WEEK 4
This week, lessons were online💻 - asynchronous. We had to go through two sets of videos📹 as a recap to our Year 1 Module : Materials for Design (MFD)👷, in order to complete our assignment.
As we documented in our Week 1 blog, we realised that MFD was the stepping stone to this current module as the knowledge we learnt in that module would be applied in ICPD to conceive an idea 💭, as part of SP's chemical engineering Spiral Curriculum.
Therefore, MFD🔨 is very relevant to Chemical product design as designing a chemical product as engineers involves 3 interrelated problems:
- Selecting a material👚
- Specifying a shape⛒
- Choosing a manufacturing process🔧
A recap on what we learnt in the previous week -> COWS decision matrix...
💥COWS decision matrix
The COWS method is described as follows:
C- Criteria. Develop a hierarchy of criteria, also known as decision model. These will be placed on the x-axis.
O- Identify the options, also known as solutions or alternatives. These will be placed on the y-axis.
W- Assign a weight to each criterion based on its importance in the final decision.
S- Rate each option on a ratio scale by assigning it a score or rating against each criterion. The score is calculated as Ratings x Weight
The scores are then evaluated, and solutions with the highest scores are the ones that best meet the criteria.
COWS decision matrix not only allows us to determine which idea is the best (done in Week 3), but it also
allows us to determine and select the best material for the product.
💥Material Selection Process
Getting this selection right the first time by selecting the optimal combination for the product design has enormous benefits to any engineering-based business. It leads to lower product costs💸, faster time-to-market⏳, and a reduction in the number of in-service failures⭐ and, sometimes, significant advantages 💪 relative to competition.
The aim of material selection process is to identify one or more materials with properties that satisfy the functional requirements of a product. Even better materials can optimise performance objectives. The most suitable material is a trade-off between requirement and numerous constraints.
As engineers we are most concerned about the material characteristics such as:
- Mechanical properties🔧 (e.g Young's modulus, Hardness etc)
- Thermal properties ♨(e.g Thermal expansion coefficient, Specific Heat Capacity, Thermal Conductivity, etc)
- Magnetic Properties
- Fabrication Properties🔨 (e.g Ease of machining, Ease of welding, Density etc)
- Environmental Properties🌱 (e.g Corrosion properties, Toxic effects etc)
💥Assignment for Group 2
Our assignment was to select materials for reusable face shield for medical workers to protect them against the Covid-19 virus😷. We have been tasked to focus on the transparent face sheet and support for the forehead. We have decided to use... (material to be chosen) as the transparent face sheet and... (material to be chosen) as the support for the forehead,
As for the final material to be chosen, we will use the COWS decision matrix to help us decide.
💫Item: Transparent sheet for face shield
Function | To protect user from airborne bacteria and prevent user from spreading bacteria as well. |
Constraints | -Light weight (low density) -Corrosion resistance to water (saliva) -Transparent -Ease of maintenance (reusable) - Comfort of material on person’s face (Flexibility) - Waterproof - Ease of cutting/manufacturing (machinability) - Cost - Non-reactive to saliva (Chemical Compatibility) |
Objectives | Maximise protection from airbone bacteria/saliva from others (Chemical Compatibility) |
Criteria Chosen:
*Note: We combined the material properties criteria with the economical criteria all in 1 COWS matrix table.
1. Density
This is an important criteria to consider because the Transparent Sheet of the face shield has to be light as it accounts for majority of the weight of the face shield which the user will put on and since the target audience is medical frontline workers, this means they will put on the face shield for majority of the day and in order to prevent discomfort, the face shield has to be light so this component has to have low density.
2. Corrosion Resistance
As the medical frontline workers will put on the face shield to cover their faces and acts as a barrier between their faces and the surrounding air to prevent them from breathing in harmful microscopic bacteria in the air, this means they will inhale and exhale against the face shield thus as they exhale, the warmer air will condense into tiny water droplets on the cool surface of the face shield in order to prevent any irritation or danger to their skin, when the water droplets of their exhaled air forms on the face shield surface, there must not be any chemical reaction between the face shield and the water droplets thus, the face shield transparent sheet component has to be corrosion resistant
3. Flexibility
As the front line workers will be wearing the face shield throughout the day, for their comfort, the face shield should be flexible.
