The processing and fabrication of plastics involve various stages. Plastics today have become inevitable in our daily life, which has caused the demand for plastic to be drastically high. Plastic industries would prefer to recycle plastic materials and convert them into something different. For example, the conversion of plastic pellets into films or the conversion of films into food containers. Recyclable plastics have been the easiest way to keep the production of plastic intact and cheap. These plastics are known as thermoplastic, but the thermosets can’t be reheated or remolded.
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Today you’ll get to know the various fabrication process of plastics and the various method that is performed during the processing. You’ll also get to know the recycling and resource recovery of plastics. Plastic processing is for thermoplastic and thermosets types of plastics.
Compounding which is the first step in most plastic fabrication procedures is the mixing together of various raw materials in proportions according to a specific recipe. The plastic resins are often time supplied to the fabricator as cylinder pellets (several millimeters in diameter and length) or as flakes and powders. Some other forms include viscous liquids, solutions, and suspensions.
Liquids can be mixed with other ingredients in a stirred tank, but certain operations require special machinery. Dry blending is a process whereby dry ingredients are mixed for further use, as in mixtures of pigments, stabilizers, or reinforcements.
Polyvinyl chloride (PVC) which is a porous powder can be mixed with a liquid plasticizer in an agitated trough called a ribbon blender or in a tumbling container. This process is also a blending process because the liquid penetrates the pores of the resin and the final mixture contain as much as 50 percent plasticizer. It’s still a free-flowing powder that appears to be dry.
Forming is the process of giving plastic various shapes, these shapes typically involve the steps of melting, shaping, and solidifying. A good example is a polyethylene pellet that can be heated above the melting temperature Tm is placed in a mold under pressure and cooled to below-melting temperature Tm in order to make the final product dimensionally stable. Although thermoplastics, in general, are solidified by cooling below Tg or Tm. whereas, thermosets are solidified by heating in order to accomplish the chemical reactions necessary for network formation.
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Extrusion is the process whereby the melted polymer is forced through an orifice with a particular cross-section (the die), forming a continuous shape with a constant cross-section similar to that of the orifice. It’s more common for thermoplastics to be extruded and solidified by cooling, than thermosets to be extruded and cross-linked by heating the extrudate.
Products that can be produced by extrusion include film, sheet, tubing, pipes, insulation, home siding, etc. In each case, their profile is determined by the die geometry, and solidification is by cooling.
Diagram of screw extruder of thermoplastic polymers
The diagram below shows the section of a screw extruder of thermoplastic polymers. It shows a longitudinal section of a screw extruder of thermoplastic polymers. Plastic pellets are fed from a hopper into the barrel of the extruder, where the pellets gradually melt by mechanical energy generated by a turning screw. The heaters are arranged along the barrel. The molten polymer is forced through a die, which then takes the shape of the final product.
Diagram of blow extruder
Unlike the screw extruder, most plastic grocery bags and other similar items are made by blow extrusion. This is a continuous extrusion of the tubing. In the process, the tube is expanded before being cooled by being made to flow around a massive air bubble. The air is prevented from escaping from the bubble by collapsing the film on the other side of the bubble.
In some applications, laminated structures may be made by extruding more than one material at the same time through the same die or with multiple dies. In this process, multilayer films are useful because the outer layers may contribute strength and moisture resistance. The inner layer may control oxygen permeability, which is an important factor in food packaging.
The blow extrusion process may form the layered films, or extrudates from three machines may be pressed together in a die block to form a three-layer flat sheet that is cooled subsequently by contact with a chilled roll. See the diagram below:
In the above diagram of blow extrusion of thermoplastic polymers, the molten extrudate is forced to pass through a tubing mandrel. It’s expanded into a balloon shape by a stream of air, drawn upward by rollers, and pinched into a collapsed sheet so that it can be cut into a number of products.
The flow through a die in extrusion often results in some orientation of the polymer molecules. The orientation can be increased by drawing – that is, by pulling on the extrudate in the direction of the polymer flow or in some other direction either before or after partial solidification.
