Many applications are of the type that push Print&Apply technology to its limit; i.e. very long labels with rotated bar-codes and high application rates on fast moving products. It is important that the potential problem areas of such an application be well understood. The following discussion outlines some of these areas.
A fundamental limitation to the Print&Apply process is the speed with which a printer can generate the label. Printing a label is composed of four time components:
- Data transfer of label format and/or variable field information
- Image processing by printer
- Printing time
- Dead time while printer waits during application process
In any application where labels are long and the application rates high these time issues become critical. Very little margin for variation is allowed for any of the four time components. Let us look at some of the implications of each of the four time components.
The control of most printers require the 2 way communication of serial data transfer. This means that transmission rates are limited from 2400 to 9600 baud, depending upon the equipment and the noise immunity of the wiring and serial standard used. Labels with large amounts of graphical information (even special fonts are graphical) require large file sizes and thus long transfer times to load a format. This can be as much as a minute on a large label with lots of graphics. Once a format is retained in printer memory, variable text fields can be inserted as a smaller transfer. These variable fields may need to be time based information such as a time and date stamp. Each change of such a variable field requires transmission time and then will force the printer to reprocess the label image.
In any instance where several printers are being accessed by a single host, this transfer time is added to by the ability of the hosts system to either write to a port or to obtain access through a network. A print spooler based system supporting multiple ports is slowed by several seconds by the spooling process and port setting (very few systems can simultaneously write to several ports); a network system depends heavily on network load as to the speed with which a remote device can be reached.
Any need for real-time information such as a weight scale input or critical time information must be done by an intelligent device on the individual applicators. If accurate time information is required then this intelligent device must have an internal clock facility.
Almost all printers will allow data to be received for the next label while a label that has been processed is being printed but the printer will not allow next label to be printed until all of the label information is completely received. Be very sure that your labelling rate does not exceed your systems ability to get the next label into the printer!
A simple character string of 12 characters plus control and command over head may take 100ms at 2400 baud for just the data alone not including time for port set-up or network access delays.
Once label data and format information is loaded into the printer buffer, the printer must convert this into a label image. This time is dependent upon the type of image and its total image area. (Remember that a 8" x 8" label with only one square inch of print is still an 8" x 8" label to be imaged by some printers) Graphics or "soft" fonts are time consuming to process; bar-codes are graphics as well and take more time than simple text but are quicker than other graphic elements because of printer firmware designs. An 8" x 8" label with mostly graphics and bar-codes can take as much as 1 minute to image on an 8 bit processor system.
Even the change of a single variable field will require the reprocessing of the label image. This processing will occur on the buffered label must be complete before the previous label is finished printing or delays in cycling rates will occur. A simple field change can add 0.3 seconds to 0.5 seconds to the label printing cycle for that first label that is changed.
The time it takes to print a label is simply a function of print speed and label length if no special movements are required such as back feeding of the label. Most printers can operate in the 6"/sec to 8"/sec print speed range. If we take the example of a 10" long label printing at 8"/sec then the printing time is 1.25 seconds.
Print speed is limited by the printer's physical characteristics and by the printer's ability to create an acceptable image quality. In the case of a rotated bar-code ("ladder" printing configuration), this can be a serious limitation. Rotated bar-codes require the printer to work very hard at processing the bar-code image; recalculating the print head control for each bar as it is printed. The thermal characteristics of the printhead, the firmware and processor of the printer and the ribbon and label quality have a large impact on how high a print speed readable bar-codes can be generated. When a "bad batch" of ribbons or labels is encountered, the only way to keep the system running is to turn down the print speed and readjust the "burn temperature".
So your system has been running well for the first two months and the printhead begins to wear or the labels or ribbon quality changes. You find that the only way to obtain a readable bar-code (ANSI grade C or better) is to slow the printer to 6"/sec. Now your printing time becomes 1.67 seconds. Will you now have to slow your production line down to allow the Print&Apply application cycle to keep up?
Most Print&Apply machines print a label and then apply the label. During the application portion of the cycle the printer is inhibited from printing because there is no applicating pad to receive the label - its busy applying the last label printed. Whether the application process is tamp or even just a non-contact, blow-on system there is time required for the application portion of the cycle. This time varies from 0.25 seconds to several seconds depending on label characteristics, distance to product and the mechanical speed of a tamp system.
