Plastic is a major part of almost every industry imaginable. From food and beverage packaging, to childrens toys, to automotive applications, plastic is all around us. Plastic is a versatile material which can be used for many purposes depending on the specific properties. This report will explore the history of plastics, the manufacturing process, and what to expect from plastics in the future. For simplicity, we will look primarily at the two of the main types of plastics: high density polyethylene, HDPE, and polyethylene terephthalate, PET. These two types of plastic have been the stepping stones for other plastic discoveries and their properties attributed to the many bottle applications. There are different processes that correspond to each plastic type depending on the specifics of the end application. Quality control and quality assurance are vital to plastic companies. These departments provide the customer with the satisfaction of a well designed product, which is why it is so vitally important. There are other problems plastic companies face on a daily basis such as change-outs, direct printing, stopping time, and damaged goods. These problems are capable of setting a company back from profit, and there are solutions being made for each of these issues. Perhaps the biggest concern regarding the plastics industry today is about recycling and what is being done to reuse and conserve our natural resources and minimize the negative impact on the environment. Recycling needs to be faced head-on and the steps to reuse and minimize waste are explored in this paper.
The History of Plastics
Plastics have always been viewed in two different ways. On the one hand, some view plastics as one of the most useful materials ever made, but on the other hand some see plastics are artificial, toxic, and detrimental to the environment. Plastic is a material that is engineered by men and can be shaped into almost any desirable form. They were designed in order to replace prominent products with an inexpensive yet efficient substitute. For centuries people have used natural resins similar to plastic, but not until the mid-1800s, did the commercial development of plastics begin (“Plastics” 563). In 1862, the first man-made plastic was developed by Alexander Parkes. This plastic, known as Parkesine, was designed to be molded and yet retain its shape when cooled, in order to replace natural rubber (Masterson). Later in the decade John W. Hyatt developed a material to eliminate the need for ivory in the manufacturing of billiard balls. In 1870 the material was patented and named celluloid, and this was the first synthetic plastic to succeed commercially (“Plastics” 563). This plastic was a breakthrough, but did have a problem with being highly flammable. Other plastics would soon be invented to fix these problems and cover a wide variety of products. In 1909, Leo Baekeland patented a resin (a material made from acids) known as Bakelite which helped pave the way for the development of commercial plastics. Bakelite opened the door to scientists to begin to understand the chemistry of plastics (“Plastics” 564).
All plastics belong to one of two groups, thermosetting plastics and thermoplastics. Thermosetting plastics are plastics that are heated but can only be molded once. Since we are covering the manufacturing of plastic bottles, we will not deal with this type of plastic. Bottles are commonly recycled, requiring the reformation of a plastic, which is the definition of thermoplastics. The thermoplastic we are most interested in is polyethylene (PE). Polyethylene was first produced in 1933 and used on radar systems during World War II to make them light enough for airplanes (Masterson). The rise of polyethylene continued after the war and became one of the most used plastics in the world.
In 1953, high density polyethylene (HDPE) was invented by Karl Ziegler and Erhard Holzkamp and used in the production of pipes (Gabriel). However, this material didnt reach success until 1956 with the breakthrough of the Hula Hoop. The toy led to the high commercialization of HDPE being used in pipes, which revealed the materials usefulness in the making of other products such as detergent and baby bottles (Ceresana Research).
Another form of PE which will be of interest is polyethylene terephthalate (PET) which was invented in 1941 by John Whinfield and James Dickson. PET was generally used in clothing but began to see more use in the manufacturing of bottles (Bellis). Not until the early 1970s did PET see use in plastic bottle manufacturing. Nathaniel Wyeth developed this bottle, by improving the method of manufacturing the bottle through blow molding. Blow molding was developed earlier in the 1940s but was inefficient because of inconsistent products. In 1973 Wyeth improved this method, leading to how bottles are currently made by stretch blow molding (Secrest).
