Why can't you make it the same way every time?
Let's say, for example, the compounding line is shut down on Friday night after producing Lot No. 1 and started up again on Monday morning to produce Lot No. 2 of the same material. Suppose that the value of a process parameter (extruder torque, pressure, melt temperature, etc.) or a product attribute (elongation, melt index, color, etc.), is different for Lot No. 2 than it was for the previous lot. It actually may be better, but in any case it is different. We'll call this a short-term reproducibility problem.
If the lots are produced one right after another, there aren't too many things that could change in this short time frame. On the other hand, if it has been weeks or months since the last lot of that product was produced, there are more factors that could change. We'll call the latter situation a problem of long-term reproducibility.
An extruder screw cannot wear out from one lot to the next when they are produced continuously, but it can wear out between production campaigns that are weeks or months apart. So what could cause the lot being compounded on Monday to be different from the one produced last Friday?
To solve this mystery, we must look at factors that the compounder controls and at those factors over which the compounder has limited or no control. The compounder has control over three things: equipment, procedures and process.
In the weekend shutdown scenario, when the line was started up again on Monday morning the extruder barrel temperatures were swinging [+ or -] 10[degrees]C from setpoint. No one in the operations department had any idea that anything had been changed over the weekend. All they could see was that the extruder was behaving quite differently on Monday than it did on Friday. But the maintenance department had worked on the cooling tower over the weekend, so they shut off the cooling-water flow-control valves to each of the extruder barrels. After completing the maintenance, the flow-control valves (which are precisely set when the PID controllers are tuned) were all fully opened rather than recapturing the former precisely tuned settings.
Similar problems can occur if electric heaters burn out during a production run on Friday. When the line is started up again on Monday morning, the extruder temperatures cannot reach setpoint.
There may also be procedural problems in the plant. Are there standard operating procedures describing how to start up, run, and shut down the compounding line---or can each operator do it his own way? There may be some details about the process that the operator controls but are not well documented. So when "Joe" goes on vacation, the process runs differently. Joe never mentioned that he intentionally plugs half the die holes to get the glass-reinforced nylon strands to run through the water bath. It's the same die plate, but it runs differently on Monday when Joe isn't here. Everything must be documented, no matter how apparently minor the detail.
Process setpoints must also be checked. They can usually be reproduced if there are quantitative means of measurement and if the measurements are actually recorded somewhere. For example, the water temperature for the cooling bath or underwater pelletizer can influence the pellet size and shape. If it is arbitrarily adjusted to some value ("cold") with no written temperature record, it will be very difficult to reproduce the same value a second time. Everything must be documented!
The compounder must also look for raw-materials and environmental changes. Raw materials are within the compounders control as long as they are within specifications for melt index. However, two lots representing the extremes of the melt-index specification range will produce different properties as well as different melt-index values for the resulting compound. If the second lot coincides with start-up on Monday morning, it's easy to point a finger at the raw materials.
Environmental conditions are also a potential culprit. When the resin comes from an outside storage silo or railcar, temperature and humidity or dewpoint can change dramatically within days and can affect the processing of hygroscopic polymers. Imagine that the resin pellets conveyed from the railcar outside are at 30 F on Monday but were at 70 F on Friday. Moisture condensation on the pellets could suddenly become a problem.
If weeks or months have elapsed since the last lot was produced, there are other potential reasons for the new lot to have different properties or to run differently. Equipment wear is the most common cause of such discrepancies, but not necessarily in the way you would expect. Perhaps machine wear is suspected, but the maintenance record shows that mechanics replaced all the screw elements and barrels just last week. The problem is that when this lot was last run, the machine was worn out. Now with new parts and tight tolerances, the "worn-out" process conditions can't be reproduced. The solution is to have routine preventive maintenance procedures scheduled at regular intervals (e.g., every six months) to monitor the wear on screws, barrels, and die plates.
Another possible problem occurring with modular twin-screws is incorrect machine set-up--using the wrong screw design, die plate, or screen pack. When the new lot comes out differently and someone checks that the correct screw design is installed, the production supervisor says, "Yes, we just pulled the screw and double-checked it against the machine set-up sheet from the last run." The problem may be that the machine set-up was incorrect the last time this lot was run, but there is no way to verify how the equipment was actually set up previously because there are no adequate records. Sign-offsheets for machine set-up and proper training of operators will prevent these types of human errors.
Raw materials can also be responsible for differences in properties or processing when there is a longer time interval between compounding lots. New lots of fillers, pigments, and other additives can behave differently, depending on moisture content, particle size (and distribution of particle size), and ash content. Certificates of analysis for current lots of all raw materials should be compared with older lots. It is also a wise idea to retain samples of all critical raw-material lots for at least two years in case problems arise with end-product performance in use.
Environmental conditions can also contribute to long-term reproducibility problems. Seasonal variation in temperature and humidity may not be that great for a plant in Phoenix, Ariz., but a compounder in Rockford, Ill., certainly feels the difference between summer and winter. Behavior of hygroscopic materials in low humidity during winter months is different from the summer months when 90% relative humidity prevails. Solution: Materials-handling systems for compounding lines should be designed to minimize the effect of the environment on raw materials--e.g., using nitrogen-purged hoppers to keep out moisture.
Minimize the risks
What can a compounder do to minimize the chances of making a different lot of the same product next time? Start by documenting the condition of the equipment (e.g., wear), the process, and the procedures used. A good preventive maintenance program is also essential to avoid having the compound properties become a function of the machinery condition. Finally, proper training of operators, mechanics, and supervisors in machine set-up helps avoid inconsistencies that will show up in the product.
Sources of Short-Term Reproducibility Problems
[check] New lot of raw materials.
[check] Procedures differ from lot to lot.
[check] Unscheduled weekend maintenance.
Sources of Long-Term Reproducibility Problems
[check] Machine wear (or repair) between production campaigns.
[check] Temperature/humidity influence on raw materials.
[check] Incorrect machine set-up, either then or now.
Adam Dreiblatt has over 20 years' experience in troubleshooting twin-screw compounding. For the past 10 years he has offered consulting and training through his own firm, Extrusioneering International Inc., Randolph, N.J. (www.b4uextrude.com). He can be reached by e-mail at firstname.lastname@example.org.
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|Title Annotation:||Troubleshooter: COMPOUNDING|
|Date:||May 1, 2005|
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