Week 4

This Blog: COWS Assignment & Reflection

 Product: Teat and cover of a polycarbonate baby milk bottle

 

Teat of a polycarbonate baby milk bottle
Function	Allows the baby to suck milk out of the milk bottle.
Constraints	High Young’s Modulus, High corrosion-resistance, Non-reactive, High Tensile Strength, High Melting Point, Availability
Objectives	Excellent corrosion-resistance & non-reactive
Material	Rubber, Silicone Rubber, Ethylene Propylene Diene Monomer

 

Cover of polycarbonate baby milk bottle
Function	Prevent dust particles and impurities from entering the teat.
Constraints	High Tensile Strength, Availability, Low Density, High corrosion-resistance, non-reactive.
Objectives	Relatively high tensile strength
Materials	Plastic, Borosilicate Glass, Stainless Steel, Gold

 Select and Evaluate Candidate Materials

Criteria Selection for teat:

The 6 selected criteria for teat are Young’s Modulus, Corrosion Resistance, Melting Point, Chemical Reactivity, Ultimate Tensile Strength, and Availability.

The corrosion resistant and chemical reactivity have the highest weightage of 30%. This is because the material for the teat is in contact with the milk, water and washing detergent. It should be non-reactive and must not form toxic product. The material used for teat should also have high corrosion resistance as it contacts with water frequently. 

The ultimate tensile strength has the second highest weightage of 15%. A high tensile strength means that the material is more durable and has a higher resistance to breaking under tension. The teat must have high tensile strength. This allows the teat to be able to withstand force applied by the baby’s teeth and the cover to not break easily when dropped.

Both young’s modulus and melting point has the same weightage of 10%. Young’s modulus is needed to check if the teat is soft and elastic. When the teat is elastic and soft, this will not hurt the baby’s teeth when bit on it and the teat will not deform.  It also gives a more real feeling of a mother’s nipple. Melting point is also used for teat, but it has a low weightage of 10% because the milk will usually be warm, which is below 100°C. Most materials would have a melting point of above 100°C. Thus, melting point is given a low weightage.

Lastly, the availability of material is given as 5%. As the baby bottle teat will be placed into the baby’s mouth, this product plays an important role which prevents the baby from getting sick. The health of the baby is much more important. Thus, we cannot substitute a material which is easier to find but has a poorer quality with a material which is harder to find but has much better qualities which can help protect the baby’s health.

Criteria selection for cover:

The 5 selected criteria for the cover are Ultimate Tensile Strength, Density, Availability, Corrosion Resistance and Chemical Reactivity.

The ultimate tensile strength has the highest weightage of 30%. A high tensile strength means that the material is more durable and has a higher resistance to breaking under tension. The cover must have high tensile strength. This allows the cover to be able to withstand dropping easily so that it will not break.

Similarly for density, it also has a weightage of 30%. The density of the material can tell us the weight of the material. As the baby does not possess much strength yet, the total weight of the bottle has to be light so that it can carry itself to drink when the baby gets old enough. Therefore, density plays a big part when choosing the desired material.

Corrosion resistance has a weightage of 20%. This is because the material for the cover is in contact with the milk if there is spillage and will also be in contact with water and washing detergent when the baby bottle is needed to be washed. Therefore, the material should be non-reactive and must not form toxic product. The material used for cover should have high corrosion resistance as it contacts with water frequently. 

Chemical reactivity has a weightage of 10%. As the cover would be in contact with water more often, corrosion resistance has a higher weightage. However, the chemical resistance is still important as milk may be spilled onto the cover. The material for the cover still has to be non-reactive and must not form toxic product.

Lastly, the availability of material is given as 10%. As the baby bottle cover is used to prevent dust or impurities from entering the teat, this product plays an important role which prevents the baby from getting sick. The health of the baby is much more important. Thus, we cannot substitute a material which is easier to find but has a poorer quality with a material which is harder to find but has much better qualities which can help protect the baby’s health.