Plastics have evolved from a little known substance into materials completely facilitated in many areas of production. Although the first substantial invention was credited in 1862, the general public didnt use the word until the mid 1930s (Masterson). Currently, in 2010, plastics have woven their way into many facets of manufacturing and production, yet these innovative breakthroughs have also brought many new challenges.
Plastics come with unique properties and colors. Their ability to be molded into nearly any shape while maintaining their strength at a low cost makes them an ideal material for many uses. From piping to bottles, plastic is a very common and highly useful material. Not every type of plastic can be applied in the same way as another type. Each type of plastic also has its own advantages and disadvantages.
An example of the different characteristics of plastics can be seen when comparing polyvinyl chloride (PVC) and high-density polyethylene. PVC is a type of plastic often found in piping materials while HDPE is a plastic that is often used in bottles containing materials such as detergent. PVC is very rigid and suited to applications requiring weathering resistance, inherent flame resistance, high gloss, abrasion resistance, and low cost (Bryce 129). HDPE, on the other hand, has a good balance of chemical resistance, low-temperature, impact strength, light weight, and low cost (Bryce 122). Both materials are useful in their own rights, but some plastics are much better than others at different tasks.
Plastic bottles are generally made from three different types of plastic. HDPE is a material generally found in detergent bottles, such as Tide or Gain. PET is a plastic often found in drink bottles. Lastly, polypropylene is a plastic often found in clear bottles with small handles, such as a hand soap refill bottle. Each of these plastics has their advantages and disadvantages, which the industry is trying to overcome so that each plastic can be used more widely.
HDPE is very commonly seen in colored, opaque plastic bottles. A large advantage of HDPE in bottle making is its ability to be layered. For example, Tropicana orange juice bottles are a multilayered, allowing the orange juice to stay fresh longer by keeping the outside environment from touching the juice contained inside. This layering process is not limited to just this bottle (Knueve).
In Tide brand bottles, there are 3 layers. The outer layer is made of virgin material, HDPE that has never been used before, and the color. The middle layer contains PCR, post consumer recycle, and regrind, which is the reground excess flash that comes off other bottles. The inner layer, or the layer that touches the product is another layer of virgin material. This layering system allows the product to be stronger and also helps keep the product safer from the outside environment (Knueve). The main reason for layering however is to consume PCR and regrind while also maintaining a protective interface layer with the product. This can also allow the color (external) and internal layers to be much thinner. A typical detergent bottle is 15% outer, 65% middle and 20% inner (Hatch).
If a material is sensitive to the outside environment or even oxygen, a six layer bottle is possible. Similar to the composition of the three layer bottle, the outside layer consists of the virgin material and the next layer is regrind. However, the next three layers is what helps give the bottle the protection to the environment. Most environmental barriers do not bind well with the HDPE, so an adhesive layer must be added to each side of the barrier layer. The inside adhesive layer is then attached to the sixth and final layer, which consists of virgin resin (Knueve).
HDPE is a good material choice because it is a low cost plastic. Often made using a blow mold process, bottles made from high-density polyethylene often show very good performance during bottle drop tests, a test for impact strength. HDPE also shows high low temperature toughness along with excellent resistance to chemicals and good electrical insulating properties (Lee 190). HDPE, also has some disadvantages as well. Due to the nature of the material, it has low clarity. Also, the neck dimensions are less accurate than that compared to PET bottles because of the blow molding process (Knueve).
PET is often found in carbonated drinking bottles, such as Mountain Dew or plastic water bottles. PET bottles are often formed in injection blow molding, as opposed to extrusion blow molding. This type of plastic incorporates stiffness and good heat tolerance (Bryce 120).
PET bottles are also very clear. When compared to an HDPE bottle, such as a milk container, you can easily tell which bottle is made from which type of plastic. Due to the injection blow mold process PET have neck dimensions that are much more accurate. Lastly, after a bottle is blown, the temperature drops much faster due to PET having much thinner walls, allowing less cooling time during manufacturing (Knueve). This also increases the rate at which you can produce PET bottles when compared to HDPE bottles.
