The Troubleshooter, Part 91: Nylon problems
Published: June 30th, 2007
Published: June 30th, 2007
Can you get rid of the swirls when molding with glass-filled PA 66?
This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Consultant Bob Hatch of Bob Hatch & Assoc. has more than 45 years of experience finding solutions to processing challenges. You can reach him at firstname.lastname@example.org.
Last month was rough on many molders, according to the calls I got. Moisture problems were popping up everywhere, thanks to the increased humidity this time of year. Just check your material dryers, get the desiccant beds, hoses, and clamps looked at, and you should be OK for a few months. I went into a molding shop the other day and found nine out of 12 dryers not performing well. Don’t panic: Just do the maintenance required for proper molding conditions in your shops.
The part I am going to review for you this month is made of glass-filled nylon—but drying was not the issue here. We had surface swirls that sometimes indicate a moisture problem but in fact are just typical of glass-filled nylon 6/6 products.
The molder sent me these parts and asked me to identify the source of the problem, which was surface swirls at each of the four gates on each of the sample parts. The molder thought he was dealing with defective material and just wanted me to verify his thoughts. After reviewing the parts, I could not verify his assessment of the problem but could offer him some suggestions.
First, surface swirls are a common problem with most glass-filled 6/6 nylons. I say most because DuPont seems to have corrected the problem by using a rubber component in its glass-filled nylon 6/6 materials that covers up the surface swirls. I am not aware of any other nylon manufacturers that use a rubber component but since I do not know every nylon manufacturer I cannot speak for them. This is just my opinion based on past experience.
So what options are available to molders using glass-filled nylon 6/6 when their customers are unhappy with the swirls? Most of the time they just change material to a glass-filled nylon 6 or 6/6/6 and the swirls generally disappear. Either of these products can usually be molded without the swirls, but watch out for slight differences in the physical properties, such as heat deflection temperatures and processing requirements.
A thick-walled nylon 6/6 part exhibited surface swirls typical of that material, which could be fixed by changing material. However, the glossiness around the gates indicated that they were sized too small.
This pretty much answered the molder’s question, but I got to looking at the parts for other problems and came up with a couple of things.
The first thing that caught my eye was that the gates were placed about halfway between the center hole and the end-of-fill area. I usually like to place gates around a part’s center hole, and in this case on top of the collar that surrounds the center hole.
This was a good place to gate for a couple of reasons. First, the wall thickness was greater in this area, so flow properties were better as it moved from thick to thin wall sections. Second, we want to flow material in one direction whenever possible. Flowing from the middle to both the inside and outside of the part sometimes causes warpage. Flowing from the center to the outside ensures the part is as flat as its geometry allows.
The glossy area around each of the gates indicated that all four gate diameters were undersized for this application. My suggestion for gate sizing with glass-filled nylon is to be sure the gate diameter is equal to 90% of the wall thickness being gated into. In this case, the gates were .080 inch each, and the wall they fed into was .160 inch thick. Using the 90% rule, the gates were too small by quite a bit. Why were they using a 50% rule here?
Probably because this rule is often applied for unfilled nylon gates.
Going by my 90% rule, these gates should have had a diameter of at least .144 inch to properly fill the .160-inch wall. And placing them on the ring around the center hole would allow for this size increase. This alternate gating area was .190 inch tall by .145 inch wide, which would enable us to use a gate that was .171 inch by .131 inch—a much better size for flowing glass-filled nylon.
Increasing the diameter of each of these gate drops would eliminate the oily-looking gloss areas around each gate, but we would still get some blush at each of the gates. To eliminate the blush we would have to switch to one of the other materials I mentioned.
I went back to the number of gates being used and planned to tell the molder that I usually use an odd number of gates (three or five) on a round part, all the same diameter and all feeding into the collar around the center hole. Sometimes I recess the gate and if I do, I also match the recess with a transition dimple just below the gate on the part core. Transition dimples are typically .050 inch deep and about .250 inch in diameter. The goal here is to maintain a nominal wall thickness through the gate area; otherwise, if we recess but don’t use a transition dimple, we end up with a thin, flow-restricting wall section to gate into.
Another problem was that the sprue O-diameter was not large enough for this runner system. The main runner was .300 inch deep in a trapezoidal shape. A trapezoidal runner is OK for a three-plate setup like this, but the feedpoint to the runner should be larger—more like .312 inch or .343 inch.
The cone-shaped feedpoint to the runner was only .190 inch in diameter, so a larger feedpoint diameter would probably quadruple the flow of material through this point. This would allow the molder to reduce his barrel heats into the 540°F range and still be able to fill and pack these parts in such a way as to eliminate any warpage and possibly speed the cycle time.
The last few comments I made notes on had to do with the proper melt temperature for a 33% glass-filled nylon 6/6: 540°F. The setpoint for barrel heaters is usually around 520°F to get a melt temperature of 540°F. Dry the material at 190°F during production and drop the heats to 175°F if you need to shut down the machine for a couple of hours.
Nylon sucks up moisture quickly this time of year, so don’t let nylon regrind sit in open containers for any length of time before you mix it with virgin material and get it back into the material dryer.
It’s also important to use a general-purpose nozzle on the molding machine for glass-filled nylons, and not a reverse-taper nylon nozzle. These reverse-taper nylon nozzles are only for unfilled nylon. I see molders making this mistake at least 10 or 15 times a year as I travel around doing seminars and consulting. Does this indicate a lack of employee training on our part? Probably.
All I have to do now is consult the free section of the ides.com database, called Prospector, and see if any other filled nylons than the ones I mentioned would offer advantages for this part.