EXTRUSION WELDING BASICS
I have, for the past several years, been teaching Concrete Protective Liner welding and Geomembrane welding both for novice welders and “old timers”. During the classroom portion, we discuss the basics of plastics, which are weldable and the basics of the welding equipment we will be using, also what is required to make a successful weld. Once we begin the welder testing portion the old adage that I mention at the beginning of each test session almost always becomes fact. That is that the person who has been welding the longest usually is the first to fail or the last one to pass the examinations.
The more years we do plastic welding the more we lose concentration on the basics taught to us “oh so many years ago”. We tend to concentrate more on the need to get the job or weld done quickly because time is money. We know subconsciously that the most expensive bad welds are those we have to re-weld after the project is finished or are discovered during the final inspection, or in the worst case, begins leaking after the structure is put into full operation, when all the equipment is at another job site and the welders are in another state. There are several reasons for this:
I have on many occasions had an examinee or welder fail a coupon test due only to two things (assuming that he has his temperatures set correctly) purge his welder and count to ten with his preheat. If he had done this, his test results would have been good.
Written by Larry Rowold - PLASTIC WELDING CONSULTANT,
1010 Vermont Ave., The Villages, FL 32159,
Polypropylene Welding Lessons Learned
During the past decade I have been asked to visit many job sites, project sites, manufacturing plants, fabrication facilities and have taught many students the fine art of Polypropylene welding. The most unique site was this past fall when I went to Kuwait.
To set the stage, the week before I went to Kuwait I had just spent 8 days on “North Slope” of Alaska, where it did get above 32° Fahrenheit. I went back to Florida, packed my bag for an extended stay in Kuwait where it only drops below the mid 90s at night.
The project involved fabricating and installing 15 mm PP liners into steel framed and lined tanks. Each tank was approx. 39 feet long, 6’ wide and 8.5’ tall. The owner was able to get PP sheet extruded into 40’ long and 9’ wide sheets which saved a considerable amount of welding time. It also presented me with many unique opportunities to use some creative wisdom and design when it came to material handling. PP has a coefficient of expansion equal to 1.8 mm/10° temperature change per linear meter of sheet stock. All of our work was done outdoors and every measurement was corrected back to an “installed in use temperature” of approx
20° C. When we arrived at the project site, the morning ambient temperature was approx 10-15° C. The sheet temperature, depending upon it’s location of standing vertical or still in the material stack, was always in need of adjustment based on the actual material temperature, the future in use temperature and the ambient temperature. For example, if the actual sheet temperature was 13°, the in use temperature was 20° and the cut line was then adjusted based on the above-mentioned formula.This presented a slight problem until we had everyone trained to make these adjustments, and then it was checked prior to cutting. The meticulousness of the workers I had on this project was beyond my greatest expectations. They would do the measuring and the calculations in pairs, and would constantly consult back with me to see if they were correct.
The sheets, being almost 40 feet in length and 9 feet wide, required a very large lifting beam with 5 cables holding steel sheet lifting “dogs” or clamps. Another factor of difficulty was that the winds were usually 15-25 mph coming from a variety of directions throughout the day. We had at our disposal a very large crane with approximately 75’ of boom, which made it possible to take the sheet from the stack, to the vertical panel saw and then into the tanks without moving the crane. Four guide ropes were used to control the sheet.
Cutting these large sheets both vertically and horizontally was accomplished by utilizing a modified panel saw. We mounted a panel saw on the side of two tanks, installed a set of lower guides made out of steel channel, installed vertical guides at every 6’ along the 80’ of saw runners and used the crane with the lifting beam to move the PP sheet through the saw. As the sheet moved through the saw we were constantly changing the cables and lifting “dogs” to meet the horizontal cuts.
Once the cuts were made, the sheet was laid down and the edges to be welded were chamfered, utilizing a hand held router. The sheets were then installed, flat sheet on the bottom, two long sides and then the two ends. To hold the sheets in place, four steel cages were installed inside the tank to keep the sheets vertical. Spacers were installed between the steel tank wall and the PP sheet to compensate for the expansion which the tank PP liner would experience when the tank would be at its operating temperature. During the morning hours, many large spacers were used but as the day progressed, the temperature increased, thus some of the spacers had to be removed. Working in extreme temperature fluctuations, expansion and contraction compensation was paramount to having a successful project.
Next came the welding. The PP panels were not always installed in the same day or the same time of the day and had to be given time to stabilize in temperature prior to dong any welding. We would install the bottom, one end and one side panel, we would tack weld them into place, and run a 4 mm hot gas bead along the bottom and the end. We would then check our tanks for leaks utilizing a spark tester. We would then install the other side, adjusting our spacers to compensate for the temperature changes, and then the other end, doing the same tack welding and 4 mm root pass and spark test.
In Kuwait, the average humidity at this time was around 20%, and whole country is sand. Sand was everywhere, including in the tanks,obviously not by design. We were constantly using vacuum cleaners to remove the sand as it accumulated inside our work areas.My welding crew consisted of a “broom man,” a “vacuum man,” a “scraper man,” a “welding rod cleaner man” and the Welder. Once the sides, ends and bottom were tacked in place, we could extrusion weld. With afternoon temperatures somewhere between 25 and 45 degrees C, it would be assumed that we should be able to run our 20 mm filet welds in a single pass without getting any voids due to weld temperature cooling/weld shrinkage. This was not the case. Now some skeptics will say that the experienced weld voids (pin holes), plastic worm holes or whatever, were caused by poor quality welding rod, moisture, not using the correct extruder, not using the correct welder technique, not using the correct welder shoe, etc. That was not the case at all. Just to elaborate a little, we used welding rod that came to us in sealed plastic bags, less than two months old and stored in a very dry conex container. We used three different extruders (manufacturers), and there is no humidity in Kuwait, and to be sure, I myself welded some.
The results were the same. The solution was very simple, protect the weld. We put a piece of foil backed insulation on the weld as soon as the weld was made and the voids disappeared. This is one of those projects that appeared very straightforward when based on the drawings and specifications. But in actuality, it required the utilization of all the basics that we were taught when it comes to welding PP in the field. Measuring and adjusting for temperatures, cleanliness, proper equipment utilization, and most of all “Team Work” is what resulted in success.
I keep waiting for the invitation to come to a project site to see how well a project has gone, and how perfect all the welds are.
Written by Larry Rowold, PLASTIC FABRICATION CONSULTANT