symptoms: Melt fracture or shear splay at the gate.

Problem: Runner feed point, nozzle, and subrunners too small; poor location for sucker pins; gates not tapered adequately; gate land too long; no venting.

Solution: Enlarge runner feed point, sprue bushing, subrunners, and gate diameters; taper gates; vent all runners and cavities.

Result: The Troubleshooter predicts no more rejects and a faster cycle.

Runner vents are generally .003 inch deep and as wide as the runner being vented. At .060 inch from the runner parting line, drop into a .040-inch-deep channel to atmosphere. Draw polish the vent lip to an A1 finish to make it self cleaning. 

For this type of material, individual part vents are only .001 inch deep and .200 inch wide. At .040 inch from the parting line, drop into a .040-inch-deep channel to atmosphere. And, of course, polish as mentioned before (Figure 3A). 

If you want to use perimeter vents for your part, you can do so by making the parting line vent depth .001 inch and running it around entire parting line (Figure 3B). Go out from the parting line with a land of .060 inch and drop into a .040-inch-deep channel to a race track that runs completely around the part cavity. Then vent the race track to atmosphere using the runner vent information I just gave you. Don't forget to draw polish this vent lip to make it self cleaning. 

In summary, all we needed to do was open up the feed point of the runner to .275 inch and the heated sprue bushing orifice to .250 inch. We could leave the main runner as it was and open up the subrunners to .250 inch deep by .300 inch wide. The gates should be tapered at the end of the drop and the gate diameters should be opened up to .125 inch. Most importantly, the molder needed to vent the runner and check the cavities for proper venting. Venting eliminates the need to slow injection speed to give air time to get out. In some cases this slower speed is what causes sink and undersized dimensions. 

I called the molder with these suggestions and he was going to get busy on the changes right away. I suppose I could wait for his feedback, but I know he'll tell me that it ran great after the changes were made, rejects are down, and the cycle is sped up by 25 percent. Now that is what I call a win/win situation. 

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