Tap your suppliers for training.
Properly trained people are more important to your goal of increasing productivity than are either raw materials or equipment. With all the changes occurring in blown film technology today, your people can never have enough training. A good way to find inexpensive assistance is to start with your equipment and resin suppliers. Many of them will assist in everything from conducting one-day seminars at your site to advising how to implement an ISO 9000 certification program.
Keep material inventory accurate and up to date.
How many times has your manufacturing plant started on an order and run short of a certain material? Even once is too much. Before set-up or even before scheduling, verify that the correct amounts of material are on hand to complete the job.
Streamline job schedules.
Material changes can be very costly, especially in coextrusion. More effective scheduling saves time, money, and purging compound. Try to schedule jobs in a sequence that minimizes changing resins, web width, or film gauge. This can't always be done because of customer demands, but start each day with a short planning session to optimize job order. Ideally, you should follow soft materials such as LDPE with a tougher, stiffer material such as HDPE. But if you're extruding the tough, stiff material and the next job requires a much softer grade, use a transition resin like fractional-melt LDPE to flush the barrel and screw. Save the cost of purging compound for tougher jobs or for when you have gel problems or carbonization.
Organize your resin changes.
For a clean transition, let resin in the extruder hopper run low but not out, while you are still running good product. Leave the extruder running at 15 rpm or more. Use compressed air to clean vacuum lines, probes, and the extruder hopper thoroughly, while letting it run empty. Hook up the connections for the next resin to be extruded. Then watch the resin level in the hopper drop until the feed section of the screw becomes visible before introducing the next resin. Slow the extruder down to 6 rpm for a minute or two to be sure the feed section is full of the new resin. Now speed up the extruder to flush it out, repeating if necessary. (Use the same procedures with purging compound.)
Identify all material-conveying lines.
Resin-feed lines from silos, daybins, and gaylords should be clearly identified for operators and technicians. The cost of material cross-contamination can be a major blow to the bottom line, costing you wasted production time and materials.
Most blown film processors still blend and feed raw materials volumetrically. But blending by weight rather than by volume is the most accurate way to control the percent of each ingredient. Batch blending with a single gravimetric gain-in-weight metering system for each extruder is a less expensive way to improve control, though continuous loss-in-weight gravimetric blending of each component at the extruder throat is more precise. Gravimetric blending is also the only way to monitor individual layer thickness in coextrusion.
Tune your processing conditions.
With the wide variety of raw materials and machinery available today, it's essential to match your process conditions to the particular resin and equipment. Temperature profiles, feed-throat cooling or heating, and screen-pack configurations are basic factors to be tuned. Try different temperature profiles to see which one maximizes feed rate and melt-temperature control. Screen packs should be changed routinely and especially before any resin change. Die lips, air ring, and IBC systems should also be routinely cleaned to remove resin build-up. Once conditions and outputs are optimized, make certain you have a record of the recipe, so you can repeat it next time.
Watch screws and barrels for wear.
Screw and barrel wear can have a major effect on pumping rate and melt-temperature control. So your screws and barrels should all be checked and measured at least once a year. Screw-to-barrel clearance is normally 0.005-0.006 in. and can be measured with special calipers. Screws worn beyond these tolerances can be rebuilt less expensively than replaced.
On the subject of barrels, grooved-feed types seem to be the best for consistent pumping rate and temperature control. But smooth-bore barrels are better for handling fluff recycling and in some cases are more accommodating for materials such as nylon and EVOH.
Maximize outer air cooling.
Blower sizes (in cfm), cooling-air temperature, and air-ring design are critical points to optimize. Dual-lip air rings will increase output and bubble stability but can have a disadvantage in limiting blow-up ratios. Air chilled to the lowest temperature the system will handle (generally 28-32 F) and correct blower sizing should significantly improve output and bubble stability.
Maximize internal bubble cooling.
Too many blown film processors don't use IBC systems to increase cooling and output. For those who don't have IBC, it's worth looking into a retrofit. New IBC designs manifold the air evenly and at much higher air exchange rates than previous designs. Again, chill your air to the lowest temperature the system will handle and increase air volume to improve output. Outer and inner air temperatures should be controlled, though some resins are more sensitive to colder or hotter air than others.
Unfortunately, many IBC systems pulse or pump while trying to control bubble diameter. Pulsing is caused by irregular or insufficient air exchange. Again, some resins are more sensitive to it than others. If pulsing isn't corrected, it can require more allowance for edge-trim waste.
Reduce edge trim.
It's harder to recycle coextruded edge trim and roll scrap because the different materials in film structures may not be compatible. Most coex films waste about 10% in edge trim. For example, if 45-in.-wide finished film has 2.5 in. of trim on each side off a total layflat of 50 in., that's a waste factor of 10% (which isn't uncommon). Edge guides, web-center guides, and good bubble stability can reduce that trim waste to 1 in. per side, a reduction in waste of over 50%. Based on your output rate, it's easy to figure what that could mean in monthly or yearly savings.
Handle finished rolls carefully.
Proper roll-handling equipment is very important to an efficient operation. Without it, goods can be damaged or improperly weighed, and employees are exposed to injury, all of which are costly. Mobile hydraulic or electric lifting devices like roll carts or scissor lifts are commonly used and relatively inexpensive. A central, dedicated area for handling, palletizing, weighing, and wrapping rolls works best for most operations.
Get the jump on maintenance.
Good preventive maintenance ensures that your equipment is in top condition to achieve optimum output and to avoid costly downtime. Since most blown film manufacturers run seven days a week, 24 hours a day, maintenance should be scheduled daily, weekly, monthly, and yearly. For example, air filters should be checked daily or weekly and gearbox oil monthly or yearly, depending on use. Keep a log of run hours on each piece of equipment to anticipate when it will require maintenance. Some computer controls will keep maintenance logs automatically. Output performance can also be tracked and compared against good maintenance records.
Know your customer.
Know as much as possible about how your customer uses your products, the type of equipment used to convert it, and how your customer's customer uses the ultimate products. Set-ups can be more efficient if you know what specs really matter the most to your customer. Knowing what your customer expects will also result in fewer returns and other unpleasant surprises. Schedule regular visits to customers and invite customers to join quality-improvement meetings at your plant. Ongoing dialogue on product improvement can be very productive.
User-friendly computer controls with a supervisory screen can show an operator what's actually happening on the extrusion line and let the operator monitor and control material feed rate, screw rpm, drive amps, melt pressure, and melt temperature. The program should also store recipes and print out historical data.
Taking the next step of automating the die, air ring, and IBC gets a bit closer to breaking the bank for some processors, but it deserves mention because the payoff is so great.
Automatic gauge control and reduction in gauge variation aren't just great sales tools and quality-control features: Tighter gauge control also results in better yield on machine time, savings in raw material, and quicker set-up times. So if you do not already have automatic die, air-ring, and IBC controls, take the savings gained from the 14 preceding tips and reinvest them in these systems.