Plastics are important in the daily lives of virtually everybody around the world and are among the most used materials among Americans. The surprising thing is not many people are aware of their importance and instead they take it for granted. They are applicable in the manufacturing of pens, clothing, and in materials used to preserve, food not to mention furniture. Given how much society relies on plastics today, this makes it difficult to comprehend the fact that plastics have only been around for a little more than a hundred years. However, plastics have made a huge impact on the quality of life and have widened the range of products that can be used. The use of sophisticated technological processes and a greater level of technological understanding of plastics in manufacturing are responsible for this.
Plastics are strong, long lasting, affordable, lightweight and are resistant to corrosion. In addition, they are materials that contain great electrical and thermal insulation properties. Polymers are very versatile, which make them applicable in a variety of products important in technological and medical advances and energy savings in addition to many other types of remuneration (Thompson et al., 2009). The result has been a great rise in the production of plastics. Plastics are present in almost every aspect of day-to-day life. For example, they are key components in telecommunications, footwear, transport, packaging materials and even in clothing. Virgin plastic polymers are normally used with various additives in order to heighten their performance. Examples of these additives include plasticizers that help make the material flexible, colorings and flame retardants not to mention thermal and ultraviolet stabilizers. The contribution of these additives in the production of plastics cannot be ignored, although there are some that can have hazardous health effects, with tributyl and lead as good examples. The way that plastics are being used and the resources that manufacturers depend on to produce these plastics have become a global concern. Another issue that has been raised is the chemical impact of the additives used in plastics production.
History of Plastic Manufacturing
John Wesley Hyatt participated in a contest in 1868 held by a billiard ball manufacturer that was looking for an alternative material to ivory, which was becoming expensive and not readily available. Hyatt came up with the idea of developing a plastic material which he called celluloid (Bryce, 1996). While he did not ultimately win the competition, his invention had profound effects that changed society.
The name Celluloid was patented and quickly flourished commercially. It proved vital in production of various products such as men’s collars and dental plates. The next few decades that followed saw the development of more types of plastics including modified natural polymers such as rayon, which is produced by cellulose material. The Belgian-American chemist Leo Hendrik Baekeland was the first to develop total synthetic plastic at the turn of the century and later sold it under the name Baekelite. The plastic industry experienced a breakthrough in terms of development of the material in 1920. The German chemist Herman Staudinger, developed the idea that plastics were a combination of big molecules that are joined by strong compound bonds. This influenced research on plastics and the designing of new products in the period between the 1920s and 1930s. Examples of these include methyl methacrylate, commonly referred to as Plexiglass or Lucite. In 1938, polytetra fluoroethylene and nylon were developed, although they were later marketed as Teflon in 1950 (Bryce, 1996).
Wallace H. Carothers of DuPont was a pioneer in the preparation of nylon, but it was considered useless in its initial form due to the fact that it was sticky and had a small amount of structural reliability. The chemist Julian Hill was the first to make the discovery concerning the use of nylon as a substitute for fiber because of the properties it had, such as its strength and silky appearance (Henty & Salamon, 1999-2000). Through experimentation, Roy Plunkett found that Teflon material can be used to make plastics due to the fact that it can withstand extreme heat and is very slippery. He did this when he while testing gaseous tetrafluoroethylene in order to prepare a refrigerant. Although Teflon is used today to provide various non-stick coating for cookware, its initial use goes back to World War II during the atomic bomb project when it was used to make gaskets inert to UF6 gas that is very corrosive and used in the purification of uranium isotopes. Natural raw materials were in short supply in many countries at the time of World War II. Germany had run out of latex and started using plastics, which led to the development of synthetic rubber to act as an appropriate substitute.
After Japan joined the war, United States was no longer able to import the silk, natural rubber and certain metals from Far Eastern countries as it had before. Americans therefore turned to the plastic industry. Nylon started being used to make fabrics while polyesters became applicable in the production of armor and other materials used during war. This boosted the volume of synthetic rubbers being produced. This continued even after the end of the war and led to many inventions.
Plastics started replacing metals in objects like safety helmets and machinery including devices that required extremely high temperatures. The German chemist Karl Ziegler developed polyethylene in 1953 and the Italian chemist Giulio followed by developing polypropylene in 1954 (Thompson et al., 2009). These two scientists were later awarded the Nobel Prize in 1963 in the field of chemistry based on their research on polymers. Polyethylene and polypropylene are the most commonly used plastics today. New inventions that involve plastics are still taking place today and this is why scientists are continuing to introduce new methods of applying plastics. In addition, there is a need to invent substitute materials for plastics.
