How plastics are made?
Plastics, also called polymers, are produced by the conversion of natural products or by the synthesis from primary chemicals generally coming from oil, natural gas, or coal.
Plastics typically have high molecular weight, meaning each molecule can have thousands of atoms bound together. When the connections of atoms result in long chains, the polymer is called a thermoplastic. Thermoplastics are characterized by being meltable. The thermoplastics all have repeat units, the smallest section of the chain that is identical. The vast majority of plastics, about 92%, are thermoplastics.
Thermoplastic and Thermoset Processing Methods
- Extrusion: This process is used to produce films, sheet, profiles, tubes, and pipes. Plastic material as granules, pellets, or powder, is first loaded into a hopper and then fed into a long heated chamber through which it is moved by the action of a continuously revolving screw. The plastic is melted by the mechanical work of the screw and the heat from the extruder wall. At the end of the heated chamber, the molten plastic is forced out through a small opening called a die to form the shape of the finished product. As the plastic is extruded from the die, it is fed onto a conveyor belt for cooling or onto rollers for cooling or by immersion in water for cooling.
- Calendering: This
process is an extension of film extrusion. The still warm extrudate is
chilled on polished, cold rolls to create sheet from 0.005 inches thick to
0.500 inches thick. The thickness is well maintained and surface made
smooth by the polished rollers.
- Film Blowing: This process continuously extrudes
vertically a ring of semi-molten polymer in an upward direction, like a
fountain. A bubble of air is maintained that stretches the plastic axially
and radially into a tube many times the diameter of the ring. The diameter
of the tube depends on the plastic being processed and the processing
conditions. The tube is cooled by air and is nipped and wound continuously
as a flattened tube. The tube can be processed to form saleable
bags.
- Injection Molding: Plastic
material is fed into a hopper, which feeds into an extruder. An extruder
screw pushes the plastic through the heating chamber in which the material
is then melted. At the end of the extruder the molten plastic is forced at
high pressure into a closed cold mold. The high pressure is needed to be
sure the mold is completely filled. Once the plastic cools to a solid, the
mold opens and the finished product is ejected. Special catalysts can be
added to create the thermoset plastic products during the processing.
Injection molding is a discontinuous process as the parts are formed in
molds and must be cooled or cured before being removed.
- Blow Molding: Blow molding is a process used in
conjunction with extrusion or injection molding. In one form,
extrusion blow molding, the die forms a continuous semi-molten tube of
thermoplastic material. A chilled mold is clamped around the tube and
compressed air is then blown into the tube to conform the tube to the
interior of the mold and to solidify the stretched tube. Overall, the goal
is to produce a uniform melt, form it into a tube with the desired cross
section and blow it into the exact shape of the product.
- Expanded Bead Blowing: This process begins with a measured
volume of beads of plastic being placed into a mold. The beads contain a
blowing agent or gas, usually pentane, dissolved in the plastic. The
closed mold is heated to soften the plastic and the gas expands or blowing
agent generates gas. The result is fused closed cell structure of foamed
plastic that conforms to a shape, such as expanded polystyrene cups.
- Rotational Molding: Rotational molding consists of a mold
mounted on a machine capable of rotating on two axes simultaneously. Solid
or liquid resin is placed within the mold and heat is applied. Rotation
distributes the plastic into a uniform coating on the inside of the mold
then the mold is cooled until the plastic part cools and hardens.
- Compression Molding: This process has a prepared volume of
plastic placed into a mold cavity and then a second mold or plug is
applied to squeeze the plastic into the desired shape. Transfer molding is
a refinement of compression molding. Transfer molding is used to
encapsulate parts, such as for semi-conductor manufacturing.
- Casting: This
process is the low pressure, often just pouring, addition of liquid resins
to a mold.
- Thermoforming: Films of thermoplastic are heated to soften the film, and then the soft film is pulled by vacuum or pushed by pressure to conform to a mold or pressed with a plug into a mold. Parts are thermoformed either from cut pieces for thick sheet, over 0.100 inches, or from rolls of thin sheet.
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