Plastic molding ” plastic chair “

Plastic Chair Manufacturing via Injection Molding
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Plastic Chair Manufacturing via Injection Molding
Introduction
The concept of chair making dates back to Egyptian era where chairs have been symbolized as a mean of power, status, and control. Moreover, the concept of throne has also originated from the very same aspect. In the modernized world, chairs have become commodities and become an essential part of almost every household. For coping up with this demand, chairs have become quite common in different varieties of materials. Among them, plastic chairs become most popular because of their low cost, lighter weight, recyclability options, and ease of transportation. With the advancement in plastic technology, plastic chairs have also evolved into more durable and resilient products having different chemical ingredients as part of manufacturing process. Injection molding is usually employed for making plastic chairs on large scale; however, there are a number of technical factors that should be considered beforehand. This paper will address the evolution of plastic chair making process along with detailed discussion about injection molding process.
Use of Raw Materials
Every aspect of manufacturing has input raw materials which are processed to form desired product. In the early production of chairs, wood and metallic alloys were quite common. However, plastic molding requires specific raw materials for catering the need for best quality product.

Polyethylene terephthalate (PET) is the most common raw material for making plastic chairs. PET is also a famous raw material for plastic bottles, which follow a similar route of production like that of plastic chairs. PET is a widely known thermoplastic that is used in a number of other processes as well. Being a part of polyester family, its fibers are also used for clothing purposes as well. Figure 1: depicts the structural formula of polyethylene terephthalate:

Figure 1: Molecular Structure of Poly Ethylene Terephthalate (Image Taken from: http://wwwcourses.sens.buffalo.edu/ce435/2001/Plastic_Microwave/MicrowaveReport_files/image004.gif)
PET possesses semi-crystalline structure that impacts good mechanical and thermal resistance. However, the presence of polar groups (benzene ring) helps it make the polymeric chairs to be electronically rich thereby creating strong intermolecular forces between each consecutive chain. The chain structure is not rigid and it has a stiffer backbone that makes it to have reduced flexibility with low flexural strength.
Nevertheless, the addition of different polymeric additives can help to tailor made properties of plastics as per need. For plastic chairs, it is essential to have thermal and oxidation resistance. In this regard, different plasticizers are used for achieving this purpose.
Injection Molding Machine and Process Parameters
Injection molding is a widely used production route used for various products that includes plastics, metals, confectionery, and elastomers. Following a pictorial representation of injection molding process:

Figure 2: Pictorial Representation of Injection Molding Machine (IELM, 2015).
Injection molding machines play a significant role in fabrication of different plastic parts as well. For plastic chair manufacturing, it is essential to know about different parts of the machine and their core purpose. An injection molding machine usually has two major units, namely, injection and clamping unit. Each of this distinction unit’s changes the properties of polymeric melt to achieve greater degree of product compliance. Material is fed into the feeding hopper either manually or using pneumatically assisted pumps. The hopper also provides metering needs for material to maintain consistency in product features. It allows precise addition of required polymeric granules and additive to achieve melt consistency. Ones the material is fed into the hopper, it is gone towards the barrel.
The barrel is a unit that allows PET granules and additives to achieve desired melt profile. Usually, screw and high-pressure rams are used for forcing the plastic melt out of barrel. The barrel is further divided into three zones (not mentioned in figure) according to melt temperature. The temperature is controlled through thermocouples fitted on barrel casing to avoid clogging and excessive melt temperatures at the end of process. For chair manufacturing, it is vital to maintain barrel temperatures in order to attain desired melt consistency. Nevertheless, based on polymeric rheology, and enhanced mixing properties, dual screws are also used instead of single reciprocating screws. Consider the following figure for the four stage cycle of injection molding operation:

Figure 3: Injection Molding Machine Operation Cycle (IELM, 2015).
In the first step, the mold cavity and the movable platen are tightened and pushed against each other for closing the cavity of mold. In the second stage, the pushing mechanism which is usually a screw pushes the polymeric melt out from the barrel and into the mold cavity with specific force and velocity of screw. The non-return valve installed at the end of screw prevents the melt from flowing in the reverse direction. In the third step, the screw is moved backward to introduce fresh molten polymer at the end of barrel for the next shot. During this stage, the solidification of polymeric melt in mold is carried out through recirculation water and other means. In the last step, the movable platen moves backward thereby ejecting the molded part out of the die for further finishing and trimming. Following is the pictorial representation of two-plate mold:

Figure 4: Two-Plate Mold (IELM, 2015).
Another part of the injection molding machine is the clamping unit that is responsible for opening and closing of the injection molding die. It is also responsible for ejection of different products as well. Normally, there are two types of clamping methods that are quite commonly used in plastic molding industries. It includes, straight hydraulic and toggle type. For plastic chair manufacturing, toggle type of ejection system is quite commonly used.
Previously, straight hydraulic clamping was used in almost every plastic industry owing to its versatility in metal molding machines as well. The hydraulic clamping system use a hydraulic fluid to create desired force, and it includes use of hydraulic piston and cylinder for creation of clamping force. The traditional two-platen clamping mode as described in figure 2 also uses the very same feature. The excessive use of hydraulic fluid to create pressures in the range of 2000-30,000 psi is quite undesirable thereby leading to shorter lifetime of hydraulic fluid in operations.

Figure 5: Hydraulic Clamping Mechanism (Rosato and Rosato, 2012).
A much developed version of clamping system uses toggle clamping. Toggle or mechanical clamps use the very mechanical feature of a linkage to create additional force to hold the mold at a desired closed position during the molding cycles. The core advantage of using toggle clamping is that it can be operated at lower hydraulic pressure as compared to conventional clamping operations. However, the core drawback of it includes the actual clamping tonnage is quite difficult to precisely calculate and modeled.

