Blow Film is the process used for making thin films products like plastic bags.
Molten material that comes out of the extruder enters a tubular die. Air is blown into the die which expand and stretches the material like blowing a balloon. This blown up material is called a film tube. The air flow pushes the tube upwards as it expand and cool. From there, it enters a sizing basket made up of small rollers to regulate the size and shape of the tube. The tube is then forced to form a flat sheet by the collapsing guides. The nip rolls presses the sheet and acts to pull the tube upwards in the entire process. The folded tube then travels down a series of rollers before it is being wind-up into rolls.
The rolls are taken to a cutting machine where it will be cut and sealed to the desired size as bags.
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The word extrusion has its roots from Greek which means push out. The extrusion process is in essence a continuous stream of material being push through a shaping tool so that whatever comes out take the shape of that shaping tool.
Take the example of a toothpaste. When the tube is squeezed, a steady flow of toothpaste comes out – taking the shape of the tube’s opening which is round. Therefore, the toothpaste that comes out will have a cylindrical shape.
Noodles and spaghetti are made the same way by mechanically pushing the dough through tiny holes in a noodle machine. So are hamburgers. Chunks of meat are fed into a grinder that chops and pushes the minced meat out of the machine is a continuous mass.
Extrusion is the most efficient and widely used process of melting plastics. It can be used as a “stand alone” machine to directly shape parts; or used as a melting device that is coupled with other secondary shaping devices.
When used to directly shape parts, a shaping tool known as a die is attached to the end of the extrusion machine. This process is called extrusion molding. The extruded part is known as the extrudate. By changing the die, different profiles of extrudate can be formed. Imagine a chef creating various decorations on a cake by squeezing icing or cream from a cream bag attached with different tips.
When using extrusion machines as a melting device, it is coupled with other shaping devices. The extrusion machine (known as an extruders) becomes an integral part of a larger machine that includes a secondary shaping device. Example of such process are injection molding and blow molding. Using the earlier example of the hamburger, the extruded minced meat is conveyed directly to a press that forms the meat into patties.
This diagram above shows a normal direct extrusion process. Plastic materials in the form of granules and pallets are fed into the hopper which falls onto the extrusion screw. The screw which turns inside the heated barrel conveys the plastics towards the die.
The plastic material is melted using external heat source and heat generated by fiction within the barrel. The screw moves the molten plastic until it exits through a hole at the end of the barrel which has a die attached to it. The die imparts a shape onto the molten plastic which is immediately cooled in a water tank to solidify the plastic. Auxiliary equipment like a Caterpillar Roller is used to pull the part way from the extruder in a continuous flow. A final equipment is installed at the end of the line to either cut-off the parts at a specific length or to wind-up (coil) the parts into rolls.
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