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 both the disposable bottle and the bottle cap. 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.
Table 5: Idea Decision Matrix (COWS) (Bottle)
Criteria | Weightages | Options | |||||
|
| HDPE | PET | Polystyrene | |||
|
| Raw Score | Weighted Score | Raw Score | Weighted Score | Raw Score | Weighted Score |
| 10% | 3 | 30% | 1 | 10% | 3 | 30% |
| 25% | 1 | 25% | 3 | 75% | 2 | 50% |
| 20% | 2 | 40% | 3 | 60% | 1 | 20% |
d. Cost | 25% | 2 | 50% | 1 | 25% | 3 | 75% |
e. Ease of manufacturing | 20% | 1 | 20% | 3 | 60% | 2 | 40% |
Score | 100% | 9 | 175% | 11 | 230% | 10 | 215 % |
Table 6: COWS bottle data
Criteria | Data | ||
| HDPE | PET | Polystyrene |
| 0.924-0.995g/cc | 1.38g/cc | 1.03-1.07g/cc |
| 60-90°C | 75-115°C | 75-100°C |
| Good | Excellent | Fair |
| 1102.6USD per ton | 1,026.90 USD per ton | 1555 USD per ton
|
| Fair | Excellent | Good |
Table 7: Idea Decision Matrix (COWS) (Bottle Cap)
Criteria | Weightages | Options | ||||||
|
| Polypropylene | LDPE | HDPE | ||||
|
| Raw Score | Weighted Score | Raw Score | Weighted Score | Raw Score | Weighted Score | |
| 30%
| 1 | 30% | 3 | 90% | 2 | 60% | |
| 25% | 3 | 75% | 1 | 25% | 2 | 50% | |
| 30% | 3 | 90% | 2 | 60% | 1 | 30% | |
| 15% | 2 | 30% | 3 | 45% | 2 | 30% | |
Score | 100% | 8 | 225% | 10 | 220% | 7 | 170% | |
Page Break
Criteria | Data | ||
| Polypropylene | LDPE | HDPE |
| 0.09 J/cm | 3.74 - 5340 J/cm | 0.02 - 5340 J/cm |
| 9.00 - 80.0 MPa | 2.80 - 56.5 MPa | 7.60 - 43.0 MPa |
| 1064 USD per ton | 964 USD per ton
| 936 USD per ton |
| Good | Excellent | Good |
Table 8: COWS bottle cap data
Identify and explain at least 5 evaluation criteria for the decision matrix. (5 marks)
Impact resistance (izod test) for the plastic bottle and the bottle cap was selected as an evaluation because when the plastic bottle drops it should not break easily.
Cost and ease of manufacturing for the plastic bottle and the bottle cap was selected as an evaluation because the plastic bottles are made to make a profit.
Density for the plastic bottle was selected as an evaluation because the plastic bottle is better when it is lighter.
Heat distortion for the plastic bottle was selected as an evaluation because the plastic bottle should not react
Chemical inertness for the plastic bottle was selected as an evaluation because the plastic bottle should not react with and chemicals in the surroundings.
Tensile strength for the bottle cap was selected as in evaluation because the bottle cap is going to be screwed on tightly, and it should not break because of that.
Assign a weightage for each criterion and explain the rationale. (5 marks)
Plastic Bottle Body
Heat distortion temperature (HDT) was given the highest weightage of 25% as users may use the bottle for hot liquids and if the HDT is low, the plastic may distort and may cause risk to the user.
Cost was given the highest weightage of 25% because if the cost of manufacturing were to be too high, the selling price would be higher to match the cost and users may avoid buying the bottle altogether.
Ease of manufacturing was given the weightage of 20% as it is linked to cost and the easier the material is to sculp or mould, the lower the cost would be.
For the plastic bottle, density was given a lowest weightage of 10% because a low amount of material is used to make the plastic bottle and the density of the plastic will not affect the weight by much.
Plastic Bottle Cap
Impact resistance was given the highest weightage (30%) as if the user were to drop the bottle cap first, the cap should have a high impact resistance to withstand the force of the impact. If the impact resistance rating was low, the cap may deform or break, causing the liquid to flow out.
Cost was given a similar weightage(30%) as if the cost of manufacturing were to be too high, the selling price would be higher to match the cost and users may avoid buying the bottle altogether.
Tensile strength was given a weightage of (25%) as the bottle cap must be able to withstand the force of screwing the bottle cap on. If the bottle cap is not able to withstand the force, the cap may leak or deform which will cause the liquid to spill out.
Ease of manufacturing was given the weightage of 15% as it is linked to cost and the easier the material is to sculp or mould, the lower the cost would be.
Score each criteria using a rating scale of 1 to 3 (i.e. 1 being the poorest and 3 being the best) with brief justification. (5 marks)
Plastic Bottle Body
Density: polystyrene and HDPE are given the best score of 3 because the density are very similar and low as compared to PET that was given the worse score of 1 which have a much higher density.
Heat distortion temperature: PET was given the best score of 3 because it has the highest maximum and minimum for the heat distortion temperature, HDPE was given the worst score of 1 because it has the lowest maximum and minimum for the heat distortion temperature.
Chemical inertness: PET was given the best score of 3 because it has an “Excellent” rating of chemical inertness, while polystyrene was given the worst score of 1 because it has an “Fair” rating of chemical inertness.
Cost: polystyrene was given the best score of 3 because it is the cheapest and PET was given the worst score of 1 because it is the most expensive.
Ease of manufacturing: PET was given the best score of 3 because it has an “Excellent” rating of ease of manufacturing, HDPE was given the worst score of 1 because it has an “Fair” rating of ease of manufacturing.
Plastic Bottle Cap
Impact resistance (izod impact test): LDPE was given the best score of 3 because it has the highest while the upper limit I=of the range is the same as HDPE, the lower limit of the range is higher than HDPE.
Tensile strength: Polypropylene was given the best score of 3 because it has the highest upper and lower limit for tensile strength, while lower limit is lower than the lower limit of HDPE.
Cost: HDPE was given the best score of 3 because it is the cheapest and Polypropylene was given the worst score of 1 because it is the most expensive.
For ease of manufacturing: LDPE was given the best score of 3 because it has an “Excellent” rating of ease of manufacturing, HDPE and Polypropylene was given the worst score of 1 because it has an “Good” rating of ease of manufacturing.
Based on this, our selected material is PET for the bottle body and LDPE for the bottle cap as it has the highest total score for the COWS matrix table.
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