For my project I was required to choose a viable material for a bicycle pump tube of dimensions diameter 20mm by 1.5mm thick wall. The design specifications given to consider were cost and quality. For the bicycle pump tubing it was important for me to consider the design parameters for such a device. The material must be flexible and ductile to enable the tubing to bend and allow ease of use when pumping the bike. The tube must also have relatively strong yield strength to prevent plastic deformation.
One of the main mechanical properties the material must have is a low value for young’s Modulus. A flexible material has a low value for Young’s Modulus. Typically a a young’s modulus value of less than 1Gpqa tends to be flexible, whilst metals which are stiff tend to have values of approximately 100Gpa. The material I have chosen for my project is PVC. The material has a young’s modulus range of .003 – .0035Gpa. Within the given design parameters cost was specified as one of the main factors to consider. It was mainly for this reason that I chose PVC as it is very cheap to produce.
Figure 1.1 Graphic representation of specific stiffness vs. Price.
From the graph above it is clear to see that in relation to price and flexibility That PVC is the best material to choose for the tubing. In relation to the environmental factors PVC is recyclable however this did not really have much impact on my decision as the various viable materials I had selected were also recyclable.
PVC had a yield strength of 10-11Mpa. This in comparison to price once again helped separate the various materials and enable me to choose PVC as it had one of the higher yield strengths within the lower priced materials
Figure 1.2 Graphic illustration of Yield strength vs. Price
PVC had a yield strength of 10-11Mpa. This in comparison to price once again helped separate the various materials and enabled me to choose PVC as it had one of the higher yield strengths within the lower priced materials. (typically less than 2Euro/Kg)
2. Process selection
For my process selection I have chosen a moulding type of polymer extrusion. Within this process a polymer or in powder or granule form is processed by rotating a screw. A funnel, or hopper, feeds this into the heating chamber of the extruder. The polymer is melted as it is heated to temperatures above 200C. Once the polymer is molten a screw forces the polymer through the shaped die head which forms the plastic into the shape. As it leaves the extruder, the plastic is cooled by water or air to give the final form. The extrusion process is cheap and for a continuous process of constant shape it is the best choice for the model.
One of the main disadvantages of this is the initial large price for the machine moulders. However this machine will last very long and the only thing that need be replaced may be different parts sch as the die castings. The length of the tubing is also not specified and this process can produce tubing for the various different lengths required.
3. Cost estimates
For calculating the volume of raw material required to produce 15,000 nits per annum, it was first required to calculate the mass per unit. For the sake of this calculation it was assumed that the length of the tube produced was 1m in length. the volume of each unit produced was calculated at .00013M^3. Using the mass, density formula the mass of each unit was calculated.
1100 Kg/M^3 * .00015M^3= .143Kg per unit
Volume required per annum- .143 * 15,000= 2145Kg of raw materials per annum
1.50 Eur/Kg * 2145 = 3217.5euro for the cost of raw materials.
To produce 15,000 per annum there is roughly 45 weeks in the year. Therefore 334 units produced per week.
334 divided by 40 hours per week gives 8.35 units per hour or 8.35 units or tube metres. One machine is sufficient as the moulding process chosen is quite quick