Before a fabrication procedure can be selected to fabricate a component many things need to be considered. The design of the merchandise. the functionality. the service conditions and the belongingss of the stuff all play a large portion when sing the fabrication procedure. This survey looks at Polypropylene Chairs used chiefly for instruction intents. It provides information that explains the intent of a chair ; the demands of this peculiar merchandise and the fabrication procedure that produces the concluding constituent.
To understand why this type of chair is made from polypropylene the reader foremost needs to understand the footing of any chair. A chair is a raised surface for a individual individual to be seated. The bulk of chairs are produced with the standard four legs but it’s the design of the chair and its intended usage that takes most consideration. The usage of the chair will find the design and the stuff used. For illustration ; is the chair to be used at a desk or for sitting at a dining tabular array to eat?
The users’ weight demands to be equally distributed over the chair to supply comfort whilst seated. If the chair is to be used for long periods of siting so it is good for the chair to be somewhat reclined. which will back up the individuals back more and take the weight from other parts of the organic structure. If a chair is excessively high so shorter people will hold their pess swinging. doing force per unit area on the articulatio genuss ; whereas lower seats cause uncomfortableness to the natess.
Looking at the demands of this type of chair we can see it is non the most comfy chair on the market but so these chairs are non used for long periods of sitting. They are chiefly used within canteens. infirmaries. libraries and schools for short sitting periods ; intending that they are mass produced. Due to the sum of chairs that are manufactured there is a demand to maintain the production cost low. They are tough chairs that can come in a assortment of colorss. can be used in or outside and have the advantage of being able to stack away ; salvaging infinite. The chair is required to keep a assortment of people with a scope of organic structure weight and tallness so they are required to be stable. strong lasting and withstand motion whilst being lightweight.
This manner of plastic chair is made of a thermoplastic polymer called Polypropylene ( PP ) . It is a thermoplastic that has a two dimensional construction. The plastic can be softened by heat and recycled. The stuff is produced by the polymerization of polymer molecules into really long ironss. The stuff is a “semi-crystalline solid with good physical. mechanical and thermic properties” . The belongingss of PP in its liquid province are defined by the length and comprehensiveness of the polymer ironss that form during the procedure. When PP is in its solid province the belongingss are based on how much crystalline and formless part signifiers from those ironss.
[ Karian. Ph. D. . Harutun G. Handbook of Polypropylene and Polypropylene Composites. 1999. New York. NY. USA. Pg 15 ]
FIG 1. Example of the concatenation of Isotactic polypropene
PP is semi-crystalline ; intending that it contains little crystals and stuff that is formless and Isotactic. The ironss are closely packed together and the sum of new wave der Walls bonding is at a maximal. doing the stuff strong as a solid object. The polymer ironss determine the weight of the stuff and the crystals within the ironss find how thick the stuff can be and in bend this will impact on how much warming is required to model the stuff.
“The crystallisability of the ironss 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 run 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 denseness. It has good opposition to impact and tire. first-class chemical opposition. a high heat opposition and an first-class wet barrier. Having good structural characteristic s makes PP a utile stuff for stiff objects. PP has a good balance between its physical and chemical belongingss.
Due to the high mold shrinking of PP it is hard to accomplish close tolerances but because the stuff is tough. resilient and has a high opposition to emphasize checking it reduces the demand for close tolerances. Having a really low denseness of 0. 90g/cm^3 ; a low cost per volume ; a broad flexibleness when it comes to plan and it besides being reclaimable makes PP an attractive building stuff. PP has the advantage of being able to organize high volume. complex forms at a comparatively low cost.
When utilizing CES package and comparing the monetary value against the break stamina it provides the user with all known stuffs. Using the bound map and using a Young’s modulus of 2GPa and a minimal tensile strength of 40MPa
Fig 2 Materials based on Price against Tensile Strength
The chairs are made of PP because the stuff is cheap. easy to clean. lightweight and lasting. The stuff can be tough and flexible with a high tensile and compressive strength. The chief ground that this stuff works good when fabricating these chairs is down to the precise control of the impact strength. they do respond when exposed to heat intending that they hold their form and supply good belongingss within the human environment. particularly when in an outside environment.
Polypropylene is most normally manufactured utilizing bulge. or injection molding.
Bulge allows highly big batches of unvarying cross sectional form points to be produced. Hot bulge is when the polymer is heated first to do it more ductile. and so ‘pushed’ through a dice. Pieces made utilizing bulge have an highly high quality of surface coating. intending they do non necessitate coating after industry.
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FIG 3. Bulge Procedure
Injection molding is achieved by runing the polymer in a barrel. and so coercing or shooting it into a mold. Some injection modeling machines are screw fed. As the piece cools. it shrinks somewhat in the mold. This can do merchandise defects particularly if the mold is ill designed.
It allows for the production of really accurately molded pieces. When the piece is removed from the mold. it will hold a contour line called a ‘parting line’ where the mold closes on it. and frequently will be marked by the ouster pin with a little circle. If the maker does non desire these Markss on their merchandise. so it will necessitate post-manufacture coating. but otherwise the coating is of a comparatively high criterion.
