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Keep shrinkage at bay by gating at the end, not the center, of a long part.

The other day a molder I have talked with on the phone but never met walked into my office. He said he was in town to visit his daughter, who attends an Iowa college, so he thought he would stop in to see if I could help him with one of his company’s recent molding challenges.

He put a part and runner on my desk and asked me to do a review for him focusing on the length and width dimensions of his part. Before getting to the dimensions of the part I could see he was also having cosmetic issues with the part’s surface. I asked him if the swirls on the surface were of any importance to the customer. He said he had been more concerned about the dimension problems and had not noticed the surface issues. He did say a better-looking surface usually makes everyone happier.

I told him the swirls are usually associated with glass-filled nylon 6/6 that is not impact modified. (He was surprised that I knew what the material was in this part since he had not told me.) Nylon 6 and 6/66 do not exhibit these surface swirls, and neither do glass-filled nylons with a rubber impact modifier—just straight 33% glass-filled nylon 6/6. This does not make the material a bad choice; it just changes the visual appearance.

When I asked what the biggest issues were for the part, he answered that the overall length was undersized, which meant the part didn’t fit its mating part. In addition, the part’s width was a little larger than they thought it should have been, but this wasn’t as important.

Back to the drawing board
I did my usual review of the part, starting with the runners and gates. The runner was a cold sprue attached to two short main runners that connected to the inside edge of the part’s middle. The sprue O-diameter was .210 inch and the nozzle orifice was .175 inch—not bad for unfilled nylon but much too small for filled nylon.

The short, stubby runners were .180 inch, which again was too small to flow glass-filled nylon through the edge gates, into a 6-inch-long part with .160-inch walls. Obviously, the present design for the sprue and runners was not going to provide the appearance or the correct dimensions required.

What to do? First, the undersized part length had to be addressed. Gate location is critical for dimension control. To keep a part flat and straight as well as produce the best overall length dimensions, we must gate the part at its end, not its middle.

I usually start on a problem like this by designing a new runner system and sizing the gates correctly. This called for a two-cavity mold instead of the existing single-cavity setup. My guest was in favor of this because he wanted to be able to mold more parts and not use any more machine time. He also indicated that the tool was originally a prototype mold.

When designing for glass-filled nylon vs. unfilled nylon, I use a flow ratio chart created by Monsanto back when it used to manufacture Vydyne nylons. (That business was spun off several years ago to Solutia Inc.) Despite its age, the chart still works, and it showed that glass-filled nylon 6/6 requires a flow ratio of 120:1, while unfilled nylon can use a flow ratio of 220:1. You can determine flow ratio by dividing the flow length of the material to fill the farthest section of the part cavities by the wall thickness of the part. According to this chart, we needed a flow ratio of 120:1 or less to be sure we could fill and pack the part from a single gate location.

In this case, we divided the part’s length of 6 inches by the nominal wall thickness of .160 inch to get a flow ratio of 37.5—far less than the maximum allowable and more than enough to tell us that we could actually fill and pack this part from either end. This would not only give us less shrinkage in the length, but also keep the part from warping or bowing. A molded part that lies flat on the table and doesn’t do an impression of a banana is a good thing in our business.

Runner redesign
Now, to lay out this runner system: We started with the level-two subrunners that fed the gate and made sure they were equal to roughly 1.5 times the nominal part thickness of .160 inch. We set the diameter at .250 inch to ensure we could pack out any voids and sink.

Since these were the only subrunners coming off each of the level-one subrunners, we needed to make the level-two subrunners the same diameter as the level-one subrunners. The main runner diameter needed to be larger than these subrunners, so we set its dimension at .312 inch. Then we called out a cold sprue with an O-diameter of .344 inch to complete our runner system.

That left us with a gate decision to make. Did we want a standard edge gate or a curved tunnel gate? Either would work with this part. The curved tunnel gate is sometimes called a “cashew” or “peanut” gate. Whatever gate we decided to use, we had to be concerned with the abrasive characteristics of glass-filled nylon. This meant the gate area material had to be hardened or a metal such as stainless steel. This metal could be inserted at the gate or the gate could be welded up with a long-wearing metal to provide abrasion protection.

The customer chose to use an edge gate design because he knew his customer was not concerned with a gate mark on either end of the part. I suspect this part was somewhat hidden in actual use.

Venting
At this point we were about halfway done. We now needed to make recommendations for the venting of this new, bigger runner system and ensure that the cavities and cores were adequately vented.

We reviewed runner venting first since we had to start somewhere. Here are my standard recommendations for runner vents:

• Must be .003 inch (easy-flow material) or .005 inch (stiffer-flow material) deep.
  
• Land length should be .060 inch.
  
• Drop into a .040-inch-deep channel to atmosphere.
• Draw polish the vent lips to an A1 or mirror finish to make them self-cleaning.
  

We vent the runner every time it turns a corner, giving us an opportunity to get rid of some of the trapped air. We like to feel a little bit of “feather” flash on the ends of the runner to indicate that the runner system has been properly vented.

Part vents are not as deep as runner vents. The type of material being molded determines the depth of a part vent. Glass-filled nylon vents are set up with a depth of .001 inch for easier-flow varieties of nylon and .0015 inch for stiffer-flowing grades. In this case I felt we should start out with vent depths of .001 inch to be safe.

We could either perimeter vent 98% of the parting line, excluding the portion of the parting line on either side of the gate, or use individual vents at a rate of one vent per parting line inch. In either case, the vent dimensions for glass-filled nylon in part walls such as these would be as follows: .001 inch deep and .200 inch wide; a land length of .040 inch for each of the individual vents; and .060-inch vent lands for perimeter venting. As with the runner vents, the vent lips must be draw polished to an A1 or mirror finish to make them self-cleaning.

Production, not prototype
We summarized these potential changes by reviewing the types of glass-filled nylon that would make the surface appearance look better. We reviewed the chance to make this prototype mold base into a two-cavity mold with an extended runner system. I cautioned him that the prototype mold base may not be usable with the new two-cavity cold runner design, and he was OK with this. (Maybe they had already been paid for a new two-cavity mold and just had not gotten around to having it built.) He also felt comfortable with my venting recommendations, so I walked with him to the door and gave him directions to Interstate 80.

It was almost a month later when I received an e-mail from him telling me about his success with the new mold. He was very appreciative of the help I gave him and invited me to visit their facility the next time I was in the area.

January, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.

About Bob Hatch
Bob Hatch is one of the leading on-the-spot problem solvers in the molding industry. Mr. Hatch spent time as the technical programs manager at Channel Prime Alliance and managed a molding operation for more than 25 years. Currently, he writes articles for Injection Molding Magazine under the pseudonym The Troubleshooter.

The Plastics Troubleshooter