Week 4

ASYNCHRONOUS LESSON 🕑

In today's asynchronous lesson, we go back to Year 1 Materials For Design (MFD) module, COWS method. Whereby we are required to complete the material selection for a certain product assigned to our group. The product assigned to our group, group 2, is the reusable face-shield used by medical workers to protect themselves against COVID-19 virus. 

1. COWS 🐮

What is COWS?

C   - Criteria

O   - Options

W  - Weightage

S    - Score

Why is _____ needed?

Criteria: ❗❕

Criteria plays an important part in COWS decision matrix, as criteria sets the minimum properties (e.g. Thermal conductivity, density, transparency, etc.) the material should have. Thus, allowing us to narrow down the materials we have searched up for/ have in mind to materials that would be suitable for our product. 

Options: 🅐🅑🅒

There will be a few options that we found/ identified that should be compared.

Weightage: 🏋

Putting weightage to each criteria, helps to identify which criteria is more important than the other.

Score: 🏆

By scoring all the options, helps us decide which of the option (material) is most suitable and should be used to design the product.

2. Group Assignment💼

Firstly, since there is 2 parts to the face-shield, the transparent sheet and the support for the forehead, we focused on 1 part at a time, and decided to start on the material selection for the transparent sheet first. 

1. Material requirement for face shield (Transparent sheet): 

Table 1.1: Transparent sheet 

Functions	To protect user from flu droplets and prevent the user from spreading flu droplets
Constraints	Light-weight (low density) Water resistance Transparent/ see through Flexibility/ ductile Non-toxic
Objectives	To maximise protection from flu droplets while remaining clear for visibility

Selection of suitable materials: 

During the selection of suitable materials, we have decided to eliminate metals and ceramics as metals are opaque (non-transparent) and have high density. While ceramics have low ductility and are very brittle. 

Thus, after researching, we decided on 3 materials. Polycarbonate, Acrylic plastic and Glass (Silicon Dioxide).

Criteria: 

1. Young’s Modulus

2. Transparency 

3. Density 

4. Ductility 

5. Water Resistance

COWS DECISION MATRIX:

Table 1.2: Options selection for transparent sheet

Criteria	Weightage (%)	Options
		Polycarbonate		Acrylic Plastic		Glass
		Raw Score	Weighted Score	Raw Score	Weighted Score	Raw Score	Weighted Score
a. Ductility	10	2	20	3	30	1	10
b. Young’s Modulus	15	1	15	2	30	3	45
c. Transparency	30	2	60	3	90	3	90
d. Density	20	2	40	3	60	1	20
e. Water resistance	25	3	75	1	25	2	50
Score	100		210		235		215

Choice of Material:

As per the COWS Decision Matrix, Acrylic Plastic is deemed the most suitable out of the three. Its high ductility/malleability is useful for the shape of the sheet and its low density will help the user be more comfortable with its weight. It is also highly transparent, meaning better visibility for the user.

Since glass and acrylic plastic are the 2 materials with the higher score, by comparing Acrylic Plastic and Glass economically, Acrylic Plastic is more cost efficient. Thus, we choose Acrylic Plastic as our material for the transparent sheet of the face shield.

After selecting the material for the transparent sheet, we then started the material selection for the forehead support!! 

2.  Material requirement for face shield (support for forehead):

Table 2.1: Face shield

Functions	To be comfortable for the user’s head
Constraints	Light-weight Flexible
Objectives	To maximise comfortability for the user to wear the face shield

Selection for suitable materials:

During the selection of suitable materials, we have decided to eliminate metals due to the low elasticity, high density, and high thermal conductivity. We took high thermal conductivity into consideration as forehead can be quite heated up when walking under the sun, thus leading to discomfort for users. As well as ceramics due to the low ductility and brittleness. Thus, we found 3 other materials for our selection of material. Nylon, Cellophane Cord and rubber.

Criteria:

1. Thermal conductivity

2. Elasticity

3. Density

4. Cost

COWS DECISION MATRIX:

Table 2.2: Options selection for face shield

Criteria	Weightage (%)	Options
		Cellophane cord		Nylon		Rubber
		Raw Score	Weighted Score	Raw Score	Weighted Score	Raw Score	Weighted Score
a. Thermal conductivity	25	3	75	1	25	2	50
b. Elasticity	25	1	25	2	50	3	75
c. Density	30	2	60	3	90	1	30
d. Cost	20	3	60	1	20	2	40
Score	100		220		185		195

Choice of Material:

As per the COWS Decision Matrix, Cellophane Cord is the most suitable out of the three materials for its low thermal conductivity and low cost. Lower thermal conductivity helps the user feel more comfortable with the head support even on hot days and low cost would mean lower selling price that is convenient for more users.