FIG 4. Injection Moulding Procedure
Some chairs could be manufactured utilizing bulge. if they were a level ‘L’ form for illustration. However most chairs are shaped for ergonomic and structural grounds. and would necessitate a more accurate fabrication procedure. This would be injection modeling. as thick prances can be included in the design to help the strength of the chair. and a comfy dip can be shaped into the dorsum of the place to do it more appealing to the user.
As chairs of this nature are designed for a really broad scope of users. and with lastingness in head. the stuff is required to be reasonably thick to suit this demand. This would decelerate the fabrication procedure with injection molding as it would take each piece longer to chill. and hence a longer delay is required before the piece can be removed from the mold. The thickness of the piece would hold small consequence on an extruded merchandise. as the dice can be made whatever size is desired. The weight of the chair would hold small bearing on industry in either of these methods. except for the sum of stuff needed.
For a batch of 1000. bulge would be an first-class procedure. as adequate stuff for the whole batch can be melted and pushed through the dice. and the pieces cut from the subdivision after bulge. Large batches can be produced really rapidly in this manner. Injection modeling would take longer. as each piece must be made separately. unless the mold is designed with several chairs in. each connected by a ‘strut’ of waste merchandise. This increases the sum of waste stuff. and the initial set up costs due to the mold design. and a larger machine being required.
There are many factors to see when organizing a constituent from composite. Failure to see these factors can drastically alter the stuff belongingss and this in bend could take to an unsatisfactory constituent. A polymer matrix for the complex is an first-class base ; their stuff belongingss already provide many favourable properties such as high corrosion opposition and low denseness. The debut of a secondary reinforcing agent will supply a synergistic consequence to the mechanical belongingss of the stuff leting a better constituent.
One of the factors that are critically of import in the production of a complex is the format of the secondary stage ; it is a cardinal factor to the stuffs belongingss of the constituent. There are three largely used signifiers for the reinforcing of secondary stages ; these are long/short fibres. or atoms.
Long fibres. besides known as uninterrupted fibres. are fibres that are layered in strips or woven into a form along an axis. The fibres by and large consist of a much stronger stuff than the polymer matrix ; as such the fibres take a much greater burden compared to the polymer matrix. This allows for a stuff with much better burden bearing belongingss. The increased ability to take burden is based upon the way of the fibres.
When bearing burden. the best mechanical belongingss are obtained in the way of the fibres. There are many different orientations used by fibres. pointing the fibres in one specific way is frequently used when the way of the force is changeless. Planar support is used when the way of force is merely along one plane of action.
Short fibres. besides known as uninterrupted fibres. are fibres that have been cut into comparatively short strands. The shorter nature of these fibres means that they can be orientated in tonss of forms and waies. Equally good as being able to be orientated in one specific way and along one plane like long fibres. they can besides be orientated indiscriminately. This random orientation allows for a unvarying distribution of the reenforcing ability of the fibres ; therfore increasing the burden bearing ability in all waies alternatively of merely one. or a individual plane.
If atoms are introduced into a polymer matrix. their effects vary greatly compared to their size.
Very little atoms act as a barrier to disruption motions ; this hardens the matrix stuff due to the fact that disruptions can non propagate as easy through the construction. Smaller atoms can non take tonss due to their size. so the burden bearing ability of the complex is based upon the matrix stuff. If larger atoms are introduced they can take some of the burden. this allows the complex of the two to take a larger burden. Due to the nature of atoms the matrix is of course able to take isotropic tonss.
[ Mikell. P. Groover. Principles of Modern Manufacturing Fourth Edition.
2011. New Jersey. NJ. USA. Pg 181-183 ]
Fig 5. A Chart Showing Long/Short fibres every bit good as Atoms.
Orientation of support is another major factor in the production of a composite constituent. Not taking into history the way of support can hold unfavourable stuff belongingss. Materials are either isotropic or anisotropic in one or more waies.
Isotropic orientation is achieved when the fibres are indiscriminately oriented. or atoms are introduced into the matrix complex. The effects of this type of orientation are that the burden can be taken uniformly in all waies. This is advantageous because it allows for a stuff that is non constrained by tonss in one way. The downside is that the stuff belongingss can ne’er be every bit strong as if they where aligned to one way.
Anisotropic orientation is when fibres are aligned to a specific way or plane. When the burden is applied in this way is highly strong. this means that an anisotropic composite stuff has highly good belongingss in the way of the fibres. The farther the load gets from the way of the fibres the less good the stuff belongingss are until the burden is perpendicular to the fiber way. When the fibre way is perpendicular to the fibres the maximal strength is the strength of the primary matrix. If the concentration of fibres is high plenty the strength in the perpendicular way 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 demoing Anisotropic and Isotropic Orientations
The chair. due to its map. has assorted bounds that need to be applied when taking into history the building of the complex.
The forces inherent in the usage of a chair are non in one way. or even one plane of action. This means that an isotropic complex is best suited for the chair. as a planar or remarkable way of fibres wouldn’t be good to the mechanical belongingss of the constituent.
