Follow These Tips to Fire Up Your Cold Blown Film Line

In an ideal world, you’d only shut your blown film line down to accommodate a changeover, and then only briefly. But most film processing plants don’t operate under ideal circumstances. Sometimes machines have to be idled for maintenance, plant shutdowns, business slowdowns and a variety of other reasons.

Starting them again after a shutdown requires finesse, as there are multiple pieces of equipment involved that all interact. The purpose of this article is to look at these components individually and provide best practices on how to get your line up and running as quickly and safely as possible.

Heating Extruders and Die: If your line has been completely cold for a day or more, the extruder and die heating process has to be carefully done to avoid heater band damage or startup could cause long delays in detecting and replacing a failed heater band. Someone should be present when the heats first come on to ensure each heater band is working. This is done by monitoring the amp draw on the heater monitor displays whether panel meters or on a visual HMI. Older heater bands can fail on initial startup so it is a good idea to monitor the heating process for 10‐20 minutes. Failures can be silent or create a noise like a small gun being shot.

Either way, someone should be present during the initial cold start to alert maintenance if something goes wrong. Remember, working on a fully heated line is a difficult process for maintenance personnel. The hotter the equipment is, the longer it takes to change out a damaged component.

Small lines are generally quick to heat up: The 2-in. die in our lab takes about 2.5 hours; a large 80-in. die will take 24‐36 hours to heat up fully. Another safety concern is the possibility of a heater band starting a fire. This is a real concern when the temperature controller that manages a heater band (there is one controller per heater zone) does not receive the temperature feedback it requires to monitor heating progress. This often happens after an unattended bubble break or an over-pressure condition that forces resin out of a burst disk. In both cases, melted resin is flowing onto the outside of the machine — after it cools, it has to be removed.

As maintenance personnel are removing the excess material, temperature feedback wires can be damaged, which breaks the feedback signal to the temperature controller. If the broken wire is left unrepaired, the next startup will result in a runaway heat zone. The temperature of the heating zone can easily exceed the resin flash point which then starts the fire or decomposes the resin inside the machine into a liquid. Both cases are particularly dangerous to the operator. Therefore, care should be taken to watch for temperature controllers that are indicating more current draw compared to the neighboring zones.

It is a good idea to warm up the gearbox oil by slowly running the nip rollers, winders, oscillating nips for 30 minutes or so.

When a film line is down for several days, particularly when there is high humidity in the factory, moisture can accumulate between the metal surfaces and the heater bands. A method to simmer the moisture away requires setting the initial heat setpoints to 212°F/100°C . This avoids steam pressure
building up under the heater bands which can break the brittle ceramic heaters. Maintaining this low temperature setting for an hour is usually sufficient. After that, increase the heat set points up in steps and avoid the temptation to set the heats well past the set point in hopes of getting the line running quicker.

If you attempt this, you are likely to overheat the material inside the extruders or die and cause it to partially decompose. Partially decomposed material produces carbons in the film that can take a considerable amount of time to clear either through purging or taking the die apart for cleaning.

Warming Driven Rollers, Oscillating Nips and the Winders: Particularly with factories located in colder climates and with older machines, it is a good idea to warm up the gearbox oil by slowly running the nip rollers, winders and oscillating nips for 30 minutes or so. The web-management systems use tension control devices that are tuned for warm operating temperatures.
Just like a car in the cold weather, it is a good idea to allow the web handling motors some time to warm up. It also lets you know if there are any issues.

Exercising Nips and Other Compressed Air Dependent Devices: It is also a good idea during this time to test the open/close functions of the primary and second nips. It happens quite often that the compressed air required to create the movements has been turned off while the line was down. The extruder and die heating process take quite a while, so make sure all the mechanical movement devices will work properly when the time comes for production.

Be sure to exercise the equipment within the safety limits and procedures established by your company. If you’ve never exercised a piece of equipment before, ask someone to show you how to do it properly and safely.

Ensure Cooling Systems Have Cold Water: Next, double check that cooling systems are up and running and water sources required for the chilled
nips, extruder freed throats and coiling coils for the air ring and internal bubble cooling (IBC) are all on and water temperature is correct. Most of the time, the chilled water temperature is between 40 and 50°F (4‐10°C). Alert maintenance if water is turned off or if water temperature indicators are trending up or staying well above the proper cooling water temperature.