In any applicator that must inhibit printing during application this is dead time that is added to the cycle. Changes in label material or humidity may require an increase in this time to get satisfactory results.
Now lets consider the cycle as a whole process:
Consider a 10" long label printing at 8"/second with a time field updated every 5 minute.
Add the whole field change time of 100ms for transmission and .5 sec for reprocessing of the image. This is necessary because few systems can monitor an applicators cycle closely enough to chose an optimum moment to make the change.
This means that a direct print and apply system must always have 2.1 seconds between product or it may mislabel or completely miss a product after each time change.
Now let us look at the introduction of other factors that typically occur in the real world of production.
The humidity climbs and you need to adjust the application cycle to allow more time for the label to settle on the vacuum grid before application. Now you need a dead time of 0.45 seconds, only two tenths of a second more.
If barcode readability becomes an issue for any reason and speed must drop to 6"/second
Now we have gone from 40ppm to 22ppm. With direct Print&Apply it is the 22ppm rate that decides whether we miss a product or catch all of them, not the average of all application times. These are still optimistic numbers.
There are ways to obtain the advantage of an average printing rates, primarily through the buffering of pre-printed labels. This is termed the "Loose Loop" technique. This allows the labels to be printed independently of the application cycle. The printer prints until the loop buffer is full. The more labels in the buffer the closer the application rates can approach the maximum output speed of the printer.
Consider a printer printing 10" long labels at 8"/second continuously. The labels print without stopping between labels. Now labels are being produced at a rate of 48 labels per minute. Even at 6"/second print speed labels are produced at 36 labels per minute. These are instantaneous print rates and are a measure of how much margin the printer has to catch up after it is delayed by a data field change. With a reasonable buffer of even 5 labels, application rates of 30 labels per minute will still allow the printer to stop printing for 4 to 5 seconds for label data changes. With a 5 label buffer a label field change once every five minutes will never limit application to less than 35 ppm even at 6"/second print speeds.
Long labels go quickly!. At 30 products per minute, a 10" long label uses up 18,000" of label stock every hour. A typical 10" diameter roll has about 10,000 of label stock depending on gauge and tightness of wind.
The normal capacity of a printer is a 8" diameter roll of labels. This might only last 20 minutes!!!
Ribbons are another supply item that will run out. They come in lengths between 300 metres and 600 metres. This will provide 35 minutes to 78 minutes between ribbon changes.
A direct Print&Apply applicating system will have to stop application during a label roll or ribbon change. This means that the production line must be stopped or product accumulated for 1 to 2 minutes. (if your applicator was already at its limit how does one clear a buffer of product).
Use of the Loose Loop technique means that labels can be printed independently of the label application process. How many labels are pre-printed is a matter of how large the buffer loop is or how large one wishes to force it to be. The loop control has the facility to be over ridden and force the printing of labels. Extra labels printed allow an operator to change a ribbon and splice on a fresh roll of labels without stopping the applicator. For a system running at 8"/sec print speed and applying at 30 ppm, the operator would need to force the system to print for 3.3 minutes to gain enough buffer to allow 2 minutes to refresh the supply of ribbon and labels. (N.B. Systems that are designed around label buffers of only 2 labels do not typically allow for the forced printing of extra labels. This means that the system must still be stopped for roll/ribbon changes.)
If label time stamping is being changed every 5 minutes a ribbon / label roll change would only offset the time stamping by 1 to 2 labels off from where the change might have occurred without a roll change. This is true if the change was made just after the change to the time stamp.
It is for the reasons stated above that the Loose Loop technique is best for many applications. Production is a process that runs 24 hours a day and suffers from conditions not always being idea. Any labeller installed in a production environment must have sufficient excess capacity to provide a margin for problems. No system should be evaluated on its highest performance or on its instantaneous rates. Long labels applied at high rates don't allow printers any room for error or even the normal operational demands. A Loose Loop Print&Apply application system affords the following advantages:
- Maximized output from printer allowing high application rates
- Applicator can apply large and long labels
- Applicator can apply large and long labels at high surface speeds
- Buffering of labels allows application during roll / ribbon changes
- Printer faults do not immediately disable application process
- Large external unwinds can easily be accommodated. (e.g.. 15" dia. rolls provide approx. 30,000" of label stock or approx. 70 to 90 minutes between roll changes for this application)