Effect of Plastic Manufacturing of the U.S. Economy
Plastics play a great role in various markets in the U.S from packaging to consumer products and electronics. Plastics have a significant impact on the American economy, as it is the third largest manufacturing industry in the nation. Research shows that the U.S plastic industry provides employment to almost nine hundred thousand workers and gets more than $380 billion from annual shipments made from plastics. More evidence of the growing rate of production and use of plastics lies in the quantity of plastic facilities in America, which has reached 18,500 and can be found all states (Henty & Salamon, 1999-2000).
There are other alternative materials to plastics which are used today, with glass, wood and polymers being good examples.
Processes of Plastic Manufacturing
Production of plastics is possible in four different categories which include getting monomer which is the raw material, synthesizing the basic polymer, compounding that polymer to form a material applicable for fabrication and finally shaping or molding the plastic to give it its final appearance. Resins, which are obtained from vegetable matter, were historically used as raw materials. Examples include cellulose from cotton, oils from seeds, furfural obtained from oat hulls and different starch derivatives (Films for the Humanities & Sciences, Films Media Group & Video Education Australasia, 2006). However, petrochemicals are used in producing most plastics today due to the fact that they are cheaper and are easily available. Researchers have the task of looking for alternative sources of raw materials due to the fact that oil sources have started to deplete over time. Examples include coal gasification.
The materials required for manufacturing of plastics
Plastic manufacturing takes place in different stages. The first stage is polymerization, which can happen in two basic methods: condensation and addition. It is possible for these to occur in liquid or gaseous phases and occasionally in a solid phase. There are times when it happens at the intersection of two immiscible liquids where there is the dissolving of the monomers. Chemical additives are an important element in the production of plastics due to the fact that they add some specific features to the final product. For example, they can act as antioxidants to prevent the polymer from being degraded by the ozone and oxygen, lubricants to help lower the rate of friction, provide protection from weathering by being antistatics and through pigment they can provide plastic with color, and they can make the polymer more flexible. Normally, composites are produced and then used to make plastics. This is done by including various reinforcements. Examples of this include carbon fibers and glass. These are important in improving strength and stability. Plastic foam is responsible for fusing gas and plastic. Polystyrene is one such example and is applicable in production of Styrofoam cups.
Shaping and finishing of plastics require various processes. Compression molding has been used for decades to turn polymers into useful materials. This is accomplished by giving plastic a particular shape with the application of pressure. A two piece mold is usually applied and one half of it is filled with plastic before the two halves are joined and the plastic is subjected to high pressure to enable it melt. Extrusion is commonly used by many manufacturers in shaping plastics. In this process, an extruder is applied in order to provide the needed force to enable the softened plastic to pass along a shaped die. This is how it acquires the various forms when it is completed. Examples of these forms include wide, tube and flat sheet.
A screw is the driving force that provides the needed constant pressure. Every extrusion product has standard cross sections. This method has a variation known as extrusion blow molding whereby a plastic tube that is manufactured from extrusion undergoes sealing around the blowing tube before expanding with the help of compressed air to form the shape of a mold. In the case of injection molding, the melted plastic is forced to go through the cold mold by at least one extruder, and this is how it acquires its shape. Injection blow molding is similar to the injection molding process, and it is applied in the production of the plastic pop bottles. This involves injection molding of a thick walled plastic tube around a blowing stick before being shifted to a blowing mold. Heat is applied to this tube again before passing air down the blowing stick to enable it to expand and take the shape of the mold.
Calendaring is another method, and its importance is felt in the sector of plastic sheets and transfer molding. A ram is significant here when it comes to pushing the softened plastic through a ram.
How the process of blow molding and the machine works
The particular product being manufactured can greatly influence the suitability of the process. Some processes can be carried out at home while others require high tooling investment. Low investment processes are generally slower because they are craft-based compared to high investment processes. It is important to note that the injection molding is only economically feasible when producing an extremely high output. The machine that carries out injection molding takes only forty seconds to transform safety granules into a safety helmet. This adds up to 2,160 helmets in 24 hours (Bryce, 1996). The fact that it is possible to share the tooling costs among the many units makes the unit price relatively inexpensive. When it comes to small runs such as 5,000 products, injection molding is not economically viable. There are methods that are cost effective during startup, although the rate of production is slow. Examples including fabrication, casting and the rotational molding. Injection molding is what most companies apply in the manufacturing plastic objects, with the exception of plastic bags.
