Articles
Intro To Polyethlene Film Extrusion
By David Banig
An extruder is a screw turning cylinder or extrusion barrel. There is a feed hopper at one end of the barrel and a specially shaped hole or die at the other, where the product comes out.
Plastics for extrusion are thermoplastics they get soft when heated and harden again on cooling. When such a material is fed into the hopper it is caught by the screw and pushed through the barrel where it gets hot and softens enough to continue out through the die. The heat is generated by friction as the screw turns in the plastic mass. The energy to melt really comes from the motor as it turns the screw. Sometimes more heat is provided by external barrel heaters and pre heated feed throat.
As the hot soft plastic comes out of the die it takes shape of the hole it passes through a long slit makes a film or sheet, a circular opening makes pipe many small holes make filaments, etc. Once out of the die the plastic must be cooled quickly by air, water or contact with metal and pulled away to be rolled or cut up to the desired dimensions.
The turning screw can be imagined as trying to unscrew itself backward out of the barrel full of material. It can't go back because a bearing holds it in place but its pushed against the material forces that material out the other end.
The die at the other end acts as a resistance. The longer and smaller it is the more screw must work to push the material out (horsepower required of the motor that turns the screw).
The extruder operator controls the temperatures of the screw and the material that enters. The barrel is divided into zones. each with its own heating and cooling controls. Barrel temperatures do have to match material temperatures but are selected as needed in each zone. The rear feed zone is especially important as it affects the feed rate and may control production rate. In the head and die there are heaters and controls also and metal temperatures are usually close to material temperatures.
Material melt temperature is measured at the output end of the screw just before the plastic enters the die. It is typically between 350 to 450 degrees F. but may be more for certain plastics. If it gets to high there maybe cooling problems of the material or perhaps chemical breakdown of the plastic.
Melt pressure is also measured at the output and of the screw and reflects the resistance of the head and die assembly. Typical pressure are between 500 psi and 5,000 psi and for tiny dies or very viscous material it may be even more.
Screw speed is selected usually as high as long as it produces good product. Typical screw speeds are between 50 and 150 RPM.
Motor amps are measured as a warning to stay below the danger limits and to show up surging which causes thickness variation in the material and unexpected changes in material viscosity (ease of flow). Amps and horsepower vary greatly with machine size from small machines 5 to 50 horsepower up to huge lines for compounding which uses 1,000 horsepower or more.
I have been involved with a number of manufacturing purchases and plant start ups and development of new packaging materials from extrusion, printing, laminating, converting, etc. for over 30 years. My goal is to give insight from my experiences to anyone that is involved in the packaging market.
For more information contact dbanig@prflexbag.com
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