PET also has its disadvantages. The biggest and most obvious problem is that you are unable to blow a handle into this type of plastic. This causes issues when bottles become increasingly large, such as gallon or larger sized containers. Recently, external handles have been produced which have created a solution to this problem .These external handles can be added during the injection blow mold process, or after the bottle has been blown. Examples of these types of bottles can be found in stores today on products such as Lipton Green Tea (Knueve).
A third type of plastic used in bottles is polypropylene which is mainly used in bottle closures. Although not common in bottles, polypropylene can be found in products that resemble PET bottles, such as a hand soap bottle. A large advantage of this material is that it can resemble a PET bottle in its gloss and clarity, but can be formed with a handle (Knueve) using an extrusion blow molding process.
Polypropylene also has good impact strength unless a low temperatures. Polypropylene bottles also have good chemical resistance, high abrasion resistance and high melt strength (Lee 194). These characteristics are very similar to that of HDPE, but in bottle drop tests, especially at low temperatures, a HDPE bottle will fare better and show more impact strength (Knueve). Each type of plastic has tradeoffs however. Although HDPE has more impact strength, it is very opaque and less shiny when compared to a polypropylene bottle.
Plastic Bottle Creation
The first common step in any bottle making process is extrusion. Extrusion is the process by which the plastic resin is mixed and melted. Similar to a meat mincer, plastic is fed from a hopper in certain predetermined quantities and is then melted. This process mixes plastic resin together to form a uniform mixture (Lee 103).
Continuous Extrusion Blow Molding (Wheel)
Extrusion blow molding is a very common technique in the creation of HDPE bottles. The process of forming the bottle in extrusion blow molding is a five step process. First, the plastic resin is melted in the extruder and mixed. This melted resin then enters a die which forms the melted plastic resin into a molten hollow tube know as a parison. The parison is then fed into a mold which clamps shut. A blow pin pierces the parison inside the mold in a section of the bottle known as flash and high pressure air is injected causing the plastic to spread throughout the mold taking on the shape of the inside of the mold. After some cooling time, the piece is ejected and the flash is trimmed (Hatch). A multilayered HDPE bottle is achieved through the use of multiple extruders each feeding into the same die forming a multilayered parison.
The rotary wheel and shuttle system are the two most common types of continuous extrusion blow molding. In a wheel process only a single parison is formed. The molds are mounted to a wheel and rotate around at a slow speed. As they travel past the extruder, the mold closes, encasing the parison. With the wheel, at any given moment, the parison is being captured, a part is being molded, a molded part is being cooled and a cooled part is being removed (Lee 109). Two major disadvantage of the rotary system is the complexity and setup of the clamp mechanism and the inability to produce calibrated neck finishes (Lee 110). Later in the paper, we discuss how the speed of change outs is improving, allowing this type of technology to become more dominant.
Continuous Extrusion Blow Molding (Shuttle)
The shuttle system follows the same principles as the rotary system. However, the molds are kept on a track. Instead of one parison being produced, the shuttle technology produces as many parisons as there are molds (Hatch). When accepting the parison, the molds clamp when the molten tube reaches the proper length. This group of molds then moves quickly back to the blowing station after the parison is cut and a blow pin enters the mouth of the bottle forming the mouth of the bottle and then blowing the remainder of the parison into the mold cavity.
Shuttle molds have a few distinct advantages over a wheel. The amount of flash produced in this method is much less because the parison length matches the bottle length. In a wheel method, the parison in-between the molds can vary because of the mold spacing around the wheel. Along with more efficient trim, a shuttle system creates a calibrated neck, where as a wheel system needs to have a separate process done to finish the neck of the bottle (Hatch).
