How to minimize waste

In an existing application at my plant, the operators are manually stopping one of our printing machines based on how much they can see left on the unwind roll. As you can guess we have alot of waste and I am looking for a cost effective way to minimize this waste. Any suggestions would be greatly appreciated.

Thanks,

What we do on our unwind stands is have a photo eye looking from one side of the roll to the other a little ways before the core. When the photo sees the reflector on the other side we turn on a beacon telling the operator to start getting ready to change the roll. We are sending a VFD a variable speed signal which we convert to how many feet have come off the roll, and this number starts accumulating once the reflector is seen. When the feet count up to a preset point the machine stops. We set the photo eye to give the operator enough time to get everythng oraganized for an orderly and efficient change of the rolls, and the "Feet till Shutdown" setpoint is adjustable in order to fine tune the amount left on the roll. Not all rolls have the same amount of feet left on them due to winding tensions, etc. Works pretty well for us.

How to go about it will depend on what's available.

For example, if the PLC already knows the line speed, then by adding a sensor that signals once per revolution of the unwind, you can calculate the unwind diameter by keeping track of the time per revolution. Knowing the diameter of the roll, you can automatically stop when reaches a minimum diameter. Of course, if you use different size cores, you will have to provide some way to compensate.

If your PLC doesn't know the line speed, you can mount a photocell that looks across the unwind and is blocked by the roll. When the roll winds down and the photocell gets cleared, you can either stop immediately or warn the operators to slow down and prepare to stop. In that case, if you use different size cores, you will have to adjust the position of the photocell when you change the chucks.

There are also ultrasonic or laser sensors that can measure the roll diameter.

I think one type of this application is the Markem 9880, we use this in our plant to print the code in plastic packing of pasta.

Here are a few useful equations to do an extremely accurate stop to diameter (assuming you can determine / calculate current diameter accurately)

First:
You need to know how long it will take to decelerate to zero speed:

Decel_Time_Minutes = (Decel_Ramp_Seconds / 60) * (Current_Line_FPM / Max_Line_FPM)

Second, you need to know how many feet will be run during the decel ramp:

Feet_To_Stop = (Line_FPM / 2) * Decel_Time_Minutes

Third, to establish the trip point to begin a stop:

Trip_Stop_Diam = SQRT( (Stop_Diam_Target_IN ^ 2) + (Feet_To_Stop * Material_Thickness_Mils * 0.048 / pi))


Then do a simple compare....
ActDiam < Trip_Stop_Diam = Trigger Line Stop.

I don't know anything about paper rolls, etc. However, I have read the replies so far and there is quite a bit of similarity to work I had done many years ago in the steel roll equipment industry.

Do machines like "accumulators" tend to work for things like this?

In other words, could you pay out the roll of paper until reaching a low point, then pay it out fast into an accumulator until the end is reached and sensed. Then you could replace the roll while the process continues at normal line speed. Just a thought.

Considering how fast paper machinery generally runs, that would be one heck of an accumulator

rdrast said:

Considering how fast paper machinery generally runs, that would be one heck of an accumulator

Click to expand...

Or the operators would have to move a lot faster than they do here!

rdrast said:

Considering how fast paper machinery generally runs, that would be one heck of an accumulator

Click to expand...

I had no idea how fast the line speed on this machine was. That parameter was not cited by the original post.

Whenever I run my money printing machine, I only run a few dozen two hundred dollar bills at a time. Otherwise, the ink smears and President Clinton's portrait looks fake...

Tim, We use splicers that will automactically splice off to a waiting roll. The festoon in the front of the splicer has the reserve paper for realing out while the brake is applied, splice and accel cycle. That way the registration and tension is held for less waste. There are some used units around, if I had the basis weight of paper, max FPM and width of paper, I could ask around for some pricing. I know where a 34" unit is currently.

A cheap and easy way of adding a roll end shut off is to mount a sensor to read the side of the roll. I used a Balluff sensor(BOS S50-PR-5-C01-PP) It has a range of 10"(approx.) This might be an issue depending on the width of your press. I wired the sensor to a relay an put the contacts of the relay in series with my stop circuit. I also added a bypass switch, so the operators can run the press with no web.

If you know the core diameter and paper tickness approx. Can you hit it with a laser and calculate how much is left until the roll runs out?

Autosplice

If you are interested we have a KTI autosplice we need to get rid of. We are running a very thick material with alot of roll changes and could not get enough use out of the machine. It will due up to 48" web widths.

A Butler or KTI splicer sounds like the way to go.

I saw this working recently

http://www.cognex.ie/solutions/checker/EN/main.asp

Its a low cost vision system (about 1600 euro i think)

It can check for patterns, brightness or contrast

You only need to attach a laptop (USB) to set it up, after that you can remove the laptop.

I am sure you could set it up to look for the core reducing to a certain size.