In a blow extrusion process, polymer molecules are oriented around the circumference of the bag and along its length. This results in a biaxially oriented structure that often has superior mechanical properties over the unoriented material.
In compression molding, a molding powder which is also known as pellets is heated and at the same compressed into a specific shape. It can be carried out on both thermoplastic and thermoset. In the case of a thermoset, the melting must be as quick as possible, because the network starts to form immediately. It’s also important that the melt filled the mold completely before solidification progresses to the point where the flow stops.
The highly cross-linked molded part can be removed without cooling the mold. Adding the next charge to the mold makes it easy for the exact required amount of cold molding powder to be compressed into a preformed “biscuit”. This biscuit can also be preheated by microwave energy to near reaction temperature before it is inserted in the mold cavity.
A heater that looks like a microwave oven, may apply up to 10 kilovolts at a frequency of one megahertz. Commercial molding machines that have high temperatures and pressures employ to shorten the cycle time for each molding. The molded article is then pushed out of the cavity by the action of ejected pins, which operate automatically when the mold opens. In most cases, pushing the resin into the mold before it has liquefied may cause undue stress on other parts.
A layer of reinforcing material must be laid down before introducing the resin in one form of compression molding. The temperature and pressure not only form the mass into the desired shape but also combine the reinforcement and resin into an intimately bound form. When flat plates are used as the mold, sheets of different materials can be molded together to form a laminated sheet.
Plywood is a good example of a thermoset-bound laminate. In this type, layers of wood are both adhered to one another and impregnated by a thermoset such as urea-formaldehyde. This forms a network of heating.
Because it is usually slow and inefficient to mold thermoplastics using compression molding. And even as the thermoplastic part will be cool before removing it from the mold, which makes it requires a cooled mass of metal that makes up the mold. The metal is then reheated for each part.
So, Injection molding is a process of overcoming this inefficiency. It looks like transfer molding in that the liquefying of the resin and the regulating of its flow is performed in a part of the apparatus that remains hot. The shaping and cooling are carried out in a part that remains cool.
In a reciprocating screw injection molding machine, material flows under gravity from the hopper onto a turning screw. This screw supplies mechanical energy, together with auxiliary heaters that convert the resin into a molten state. During this time, the screw retracts toward the hopper end. The screw moves forward, making it act as a ram when a sufficient amount of resin is melted. This forces the polymer to melt through a gate into the cool mold. The plastic will be solidified in the mold, then the mold will be unclamped and opened. The article will be pushed out from the mold by automatic ejector pins. The mold will then be closed and clamped and the screw turns and retracts again so that the same process can occur.
The above explanation is the working operation of an injection molding in plastic fabrication. See the diagram below!
In the above diagram, the working operation of injection molding of thermoplastic polymers is described.
Reaction injection molding
Reaction injecting molding is a type of molding or network-forming thermoset, polyurethane. It’s often performed on automobile parts such as bumpers and inside panels. The molding process is abbreviated as RIM. The two liquid precursors of polyurethane are a multifunctional isocyanate and a prepolymer. A low-molecular-weight polyether or polyester bearing a multiplicity of reactive and end-groups like hydroxyl, amine, or amide.
The presence of a catalyst such as a tin soap, causes the two reactants to rapidly form a network joined by urethane groups. This reaction takes place so rapidly that the two precursors have to be combined in a special mixing head and will be introduced into the mold as soon as possible.
However, the product requires very little pressure to fill and conform to the mold, once it enters the mold – mostly if a small amount of gas is evolved in the injection process. The polymer volume will be expanded and reduce resistance to flow.
Because of the low molding pressures, a relatively lightweight and inexpensive mold can be used. Even when large parts such as bumper assembles or refrigerator doors are to be produced.