TEAT		Options
Criteria	Weightage	Nitrile Rubber		Silicone Rubber		Latex
		Score	Value	Score	Value	Score	Value
Young’s Modulus (MPA)	10%	1	3	3	950	2	100
Corrosion Resistance	30%	3	A	2	B	3	A
Melting Point (°C)	10%	1	120	3	1414	1	120
Chemical Reactivity (Fatty Acids and Lactic Acid)	30%	1	C, D	2	C, A	2	C, A
Ultimate Tensile Strength (MPA)	15%	1	15.2	3	82.6	2	17.5
Availability	5%	3	-	2	-	2	-
Total	100%	170		235		220

 

COVER		Options
Criteria	Weightage	Polyethersulfone		Borosilicate-Glass		316 Stainless Steel		Gold
		Score	Value	Score	Value	Score	Value	Score	Value
Ultimate Tensile Strength (MPa)	30%	2	143.25	1	81.6	3	586	2	120
Density (g/cc)	30%	3	1.53	3	2.4	2	7.95	1	19.2
Availability	10%	3	-	3	-	3	-	1	-
Corrosion Resistance	20%	3	A	3	A	3	A	3	A
Chemical Reactivity (Fatty Acids and Lactic Acid)	10%	3	A, B	3	A, B	3	A, B	3	A, B
Total	100%	270		240		270		190

The best 2 materials for teat are Silicone-Rubber and Latex.

The best 2 materials for cover are Polyethersulfone and 316 Stainless Steel.

 

Select and Evaluate most economical material

Criteria Selection:

The 3 selected criteria are Cost, Density, Machineability. The cost has a high weightage of 50%, density and machineability has a weightage of 25% each.

Cost has the highest weightage as it must be cheap so that it can be mass produced and make maximum profit.

Density and machineability has the same weightage as they are equally important. Density in this case tells us the transportation cost. A material with higher density will cost more for transportation. Machinability refers to how easy a material can be shape and machine into what we need. Thus, both density and machineability has the same weightage of 25%.

 

 

TEAT		Options
Criteria	Weightage	Silicone-Rubber		Latex
		Score	Value	Score	Value
Density (g/cc)	25%	2	2.25	3	0.92
Cost ($/lb)	50%	2	25.00	3	1.08
Machineability	25%	2	-	3	-
Total Score	100%	200		300

 

 

COVER		Options
Criteria	Weightage	Polyethersulfone (PES)		316 Stainless Steel
		Score	Value	Score	Value
Density (g/cc)	25%	3	1.53	1	7.95
Cost ($/lb)	50%	2	14.00	3	0.84
Machineability	25%	3	-	1	-
Total Score	100%	250		200

 

In conclusion, latex is use as material for the teat, and polyethersulfone (PES) is used as the material for the cover.

 

Latex has high young’s modulus which means that it is elastic. This allows it to withstand biting and pulling from the baby, which can also prevent the baby from hurting its teeth as it is soft. It also has a high corrosion resistance, which means that it will not corrode when water is introduced. This makes washing the teat possible with water. Latex is also relatively inert so washing detergent and milk will not chemically affect it. Although it has a C grade towards fatty acids, fatty acids composition is very little inside milk, thus it will not affect much. It also has a decently high melting point, at 120°C, which makes it suitable to be used as warm milk will be flowing through it. The warm milk will not exceed 120°C thus latex low melting point is acceptable. Latex also has a high ultimate tensile strength which increases its resistance to wear and tear, making it more durable. Latex is a readily available material which makes it cheap and easy to obtain. Besides being cheap, latex is also very light as it has low density which makes it suitable for a baby to use as baby's do not have much strength. A low density also means that the transportation cost will be lower, therefore, companies get to save more on expenses. Machinability rating for latex is also very high which is also very important as teats comes in all shapes and sizes and hence needs to be easily moulded into whatever form is needed. Therefore, latex is chosen as the material for the teat.

