Week 4 Activity

 Roy, Nigel ,Bjorn, Vernon

Group 3 DCHE2A02

Topic: Material Selection for Inner Layer of Thermoflask

For week 4, we went to revisit our last year module which was the materials for design and we were tasked to find the right material for the inner layer of a thermoflask. In materials for design, we needed to find the right requirements for the material and from there find different suitable materials. We then use the COWS matrix (Criteria, Options, Weightage and Score) to determine the best material. Here is our findings.

Material Requirements

In order for the liquid inside the thermoflask to remain hot/cold, the inner layer of the thermoflask must reduce the amount of heat transfer from the liquid to the surrounding. The inner layer thus should have a low thermal conductivity and a high melting point.

 

Since the inner layer will be in contact with water, it must resist corrosion by any liquid put into it, water and acids from soda drinks.

It will be used for drinking, so it must be non-toxic and portable. It also must be durable to withstand drops.

Function	To contain liquid and to keep liquid hot/cold for a long period of time
Constrain	High Thermal Conductivity Corrosion Resistance  High Melting Point (Over 100 Degrees Celsius) Light-Weight Non-Toxic Durable to withstand drops and scratches
Function	Minimize Heat Transfer Between Liquid and Surroundings

Materials Properties

1. Poor thermal conductivity 

2. Corrosion resistant / non-reactive to drinks

3. Non-toxic

4. High melting point

5. Light weight / low density

6. Ease of machinery and welding / ease of fabrication

7. Durability

Materials selected

1. Borosilicate glass with outer aluminium coating

2. Borosilicate glass

3. 18/8 food grade stainless steel (SUS304)

4. High density polyethylene

Criteria	Borosilicate glass with aluminium coating	18/8 food grade stainless steel	High density polyethylene
Thermal conductivity	1.10 W/m.K	16.2 W/m-K	0.288 - 0.480 W/m-K
Melting point	800°C	1400 - 1455 °C	118 - 137 °C
Density	3g/cc	8.00 g/cc	0.924 - 0.995 g/cc
corrosion resistance	Great	Great	Great
toxicity	Non-toxic	Non-toxic	slightly toxic
Durability	Decent	Very good	Decent
Machinability	Poor	Good	Decent

COWS Matrix

Toxicity and corrosion resistance are given the highest weightage for its high importance in an inner lining of thermoflask. The material should be non-toxic as the inner layer will be holding liquids which will be consumed, and thus must be food grade. Corrosion resistance is important as the inner layer will be in contact with liquids containing water,  mild acids and salt. The material must be able to withstand corrosion.  Thermal Conductivity is given the second highest weightage as it is low in order to keep the liquid in the flask hot/cold for a long period of time.

Criteria	Weightage	borosilicate glass with outer aluminium coating	18/8 food grade stainless steel	High density polyethylene
Thermal conductivity	20%	2	1	3
Toxicity	25%	2	2	1
Melting point	15%	2	3	1
density	5%	2	1	3
Durability	5%	1	3	1
Corrosion Resistance	25%	3	3	3
Machinability	5%	1	3	2
Total Score		215	225	205

With 18/8 Food Grade Stainless Steel obtaining the highest score of 225, it is selected as the most suitable material for the inner lining of the thermoflask. Its value lies in its high melting pint, high durability, high corrosion resistance and its high machinability.

Matweb.com. n.d. 304 Stainless Steel. [online] Available at: <http://www.matweb.com/search/DataSheet.aspx?MatGUID=abc4415b0f8b490387e3c922237098da> [Accessed 14 May 2021].

Matweb.com. n.d. Overview of materials for High Density Polyethylene (HDPE), Injection Molded. [online] Available at: <http://www.matweb.com/search/DataSheet.aspx?MatGUID=fce23f90005d4fbe8e12a1bce53ebdc8> [Accessed 14 May 2021].

Matweb.com. N.d. Borosilicate Glass. [online] Available at: <http://www.matweb.com/search/DataSheet.aspx?MatGUID=b0dbbac859444ffe98307f24ffd4c6a2&ckck=1> [Accessed 14 May 2021]