Blow molding is employed for thermoplastic containers that require a small part of its development. In the technique, a thermoplastic hollow tube, the parison, is formed by injection molding or extrusion. In heated form, the tube is sealed at one end and then blown up like a balloon. The expansion will then be carried out in a split mold with a cold surface; as the thermoplastic encounters the surface, it cools and becomes dimensionally stable.
The parison itself can be programmed as it’s formed with different wall thicknesses along its length so that it can be expanded in the mold. This will allow the final wall thickness to be controlled at corners and other critical locations. In the process of both in diameter and length, which is known as stretch blow molding, the polymer is biaxially oriented, which enhances the strength. The crystallinity is enhanced in the case of polyethylene terephthalate (PET).
blow molding of plastic containers
the above diagram explains the blow molding of plastic containers. Counterclockwise from the top, a molten polymer is extruded into a hollow tube-shaped parison. A split mold is closed around the parison, which is expanded against the sides of the mold by a stream of air. As soon as the plastic solidifies, the mold will open and the shaped bottle released. Take a close look at the image to get the knowledge.
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Conventionally, blow molding is employed to produce bottles of polypropylene, polyethylene, polystyrene, polycarbonate, PVC, and PET for domestic consumer products. The molding method is also used to produce fuel tanks for automobiles. in a situation where a high-density polyethylene tank is required, the blown article may further treat with sulfur trioxide in order to improve the resistance to swelling or permeation by gasoline.
Casting and dipping
Since all forming process does not require high pressures. The material to be molded might have stable liquid, if it happens to be so, simply pouring (casting) the liquid into a mold may be sufficient. This is because the mold need not be massive, even the cyclical heating and cooling for a thermoplastic is perfectly done.
A good example of a cast thermoplastic is a suspension of finely divided, low-porosity PVC particles in a plasticizer such as dioctyl phthalate (DOP). The suspension will form a free-flowing liquid (a plastisol) that is stable for months. But if the suspension (i.e., 60 parts PVC and 40 parts plasticizer) is heated to about 180 0C (356 0F) for five minutes. This will cause the PVC and plasticizer will form a homogeneous gel that will not separate into its components when cooled back to room temperature.
Thermoset material can also be cast, for instance, a mixture of polymer and multifunctional monomers with an initiator can be poured into a heated mold. The article can then be removed after the completion of the polymerization. In addition, a transparent lens can be formed in this way using a diallyl glycol carbonate monomer and a free-radical initiator.
When trying to create an article with hollow parts, rotational molding is perfect. It is achieved when a split mold is partially filled with plastisol or a finely divided polymer powder. The rotation of the mold while heating converts the liquid or fuses the powder into a continuous film on the interior surface of the mold. The hollow part can then be removed when the mold is cooled and opened. Toys like balls, dolls, etc. are articles that can be produced with this molding.
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Thermoforming and cold molding
A sheet of thermoplastic can be heated above its Tg or Tm so that it can form a free, flexible membrane as long as the molecular weight is high enough to support the stretching. In this heating state, a vacuum will be used to pull the sheet into contact with the cold surface of a mold, where it will be cooled below Tg or Tm. it then becomes dimensionally stable in the shape of the mold. This molding process is often used to produce cups for cold drinks in the form of polystyrene or PET.
Thermoplastics can be formed into new shapes without heating. This can be achieved by the application of sufficient pressure; thus, it’s known as cold molding. This technique is used to produce margarine cups and other refrigerated food containers from sheets of the acrylonitrile-butadiene-styrene copolymer.
Foaming in the processing and fabrication of plastics is also known as expanded plastics. It has inherent features that make them suitable for certain applications. For example, the thermal conductivity of foam is lower than that of the solid polymer. In addition, a foamed polymer is more rigid than a solid polymer for any given weight of the material. Lastly, foams will collapse by compressive stresses while absorbing much energy, which is why it’s advantageous in protective packaging. These properties are widely required to fit various applications by the choice of polymer and by the manner of foam formation or fabrication. Home insulation (polystyrene, polyurethane, phenol, formaldehyde) is the largest market for foamed plastics. And also, the packaging includes various disposable food and drink containers. The diagram below shows polystyrene packaging.