There are times when the final plastic product comes out with marks. This is especially so when a process like injection molding is used. There are normally two varieties of marks left, which are the ones from the sprue and ejector pin marks. The time that the plastic goes inside the mold is where the sprue marks appear because of the tail that breaks off. When the molding is taken away, smooth and circular marks appear from the ejector pin. When thermosetting plastics, it is necessary to be aware that it is not possible to thermoform them. It was only from around 1960 that the use of injection molding started being applied.
Blow molding started being applied in conjunction with cellulose nitrate in 1881. Its tooling cost is relatively high and its production volume is high as well. Blow molding is used for hollow articles that usually have small diameter openings compared to the body. Examples of this include containers and bottles. This method is commonly used for high density polyethylene terephthalate and polythene, and it forms marks at the point where the mold parts are combined. In the case of blow molding processes, it is necessary to prepare a tube in which hot air is blown into and semi-molten plastic then expands and fills a cavity that the two part form. The two parts are usually mold and plastic. From here, it is possible to injection mold into the tube in order to allow for the formation of a thread used for the lid or any other detail . It is also possible to extrude it as a tube, which is then pinched at one end before being expanded again to be able to fill the two part metal mold cavity. The mold walls can hold the key to the particular texture that someone wants to achieve.
The casting process has been used for decades with traditional materials for example metals. It involves a craft process, and substances can be added to the liquid when it is solidifying. It is possible to carve the cast form and manipulate the end result of the open casts by using the curing process. It is commonly used to make plastics from polyurethane and polymethyl methacrylate. The casting process is important in making preformed shapes such as tubes, radio housings, paperweights, rods and furniture. In casting process, the plastic which is in liquid form is poured on an open mold.
How the process of blow molding is carried out
Blow molding is a manufacturing process in which a heated plastic tube is inflated until it occupies an entire mold and takes up the desired shape all in a bid to form a hollow plastic part. A thermoplastic in the shape of granules and pellets is the raw material used in this process. The thermoplastic is melted and a hollow tube referred to as parison is made. A parison can be created in various ways. After formation of the parison, it is put in the middle of two mold halves before being inflated with pressurized air to the point where it takes up the inner shape of the existing mold cavity. The standard pressure under which this takes place is between 25 to 150 psi (Cook & Society of Manufacturing Engineers, 1998). The part is left to cool for some time and then the separation of the mold halves takes place and the needed part is removed.
Plastic blow molding has undergone technological advancement since the time when glass blowing was first used. Extrusion blow molding is the principle process under plastic blow molding even though other process such as biaxial stretch blow molding and injection blow molding exist. All these elements involve the use of injection, extrusion or even both of them in some cases. Although these processes may be a bit different, they have common distinct production stages. These include the stages of melting the resin, and the production of parison which is common with most of the blow molding processes which can be perform production in relation to biaxial stretch blow molding. The inflation and cooling processes involved are also the same, including the removal of the product from the mold. Extrusion blow molding contains another stage that does not exist in other forms of blow molding, which is trimming the final product.
Plastic co-extrusion refers to the process of pressing at least two materials together through the same die in order to come up with a single piece. Combining multiples plastics can lead to a product that has different properties from the ones of a single material. Co-extrusion solves some of the challenges previously experienced in manufacturing. For example, when radiopaque plastic is coextruded to make a catheter, it comes with an improved quality of x-ray due to the fact that the catheter is able to go through the vein and not interfere with catheter’s effectiveness. Co-extrusion can also use recycled and scrap materials, which can end up lowering the cost. This can be done for projects that are very diverse, such as those that involve air blown food containers and structural and tubing components.
In co-extrusion, there are several extruders used to come up with encapsulated parts. There are times when a single cycle can contain more than five different materials and in this case, every extruder has a role to play in terms of the exact amount of molten plastic it will give for the operation. In each ordinary plastic mixing, every particular plastic is able to maintain its original properties, although they are mixed to form a compound material part. If they are mixed before extrusion, the individual materials may have altered features even though the final product is homogeneous. The co-extrusion method is not suitable for every type of plastic because there are some polymers that will fail to stick to others. There are cases where a conductive middle layer can be the solution. Plastics whose melting temperatures differ greatly are also not recommended for co-extrusion because the material with the lower melting temperature will succumb to degradation. In addition to these, acetals and PVC materials are not a good combination for co-extrusion because they can cause violent reactions when combined.
