There are no toxic plastics. None of them. This will be my intro to everything I do that allows it. If you don’t believe me, you are in the majority, and that’s the problem. Find out the truth and try to convince others. You’ll find out how a missionary feels in a hostile land.
Now, to extrusion. Our machines may have a lot of information on them, or none at all. Get it into your hard drives while you can, before it disappears or you can’t read it reliably any more, as you may need it, maybe now, maybe later. This is what you may find.
Extruder ID plate
There should be a plate on the body of the extruder, which identifies the maker, size, model designation, serial number and sometimes sales order and date of manufacture. This will help identification if long-distance service is needed. Today’s communication possibilities make service easier, provided you can get the right person on the other end of the line.
Somewhere on this plate (or another) may be the gearbox reduction ratio, the amount and type of lubricant needed, and also the thrust bearing capacity or an estimated bearing life at standard conditions, typically 5000 psi and 100 rpm. From either the capacity or standard life, you can calculate your own projected bearing life based on expected rpm and pressure. The numbers may be very high—20 to 200 years, for example—as it is easy and relatively cheap to overdesign a bearing, so that its failure is usually a result of lubrication failure and/or misalignment. When you can’t find a number, or there is reason to believe the original bearing has been replaced, find out who made the bearing and get an estimate of its capacity, using photos of the bearing, if needed, to identify the maker or to help them get the right numbers.
Some extruders show information about the electrical heating (power, voltage), which will be useful to the person who installs the line or moves it later. Heater data are also needed for separate heaters on the head zone and, especially, for large dies, which lose a lot of heat by radiation during operation and need to be preheated before startup. I’m less worried about the cost of heat loss (calculate it before you worry too much), more so about requiring long startup times, especially with resin in the system that can cook and degrade while waiting for the head and die to get hot enough long enough.
Don’t forget the heads, accessories (screen changers, gear pumps) and the dies. They may not have information visible on them, but it’s still important to know the basics, such as makers, model numbers, dimensions and heater capacities.
This plate will show the maker, type of motor and other identifiers that you’ll need to get service and replacement parts. Look for serial numbers (common) and dates of manufacture (rare but may be coded into the serial number). Find the four key numbers:
- Power, in HP (horsepower) or kW (kilowatts). One horsepower is 746 watts or 0.746 kW, so 4 HP equals 3 kW, close enough for most work. Remember that this is maximum power under ideal conditions, and that the available power may be lower at less-than-top speeds. Motors are different, but it will pay to learn what you have driving each line, and how to find out if you need it all or can use it all.
- Top speed, typically but not always around 1750 rpm. (Initial U used on some German motors. ) This is the top speed of the motor, which is converted through gears (and sometimes pulleys) to the much-lower screw speed. The reduction ratio of the gears is sometimes also visible on the ID plate or a separate plate on the gearbox. Don’t overlook pulleys, which change things. The best way to get this ratio is to measure motor-shaft speed (stroboscope) and screw speed (count rpm, unless you really trust your display) at the same time. Some DC motors have two numbers on their plates, such as 1750/2100. The first is normal top speed at full voltage—the speed varies directly with voltage, so it works like a dimmer switch. The second is a higher speed possible by weakening the magnetic field that the rotor turns in. This field-weakening system can give you more rpm but no more power, so it’s useful only if full speed still leaves some power unused, you have the cooling and takeoff to handle the increased rate and the sales to support it.
- Maximum voltage. This is critical for DCs, as screw speed is proportional to voltage. The relationship is more complicated for ACs, and power factor gets involved in costs—necessary to know if you have one and simple to explain, but too much for here.
- Maximum current (Ioad, amperes). This protects the motor, as too many amps mean a soon-to-burn-out motor and no production. Often, the motor will smoke before it stops, so consider smoking motors very seriously. It is easy to put alarms on the ammeters, but it is also easy to forget about doing it.
Although I’m concerned with primary drive motors here, the same concern holds for others in the system, especially drivers of gear pumps, which control production rate.
Know if you have manuals for each line, and, if so, where they are kept and where the backups are. And we can’t assume that each machine is still the same as it was when sold, as people replace screws, motors, even barrels and heaters. Unless you’ve been with the machinery since “birth” and have a good memory and records, treat data for old machines as probable, but not certain. Certainty, it’s been said, is the hallmark of the fanatic, as the need for certainty drives people to believe and say whatever is necessary to avoid looking “wrong.” The Art and Science of Doubt should be a required course in engineering schools, along with statistics and design of experiments . . . and the psychology that explains plastophobia.
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 Chicago on April 5, Toronto on June 6 and Cincinnati on June 28. E-mail him at the address listed above for more information.