Injection Reheat Stretch Blow Molding
Injection reheat stretch blow molding is often used for PET bottles. This technology is a two-step process. First, the molten resin is injected into a mold cavity which forms the threads, neck and body. This shape is then transferred to a different mold where it is expanded forming the shape of the bottle (Lee 124). The first step creates what is known as a preform. These preforms do not need to be used directly after they are formed. Using quartz lights, the preforms are re-heated until they are in a pliable state where a rod then stretches them downward while a blast of high pressure air expands the bottle into the shape of the mold (Lee 124).
Significant engineering goes into the design of the preform. The plastic distribution in the final bottle is obtained through the plastic distribution in the preform as well as selectively reheating the preform. The hotter areas after reheating will flow more producing thinner areas. Through the combination of preform design and reheating you can redistribute plastic to areas of the bottle that need extra strength and thin out areas of the bottle dont need the added strength. Typically the shoulder and base of the bottle have thicker areas to produce a bottle that can carry sufficient top load (Hatch).
Injection stretch blow molding
This process is normally considered a one step process because the preforms are produced in groups equivalent to the number of molds. This process is intermittent so instead of the preforms being manufactured and stored for later use the preforms are produced within the bottle blower. This technology is much slower that the process described above but has several inherent benefits. The first benefit is that you can produce wide mouth containers without concern for the injection mold efficiencies. In normal injection blow molding operations, the bottles have a smaller neck and some machines can create up to 144 preforms at one time. However, with a wider mouth bottle, there is less room to make each preform. Therefore, creating preforms to use at a later time, which is the reheat stretch blow process, it is much more efficient to blow directly after forming the preform. Secondly, because of the slower blowing speed this technology is well suited for smaller volume products where production better matches the demand (Hatch).
The traditional way to ensure that the customer received an error free product is quality control testing. In this process the products that are already manufactured undergo certain tests to determine whether or not the products meet the standards required. It is common in manufacturing that only a sampling of the products are tested not every individual item. In some facilities it would not be economical or efficient to test every single item so random samples must be used (Kalpakjian 1073). An example of a quality control test in the manufacturing of plastic bottles is to verify the thickness of the bottle walls at various locations. It would be difficult to measure the thickness at all locations using a caliper so an alternative is to slice the bottle into sections. The sections are cut using hot wires to ensure a clean cut and the bottle is always located in the same position in order to make sure the resulting sections are always the same. The sections are then weighed and compared with standard values to ensure that the plastic is spreading appropriately throughout the mold (Knueve). It is not expected that all sections would weigh the same since specific areas need to be stronger than others such bottle shoulder or bases. Automated inspection is another process that is very prevalent in the manufacturing of products. Sensor systems are designed that measure relevant parameters of the products as they proceed through the production line. Since the products are inspected during the manufacturing process any defective ones can be removed before they reach another manufacturing process (Kalpakjian 1070). The advantage to this is that time is not wasted performing another process on an item that has already been deemed unacceptable. A problem with this process of quality control is that there is always the chance that defective products could reach the customer.
Plastipak employs multiple quality control stations throughout their HDPE production lines to ensure that all the products that reach the consumer are of the highest quality. The main purpose of a bottle is to contain a liquid so naturally one of the most important tests along the production line is the leakage test. The bottles will be used to contain liquids but it would not be efficient to take the time to fill the bottles to capacity, ensure there are no leaks, and drain them. Also that method could lead to potential contamination and the need to completely dry the internal surface of the bottle. The solution to this method is to test for leaks with air pressure. There are multiple nozzle heads at this station and a revolving loop is incorporated into the production line at this station. When the bottle reaches this station an air tight nozzle is applied to the opening of the bottle and air is pumped into the bottle until it reaches a certain pressure (Knueve). The revolving loop allows multiple bottles to be tested at one time to ensure production is not slowed. The bottle then proceeds through the loop and if the pressure holds the bottle proceeds down the production line. If the bottle does not hold the specified pressure the bottle is taken off the line after it exits the test station by a burst of air from a nozzle locate on the side of the production line (Hatch). In order to ensure that this quality control station is functioning properly it is periodically challenged. Plastipak originally created calibrated bottles by drilling a small hole in the bottle. They recognized that not only does this create a waste of material but also leads to the chance that if the test machine is not functioning properly a defective bottle would reach the customer. The solution to this problem was to modify the machine by creating a leak of pressure (Knueve). When the test station is going to be challenged a solenoid is adjusted so that it leaks air which creates the drop in pressure which in turn triggers the rejection of the bottle after it exits the loop. After the challenging of the test station is complete the bottles that were rejected while the solenoid was leaking can be run through the test station again eliminating the unnecessary waste of materials or risk of the challenge bottles being shipped to the customer (Knueve).