Most of the blow molded products are normally used in the cosmetic and food industry. When seeking to improve the shelf life of a product, a barrier layer can be introduced and coextruded in the plastic and this helps in the leaching of aromas, gases and moisture from the container. Coextruded products are also used by breweries to make beer products due to the fact that they possess non-porous properties that are similar to the ones of aluminum and glass. Another industry that uses co-extrusion too is the energy industry. They contribute towards satisfying particular emission standards. In order to manufacture vapor-proof fuel tanks, which can satisfy the environmental requirements put in place, co-extrusion of polyethylene and ethylene Vinyl (EVOH) alcohol has to take place. EVOH is normally used in various items such as ketchup, baby food bottles and mayonnaise because it is a barrier material that is all-purpose. It can be placed in the middle of twin layers of substrate even though it is considered food-grade. EVOH can also be used to manufacture transparent containers used for pharmaceuticals and vitamins.
The parts derived from blow molding process are normally plastic, thin walled and hollow. Examples include containers and bottles, which come in various shapes and sizes. The largest share of products manufactured through blow molding is occupied by disposable containers that are used to package liquid consumer goods. However, there are other items, including huge shipping drums, hulls used to make small boats and sailboards, toys, automotive gasoline tanks, tubs, storage tanks and plastic drums. For the case of the small boats, the manufacturing of two boat hulls takes place through single blow molding before they are cut to form two open hulls. There are various thermoplastic materials that can be used to create low molded parts, including high and low density polyethylene (LDPE and HDPE), polyvinyl chloride (PVC) and polypropylene (PP) (Sinotech, 2010).
As mentioned earlier, the parison can be formed from different types of blow molding. Extrusion blow molding involves passing molten plastic through a die head, and this is done by using a rotating screw. The parison is placed vertically in the middle of two of the open mold halves to enable them close on the parison. When they do this, they blow pin it. The blow pin provides passage for the pressurized air in order to inflate the parison. Extrusion blow molding is the most popular type of molding used by many manufacturers. It is applied when manufacturing simple parts but in large volumes.
Injection blow molding, on the other hand, involves injection molding of molten plastic within a parison mold in the central part in order to come up with the hollow parison. Opening the parison mold leads to the transfer of the core and parison inside the blow mold and are firmly clamped. The core responds by opening to enable the inflation of the parison by the pressurized air. This method is least commonly used due to the fact that it has a lower production rate. However, it can create more complicated parts, and does so with a greater amount of accuracy. Injection blow molding is normally used when manufacturing small complex bottles. An example of this include the ones used in medical applications.
Stretch blow molding helps to form the parson in the same manner that injection blow molding does. However, after it has been taken to the blow mold, heated and downward stretching takes place in the core and then it is inflated. The importance of stretching it is to provide the plastic with a superior amount of strength. This method is normally applied in the case of parts that must be subjected to some internal pressure or the ones that need to be very durable. Soda bottles are a good example of this.
Compression molding is mainly used to process thermosetting polymers. It involves enclosing a polymer that is performed and premeasured inside a closed mold. Heat and pressure is then applied to the polymer until it assumes the form of the mold cavity, enabling it to cure too. Compression molding undergoes a longer cycle compared to injection molding. It is difficult to manufacture intricate parts and those with very close tolerances using this method. However, it has some advantages, including the fact that it needs a low capital cost because the equipment and tooling required for this process are cheaper and simpler. It accounts for less material waste, is adaptable to fast automation and it can be used to mold bulky parts.
Blow molding method has the advantages of fast production rates, low cost inters of the tools and die and the fact that it is able mold together complex shapes. However, the use of this method is only limited to tubular or hollow shapes.
Casting is simply about solidifying molten plastic/metal poured inside a mold or injected inside a die with a cavity containing the part one wishes to produce. This method is normally applied in the manufacture of tubes, fixtures, sheets, trial jigs and rods. It is also important in insulating electrical components. This is a simple process and there is no need for any external pressure or force. Liquid plastic is placed inside a mold until it is full before being heated to cure. After this, the material will be isotropic, meaning all the directions will have regular properties. Examples of plastics that can be used include epoxies, nylon or PVC, polypropylene and polyesters. Casting method has several benefits including the ability to create big parts that have thick cross sections, low molding costs, the convenience it provides with regard to low volume production and the fact that it provides a good surface finish. The major disadvantage i the fact that this method is limited to manufacturing of relatively simple shapes. In addition to that, it can become uneconomical when involving high production rates. When looking for a faster way of shaping metal to form a particular part, then casting is the best technique (Cook & Vaccari, 1999).