Figure 6: Toggle Clamping Method (Rosato and Rosat, 2012).
A relatively smaller hydraulic cylinder for closing the clamp that travels at a constant speed. The core advantage of this mechanical linkage is quite huge that can produce high tonnage using a smaller cylinder. The single toggle clamp applies a clamping force by the amplification of force applied on it. It is around 15-20 times for the single toggle clamp, and it can be as much as 25-50 times for double type. Hence, for a toggle with mechanical advantage of 20 can produce a 100 ton of clamping force for single toggle in contrast with 5 tons of applied force. The toggle clamping systems have also developed with the toggle clamping force of 200 tons which has been reduced to 70 tons with different process innovations. Considering single and double clamping, the power consumption from the double toggle machine is lesser as compared than single toggle machine. Nevertheless, double toggle machines are substantially costly as compared to single toggle (Rosato and Rosato, 2012).
Costing and Financial Outlooks of Chair Injection Molding
As per the SMEDA (2015) pre-feasibility assessment has showed, a total of around USD 400 thousands is required for the startup. It includes construction cost, injection molding machine cost (considering refurbished machinery), metal molds, working capital, and raw material inventory to name a few. These are some of the core costs that are faced by the manufacturer at the initial level of construction and erection. Considering the European and Chinese machinery, the desired IRR (Internal Rate of Return) should be atleast 25% for profitability and project feasibility. The costing can also be minimized if the industrialized or free-zone areas of the United States are targeted. The cost of exportation is greatly reduced while operating under that domain.
Besides mentioned direct costs, it is essential to keep tabs on indirect cost as well. Indirect cost mainly involves electrical and mechanical maintenance costs together with cost associated with machine downtime and cooling cycle time of formed plastics. Considering the injection molding machine, the process parameters are either controlled through computerized or analog controlling means (SMEDA, 2015). Better process control can be achieved through DLC (Distributive Logic Control) in order to achieve better product compliance. However, it is significantly costly as compared to conventional PLC (Programmable Logic Control). Detailed process flowchart of adopting a CAM model is also by Wong et al. (2004).
Finance is the biggest constraint for the company to achieve break-even point at the earliest possible. For that, the sales of plastic chairs should be justified with the investment and production costs associated with the startup. Besides these core costs, the cost of utilities should also have to be taken into consideration. The costs associated with electricity, and water consumption is surely significant one. However, the plastic chair making company can also generate electricity at their site to reduce electricity costs significantly. However, it would burden the firm with additional costs that include gas utilities.
Mold designing is another core cost that should be borne by the company. For ease of operations, it is essential to have atleast two molding dies in case of any malfunction and/or chocking of melt. Cleaning of sprues and runners is also essential to be conducted at regular intervals for better operational feasibility. Following figures shows a clear depiction of pictorial representation of injection molding plastic chair mold:

Figure 7: Plastic Chair Mold (Image taken from: http://www.lulusoso.com/upload/20120714/plastic_mould_injection_mould_plastic_injection_mold.jpg_250x250.jpg).
Technological Innovation in Chair Making Process
During the course of history, chair making process have been developed considering material of construction as well as addition of different core ingredients. At first, the molding begin with a simple process of hand laying the material within the molds, the transfer of resins and subsequent trimming of excessive edges and material after completion of molding operation. Nevertheless, this process has exposed a number of different health hazards and can cause the shell to crack in case of any mishandling. With the development of injection molding machines, it became possible for chairs to become more flexible and durable. Robin Day has incepted the usage of polypropylene usage in furniture with PP chair production in early 1963. In this era, the plastic chair production has gained a boom and different companies started introducing aesthetically pleasant designs for plastic chairs (Parsons 100-102).
The core disadvantage faced by such mass production was Plastic non-biodegradable nature. For that Yang et al. (13776-13784) have introduced a method of making plastic material biodegradable. The core aspect of innovative plastics and its usage on industrial scale has gained substantial popularity owing to the harmful impact of plastic on environment.
Conclusion
All in all, plastic molding machines are used for a number of potential applications. It also includes manufacturing of plastic chairs on large scale. However, it is essential to consider issues faced by nature because of massive plastic production as well. Hence, a proper disposal and recycling system should have to be implemented to make the process greener with the passage of time.

References
IELM,. (2015). Lecture 7. Plastic Part Manufacture, Injection Molding. Retrieved 28 November 2015, from http://www.ielm.ust.hk/dfaculty/ajay/courses/ieem215/lecs/6_plastics.pdf
Parsons, Tim. Thinking: Objects: Contemporary approaches to product design. AVA publishing, 2009: 100-102.
Rosato, D. V., & Rosato, M. G. (2012). Injection molding handbook. Springer Science & Business Media.
SMEDA,. (2015). Pre-Feasibility Study: Injection Molding Plastic Products. Retrieved 28 November 2015, from http://www.smeda.org/index.php?option=com_phocadownload&view=category&download=1218:injection-molding-plastic-products-rs-1-2-million-mar-2005&id=123:archive-pre-feasibilities&start=40&Itemid=553
Wong, C. T., Sulaiman, S., Ismail, N., Hamouda, S., & Magid, A. (2004). Design and Simulation of Plastic Injection Moulding Process. Pertanika Journal of Science & Technology, 12(2), 85-99.
Yang, Jun, et al. “Evidence of polyethylene biodegradation by bacterial strains from the guts of plastic-eating waxworms.” Environmental science & technology 48.23 (2014): 13776-13784.