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 how much heat energy is required to melt them”.
[Karian, Ph.D., Harutun G, Handbook of Polypropylene and Polypropylene Composites, 1999, New York, NY, USA, Pg 17]
PP is stiff, with a low density. It has good resistance to impact and fatigue, excellent chemical resistance, a high heat resistance and an excellent moisture barrier. Having good structural characteristic s makes PP a useful material for rigid objects. PP has a good balance between its physical and chemical properties.
Due to the high mould shrinkage of PP it is difficult to achieve close tolerances but because the material is tough, resilient and has a high resistance to stress cracking it reduces the need for close tolerances. Having a very low density of 0.90g/cm^3; a low cost per volume; a wide flexibility when it comes to design and it also being recyclable makes PP an attractive construction material. PP has the advantage of being able to form high volume, complex shapes at a relatively low cost.
When using CES software and comparing the price against the fracture toughness it provides the user with all known materials. Using the limit function and applying a Young’s modulus of 2GPa and a minimum tensile strength of 40MPa
Fig 2 Materials based on Price against Tensile Strength
The chairs are made of PP because the material is inexpensive, easy to clean, lightweight and durable. The material can be tough and flexible with a high tensile and compressive strength. The main reason that this material works well when manufacturing these chairs is down to the precise control of the impact strength, they do react when exposed to heat meaning that they hold their shape and provide good properties within the human environment, especially when in an outside environment.
Polypropylene is most commonly manufactured using extrusion, or injection moulding.
Extrusion allows extremely large batches of uniform cross sectional shape items to be produced. Hot extrusion is when the polymer is heated first to make it more malleable, and then ‘pushed’ through a die. Pieces made using extrusion have an extremely high quality of surface finish, meaning they do not require finishing after manufacture.
FIG 3. Extrusion Process
Injection moulding is achieved by melting the polymer in a barrel, and then forcing or injecting it into a mould. Some injection moulding machines are screw fed. As the piece cools, it shrinks slightly in the mould. This can cause product defects especially if the mould is poorly designed.
It allows for the production of very accurately shaped pieces. When the piece is removed from the mould, it will have a contour line called a ‘parting line’ where the mould closes on it, and often will be marked by the ejector pin with a small circle. If the manufacturer does not want these marks on their product, then it will require post-manufacture finishing, but otherwise the finish is of a relatively high standard.
FIG 4. Injection Moulding Process
Some chairs could be manufactured using extrusion, if they were a flat ‘L’ shape for example. However most chairs are shaped for ergonomic and structural reasons, and would require a more accurate manufacturing process. This would be injection moulding, as thick struts can be included in the design to aid the strength of the chair, and a comfortable dip can be shaped into the back of the seat to make it more appealing to the user.
As chairs of this nature are designed for a very wide range of users, and with durability in mind, the material is required to be fairly thick to accommodate this need. This would slow the manufacturing process with injection moulding as it would take each piece longer to cool, and therefore a longer wait is required before the piece can be removed from the mould. The thickness of the piece would have little effect on an extruded product, as the die can be made whatever size is desired. The weight of the chair would have little bearing on manufacture in either of these methods, except for the amount of material needed.
For a batch of 1000, extrusion would be an excellent process, as enough material for the whole batch can be melted and pushed through the die, and the pieces cut from the section after extrusion. Large batches can be produced very quickly in this way. Injection moulding would take longer, as each piece must be made individually, unless the mould is designed with several chairs in, each connected by a ‘strut’ of waste product. This increases the amount of waste material, and the initial set up costs due to the mould design, and a larger machine being required.
There are many factors to consider when forming a component from composite. Failure to consider these factors can drastically change the material properties and this in turn could lead to an unsatisfactory component. A polymer matrix for the composite is an excellent base; their material properties already provide many favorable attributes such as high corrosion resistance and low density. The introduction of a secondary reinforcing agent will provide a synergetic effect to the mechanical properties of the material allowing a better component.