Casting process used to make polymer plastics
Resin casting is basically used in the case of small scale production such as manufacturing prototypes and dentistry. Amateur hobbyists who have a small amount of the first investment can do it. It is important in producing models, collectible toys and jewelry at a small scale. Gravity casting is by far the simplest type of casting process, and it involves pouring the resin inside the mold before being pulled down by gravity. Air bubbles are sometimes formed during the process of mixing the two part resin, but these can be eradicated in the vacuum chamber. With the help of open molds, casting can take place in the vacuum chamber to help eradicate the bubbles. Alternatively it can be done inside a pressure pot where their sizes can be significantly reduced until they become almost invisible. Centrifugal force, pressure or both of them together can be applied to expel the liquid resin on the mold’s details.
It is possible to use thermoset liquid molding to obtain parts that are optically clear even though it is not easy to do. This is especially so if the particular cast has some complicated details. This is where today’s advanced machines come in. Thermosets molds and prototypes are normally applicable in almost all areas of the manufacturing industry once the right amounts of resins and processes are combined together. Prototyping and production involve a variety of formulations and chemicals. Pressure casting and vacuum degassing are mostly used when manufacturing castings that are clear and free of all voids and bubbles. It requires some extra time and energy and there is also the possibility of production rejects. Alternatives are vibrating the mold and also heating resin. An advantage of this particular procedure is the fact that it gives air bubbles room to escape easily and also to relieve tension in the process of filing the mold. Although vibrating and heating can lead to much better results, it is not considered failsafe. It is important to ensure that mold and cast material are compatible for the case of water-clear casts no matter the particular process you choose. It should not be assumed that any mold and cast material can do. For example, there are release agents and mold materials that are incompatible with aliphatic.
The type of release agent chosen is also important to be able to prevent the issue of surface defects and tackiness. Silicone based release agents are not the best with regard to clear resins because they react poorly with them leading to cure-inhibition and various forms of defects. This is the reason for many silicone based agents choosing silicone mold in order to prevent the process of releasing even if it has challenges with regard to casting clear resins. Polyvinyl Alchohol (PVA) qualifies as release agent that is considered to be failsafe. This consists of one part liquid that is brushed or sprayed in order to create a film over the part that is non- reactive. RTV silicone rubber is mostly preferred by many liquid molders even whether in platinum or tin form. This is due to their flexible and self-releasing properties. The biggest challenge with using silicone molds to cast clear resins is that after de-molding, the part surface can become uncured or tacky. Cure inhibition is the term used to refer to this phenomenon and is one huge challenge that has limited solutions. It is crucial to post cure silicone mold before using it in order to remove part of the acids and natural oils found on the surface. These are the substances that make it difficult for the clear resins to fully cure. The unfortunate thing is that it is not always possible to apply post curing especially in cases where there are extraordinarily big molds.
Generally, casting requires hands-on labor and this result in a fairly high final cost different from injection molding which only requires a high initial cost but the high volume produced by the mold helps to greatly lower the final cost per unit.
Plastic Extrusion Method
Let’s overview the process of extrusion for manufacturing of plastic. Plastic extrusion is a very common method in the plastics industry today whether it is for manufacturing thousands of drinking straws or making yards of pipe tubing. This is due to the fact that it is an easy process to carry out and is readily available. It is highly recommended for applications involving a final product that has a regular cross-section (Sinotech, 2010). The fact that it has high production rates and low cost has made it the popular choice when it comes to products like weather stripping, piping, adhesive tape, plastic sheeting and wire insulation.
Before beginning the process of extrusion process, it is important to understand the right machinery and supplies needed in order to make the whole process possible and efficient. This process specifically requires a plastic extruder machine, which is a simple device that will ease the entire process from the beginning to the end. The main components making up a plastic extruder are barrel, screw drive motor, hopper and screw drive. The next component that is required for the process is raw thermoplastic material. Most of the plastic extrusion operations use small solid beads known as resin plastic to enable easy loading and rapid melting times. Examples of the common plastic materials applicable in this process are polyethylene, ABS, high impact polystyrene (HIPS), polypropylene and PVC (Thomasnet.com, 2015). The last component needed for this process is the die. The die acts as the mold and enables the molten plastic to flow evenly. Dies are normally custom made and sometimes need extra lead time before starting the manufacturing process.
