Material analysis
A very important aspect of The Twist is the material that the bottle is going to be made of. By setting requirements, failure modes and load conditions, many materials were eliminated. In the end, 2/3rd of the bottle is going to be made of Polymethyl methacrylate (PMMA), which is going to be the transparent bottom half of the twist. Next to this, 1/3rd of the bottle is going to be made of Polypropylene (PP).
Failure modes
Failure modes describe the most relevant ways in which the product can fail mechanically during normal use. It is important to analyze these, in order to prevent them from occurring.
Load conditions
In order to have an indication of what the material properties of the material should be, some load cases can be analyzed first. These can be used to determine the minimal values of what the Young’s modulus, yield strength and shear strength, for example, should be.
Under normal circumstances, load cases could be the twisting of the mechanism or dropping the bottle on the floor from a height. Furthermore, the thermal failure modes should be taken into account.
With these load cases, minimum values for properties are found. These are used in CES Edupack to select a material. The following requirements are added in CES:
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Maximum service temperature must at least be 50°C.
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Minimum service temperature must at most be -20°C.
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Acceptable or excellent for polymer injection molding.
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Acceptable or excellent resistance to water (salt and fresh).
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Acceptable or excellent durability against weak bases and acids.
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Good or excellent durability against UV-radiation.
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Transparent material for the bottle, opaque for the lid.
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Should be recyclable.
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Minimum tensile strength of 11.2 MPa.
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Minimum yield strength of 9.2 MPa.
Weighting table
A weighting table has been made for making a final material choice. A performance index of density against price per cubic meter is used to select the lightest and cheapest material. Some of the best materials are then weighed in a table. The weighing factors initially were set equal. Then, it was considered which properties were more important than others. Since several analyses on Moldflow are done for the opaque part, factors as warpage and cooling time are also taking into consideration. For the transparent part, only the processability as stated in CES Edupack is used.
Conclusion
In the end, PMMA (polymethylmethacrylate, impact modified) scores the best for the transparent part. For the opaque lid, PP (polypropylene, impact copolymer, UV stabilized) scores the best. Both are impact modified. This means that their toughness increases. This results in resistance to impact. The polypropylene is also UV stabilized. This results in better resistance to sunlight.
PP is a material that is frequently used for food and drink containers. This is an extra confirmation for the material choice. Common drink bottles, not the ‘’throw-away’’ ones, are regularly made of this material. This can be recognized by ‘’PP’’ or ‘’5’’ embedded somewhere on the plastic.