Ensure Blenders Are Ready to Operate: Blenders have many electromechanical devices that often do not operate properly after being shut off for an extended period of time. As you are preparing the new resin recipe for first job after startup, make sure all the weigh hoppers are working, the vacuum loader is moving resin and not just air and hopper mixers are indeed mixing.

Preparing the Air Ring: If the previous shift left solid material on the air ring, the best bet is to allow time for the heating die to soften the material until it is pliable and easy to remove. Generally, it is best to leave the air ring blower running at a very slow speed (10‐20%) but not off. This prevents the air ring from overheating, which makes startup very difficult until it cools back down to a proper temperature. Clean the air ring (and IBC cooling) openings of all leftover resin and silicone grease. Anything blocking the openings will affect the gauge profile regardless if you have an auto gauge control system or not.

Once it is all clean, make sure you know the position of any air ring or IBC adjustment. If there was work done on the air ring or IBC, the maintenance personnel likely did not reposition the adjustments to proper production positions. The most important adjustment is the lower lip adjustment. For most air rings, getting the correct adjustment is easy to achieve by rotating the adjustment clockwise until the adjustment stops. Then rotate the adjustment counterclockwise between 1 to 2 turns, depending on the air ring. It will be a good starting point.

It is a great idea to listen and remember how the blown film line sounds when running properly.

On smaller dies, it is often helpful to use food-grade silicone grease thinly coated over the lower lip area and the lower portions of the forming cone. This prevents the film from sticking to the metal parts of the air ring during startup. Just make sure to not glob it on and block an airflow path. Blocking the airflow path in any way will reduce the film cooling rate and cause the film to be thinner in the area of the blockage.

Don’t Start the Oscillating Die Until Line Is Running Stable: Most lines with an oscillating die have a high-pressure shutdown or a low-temperature shutdown. In either case, the idea is to prevent the die from oscillating until after the polymer is soft enough inside the die to easily move with the changing flow paths as the die oscillates. Without this safety element, it
is likely the oscillating die drivetrain and possibly die seals would be damaged.

The best bet is not to start the die oscillation directly after a cold start until it is obvious the line is running in a completely stable manner. This can take 30‐60 minutes after the line is started before all the internal metal surfaces are at the correct temperature. Repair of an oscillating die drivetrain or seals can take several days to complete, so it is important to be sure the process is fully stable both heat wise and bubble wise before starting the die oscillator.

Listen to the Line: Running a blown film line takes all the human senses except taste. The one sense required which may be less obvious is hearing. It is a great idea to listen and remember how the blown film line sounds when running properly. There are many motors used in running a blown film line and each has a unique sound.

The sounds of sudden acceleration or deceleration are most important as these sounds should be absent during production. So, when starting the line, pay attention to how the extruders sound when the production rate is increased. The same applies when slowing the line down, pay attention to how the tones emitted by the motors change in pitch and volume. The reason for this suggestion — the quicker you can react to a sudden acceleration or deceleration, the less potential damage will be done to the production or worst case, the equipment.

Operators should not have music in their ears when running a blown film extrusion machine. They should have hearing protection, but this won’t prevent them from hearing changes in how the machine is running — if they are listening.

Stopping the line: The fastest way to stop a line is to activate the emergency stop. In a safety condition, this is the best way to stop the line for the sake of operators in harm’s way. But the emergency stop should never be used
as the standard method for stopping the line. Under normal conditions, the best method is to slow the extrusion rate to zero. In fully automated systems, this also slows down the downstream equipment. In a manually controlled system, the operator is responsible for slowing down the downstream equipment.

Under nonemergency conditions, slow down the extruders to zero over the span of a minute or two. Once that is complete, slow down the nips and winders, then the air ring and IBC until all are stopped. Clear any unwanted film or excess polymer form the outside of the machine before restarting.

ABOUT THE AUTHOR: Daniel R. Joseph founded D.R. Joseph, Arlington, Texas, in 1987 to supply the blown film industry with engineering support. That idea grew over the years into patented innovations, which resulted in DRJ becoming a global leader in blown film bubble-management systems. His current activities include R&D efforts for product development and training systems.  Contact: 817-987-2030; danielj@drjosephinc.com; drjosephinc.com.