Plastic extrusion process is aimed at changing solid plastic materials to liquid state, which is later reconstituted to form finished products. The first stage involves feeding of pellets gravity acting as raw resin through a hopper and then inside a jacketed screw. If the resin doesn’t have the additives needed for the specific application you are carrying out then you can add them to the hopper. Examples of these additives include anti-oxidants, UV inhibitors and colorants among others.
The resin travels within the barrel and during this time, it is exposed to very high temperatures to the point of melting. Barrel temperatures vary according to the particular type of thermoplastic used and they are normally between 400 and 530 oF (Cook & Vaccari, 1999). When the molten plastic comes out of the barrel, it enters a screen pack leading to the feed pipe attached to the die. The screen helps to eliminate contaminants that may be available within the molten plastic. The screen is able to do this due to the reinforcement received from the breaker plate as a result of the high pressures contained inside the barrel. Uniform melting is required when the proper amount of back pressure is applied, and this can be done by manipulating the number and porosity of screens. The molten metal moves from the feed pipe into the die cavity which helps it to cool and harden. A sealed water bath can help to speed up the cooling process. Cooling rolls can be used to do this instead of water bath when it comes to plastic sheeting extrusions.
After this, a die forces the molten material through it in order to give it a particular cross section shape and it creates parts that have an impending extensive range of lengths. Plastics are converted from solid to liquid state and then back again during the extrusion process without interfering with their distinctive properties.
Many extruders contain barrels that experience a gradual heat increase from the loading part up to the feed pipe in order to give room for gradual melting and greatly reduce the rate of degradation. This is why it is possible to ground and re-extrude scrap parts with the smallest amount of degradation. This has made extrusion a popular method with regard to recycling or reducing plastic waste (Thomasnet.com, 2015).
When carrying out the plastic extrusion process, it is important to maintain the right temperatures and melting rates with regards to the resin. Optimal temperature is important in giving the plastic maximum uniform fluidity and greatly reducing the likeliness of the final product having stress and warping. There are variables that accumulate inside the extruder barrel which prevent temperatures from remaining stable. Friction and pressure are some of those variables. It is therefore important to monitor the heaters, whether they require raising, lowering or shutting to be able to achieve constant heat inside the extruder. Cast in jackets and cooling fans are important in maintaining the right level of extrusion temperature. The size and design of the screw used is important in this process because it greatly determines the feed and heating rate among various integral extrusion factors since this is the only part that moves in the plastic extruder. There are factors that help to determine how the diameter and length of the screw will be and examples include the particular raw plastics you use and the quantity of both the pressure and resin that is needed.
There are several applications where specialized extrusion processes are needed to get the right results or to accelerate the production process. Blown film extrusion, overjacketing, coextrusion and tubing extrusion are examples of some of the specialty extrusion processes. Blown film extrusion is the method used in fabrication of plastic film products like food storage and grocery bags. The die in this case has a straight cylindrical design in charge of pulling the molten plastic up during the process of formation and cooling. Coextrusion just like the name suggests involves extrusion of many layers consecutively. A minimum of two extruders have the task of feeding the various plastic types inside one extrusion head. In overjacketing, extrusion is used in coating a substance in protective plastic coating. The common application of this method is in cable and exterior wire jacketing. Tubing extrusion takes place just like traditional extrusion except for the fact that the die is inclusive of interior mandrels or pins which are important in aiding in the manufacture of void plastic materials.
It is common to find several manufacturers of plastic resin selling new and recycled goods. They re-melt extruded scrap and then return it pellet form in order to make the recycled products. These manufacturers normally obtain obsolete parts, unused resins or the excess supply from the production runs. This kind of recycling is greatly encouraged because it helps eradicate industrial plastic waste in a valuable and lucrative way. With regards to recycling, it is possible to obtain virgin thermoplastic resins from the laboratory certifications of purity which come with standard technical grades for general use. Biodegradable plastics are becoming more common particularly in the case of blow molded bottle production. Resin manufacturers can add a few things to the material such as enhancers, colorants or flock when shipping them to the fabrication companies which use them in their hoppers and are extruded without further pretreatment.
The interior of the die is where the product takes its ultimate design and this is why the die used in plastic extrusion is important. The plastic gets in to the front extension of the die through the metering section and then proceeds to the last part of the extruder barrel. It then flows around a mandrel hanging in the middle part of the channel. At the end of the mandrel there is a pin and land size for the product that are both removable. This makes it possible to either reconfigure the die or substitute its worn parts for new ones. Pressurized air is let inside the mandrel’s support and comes out through the die pin. The work of the airflow is to ensure the product does not collapse when exiting the die. After this, the component is supposed to go through post treatment.
