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June 7, 2001

6 Min Read
The Troubleshooter, Part 48: Acetal's special needs

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.

This month, as I sat wondering what might make a good story for this issue, a package arrived that fit the bill exactly. The material was a copolymer acetal, but it could just as easily have been a homopolymer. I could see a four-cavity, cold runner, two-plate mold, and small parts. A metal insert was molded into each part, making them look like little metal rods with molded-on bushings. The complaint was that the levels of molded-in stress were too high and the dimensions were going out of the acceptable tolerance range. 

I could see the runner system was way too small for conveying the acetal to the cavities. The parts were edge gated and the toolmaker had put in very shallow flat runners. The main runner was only .055 inch thick and the subrunners were .045 inch. The sprue appeared to be big enough, but it had been cut in half for shipping, so I couldn't determine the actual sprue OD. 


New measurements: 
Nozzle orifice: .175 inch
Sprue: .200 inch
Runner: .150 inch
Subrunner: .125 inch
Gate depth: .042 inch
Gate width: .075 inch
Land length: .021 inch

Part dimensions were out of spec on these parts because the runner system and gates were too small for acetal.

I could also tell that the gates were much too small for these parts. One of the basic rules of designing for acetal is that it is OK to use small runners, but the gates need to be big. This rule is based on the volume of material needed to fill and pack the cavities and adjust for the quick freeze time of acetal. 

Another rule to remember is that when using an edge gate, that gate should be fed with a full round runner. Also, the gate needs to come off the center—not the edge—of the full round runner. The gate needs to be sized for the part wall thickness, with an emphasis on depth and land length. For parts like these with .080- and .090-inch wall thicknesses, I typically suggest a gate depth of .060 to .075 inch. 

Since the parts do not require a lot of volume, I would make the width of the gate the same as the depth and then use a land length of .030 inch. But as I looked at the parts again, I could see that the ring they were gating into was not as thick as other areas on the part, which changed my gate sizing numbers. The rib that was gated into was only .055 inch thick, so my gate depth would be 75 percent of this number, or .042 inch. 

Since gating was into the thin rib, I figured I would compensate by making the gate wider than I normally would to allow for the volume requirements of the thicker wall sections. Because of this, I would recommend a gate width of .075 inch and a land of half the depth, or .021 inch. At these sizes, the gate was .042 inch deep and .075 inch wide, with a .021-inch land length. 

When designing for acetal it is OK to use small runners, but the gates need to be big.

Next, I sized the runner, sprue, and nozzle orifice. I always start with the runner that feeds the gate, and I wanted this runner to be 1 1/2 times larger in diameter than the thick section of the part. In this case, I could see that .080 inch was the thickest the wall got, so I made the subrunner that fed the gate .125 inch. The main runner that fed the subrunners needed to be somewhat larger, but not so big as to create extra regrind. Therefore, I would suggest a main runner diameter of .150 inch. The sprue OD would then need to be .200 inch and the nozzle orifice would be .175 inch. 

Now all that was left was to vent the runner and the parts. Runner vents should be .003 inch deep, as wide as each of the runners being vented, with a land length of .060 inch out from the parting line, dropping into a .040-inch-deep channel to atmosphere. 

The sprue puller can also be vented. In this case, .0005 inch per side could be taken off of the end of the pin for a .060-inch land length. A .040-inch-deep groove should then be milled from that point to the head of the sprue puller core pin. 

Part vents were only .0005 inch deep and from .150 to .200 inch wide, depending on the part geometry. It was necessary to go out .040 inch from the parting line with the vent lip and drop into a .040-inch-deep channel to atmosphere. To promote self cleaning, the vent lips and the end of the pin should be draw polished. 

Acetal Tips 
One thing to remember about acetal is that copolymer acetal needs to be dried at 180F for 3 to 4 hours prior to molding. Copolymer regrind needs to be dried at 230F and then kept dry with the virgin material at 180F. (I have been told by manufacturers of homopolymer acetals that they do not like to have their material dried. Something about the drying changes the natural color from milky white to a light pink.) 

Troubleshooter's Notebook

Part: Copolymer acetal, insert molded part.

Tool: Four-cavity, cold runner, two-plate mold.

Symptoms: Molded-in stress was too high; dimensions out of tolerance.

Problem: Runner system was too small for acetal.

Solution: Size the gate for the part wall thickness, enlarge gates, add venting to runner and parts.

Result: Problems solved, molder pleased. 

Also, if you want a glossy surface on your acetal parts, inject fast; for a low-gloss finish, inject slow. A hot mold (180F) is used to mold a more rigid part and a cold mold (50F) is used to mold more flexible parts. 

Using stainless steel for mold construction is recommended to counteract the corrosive properties of acetal, especially in the poorly vented areas of the mold. Some toolmakers like to chrome plate other steels for acetal, but when acetal degrades it turns into hydrochloric acid, which is a natural chrome stripper—the chrome just flakes off. 

Any time you see blush at the gate on an acetal part, the gate is too small. Typically, processing people raise material melt temperatures and injection pressures to higher-than-needed levels to get the parts to fill and pack when the gates are too small. The result is that the parts have a lot of molded-in stress, and close tolerances are difficult to achieve or maintain. 

One problem with insert molding—the process used with these parts—is that sometimes the metal inserts need to be preheated to about 150F so that the insert cools at about the same rate as the acetal. Under these ideal conditions you probably won't see any cracking of the acetal around the insert as the parts cool. There wasn't any cracking in this case, but it can happen. 

High molded-in stress and lack of dimensional control are exactly what happened to these parts. I passed my suggestions on to the molder and a couple of days later he called, saying everything was going much better and that he was actually enjoying molding again. 

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