Gross Versus Net

 In Extruder Screws, Extruders, Preventive Maintenance & Tech Tips

Several years ago, a customer had ordered a new screw for their thin sheet process. They requested a throughput rate of 1600 lb/hr.

The new screw was designed so that 90% of horsepower available was consumed, which resulted in a maximum throughput of approximately 1800 lb/hr at full screw speed.

Shortly after the customer had received and installed the new screw, they called and complained that they were only getting 1200 lb/hr when they had requested 1600 lb/hr. We spent several phone calls trying to diagnose the problem and nothing seem to change the throughput rate of the screw, it stayed at the 1200 lb/hr.

It was then decided that I would have to travel to the customer’s Chicago facility and resolve the problem. After I had received much grief from the company’s General Manager upon my arrival, we went out on the factory floor.

The first thing that I told the customer that we needed to do was to shut the line down so that we could do a rate check directly from the lips of the sheet die. So we did our rate checks at 25, 50, 75 and 100 percent screw speeds. And the rates were within 2% of the theoretical calculated design rates. I was totally baffled as to why we got the expected rates at the die but the GM was complaining that they were only getting approximately 75% of the theoretical rate.

So it was decided to string the sheetline up to see if that gave us any indication as to why they were not getting the full throughput of the screw that they expected.

After stringing up the line, the operator pulled the slitter knives into position. Immediately I knew what was the problem. The knives were set to slit the 48” sheet to 36” wide and they had 6” of trim on each side of the sheet. Well, 36” divided by 48” equals 75% and 75% time 1600 lb/hr equates to 1200 lb/hr!!!

The throughput rates that the GM was seeing in his production report every day was the NET weight excluding the 12” of trim. I had to explain to him that we determine the GROSS throughput rate as what comes out of the die, not what he winds into his rolls.