Extrusion basics: How much do you love me? Evaluating a used extrusion line

First, my monthly reminder: There are no toxic plastics, none of them. When you meet anyone who thinks otherwise, try to find out why. Ask for statistics, toxicity data and their sources. You probably won’t get any, or they will be selective or come from questionable sources. Anything you think is really significant, please let me know.

Now, to extrusion. How much do you love me? My favorite answer to this often-serious question is, “11 on a scale of 10.” That may make the asker feel more secure, but doesn’t get to the heart of the matter. Some things can’t be easily quantified. Love is one of them; the value of a used extrusion line is another.

Sometimes I get asked to estimate value or capacity, or both, perhaps for legal purposes or by someone wanting to sell or buy a used line. My easy answer to a buyer is to invoke the rule of thirds.

First, find out the cost of a new line of equal size (diameter and L/D) and power (kW or HP), if possible from the same company that made the used one. Then plan to pay one-third of that price to buy the used line. If it’s selling for less, it may be very old or in poor condition, or both. It may well be a bargain, but such a line requires a visual inspection and should be competently tested before going further. Like a racehorse, it’s no bargain if it can’t run. If the price is more than one third, find out why. There may be a good reason: The machine may be relatively new and is being sold because the company has folded or decided to move far away, or it lost some business and the machine is now redundant.

Then, allow another third to fix what’s broken, often electricals, and shipment costs.

The final third is the savings achieved by buying used.

But it usually isn’t that easy. A line may be more valuable depending on how soon it can be put into operation. In some cases, where the line is still at its owner’s plant, it might even be leased and run there to get seasonally needed production right away, and then sent to its new location in the off season. If you do this, consider the possibility of leaving the line in place and having the owner continue to make your product under lease or some other creative financing. This is particularly attractive if you don’t do extrusion yet, as it saves you the hassle and cost of buying and storing resins and additives, finding employees and running 24-hour days. Like a car rental, such agreements must deal with responsibility if the line breaks down or the product doesn’t meet predetermined standards.

Another way to estimate value is to go online and look for similar equipment on websites of used-extrusion-line dealers. Prices are not usually posted, but you may encounter friendly salespeople who believe that if they quote prices, you may buy from them rather than someone else. In evaluating prices, make sure they include location, options to inspect and test, packing costs, availablility and payment terms—Now? On receipt? After successful operation? I ask for age, but I also try to see the machine nameplates, which may show the year of manufacture or at least a serial number that will allow me to check with the OEM.

As for capacity, there are many limits, and they often depend on the product to be made as well as the equipment, so a combination of experience and data are needed. If I have to make an experienced guess, I use my table of limits for different resins: 4 lb/hr per HP for HDPE; 5 for PP; 6 for LDPE; 8 for PS; and 10 for PVC. But that is only the beginning. We may not always be able to get full power from the motor because of the speed reduction ratio. Also, higher viscosity (lower melt index) may need more power, but that may be countered by preheated feed and more barrel/die heat.

Downstream equipment (line speed) may control the production rate, and this usually is limited by cooling. This may depend on roll size in a flat film/sheet line, or tower height in blown film, but limits can often be raised by modifications such as internal bubble cooling in blown film, added surface agitation in water-cooled profiles and sometimes just by cleaning the water passages in the rolls. 

Cooling is also affected by melt temperature, so controlling that temperature by conditions or screw design will affect maximum salable output. I include the word salable, as product value depends on control of thickness and physical properties, which may deteriorate at high production speeds. In short, 70 lb salable is worth more than 100 lb of scrap.

Allan Griff is a veteran extrusion engineer, starting out in tech service for a major resin supplier, and working on his own now for many years, as a consultant, expert witness in law cases, and especially as an educator via webinars and seminars, both public and in-house. He wrote the first practical extrusion book back in the 1960s as well as the Plastics Extrusion Operating Manual, updated almost every year, and available in Spanish and French as well as English. Find out more on his website, www.griffex.com, or e-mail him at [email protected].

Griff will present live seminars in Cincinnati on June 28 and in Costa Mesa in Southern California on July 31. E-mail him at the address listed above for more information.