Another quality control station on the production line ensures that the labels are correct and in the right position. As the bottles travel down the line they pass through a device that takes an image of the bottle with the label on it. The software in the station is designed to recognize that specific portions of the label are orientated at specific locations on the bottle (Knueve). Once again if the bottle does not pass this test an air nozzle is located where the bottles exit the station and a burst of air is used to remove the bottle from the line. Much like the leak test station the label verifier must be challenged in order to ensure that it is functioning properly. Some defective label bottles are purposely sent down the production lines when the system is being challenged. Some examples that would be rejected are bottles with no label, skewed label, or the wrong label (Knueve). At Plastipak the tolerances are set so low that often the few bottles rejected in this station would still meet the standards set forth by the customer (Hatch).
There are certain bottles that are manufactured at Plastipak that require exterior pouring spouts attached to them. The bottles travel down the production and the spouts are secured to the bottles with an adhesive (Knueve). The spouts must be orientated in a certain direction to allow the bottle to be poured properly. A test station is situated immediately after these spouts are applied. There are many sensors located at this station to ensure that the spout will function properly. One of the sensors monitors the temperature of the spouts before they are placed on the bottle because if they are not at the correct temperature it will not seal correctly (Hatch). Immediately after the spout is placed on the bottle there is another set of sensors to ensure it was attached correctly. Two sensors located at specific locations measure to make sure the spout is orientated in the correct plane. A third sensor at the same location is used to make sure there are no gaps between the bottle and the spout (Hatch). Finally there is a sensor that verifies the spout is the correct color. When Plastipak challenges this station they purposely send bottles through the station with spouts not positioned correctly or no spouts at all (Knueve). They also send bottles with the wrong color spout down the production line. The color recognition sensor is critical because they do not manufacture the spouts (Knueve).
An alternative to a quality control system is quality process control (Quality Assurance). In a quality process control method the focus is shifted from the items produced to the processes that make the items. If the process can be fine-tuned to produce exactly the same product consistently and correctly there would be no need to check that it is to specifications and defect free. In an ideal world the production lines could be set once to produce a perfect product not only today but two years later. The reality is that things such as equipment failures, variations in the material, or unauthorized adjustments among other things could lead to the production of defective items (Gordon 424). The solution to this problem is that the processes must be monitored to ensure they are being performed within acceptable limitations. All of the monitoring systems can be networked that allows management to easily observe whether or not the processes are being performed correctly which in turn tells them high quality products are being produced (Gordon 286). The system also has the ability to alert the operators when a process is not being performed correctly and in some cases actually shut down the production line if necessary (Gordon 287).
The temperature of the resin right before the bottle is formed is an example of a key process that could be monitored. The obvious problem is that the products produced while the temperature is decreased more than likely will be defective. Another potential problem is that it could damage the molds and machinery further down the production line. If the company relied on the old method it would take the operator recognizing the error or it reaching a quality control station further down the line which would still require someone to recognize the problem and take action. In the new process control system the temperature drop could be easily detected by a thermometer and the line could automatically be shut down before a catastrophic failure. The quality process system would enable the company to streamline their production lines. Streamlining is eliminating as many activities from the production process that do not add value to the product. While the quality control checks are necessary in situations; they do nothing to add more value to the product.