An isopentane can be dipped into polystyrene pellets at room temperature and modest pressure. when these pellets are heated, they can be made to fuse together at the same time that the isopentane evaporates. This will foam the polystyrene and cool the assembly at the same time.
Pellets are usually performed to some extent before being put into a mold when forming a cup or some form of rigid packaging. The isopentane-impregnated pellets can also be heated under pressure and extruded, where we obtain a continuous sheet of foamed polystyrene. This can then be shaped into packaging, dishes, or egg cartons while it is still warm.
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We can also produce structural foams by injecting nitrogen or some other gas into a molten thermoplastic like polystyrene or polypropylene. This will then be performed under pressure in an extruder. Foams produced in this way are denser than the above-explained ones, however, they have excellent strength and rigidity, which makes them suitable for furniture and other architectural uses.
Another way of making foams of a variety of thermoplastics is to employ a material that will decompose and generate gas when heated. To make a more effective blowing agent, the material should decompose at about the molding temperature of the plastic. It should also decompose over a narrow temperature range, evolve a large volume of gas, and finally, be safe to use. A popular commercial agent often used is azodicarbonamide. It is usually compounded with some other ingredients in order to modify the decomposition temperature and aid in the dispersion of the agent in the resin.
Azodicarbonamide of one mole (116 grams) creates about 39,000 cubic cm of nitrogen and other gases at 200 0C. So, when one gram is added to 100 grams of polyethylene will result in foam with a volume of more than 800 cubic cm. Polyethylene, polypropylene, polystyrene, polyamides, and plasticized PVC are polymers that can be foamed with blowing agents.
Just as explained above in reaction injection molding, the rapid reaction of isocyanates with hydroxyl-bearing prepolymers creates polyurethanes. These materials can also undergo foaming by incorporating a volatile liquid. It is capable of evaporating under the reaction of heat and foams the reactive mixture to a high degree. The components chosen determine the rigidity of the network, especially the prepolymer.
Hydroxyl-terminated polyethers are used to prepare flexible foams, which are used in furniture cushions. On the other hand, a hydroxyl-terminated polyester is popularly used for making rigid foams such as those used in custom packaging appliances. Because of the excellent adhesion of polyurethanes to metallic surfaces. It is used in some novel applications such as filling and making certain rigid aircraft components such as rudders and elevators.
Another rigid thermoset that can undergo foaming is based on phenol-formaldehyde resins. The final stage of network formation is brought about by the addition of an acid catalyst as a volatile liquid is present.
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Plastics are often time joined by welding, in the same manner as metals. Surfaces are joined by bringing them in contact with one another and heated by conduction or by dielectric heating. A good example is PVC and polyethylene tanks and ductwork. Heat sealing of bags made from tubes of blow-extruded polyolefins such as polyethylene and polypropylene often requires contact with a hot sealing bar.
PVC with high enough dielectric loss heat can be generated throughout the material by exposure to a high-frequency, high-voltage electric field.
Thermoplastics and thermosets of rigid parts can be machined by conventional processes like drilling, sawing, sanding, turning on a lathe, etc. often time, glass-reinforced thermosets are machined into gears, pulleys, and other shapes, especially when the number of parts will not work well on mold metal. Sheets of thermoplastics and thermosets can be stamped out (die-cut) into various forms. A good example is a cup made by vacuum forming, that is, it is cut out of the mother sheet using a sharp die. Thermoplastic such as polystyrene, the scrap sheet leftover can be reground and remolded.
Often time in the fabrication of plastics color is added in the form of a pigment or dye throughout the plastic article. There are many applications where a surface coating can serve for protection or decoration purposes. An automobile bumper, which is produced by reaction injection molding can be painted to unite with the rest of the body. The coating can only be applied to plastics so that the solvent used does not cause swelling of the underlying substrate. This is why latex dispersion paints are helpful even though surface treatment is required to provide good bonding with these materials.