Due to the complex that is being used it must hold an isotropous orientation so merely discontinuous fibres or atoms can be used ; uninterrupted fibres can non be used practicably in an isotropous orientation.
A short fibre based 2nd stage would increase the stamina of the constituent every bit good as increasing the stiffness and strength in all waies. These belongingss seem more favourable than those of particulates which increase weariness strength. Short fibres in a random orientation would be good for a place ; this would let it to take impacts during its service every bit good as staying in form and defying bending.
[ Mikell. P. Groover. Principles of Modern Manufacturing Fourth Edition. 2011. New Jersey. NJ. USA. Pg 181-183 ]
The length of the short fibres would be above the critical length value ; this is based on the diameter of the fibre. the ultimate tensile strength of the fibre and the shear strength of the matrix-fiber interface. The diameter of the fibres should be every bit little as possible to let 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 complex is the merchandise of: ( Volume of Fibers/Volume of Composite ) .
It is a manner of showing the sum of fibre in a composite. it is besides really utile in polishing the belongingss of your complex.
The volume fraction for our complex should be greater than the critical
A volume fraction lower than this means that the matrix would interrupt before the fibres could back up the burden applied. This can be given by:
Vcritical = ( ?m* – ?’m / ?f* – ?’m )
?m* = Matrix Ultimate Tensile Strength
?’m = Matrix Yield Strength
?f* = Fiber Ultimate Tensile Strength
Once a value for Vcritical is determined. this can be used as a footing for finding the volume fraction you want. Any volume fraction must be above Vcritical so that the matrix is really strengthened by the add-on of the fibres. You can go on adding a higher concentration of fibres until you begin to imbrittle the polymer matrix composite due to the extra fibre content.
Without cognizing the exact fibre and matrix picks it would be difficult to foretell the volume fraction this would happen at. as a guideline the value would be any value greater than 0. 7.
Therefore the volume fraction should lie above Vcritical. To determine the best value for the volume fraction you would hold to make proving on the complex at assorted volume fraction values and utilize the consequences to find which would be the best for the chair constituent.
Using a indiscriminately oriented short fibre composite would heighten the stuff belongingss.
The random orientation would do it able to manage forces from all waies and the short fibres would increase assorted mechanical belongingss such as stamina. tensile strength and weariness strength. This material complex would bring forth a really light and strong construction which should be able to defy alot more than the polymer entirely. The synergistic consequence of this composite allows it to achieve a much higher specific strength than the polymeric design. It besides attains a much higher stiffness than the original design ; leting it to keep its form more systematically during its service clip. and it besides means it is less likely to neglect due to a sudden daze.
The increased weariness strength would intend that the stuff can keep its service for a longer period of clip due to its ability to defy cleft extension and it’s opposition to mechanical defects constructing up over clip.
In subdivision 2. it was decided that the best procedure chosen for bring forthing the chairs was bulge casting over injection casting as it was a faster procedure and allowed for less waste stuff.
This procedure is non suited for the add-on of short fibres excessively the polypropene. alternatively strengthened reaction injection modeling would do a better pick.
Reaction injection casting is a low force per unit area procedure used to bring around thermosets that require a chemical reaction instead than heat. This procedure requires two reactive ingredients which are assorted and injected into the cast pit where the hardening and hardening procedures occur quickly. This procedure can be used for big parts every bit good as complex forms.
When reenforcing fibres are used in the mixture the procedure is called strengthened injection casting.
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FIG 7. Image OF REINFORCED REACTION INJECTION MOLDING PROCESS
The capital get down up cost for this procedure is high but due to the low force per unit areas that are used in the procedure it balances out cost. The initial batch size chosen for the production of the chairs was 1000 units. This figure works with strengthened reaction injection casting as it allows for economic bath sizes between 100 and 10000 units. so even if batch sized was reduced by a ten percent of its original size or increased by 10 times the size this procedure can still be economical to the maker.
[ 27 Mar 2012 ]
[ 27 Mar 2012 ]
Stephen Pheasant. 2nd Ed ; Bodyspace: Anthropometry. Biotechnologies and the Design of Work ; Taylor & A ; Francis Ltd
[ 2011 ]
[ 20 Feb 2012 ]
FIG 7. ( hypertext transfer protocol: //composites. owenscorning. com/processes/Reaction_Injection_Molding. aspx )
hypertext transfer protocol: //staff. bath. Ac. uk/msscrb/dent. pdf – equation 5. page 1657
FIG 1 – [ Polypropylene Molecule. Chempolymerproject. [ Online ] Available: [ hypertext transfer protocol: /…/Polypropylene-B-CABA ] [ 28 Mar 2012 ] ]
FIG 3 – [ Extrusion of a Round Blank through a Die [ Online ] Available: hypertext transfer protocol: //en. wikipedia. org ( 2011 ) ]
FIG 4 – [ Injection Moulding Machine [ Online ] Available: hypertext transfer protocol: //www. engineerstudent. co. United Kingdom ( 2011 ) ]