In the system of quality processes the quality checks that you perform actually turn into quality assurance. Since the process is being monitored to ensure correct items are being produced all quality tests that are being performed through the production line are more out of a need of assurance compared to necessity (Hatch). The question is then posed why are the production line inspections still necessary if in theory every item produced in this system is defect free. The reality is there will always be a minute amount of variation in the products being produced. The variation could be from a range of things for example quality of incoming material or wear on the machines being used (Kalpakjian 1073). There are a multitude of benefits from putting the quality into the processes instead of waiting until the product is manufactured to be concerned with quality. In the manufacturing of plastic bottles it will eliminate the amount of scrap and waste materials produced (Hatch). Also it will increase the reliability of the machines which improves efficiency that naturally leads to more profit. Most importantly quality process control will lead to an increase in satisfied customers (Gordon 573).
Plastipak created a system that integrates all of their process control methods into a single entity. When they first began developing the system their core goals were to simply obtain a count of the bottles produced and the amount of downtime (Knueve). Plastipak realized that there was the potential to go a step further and monitor individual processes within the production line. A computer system was developed that allows management to view the status and performance of many different components of the production from any computer with internet access. The system also has an automated email system that alerts the appropriate management positions when significant malfunctions occur or there is a period of extended downtime (Knueve). A few examples of processes that are monitored are air pressure, water temperature, plastic temperature, downtime frequency and length, and reject count at different steps (Knueve). The different test stations described earlier at Plastipak now are a means of assurance since the products produced meet the required specifications.
The introduction of plastics into manufacturing and production has brought about a disposal problem. Being so successful, PET has attributed to this problem greatly. Since a large number of bottles are being produced, once used up these bottles create large amounts of waste because most plastics do not readily break down. These products usually were thrown away, but because of environmental concerns, active solutions were sought out. Recycling is one of the most pertinent actions taken to decrease the amount of waste created by plastic. It is a process designed to recover and reuse material, which helps conserve raw materials and keeps those materials out of landfills (“Recycling” 184). Instead of throwing away the bottles, people instead are encouraged to recycle them, in which case the bottles will make be used again in the manufacturing process of the same or other products. There are typically two types of recycled material used by industries: materials that have been used by consumers and waste materials from the production process. However both of these materials can be reused in the production process in much of the same manner. These materials are usually separated by type and then broken down in different ways for reuse. Thermoplastics are re-melted and reformed into new products, or new bottles. Thermosetting plastics are usually grounded into powders or shredded, in which these are used for other products such as insulators in clothing (“Recycling” 184). Recycled PET was used in many other products until 1991, when the first PET bottle was made from recycled PET. This bottle was made by Coca-Cola and Hoechst Celanese Corporation and consisted of 25% recycled PET (Secrest). This paved the way for other measures taken to increase the efficiency of recycling plastics.
The process of recycling PET and HDPE products has been refined and engineered to get the most use out of recycled products. Typically the process is as follows:
1. All of the recycled products and waste are collected together and sent into large bales.
2. The bales are shipped to where a bale breaker which rips apart the bales.
3. The bottles are sorted by resin and color.
4. These are shredded into tiny flakes.
5. The flakes are cleaned and melted.
6. The plastic is extruded into pellets which are used in the process of forming new plastics.
This completes the process of reusing the recycled plastic, where the cycle starts over for the manufacturing of plastic products (How Plastic Bottles Are Recycled).
Unfortunately, recycled PET encounters problems for the manufacturing process. Although the cycle is an advantage for the environment and limiting waste, recycled PET is typically more expensive to use and has lower quality than virgin PET. Recycled PET also is regulated by the FDA which creates more restrictions (Koester). Although it would be environmentally friendly to use more recycled material in each product, it is not the most efficient decision because there are limitations involved. One reason is that the recycled material is generally more expensive. The curbside programs designed to recycle materials are not available everywhere and completely adequate. If these programs could be improved then the materials price may decrease and the quantity may increase. Also, the product needs to have a balance of ingredients, because there are limitations to the amount of each ingredient. If a bottle was
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