 

 

PES has a decently high ultimate tensile strength which increases its overall durability and resistance to wear and tear and dropping. It is also a commonly used material for baby feeding bottles which makes it readily available. In addition, PES is light, making it a good choice for baby’s who lack strength. A high corrosion resistant means that washing the cover will not form toxic product. The chemicals commonly found in milk (fatty acids and lactic acids) also do not react with PES to form toxic product. Besides these functional properties, PES is also a great option from an economic standpoint as it is light and strong, reducing transport cost, it is a cheap material in general and is very easy to machine i.e. it is very mouldable. Therefore, PES is chosen as the material for the cover. Moreover, baby feeding bottles are made using PES plastic, which does not contain BPA. BPA is a compound which some researches claim is detrimental to human health.

References

-          Coleparmer.com. n.d. Chemical Compatibility Database from Cole-Parmer. [online] Available at: <https://www.coleparmer.com/chemical-resistance> [Accessed 11 May 2021].

 

-         latex, E., n.d. Elasticity of latex. [online] Chemistry Stack Exchange. Available at: <https://chemistry.stackexchange.com/questions/20008/elasticity-of-latex#:~:text=In%20'engineering%20materials%20terms'%20the,modulus%20of%20about%200.1%20GPa.> [Accessed 12 May 2021].

 

-          Matweb.com. 2021. Online Materials Information Resource - MatWeb. [online] Available at: <http://www.matweb.com/> [Accessed 11 May 2021].

 

-          Mykin.com. 2021. Rubber Chemical Resistance, Rubber Chemical Compatibility, Page 3 - Mykin Inc. [online] Available at: <https://mykin.com/rubber-chemical-resistance-chart-3> [Accessed 11 May 2021].

 

-          n.d. [ebook] Available at: <https://www.flinnsci.com/api/library/Download/8c21e94f13bb429c90501abbe58400c5#:~:text=Rubber%20latex%20begins%20to%20melt%20and%20decompose%20at%20approximately%20120%20%C2%B0C.> [Accessed 12 May 2021].

 

-         Techiescientist. n.d. Does Silicone Melt? - Techiescientist. [online] Available at: <https://techiescientist.com/does-silicone-melt/#:~:text=about%20the%20silicone.-,Does%20silicone%20melt%3F,hardly%20looses%20its%20physical%20properties.> [Accessed 12 May 2021].

Learning Points:

COWS Matrix is used for material selection which aims to seek the best match between design requirement of a product or process and material attributes. The COWS matrix stands for Criteria, Options, Weightage and Score. Before using COWS matrix, we have to define the product function, define the product requirements, translate product requirements into material properties, identify probable candidates materials and then we can use COWS matrix to select most suitable material(s). 

COWS matrix helps to score the identified probable materials using the relevant criteria. We can obtain the data for each criteria for each material candidate using websites such as "matweb and "coleparmer". Matweb gives us the individual material properties and coleparmer can give us the material compatibility.

COWS matrix is feasible as it is easy to use and allows us to easily identify the best material for the given criteria and purpose. 

COWS matrix allows the user to contextualize certain criteria for certain usage. For example, to put into context, the user needs a material that can withstand high pressure and temperature. The user can use COWS matrix with the criteria of able to withstand high pressure and temperature to get the best material.

Basis of scoring: To score the probable materials, we give them a score of "1" to "3", with "3" being the best and "1" being the worst. After scoring all the materials based on the criteria, we can add up their total score. The material with the highest scoring is selected to be the best material for the given product.

2) Group Assignment

For the group assignment, our group was given the product of a teat and cover of a polycarbonate baby milk bottle. Firstly, we had to define each product function, and the product requirements. Next, we then translated the product requirements into material properties. An example is given below: 

Teat of a polycarbonate baby milk bottle
Function	Allows the baby to suck milk out of the milk bottle.
Constraints	High Young’s Modulus, High corrosion-resistance, Non-reactive, High Tensile Strength, High Melting Point, Availability
Objectives	Excellent corrosion-resistance & non-reactive
Material	Rubber, Silicone Rubber, Ethylene Propylene Diene Monomer

After listing out the functions, constraints, objectives and the probable materials, we can then use COWS matrix to identify the best material to be used. We had to score the materials candidates using data we found online. After getting all the data, we summed up their individual scores and the material with the highest score is selected as the best material for the product.