Although the previous segment had covered the role of recycled plastics within the manufacturing process, this section is dedicated to the plastic manufacturing process as a whole, and not just limited to recycled polymers. This will be a continuation from the section titled How Plastic is Made.
Additives
Plastics are formed and generated within large reactors where atoms and chains and combined to generate the polymers that we use to make plastic which is used for the molding of the everyday products. This chemical breakdown and composition of plastic was covered previously and is that first jump to the manufacturing floor. Additives can be included into a polymer either when they are being created in their reactors, or when they are being processes into finished goods. Additives are essential for plastic manufacturing because they allow the polymer to be mechanically, physically, or chemically altered. Additives have another crucial role to play within the manufacturing process and can allow the polymer to be resistant to light degradation, heat, bacteria, change the melting point, add color, surface appearance, friction reduction, and flame retardancy. There are 8 main types of additives:
- Antioxidants – these are implemented for plastic processing and outside applications for weathering resistance
- Colorants – coloring for plastic parts
- Foaming agents – for expanded polystyrene cups and building board and for polyurethane carpet underlayment
- Plasticizers – for wire insulation, flooring, gutters, and films
- Lubricants – used for making fibers
- Anti-stats – reduces dust collection by static electricity attraction
- Antimicrobials – utilized for shower curtains and wall coverings
- Flame retardants – improves the safety of wire, cable coverings, and cultured marble
Plastic Manufacturing Types
After the plastic has been created with its molecular structure it can be one of two types based on how it was processed which are thermosets or thermoplastics. One can conceptualize what a thermoset is from its name which contains the word “set”. Polymers that are thermosets are solid after they are heated and manufactured which cannot be softened again once they are set into their molds. Thermosets are valued for their high durability and are utilized for a variety of applications. Thermoplastics, on the other hand, are made to be more malleable and are held together by a weak bonding force which is softened by high heat, but, can return to any form when cooled. Thermoplastics are great for the recycling process as once they are softened by a heat source, the resin can be reshaped by extrusion, molding, or a pressing procedure. Below are examples of some applications of thermosets, and thermoplastics:
Thermosets:
- Polyurethanes – mattresses, cushions, and insulation
- Unsaturated Polyesters – boat hulls, bath tubs and shower stalls, and furniture
- Epoxies – adhesive glues, coating for electrical devices, and helicopter blades
- Phenol Formaldehyde – plywood, electrical appliances, and circuit boards
Thermoplastics
- Polyethylene – packaging, electrical insulation, water bottles, milk jugs, film packaging, house wrap, and agricultural films
- Polyproline – carpet fibers, automotive bumpers, and microwave containers
- Polyvinyl Chloride – sheathing for electrical cables, floor/wall coverings, siding, and automobile instrument panels
Plastic Manufacturing Methods
The various methods of manufacturing plastics into a final product are as diverse as the polymers themselves. Plastic manufacturing can fit numerous needs by utilizing a plethora a methods that are well tailored to satisfy the demands of a consumer market, or meet the finished good that is required. These methods of manufacturing plastic product are: extrusion, calendering, film blowing, injection molding, blow molding, expanded bead blowing, rotational molding, compression molding, casting, and thermoforming.
Extrusion
The extrusion process is used to manufacture products within the categories of films, sheets, profiles, tubes, and pipes. To begin the extrusion process plastic material formed after the recycling process, or virgin plastic creation, like granules, pellets, and powder is loaded into a large hopper, which feeds the material into a heated chamber where it is moved around by a continuously revolving screw. The chamber where all of these actions take place is a cylinder referred to as an extruder. Extruders can contain two or more revolving screws within them which helps to melt the plastic. Once the plastic gets to a molten state, it will be guided towards the end of the extruder where there is a small opening, called a die, which will form the shape of the finished product. Once this process is complete, the plastic is then moved by conveyor belt towards a cooling station which is comprised of cooling rollers, or the product will be immersed in water. Examples of product that can be produced by this method are: lawn edging, pipe, film, coated paper, electrical wire insulation, plastic lumber, and widow trim. Thermoplastics are processed by utilizing this manufacturing process (How Plastic Recycling Equipment Works).
Calendering
Calendering is a continuous process that is derived from the film extrusion category. Once the film is released from the extruder, it is chilled on polished cold rolls. This procedure will form sheets anywhere from .005 to .5 inches in thickness. The polished rollers allow the surface of the film to remain smooth throughout the cooling process. When a resin is sent throughout the calendering process, it is done because calendering has the capability to handle high output and low melt strength, specifically. Examples of what products, or categories, are calendered are: heavy polyethylene films used for construction vapor, liquid barriers, and high-volume PVC films (How Plastic Recycling Equipment Works.).
Film Blowing
This process is used to make an extreme level of thin plastics. Film Blowing will continuously expel a vertical ring of semi-molten plastic in an upward path. A bubble of air is then formed which will elongate the plastic axially and radially into a tube which is many times the diameter of the polymer ring being formed. The diameter of the tube is dependent on what exact plastic is to be formed and the product that is the result. Towards the end of the process the tube is cooled by air, nipped, and wound continuously as a flattened tube. It is this very tube that can be utilized, or processed, to form sealable bags, and a slit to form rolls of film with a thickness rating of approximately .0003 to .005 inches. Additionally, multiple different resins cannot be incorporated in the layers of the film during the manufacturing process (How Plastic Recycling Equipment Works).