A polymer-matrix composite is applied to a number of plastic-based materials in which several phases are present. It is often used to describe systems in which a continuous phase (the matrix) is polymeric and another phase (the reinforcement) has at least one long dimension. Major classes of composites include those made up of discrete layers (sandwich laminates) and those reinforced by fibrous mats, woven cloth, or long, continuous filaments of glass or other materials.
Plywood in the form of a sandwich construction of natural wood fibers with plastics. The layers are easily distinguished and are both held together and impregnated with a thermosetting resin such as urea-formaldehyde. A decorative laminate can consist of half-dozen layers of fibrous kraft paper (similar to the paper used in grocery bags) together with surface paper with a printed design. The cross-linking reaction of both plywood and paper laminate is carried out with sheets of the material pressed and heated in large laminating presses.
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Even though other fibrous materials like carbon, boron, metals, and aramid polymer can be used as fibrous reinforcement, fiberglass is the most common type. It is supplied as mats of randomly oriented microfibrils, as woven cloth, and as continuous or discontinuous filaments.
Recycling and Resource Recovery of Plastic
Recycling and recovering plastic material are important stages that can never be overlooked. This is why a favorable method of disposing and recycling is employed. Just like other materials such as paper, glass, and aluminum containers have been recycled to some extent for several years. Plastic recycling has also become common since it can be re-used and can serve some other positive benefits.
Although, there are various technical and economic problems in the recycling of plastics which are generally categorized into two;
- Identification, segregation (or sorting), and gathering into central stations.
- The economics of recovering value.
Identification, segregation, gathering
Plastics are the common form of packaging most commodities today. Lately, most recycling efforts have focused on containers. Almost all bottles, food trays, cups, and dishes made of the major commodity plastics bear an identifying number enclosed in a triangle together with an abbreviation.
In most localities, consumers are encouraged to return empty beverage containers to the place of purchase. They are required to pay a deposit on each unit at the time of purchase. The method helps to solve two of the major problems associated with economic recycling. The method works because consumers are seeking the return of the deposit, which does the sorting and the stores gather the plastics into central locations. This law of deposition has drastically decreased plastics in roadside litter. In addition, the system has helped to raise the recycling rate of plastic bottles, especially those made of polyethylene terephthalate (PET) and high-density polyethylene (HDPE). With this, more than 10% of all plastic products are recycled after first use.
On the other hand, most plastics are used in long-term applications such as appliances, construction, and home furnishings. This has made recycling very difficult.
The economic recovery of value
The thermoplastic material can be recycled more readily than thermosets, even though there are some limitations. First, recyclable plastic may be contaminated by nonplastics or by different polymers making up the original product. Also, within a single polymer type, there are differences in molecular weight. For instance, a supplier of polystyrene may produce material of high molecular weight for sheet-formed food trays, since its forming process favors a high melt viscosity and elasticity.
Supplier may offer low-molecular-weight polystyrene for the injection molding of disposable dinnerware. This is because injection molding works best with a melt of low viscosity and very little elasticity. If the polymers of the above products are mixed in a recycling operation, the mixed material will not be very suitable for either of the original applications.
Another common issue with recycling plastics is the mixing together of pigments or dyes of different colors. Another is the problem of quality control since almost all plastics change either slightly or greatly due to the result of the use or initial fabrication. For instance, some plastic undergoes changes in molecular weight because of the cross-linking or chain scission (breaking of the chemical bonds that hold a polymer chain together). Others may undergo oxidation, which is another common reaction that can change the properties of plastic.
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In this in-depth article on the processing and fabrication of plastics, we’ve seen the various method involved. Where we explained compounding, forming, extrusion, and molding. We enlighten on the various types of plastic molding and their operation, where we explained injection molding, reaction injection molding, blow molding, and casting and dipping. We also understand the term foaming in the fabrication of plastics, and finally, we learn the finishing operations that are applicable in plastic fabrication.
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