102EMM Coursework 2; Manufacturing Case Study
James Yerbury (3160565), Russell Williams (3523106), Steve Collingwood (3067437) & Karen Whitehead (3785276)
Before a manufacturing process can be selected to manufacture a component many things need to be considered. The design of the product, the functionality, the service conditions and the properties of the material all play a big part when considering the manufacturing process. This study looks at Polypropylene Chairs used mainly for education purposes. It provides information that explains the purpose of a chair; the requirements of this particular product and the manufacturing process that produces the final component.
To understand why this type of chair is made from polypropylene the reader first needs to understand the basis of any chair. A chair is a raised surface for a single person to be seated. The majority of chairs are produced with the standard four legs but it’s the design of the chair and its intended use that takes most consideration. The use of the chair will determine the design and the material used. For example; is the chair to be used at a desk or for sitting at a dining table to eat?
The users’ weight needs to be evenly distributed over the chair to provide comfort whilst seated. If the chair is to be used for long periods of seating then it is beneficial for the chair to be slightly reclined, which will support the persons back more and remove the weight from other parts of the body. If a chair is too high then shorter people will have their feet dangling, causing pressure on the knees; whereas lower seats cause discomfort to the buttocks.
Looking at the requirements of this type of chair we can see it is not the most comfortable chair on the market but then these chairs are not used for long periods of sitting. They are mainly used within canteens, hospitals, libraries and schools for short sitting periods; meaning that they are mass produced. Due to the amount of chairs that are manufactured there is a requirement to keep the production cost low. They are tough chairs that can come in a variety of colours, can be used in or outside and have the advantage of being able to stack away; saving space. The chair is required to hold a variety of people with a range of body weight and height so they are required to be stable, strong durable and withstand movement whilst being lightweight.
This style of plastic chair is made of a thermoplastic polymer called Polypropylene (PP). It is a thermoplastic that has a two dimensional structure. The plastic can be softened by heat and recycled. The material is produced by the polymerisation of polymer molecules into very long chains. The material is a “semi-crystalline solid with good physical, mechanical and thermal properties”. The properties of PP in its liquid state are defined by the length and breadth of the polymer chains that form during the process. When PP is in its solid state the properties are based on how much crystalline and amorphous region forms from those chains.
[Karian, Ph.D., Harutun G, Handbook of Polypropylene and Polypropylene Composites, 1999, New York, NY, USA, Pg 15]
FIG 1. Example of the chain of Isotactic polypropylene
PP is semi-crystalline; meaning that it contains small crystals and material that is amorphous and Isotactic. The chains are closely packed together and the amount of van der Walls bonding is at a maximum, making the material strong as a solid object. The polymer chains determine the weight of the material and the crystals within the chains determine how thick the material can be and in turn this will impact on how much heating is required to mould the material.
“The crystallisability of the chains is one factor that determines how thick the crystallites will be and the thickness of the crystallites determines...
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