One of the factors that are critically important in the production of a composite is the format of the secondary phase; it is a key factor to the materials properties of the component. There are three mostly used forms for the reinforcing of secondary phases; these are long/short fibers, or particles.
Long fibers, also known as continuous fibers, are fibers that are layered in strips or woven into a pattern along an axis. The fibers generally consist of a much stronger material than the polymer matrix; as such the fibers take a much greater load compared to the polymer matrix. This allows for a material with much better load bearing properties. The increased ability to take load is based upon the direction of the fibers.
When bearing load, the best mechanical properties are obtained in the direction of the fibers. There are many different orientations used by fibers, orienting the fibers in one specific direction is often used when the direction of the force is constant. Planar reinforcement is used when the direction of force is only along one plane of action.
Short fibers, also known as continuous fibers, are fibers that have been cut into relatively short strands. The shorter nature of these fibers means that they can be orientated in lots of patterns and directions. As well as being able to be orientated in one specific direction and along one plane like long fibers, they can also be orientated randomly. This random orientation allows for a uniform distribution of the reinforcing ability of the fibers; therfore increasing the load bearing ability in all directions instead of just one, or a single plane.
If particles are introduced into a polymer matrix, their effects vary greatly compared to their size.
Very small particles act as a barrier to dislocation movements; this hardens the matrix material due to the fact that dislocations cannot propagate as easily through the structure. Smaller particles cannot take loads due to their size, so the load bearing ability of the composite is based upon the matrix material. If larger particles are introduced they can take some of the load, this allows the composite of the two to take a larger load. Due to the nature of particles the matrix is naturally able to take isotropic loads.
[Mikell, P. Groover, Principles of Modern Manufacturing Fourth Edition,
2011, New Jersey, NJ, USA, Pg 181-183]
Fig 5. A Chart Showing Long/Short fibers as well as Particles.
Orientation of reinforcement is another major factor in the production of a composite component. Not taking into account the direction of reinforcement can have unfavorable material properties. Materials are either isotropic or anisotropic in one or more directions.
Isotropic orientation is achieved when the fibers are randomly oriented, or particles are introduced into the matrix composite. The effects of this type of orientation are that the load can be taken uniformly in all directions. This is advantageous because it allows for a material that is not constrained by loads in one direction. The downside is that the material properties can never be as strong as if they where aligned to one direction.
Anisotropic orientation is when fibers are aligned to a specific direction or plane. When the load is applied in this direction is extremely strong, this means that an anisotropic composite material has extremely beneficial properties in the direction of the fibers. The further the load gets from the direction of the fibers the less beneficial the material properties are until the load is perpendicular to the fiber direction. When the fiber direction is perpendicular to the fibers the maximum strength is the strength of the primary matrix. If the concentration of fibers is high enough the strength in the perpendicular direction can be significantly less than the primary polymer.
[Mikell, P. Groover, Principles of Modern Manufacturing Fourth Edition, 2011, New Jersey, NJ, USA, Pg 181-183]
Fig 6. A Chart showing Anisotropic and Isotropic Orientations
The chair, due to its function, has various limits that need to be applied when taking into account the construction of the composite.
The forces inherent in the use of a chair are not in one direction, or even one plane of action. This means that an isotropic composite is best suited for the chair, as a planar or singular direction of fibers wouldn’t be beneficial to the mechanical properties of the component.
Due to the composite that is being used it must have an isotropic orientation so only discontinuous fibers or particles can be used; continuous fibers cannot be used feasibly in an isotropic orientation.
A short fiber based second phase would increase the toughness of the component as well as increasing the stiffness and strength in all directions. These properties seem more favorable than those of particulates which increase fatigue strength. Short fibers in a random orientation would be beneficial for a seat; this would allow it to take impacts during its service as well as remaining in shape and resisting bending.
[Mikell, P. Groover, Principles of Modern Manufacturing Fourth Edition, 2011, New Jersey, NJ, USA, Pg 181-183]
The length of the short fibers would be above the critical length value; this is based on the diameter of the fiber, the ultimate tensile strength of the fiber and the shear strength of the matrix-fiber interface. The diameter of the fibers should be as small as possible to allow for a higher tensile strength.