The product goes through a vacuum chamber after leaving the die, and sizing rings are used to pull it in here. The mixture of the air pressure and vacuum pulling makes the plastic take up the shape of the specific sizing rings. When the sizing rings are worn out then they affect the product by leaving some longitudinal scoring behind. The vacuum chamber is filled with water which is important in cooling the plastic to form a hard solid. Belted runners are used to pull the product after it has cooled and cut in to the desired length or coiled up to form a spool.
Rotational molding is often referred to as “rotomolding” or “rotamolding” and it is simply a process in which hollow plastic products can be produced by using thermoplastics or thermosets rarely (Henty & Salamon, 1999-2000). Extra post molding operations come into play to enable manufacturing of complex components in order to put this method on the same level with the rest of the molding and extrusion methods. Rotational molding is different from the rest of the processing methods in the sense that the stages the product goes through such as melting, shaping, heating and cooling all take place after the polymer has been positioned in the mold. This means that there is no external pressure applied in the formation and it comes with some advantages. It has very few design constraints, is economical to produce large products, its mold costs are comparatively low, it does not leave polymer weld lines and the products are stress free.
The rotational molding process and the machine used to perform it
The process of rotational molding is divided in to four categories: charging mold, heating and fusion, cooling and unloading or demolding. In the charging operation, a prearranged quantity of polymer powder is positioned within the mold and the mold is then shut, locked and placed inside the oven. The powder is then pre-compounded to form any color of choice. When the mold is placed inside the oven, it is time to proceed to the heating stage. It rotates about two axes on a relatively low speed of below 20 rpm which tumbles the powder (Henty & Salamon, 1999-2000). Gravity and not centrifugal force is applied here to enable the mold surfaces to have uniform coating. Convection, conduction and radiation are the methods used to heat the ovens. The powder starts melting with growing hotness of the mold and it attaches to the interior walls of the mold as it does this. The powder forms a regular kind of coating on the surface during the melting. After satisfactory melting of the powder, it can be allowed to cool and there are various ways of doing this such as through water, air or both of them combined.
The polymer then solidifies and takes up the desired shape. The polymer needs to be unloaded or demolded after cooling sufficiently. At this point it should be easy to handle and able to retain its shape. It is as simple as opening the mold and taking out the product. It is possible to put some powder inside the mold again and repeat the cycle.
The typical materials normally used in this process include Polypropylene, Polyvinyl Chloride, Polyethylene and Ethylene Vinyl. Examples of the typical products produced from rotational molding include traffic cones, manhole inspection chambers, pallets, rainwater tanks, children’s playhouses, canoes and kayaks, children’s playhouses, diesel fuel tanks and slides, climbing frames, small swimming pools, automotive dashboard, luggage pieces, fashion mannequins, buoys and various flotation devices, large industrial barrels, portable outhouses, garbage cans and septic tanks. Rotational molding is mainly used for thermoplastic polymers, elastomers and thermosets application are gaining popularity. Although this process is considered a substitute to blow molding with regards to producing large hollow shapes. Rotomolding works well with large parts, more difficult geometries and lower volumes of production compared to blow molding.
Although the rotational molding is made up of a long cycle, it has the advantages of enabling molding of complex parts, providing unrestricted freedom to product design and the fact that it does all this by using machinery that are low cost makes the process itself cost effective. In order to balance the merits and demerits of this method, it often takes place on multi cavity indexing machines. An example of this is the three station machine, which is structured in such a way that three molds work together through three workstations enabling all the molds to work simultaneously. The primary workstation is for unloading and it is the part where unloading from the mold takes place for the finished part. The powder needed for the subsequent part is then loaded on the cavity. The next station has a heating chamber for heating the mold using hot air convention as it rotates simultaneously. The temperatures in the chambers are normally between 3750-7000 F depending on factors such as the particular item being molded and the type of polymer (Sinotech, 2010). The third station is in charge of cooling the mold and it does this with the help of forced water spray or cold air. The plastic mold inside it is then able to solidify.