4. Transparency
As the front line workers will be wearing the face shield throughout the day to protect them from the external environment biologically, they need to be able to see through the face shield in order to navigate through the day thus it must be highly transparent.
5. Cost
The cost of the material to make the transparent part of the face shield should be affordable so that the manufacturing would be cost effective and profitable. The overall price of the face shield for a consumer decreases also making it more prone to purchase.
6. Machinability
The ease of the making the transparent sheet of the face shield from the material shield should be high so that the manufacturing would be cost effective and profitable. The overall price of the face shield for a consumer decreases also making it more prone to purchase.
Ranking:
Scale: 1 (worst) to 3 (best)
Options | |||||
Criteria | Weightage (%) | (PET) Polyethylene Terephthalate | Polycarbonate | Polyester | Polyvinyl Chloride (PVC) |
Density | 20 | 2 | 3 | 1 | 1 |
Corrosion Resistance | 25 | 3 | 3 | 3 | 3 |
Flexibility | 15 | 2 | 1 | 3 | 2 |
Transparency | 15 | 3 | 2 | 3 | 1 |
Cost of material | 15 | 2 | 1 | 3 | 2 |
Machinability | 10 | 2 | 2 | 2 | 2 |
Total | 100 | 240 | 215 | 250 | 190 |
Since Polyester has the highest score of 250, it is most suitable as material for the transparent sheet of the face shield due to its High corrosion resistance to water, High flexibility, High transparency, Low Cost, relatively good machinability. Although its density is higher than the other materials, due to the above mentioned excellent properties, it is the best material still.
💫Item: Support for forehead
Function | Provide user with a support for the face shield while wearing it |
Constraints | -Light weight (low density) -Corrosion resistant to water (sweat) -Ease of maintenance (reusability) -Flexibility -Machinability -Cost |
Objective | Maximize comfortability (flexibility) |
Criteria Chosen:
*Note: We combined the material properties criteria with the economical criteria all in 1 COWS matrix table.
1. Density
This is an important criteria to consider because the face shield has to be light and this is the component which the user will put on against his/her head and since the target audience is medical frontline workers, this means they will put on the face shield for majority of the day and in order to prevent discomfort, this component has to have low density.
2. Corrosion Resistance
As this component of the face shield would be put against their foreheads, if they sweat of if the external environment is humid, the water contact with this component must not stimulate a chemical reaction to prevent any skin irritation to them.
3. Flexibility
As the front line workers will be wearing the face shield throughout the day, for their comfort, the face shield should be flexible and so should this component.
4. Cost
The cost of the material to make the support for the forehead part of the face shield should be affordable so that the manufacturing would be cost effective and profitable. The overall price of the face shield for a consumer decreases also making it more prone to purchase.
5. Machinability
The ease of the making the support for the forehead part of the face shield from the material shield should be high so that the manufacturing would be cost effective and profitable. The overall price of the face shield for a consumer decreases also making it more prone to purchase.
Ranking:
Scale: 1 (worst) to 3 (best)
Options | ||||
Criteria | Weightage (%) | Polyether foam | Latex Foam | Polyester Foam |
Density | 25 | 3 | 2 | 1 |
Corrosion Resistance | 15 | 3 | 3 | 3 |
Flexibility | 30 | 3 | 1 | 2 |
Cost of material | 20 | 1 | 2 | 3 |
Machinability | 10 | 3 | 3 | 3 |
Total | 100 | 260 | 195 | 220 |
Since Polyether foam has the highest score of 260, it is the best material for the Support for forehead component of the face shield. This is due to its low density (thus it is lightweight), high corrosion resistance to water, high flexibility and excellent machinability. Although its Cost is higher than the other material options, however due to the above-mentioned excellent properties, it is still the best material.
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