MMP Final ReportPolyester Film Metallization Process
Farhan Zafar Khan - 1445108BhaweshRathi - 1345141
Submitted toMiss TanzilaYounasContentsIntroduction1Film Extrusion1Description of Polyolefin and polypropylene1Production of polyolefin (specifically LDPE)1Blown Film Extrusion Process2Description of process2Blown Film Extrusion Equipment2Film Extrusion Equipment2Film Forming Dies5Cooling Systems7Windup Equipment7Metallization Process8Description of metallization process:8Applications:8Description of the metallizer platform:9Coating trials conducted:9Optical Density Measurements:10Barrier measurements:10Conclusion10Works Cited11
IntroductionThe purpose of this report is to describe the process for the production of metallized thin films used commonly in a variety of packaging applications from food to decorative wrapping papers. Film Extrusion(Production of Polypropylene film by extrusion BOPP and BOPET processes)Description of Polyolefin and polypropylenePolyolefins are plastic resins which are polymerized from petroleum based gases. The two principal gases are ethylene and propylene which are used to make polyethylene and polypropylene respectively.Polyolefins are classified as thermoplastics, which means that they can be melted, set and then melted again. In comparison, thermoset plastics cannot be remelted once they have been set.Most polyolefin resins for film extrusion generally are used in pellet form. The pellets are about 1/8 inch thick and 3/16 inch in diameter, usually somewhat translucent and white in color. Polyolefin resins sometimes will contain additives, such as thermal stabilizers.Production of polyolefin (specifically LDPE)Polyolefins are plastics made from high purity alkenes such as ethylene, propylene and butane. These gases are obtained from purification of gas from gas fields, or from refineries as by products. For high quality end products it is important that high purity gases be used as raw materials.LDPETo make LDPE (Low Density Polyethylene), high pressure,high temperature polymerization reactors are used. Ethylene gas, pumpedinto the reactors, is activated by acatalyst and polymerizes intopolyethylene. The LDPE formedflows to a separator where unused gas is removed. Next, the LDPEgoes to a compounding extruderwhere additives are added prior topelletizing.
Figure 1: LDPE production processBlown Film Extrusion ProcessDescription of processThe film extrusion process occurs in a few steps. The resin in pellet form is added to an extruder, which melts and mixes the resin into a continuous form ready to be molded into a sheet. The extruder is heated in stages so that the resin is gradually melted, and a screw in the extruder helps to mix the resin and push it forward into the adapter. The adapter is a connection point between the extruder and the die. The die is the main step in this process. It is used to take the molten resin and convert it into sheet form. Many different types of dies exist and they are used for a variety of different compounds. The two most common ones, blown film dies, and cast film dies are discussed in the next section. After the resin has been converted to sheet form, it goes through a cooler to undergo sufficient cooling, and is then passed through a winding setup to be collected into rolls. Details of these equipment and processes are also discussed in the following section.Blown Film Extrusion EquipmentThere are two basic methods used in the production of film, cast film, and blown film extrusion. In both of these methods, the resin is melted using heat and pressure in an extruder and then forced through a narrow slit in the die. The slit may be either a straight line or a circle. The resulting thin film is either in the form of a sheet or a tube. The equipment used in this process is described below.Film Extrusion EquipmentExtruderAn extruder consists of a hopper, a heated barrel, a rotating screw, a screen changer, a die adapter and a base.
Figure 2: Standard extruder setupHopperResins are droppedinto the extruder feed throatthrough a round or square funnel,called the hopper. An automaticloader on top of the hopperperiodically feeds resin into it.Two basic types of automatichopper feeding systems exist:volumetric feeders, which refill the hopperon a schedule based on theextrusion systems output; andgravimetric feeders,directly feed resin into theextruder.The hopper throat can be cooled using water to prevent pellets from sticking together. If an extruder is run at heat then during shut down, the feed screw must be kept turning until all of the resin has been cleared out, to prevent it from sticking and bridging over the gap. BarrelA barrel is a long tunnel where the heating processes for the resin take place, before sending it to the die to be shaped. Most barrels have smooth bores. However, for some low melting point resins, the mouth of the barrel may be water cooled and have helical grooves. This allows low melt resins to be processed at high extrusion rates.