[Mikell, P. Groover, Principles of Modern Manufacturing Fourth Edition, 2011, New Jersey, NJ, USA, Pg 181 Figure 8.15]
The Volume fraction of a composite is the product of: (Volume of Fibers/Volume of Composite).
It is a way of expressing the amount of fiber in a composite, it is also very useful in refining the properties of your composite.
The volume fraction for our composite should be greater than the critical
A volume fraction lower than this means that the matrix would break before the fibers could support the load applied. This can be given by:
Vcritical = (σm* – σ’m / σf* – σ’m)
σm* = Matrix Ultimate Tensile Strength
σ’m = Matrix Yield Strength
σf* = Fiber Ultimate Tensile Strength
Once a value for Vcritical is determined, this can be used as a basis for determining the volume fraction you want. Any volume fraction must be above Vcritical so that the matrix is actually strengthened by the addition of the fibers. You can continue adding a higher concentration of fibers until you begin to imbrittle the polymer matrix composite due to the excess fiber content.
Without knowing the exact fiber and matrix choices it would be hard to predict the volume fraction this would occur at, as a guideline the value would be any value greater than 0.7.
Therefore the volume fraction should lie above Vcritical. To ascertain the best value for the volume fraction you would have to do testing on the composite at various volume fraction values and use the results to determine which would be the best for the chair component.
Using a randomly oriented short fiber composite would enhance the material properties.
The random orientation would make it able to handle forces from all directions and the short fibers would increase various mechanical properties such as toughness, tensile strength and fatigue strength. This material composite would produce a very light and strong structure which should be able to withstand alot more than the polymer alone. The synergetic effect of this composite allows it to attain a much higher specific strength than the polymeric design. It also attains a much higher stiffness than the original design; allowing it to maintain its shape more consistently during its service time, and it also means it is less likely to fail due to a sudden shock.
The increased fatigue strength would mean that the material can maintain its service for a longer period of time due to its ability to withstand crack propagation and it’s resistance to mechanical defects building up over time.
In section 2, it was decided that the best process chosen for producing the chairs was extrusion molding over injection molding as it was a faster process and allowed for less waste material.
This process is not suitable for the addition of short fibers too the polypropylene, instead reinforced reaction injection molding would make a better choice.
Reaction injection molding is a low pressure process used to cure thermosets that require a chemical reaction rather than heat. This process requires two reactive ingredients which are mixed and injected into the mold cavity where the curing and solidification processes occur rapidly. This process can be used for large parts as well as complex shapes.
When reinforcing fibers are used in the mixture the process is called reinforced injection molding.
FIG 7. IMAGE OF REINFORCED REACTION INJECTION MOLDING PROCESS
The capital start up cost for this process is high but due to the low pressures that are used in the process it balances out cost. The initial batch size chosen for the production of the chairs was 1000 units. This figure works with reinforced reaction injection molding as it allows for economic bath sizes between 100 and 10000 units, so even if batch sized was reduced by a tenth of its original size or increased by 10 times the size this process can still be economical to the manufacturer.
[27 Mar 2012]
[27 Mar 2012]
Stephen Pheasant, 2nd Ed; Bodyspace: Anthropometry, Ergonomics and the Design of Work; Taylor & Francis Ltd
[20 Feb 2012]
FIG 7. (http://composites.owenscorning.com/processes/Reaction_Injection_Molding.aspx)
http://staff.bath.ac.uk/msscrb/dent.pdf – equation 5, page 1657
FIG 1 – [Polypropylene Molecule, Chempolymerproject, [Online] Available: [https:/…/Polypropylene-B-CABA] [28 Mar 2012]]
FIG 3 – [Extrusion of a Round Blank through a Die [Online] Available: http://en.wikipedia.org (2011)]
FIG 4 – [Injection Moulding Machine [Online] Available: http://www.engineerstudent.co.uk (2011)]