Week 4

13/5/2021

🕤 Activities Timeline 🕐

Deciding object criteria and weightage

Research on suitable objects

Applying COWS decision matrix for the selected materials

Material Selection Assignment

We are tasked to select materials for the body and cover of the thermoflask.

• The inner layer that will come into contact with the hot fluid
• The cover

Material Selection

We decided to use metals for the inner layer that would come into contact with the hot water.
The three metals we decided to use are:

1. Stainless steel 316
2. Stainless steel 404
3. Aluminum

For the cover of the thermoflask, we decided to use polymers as they are easily molded and shaped. The three materials we decided were

1. Polypropylene
2. Polycarbonate
3. Low Density Polyethylene (LDPE)

Cows Decision Matrix

Table 1.1 Material Property Selection Table for Inner Layer of Thermal Flask

Criteria	Materials
	Stainless Steel - 316	Stainless Steel - 304	Aluminium
Thermal Conductivity (W/m-k)	16.2	16.2	25
Melting Point (℃ )	1379- 1399	1400-1455	660.3
Corrosion Resistance	A- Excellent	A- Excellent	A- Excellent
Density (g/cm3)	7.99	8.00	3.90
Thermal expansion Coefficient (µm/m.℃)	16.0- 19.9	17.3- 18.7	8.10
Tensile Strength (MPa)	579	505	240

Table 1.2 Material Selection Table for Inner layer of Thermal Flask

Criteria	Weightage	Material
		Stainless Steel - 316	Stainless Steel - 304	Aluminium
Thermal Conductivity (W/m-k)	30%	2	2	3
Melting Point (℃ )	15%	2	3	1
Corrosion Resistance	20%	3	3	3
Density (g/cm3)	10%	1	1	3
Thermal expansion Coefficient (µm/m.℃)	10%	3	3	1
Tensile Strength (MPa)	15%	3	3	1
Total	100%	235	250	220

Conclusion : 

Among these three materials, we choose Stainless Steel -304 as our material for the inner layer of thermal flask. This is because it poses sufficient thermal conductivity, moderate density, high melting point, high corrosion resistance, high thermal expansion and also high tensile strength. 

Table 1.3 Material property selection for Cover of Thermal Flask

Criteria	Weightage	Material
		Polypropylene	Polycarbonate	Low Density Polyethylene (LDPE)
Thermal Resistance (1/Thermal Conductivity) (m2-K/W)	40%	0.012	0.642	3.030
Melting Point (℃ )	20%	195	200	105
Corrosion Resistance	20%	A- Excellent	A- Excellent	A- Excellent
Tensile Strength (MPa)	20%	9.00-80.0	25.0-586	2.80-56.5

Criteria	Weightage	Material
		Polypropylene	Polycarbonate	Low Density Polyethylene (LDPE)
Thermal Resistance (1/Thermal Conductivity) (m2-K/W)	40%	1	2	3
Melting Point (℃ )	20%	3	3	1
Corrosion Resistance	20%	3	3	3
Tensile Strength (MPa)	20%	3	2	2
Total	100%	220	240	240

Table 1.4 Material Selection Table for Cover of Thermal Flask

Conclusion: Among the 3 materials, we have selected Polycarbonate for the cover of the thermal flask. As it best fits the criteria. Even though LDPE has the same score as Polycarbonate, Polycarbonate is still a better choice as it has a much higher melting point compared to LDPE.

---------------------------------------------------💡Group Reflection 💡----------------------------------------

This week, we were able to make use of the COWS decision matrix to aid us in material selection for the assignment. At first, we had to decide on certain criteria that the object may have possessed. Then, with the selected criteria, we had to decide on the weightage on each of the criteria based on the importance of the criteria. Afterward, we listed out different materials that could be used for the object. After much discussion, we decided on 3 materials . Then we ranked our materials on the cows decision matrix before deciding the most suitable material for the respective parts.