Figure 3: Barrel with screw.HeaterFor fast extruder start-up,barrel heating is necessary andusually done by electrical barrelheating bands. They are PID controlled, respondrapidly, and are easy to adjust.The heater bands are distributedalong the barrel length in zones. Generally, the barrelis divided into three to six zones.Blowers in each zone decrease theheat when necessary to rapidlycool the barrel when the extruder isto be shut down. Water-cooledzones also are used by somemanufacturers for faster heattransfer.Thermocouple Thermocouples are inserted in the barrel wall, and insome cases into the melt, tomonitor processing temperature.Signals from the thermocouplesactivate temperature controllingmechanisms to regulate the heaterbands and cooling devices.ScrewA motor-driven screw rotateswithin the barrel. Screw speedsrange from 50 to250 rpm. As the screw rotates, the resin is forced forward, the screwchannels become shallowerand the resin is heated, compressed, melted and mixed. The four basic sections of the screw are shown in the figure below.
Figure 4: Different sections of a screwA long, properly designedscrew allows better melting andmixing of the resin, as well asbetter film appearance, closergauge tolerance and increasedproduction rate. Screws are specifiedby their length to-diameter(UD) ratio and compression ratio.Ideally, the screw should be at least24 times, as long as its diameter. A larger UDratio allows enhanced mixing. Thecompression ratio is the ratio of thechannel volume of one screw flightin the feed section to that of onescrew flight in the metering section.Screen Pack and Breaker PlatesAfter traveling through the screw, the molten resin passes through a screen pack, the supporting breaker plate and through the adapter to the die.The functions of a breakerplate are to: Support the screen pack Reinforce the screen packs action Develop back pressure Straighten out the spiral flow ofthe melt caused by the screw Serve as a melt seal betweenthe barrel and the adapter(which a loose breaker platecannot do) Help align the barrel andadapter.AdapterThe adapter guides the resin melt from the barrel to the die as quickly and uniformly as possible. Uneven flow would mean dead areas where some of the melt could be held back, excessively heated and decomposed chemically. A hinged collar generally serves to attach the adapter tightly to the barrel. Adapter heaters keep the adapter at a specified temperature. Maintaining the temperature of the melt when it leaves the extruder to be formed into film is very important. Melt temperatures can be checked by using athermocouple that extends throughthe adapter or die wall and into themelt stream.
Film Forming DiesThe film forming die is attached to the adapter. Its purpose is to smooth and complete melt flow, thus preventing resin degradation from overheating. The functions of the die are to: Force the melt into a form approaching its final shape. Maintain the melt at a constant temperature. Meter the melt at a constant pressure and rate to the die land for uniform film gauge, with allowance for gauge reduction.The die consists of a body, mandrel or pin, heaters and lands. The die lands decrease the speed of the melt flow and build-up back pressure in the die and adapter. The die mandrel can beadjusted to change the die openingin order to control gauge uniformity.Most film extrusion dies are dividedinto heating zones and die heatersare automatically controlled.Many different types of dies are available for the extrusion process. Some of them are: Blown Film Dies Rotating Dies Coextrusion Blown Film Dies Cast Film Dies Coextrusion Cast Film DiesBlown Film DiesIn blown film extrusion, the melt is forced through spiral grooves around the surface of a mandrel inside the die and extruded through the circular die opening in the form of a thick-walled tube. The tube is expanded into a long bubble of desired diameter and decreased thickness. This expansion results from the volume of air inside the bubble, which is introduced into the tube through the center of the mandrel. Blown film dies have a number of circular heating zones. Most blown film dies for polyethylenes are positioned vertically to push the tube upward. However, downward extrusion also occurs. The gap between the mandrel and the die ring ranges from 0.5 to 3 mm. In some dies, this opening can be changed by moving the mandrel lengthwise in the die. Most dies require a change in mandrels in order to adjust the die gap opening. This adjustment changes the extruded tube thickness. A wider ring opening increases output slightly, but due to uneven flow, it may make gauge control more difficult. It also tends to promote film snap-off, particularly when the film is drawn down to a gauge of less than 13 microns. Blown film dies can be as large as 80 inches in diameter, producing film with 125 inches or more of flat width.