posted February 17, 2004 07:19 AM               

High viscosity nylon must be super dry in order not to have any loss of viscosity while melt processing. You are looking at requiring moisture contents below 0.1% . In the old processed nylon you had, the solid state polymerization would dry out the nylon for you. Now you need to look closely at the delivered moisture content of the nylon you are purchasing and your handling procedures so that you keep it very dry. This is not an easy task as nylon will absorb water from the air very quickly. Nor is it easy to dry nylon once it has absorbed water.

A high vacuum vented extruder will help with minor moisture absorbtion problems, but is not the best solution as some changes in molecular weight distibution will still occur.

posted February 18, 2004 10:30 PM               

welcome to the single screw forum

When you say you've tried drying, what sort of dryer, temps residence time and transfer method (from dryer to extruder throat)

posted February 18, 2004 11:45 PM               

I have tried many things to keep the material dry. The only dryer units we have are dessicant dryers, but we have made sure that the dessicants are changed and the dryers are maintenanced on a regular basis. We make sure that the dryer dewpoint is kept at -40 and that there are no leaks in the system. We originally would take material out of a dryer and place it in the hopper. When this seemed to be the problem we mounted a dryer hopper directly to the machine. Once again we made sure the dryer was working properly and the loop was as sealed as possible. We then trickled some nitrogen into the throat to help keep the material dry. The first time we tried this it worked for a while, but when we tried to run production for an extended period of time the melt took a nose dive again. At first I thought maybe the material wasn't drying long enough but the residence time of the material in the hopper is over four hours. This was material directly from a sealed bag and each lot is tested when received. The dryer is set at 180 degrees F which is the recommended drying temp. We have also tried the enhancing process that we did with the previous material, Zytel 101. This helps because it does dry the material but there are still problems there and for cost cutting reasons we are trying to get away from this practice. All the info we receive says we are doing the correct things to process this material but I guess we are missing something. We also run some GS and Nylatron profile shapes and have received a few complaints about the parts being brittle. I think that covers it. Thanks for the welcome.

posted February 19, 2004 07:35 AM               

You should check the following items;

The moisture content of the material going into the extruder. Try sampling when the process has gone bad. Needs to be 0.1% or below. Situations where the starting moisture is high and the pellets are large may result in a situation where 10 hours or more of drying is required with a dessicant bed dryer. This is the most likely cause of your problem.

Screw and barrel wear. High viscosity materials can be overly stressed in wear gaps.

Check the melt temperature with an IR gun. High temps can degrade the resin.

Check the residence time in the process. Extrusions tend to run the extruder slow. In this case a little moisture can go a long way. Try a smaller extruder and reduce the residence time.

posted February 19, 2004 02:50 PM               

I think Tom is on to it with his advice to increase the residence time, I'd be inclined to run the dryers hotter also (105C/221F).

For me, the clue to this is that the material extrudes acceptably at the start, when presumably the residence time is greatest. Drying residence time is a hard one to get right. You might want to look at vacuum dryers, perhaps try and get a loaner from someone in your vacinity.

posted February 21, 2004 09:01 AM               

I've seen this more often that not.