Injection Molding
Perhaps one of the more common manufacturing methods for plastic products, injection molding, has the capability to produce complex three-dimensional parts, and products, with a high productivity rate and stunning quality. Predominantly injection molding is used with thermoplastics but also has the capacity to process thermosets and elastomers (How Plastic Recycling Equipment Works). The process of injection molding is simple, yet highly affective. Injection molding begins with plastic material being fed into a hopper, which feeds into an extruder. An extruder screw then pushes the plastic through a heating chamber where the polymer is melted. At the end of the extruding machine the melted plastic is then forced, at high pressure, into a closed cold mold. The reason why the pressure is at high level is to assure that the mold is filled appropriately. After the plastic is cooled it then turns into its solid form and is ejected as a finished product from the mold. This process is widely used; some common products that are produced through this are: butter tubs, yoghurt containers, bottle caps, toys, fittings, lawn chairs, and a wide range of plastic products (How Plastic Recycling Equipment Works). If thermosets are to be processed using this method, a catalyst must first be added to generate products like silicone rubber parts. Productivity can be determined by cycle; each cycle is defied within how many products can be produced within a timespan.
Blow Molding
This process can be used either with injection or extrusion molding. The extrusion blow molding process is rather basic, and follows a similar pattern to extrusion molding. Once the die forms a semi-molten tube of thermoplastic material, a mold that is cooled is wrapped around the thermoplastic compressed air is then blown into the tube which will conform the contents of the interior which will later harden and stretch. Breaking down the extrusion blow molding process, it becomes more simplified. Essentially, the process is to produce a uniform melt, then manipulate it into a tube, and blow the molten plastic into the desired shape of the product. Blow molding exists to produce plastic products which are hollow in nature. The benefit to using this method is that it produces hollow shapes without the need of joining two or more separately injection molded parts. Blow molding is widely used to produce products like: commercial drums, milk bottles, and carbonated soft drink bottles. Carbonated soft drink bottles are made with another form of blow molding. In making carbonated soft drink bottles, the first step is to utilize a blow molding technique and then to injection mold a intermediate shape called a “perform”, heat the perform until it is an easily malleable form, then blow the soft plastic into the shape of the desired product while it is in a cold mold (How Plastic Recycling Equipment Works).
Expanded Bead Blowing
This process is widely utilized to make every day polystyrene cups, or more commonly referred to as Styrofoam cups, or foam like food storage containers which are commonly utilized within the consumer market for everyday needs. This bead blowing procedure begins, where most processes end, with the placement of carefully measured plastic beads into a preset mold. These plastic beads, or pellets, contain pentane but could also house and other blowing agent or gas which is dissolved in the plastic. The closed mold is then heated to soften the plastic while the gas is given the opportunity to expand within, or the blowing agent becomes active generating the gas needed to fill the mold. Concluding the process is a closed and fused cell structure of foamed like plastic which has high capability to form to the preset molded shape.
Rotational Molding
This method of manufacturing requires a two-axel machine to rotate each axis simultaneously with the plastic mold mounted within. Either solid, or liquid resin can be placed inside of this mold, and then heat is applied. The rotation allows for the resin to be distributed in a uniform coating within the mold, the plastic coating is then cooled and hardened. This dual axel spinning method allows for the creation of hollow plastic products like: shipping drums, storage tanks, furniture, and toys.
Compression Molding
The method of compression molding has an already prepared volume of plastic placed into a cavity of a mold which is then compressed by a second mold which conforms the plastic into the shape required. This method can either be automated or manual, but if done manually would require a large amount of physical labor to accomplish. Predominantly, the plastic formed as a result is a semi-cured thermoset. Some products that can be produced using this method are: automobile tires, thermoplastic or thermoset mats used for boat hulls. Plywood, for example, can be created using the compression method. The wood veneer can be lathered with a catalyzed thermoset phenol formaldehyde resin, which can then be compressed and heated causing the thermoset to become rigid and non-melting.
Casting
Casting is a method that has been utilized for many years, and not just for plastic products. Within plastic manufacturing, casting is a low pressurized pouring of liquid plastic into a preset mold. Catalyzed thermosets can be manipulated into a plethora of shapes which can be well suited for a multitude of consumer, or commercial needs. For example, molten polymethyl methacrylate thermoplastic can be created into windows for aquariums. Casting can generate plastics with a thickness of .500 inches and up.
Thermoforming
During thermoforming, thermoplastic films are heated to a point of softening. Once this point is reached, the film is then pulled up by a vacuum or can be pushed by a pressurizing technique to a molded shape; additionally, the softened film can also be pressed by plug into a mold. The film like material that can be utilized in this process to produce products are from cut pieces of thick sheets that are roughly .100 inches thick, or rolls of thinner sheets. Finished goods are then cut from these sheets, while anything that has gone unused can be recycled for a future project. Generally, thermoforming is an automated process which can manufacture products like: food containers and various individual items of high precision.
Citations:
How Plastic Recycling Equipment Works. (n.d.). Retrieved from https://www.thomasnet.com/articles/plastics-rubber/plastic-recycling-equipment