There are various properties of rotational molding, with rigidity being one of them. The density of the polymer that is being used, the rate of cooling and thickness of the wall molding are important in determining the rigidity of an article that is rotationally molded and produced using Alkatuff LLDPE resins. The more density applied, the greater the stiffness. Slow cooling helps to encourage a greater molding density which is what leads to the stiffness. In order to achieve extra rigidity, then rims or ribs have to be included in the articles’ design. The type of polyethylene used, design of molding and rotational molding cycle contribute to the toughness of the articles and this can be measured using the environmental stress crack resistance (ESCR) in addition to impact properties of low temperatures. The MFI and polyethylene’s density greatly determine these properties. In order to achieve maximum toughness, low density and low MFI are recommended. A medium density grade with regards to high MFI which is normally over 20g/ 10 min has a high possibility of brittle failure. Regulating heating and cooling cycles involved in the process is a great way of maximizing the stiffness of rotational molding.
It is important to maintain adequate heating time and temperature in order to achieve the greatest level of fusion of the powder. If the aim is to achieve lower densities that lead to higher ductility, then it is important to apply rapid cooling. Moldings can show signs of premature failure when exposed to particular situations for example in cases of sharp corners, notches, molded parts having rough edges or I case the molded articles is exposed to extreme levels of stress rates, stress in the stress cracking liquids or impact experienced at low temperatures. However, research has shown that most of the moldings produced from Alkatuff LLDPE have lesser chances of breaking when in service.
Injection molding is the most common method with regards to manufacturing of large volumes of plastic products, and this is due to the fact that it has superior power when it comes to product’s dimensions and has high production rates. This method transforms the plastic material from the raw form to the mold and it does this step by step.
Injection molding machine and the process it uses to manufacture plastics
In injection molding (IBM), an injection molding machine is used to manufacture the plastics instead of extruder, which normally produces the precursor. In this method, the precursor is referred to as the preform instead of parison like with the other methods. IBM is often preferred over EBM due to the fact that a more standardized wall thickness can be achieved with it after designing the perform shape. The process carried out here involves blowing a molten thermoplastic on the interior parts of the walls in a female mold cavity before letting it chill to form a rigid solid product. An essential injection unit together with multi-impression mold assembly present in the IBM machine allows the mounting of the mold cores in this process, and this takes place on a rotary table. Its cores act as both the index and the blowing pins required in the basics of blowing, ejection and injection in 120 degrees steps.
Research conducted show that injection molding process accounts for 30% of the entire quantity of plastic products manufactured (Cook & Rubin, 1998). This process consists of six steps after making the prototype in order to come up with the final product. The first step is clamping the mold, and this is done by the clamping unit. An injection machine is made up of three standard parts including the clamping unit, the mold and the injection unit. The clamp is responsible for holding the mold during injection of the melted plastic. The mold is subjected to pressure at the time of cooling of the injected plastic. The next step is injecting the melted plastic, and this involves feeding polymer either into a cylinder if it is in powdered form or in pellet from a big hopper. It is then heated into a molten state that can be easily pushed inside the mold. This plastic is then pushed through a split mold cavity where it hardens due to the pressure it is subjected to. The next part is the dwelling phase. This consists of confirming whether every cavity in the mold contains melted plastic. After this is done, then the cooling process commences and this goes on until the plastic hardens. It is then time to open the mold and get the finished product which is a newly formed plastic. If there is some additional plastic in the mold, then it is cleaned off.
Injection molding has several advantages including low labor costs due to the fact that the achiness do most of the work, greater ability to reproduce complex details, accommodate high levels of production and it is associated with an excellent surface finish. In addition, there are lower scrap costs when the injection methods used due to the fact that there is accurate manufacturing of the mold. It has limitations too such as the fact that the first tool and die costs are high and it is quite expensive for small runs. Small companies can face challenges when they want to use this method due to the fact that the equipment is expensive.
The type of material chosen is very important in injection molding because it will determine quality and characteristics of the final product. There are thousands of varieties of resin grades and the wrong choice can turn out to be disastrous. It is important to carry out sufficient research and consultations especially for someone who is not familiar with plastic part of design. The application of the part normally helps to make the material selection. It should be noted that different materials have different shrink rates and so altering them after construction of the mold increases the chances of inconsistencies in the geometry. This is why it is advisable to check the particular molder before making any changes in material.
Plastics are used in the manufacturing many of the things society uses everyday and being in a world without them is imaginable. This paper has clearly established the importance of plastics and the different areas of life that they are applicable. Different types of plastics are needed in various sectors and this is why there are a variety of methods such as injection molding, extrusion, casting and blow molding that are used due to the different properties that they establish in the final product. It therefore becomes easier to choose the type of process that is most suitable for a particular application. Research and studies are being conducted to develop new innovations in plastics production that will be able to keep up with the high demand for them and also enable safer ways of disposal.