Material selection was performed on all plastic casing parts, i.e. the plug, socket, shutter assembly, isolation sheet and button. These parts must have an insulation material and the same material selection was performed on these parts.
The material selection was divided into five phases; translation-, screening-, ranking-, documentation-, and final selection phase.
Translation phase
β’ Definition of function:
- The function of the Smart Plug is to integrate an appliance to a smart home network
- The function of the casing must to shield the PCB assembly as well as protect the user from potential electrical harm.
β’ Constraints:
- The structure of the casing must be able to withstand loads which potentially could cause a size reduction as presented in figure 4.45, i.e. high compression strength
ππ‘ >πΉ
π΄ (4.1)
Figure 4.44. Load condition
β’ Aim:
- Minimize carbon dioxide emissions,
π = πΆπ2β π΄ β πΏ β π β π½ (4.2) - Minimize energy consumption
π = π»πβ π΄ β πΏ β π β π½ (4.3)
β’ Free variables:
- Selection of material - Area of the Smart Plug
β’ Deriving the performance equation, P:
Rewriting (4.1) π΄ = πΉ
β’ Obtained merit indices:
(4.5) gave carbon dioxide index: π1 = πΆπ2β π
ππ‘ (4.7)
(4.6) have energy consumption index: π2 = π»πβ π
ππ‘ (4.8)
F
Screening phase
By defining additional restrictions, the amount of unsuitable materials could be reduced, resulting in the material map only displaying relevant materials.
Further restrictions:
- Material must be flame retardant. It is not possible to specify this requirement further in CES EduPack which makes it critical to verify that the proposed materials can satisfy the V-0 requirement.
- Electrical conductivity of the material can maximum be 1.724 %IACS. This since insulating materials must have an insulation resistance of minimum one megohm per 1000 volts of operating voltage, with a minimum value of one megohm [43].
- Material must be recyclable
Rank and material maps
When multiple aims are present, a trade-off analysis can be performed in order to create material maps [38]. This was done in the material database CES EduPack.
By inputting the merit indices and the further restrictions to CES EduPack, a material map was developed in a trade-off analysis where the most environmentally friendly materials with regard to manufacturing of the Smart Plug emerged, see figure 4.45.
Figure 4.45. Merit index 2 is plotted against merit value 1.
The materials which minimizes both the merit indices in figure 4.45 are the possible best suited materials produced from the trade-off analysis.
The leading materials from figure 4.45 was identified and its properties displayed in table 4.3
Table 4.3. Selected material candidates
Flame retardant UV radiation PLA (Flame retarded) Depending on wall thickness Good
PP (homopolymer, flame retarded HB) Depending on wall thickness Poor PSU (flame retarded) Depending on wall thickness Fair ABS+PC (flame retarded) Depending on wall thickness Fair PC (low viscosity, molding and extrusion,
flame retarded)
Depending on wall thickness Fair PP (impact copolymer, flame retarded) Depending on wall thickness Poor ABS+PVC (flame retarded) Depending on wall thickness Fair AES (flame retarded) Depending on wall thickness Good PVC-elastomer (Shore A75, flame
retarded)
Depending on wall thickness Fair TPU (Ether, aromatic, Shore A85, flame
retarded)
Depending on wall thickness Fair
Since the product will be a consumer product, poor UV-radiation quality will be dislodged from the proposed materials. With poor UV-radiation quality, discolorations to a products exterior design is common and can occur rapidly and should therefore be avoided [44].
Table 4.4. Selected material candidates
Density
By using the data in table 4.4, the material candidates merit indices were calculated, see table 4.5 for calculations. The weight in table 4.5 were calculated by using equation 3.2. Attempting to select a material which minimizes both the carbon dioxide emissions as well as the energy consumption is a conflicting aim. Since both aims are prioritized equally, the weight factor, wi, was set to 0.5 for both merit indices.
Table 4.5. Calculated merit indexes and weight
M1=CO2βΟ/Οt M2=HpβΟ/Οt W1 W2 W weight, i.e. flame retardant Polylactic acid (PLA), flame retardant polysulfone (PSU) and flame retardant ABS+PC
Documentation of selected materials
Since the material map only regards the process of manufacturing the material, a documentation containing the processing energy and carbon dioxide footprint for product manufacturing was implemented. The processing energy and carbon dioxide footprint were compared for the three materials with the best weight from table 4.5. This was performed since processes are inevitably linked to materials. When designing for minimal environmental impact, both the primary production and, in this case, the polymer molding energy respective polymer molding CO2
should be studied.
Table 4.6 presents the processing energy and carbon dioxide footprint for flame retardant PLA, PSU and ABS+PC.
Table 4.6. The processing energy and carbon dioxide footprint for flame retardant PLA, flame retardant PSU and flame retardant ABS+PC
FR-PLA FR-PSU FR-ABS+PC
Polymer molding energy [MJ/kg] 14.35 28.2 20.2
Polymer molding CO2 [kg/kg] 1.08 2.12 1.52
According to table 4.6, molding a product with FR-PLA requires approximately 50 % less energy while it simultaneously produces approximately 50 % less CO2 compared to a product molded with FR-PSU. Similarly, a product production requires approximately 30 % less energy while simultaneously producing approximately 30 % less CO2 when the product is produced with FR-PLA instead of FR-ABS+PC.
Final material selection
Flame retardant PLA is a renewable polymer that has the lowest weight according to the calculations presented in table 4.5. It also has the lowest processing energy and carbon dioxide footprint and is thereby concluded to be the most environmentally friendly material with regard to manufacturing of the Smart Plug.
4.5.5.1 PLA-FR
PLA is a biodegradable thermoplastic polymer made with two monomers; lactic acid and lactide [45] [46]. PLA is a bio-based plastic material made from plants. The polymer is obtained from renewable resources like corn or sugar cane [47], [48]. A few properties of PLA are presented in table 4.7.
Table 4.7. Properties of PLA
Material properties Average value
Youngβs modulus 3.35 GPa
Yield strength 63.5 MPa
Compressive strength 76.2 MPa
Fracture toughness 4.11 MPaβm
Glass transition temperature 82.1ο°C
PLA has good physical properties, such as high Youngβs modulus and strength. The material has good thermoplasticity and biocompatibility and has a low toxicity. PLA is widely used in many fields, such as biomedicals where they are used in medical implants, orthopedic devices, drug delivery systems etc [49], [50].
Due to PLAs relatively low glass transition temperature, see table 4.7, the polymer is unsuitable in high temperature applications. The polymer has a maximum service temperature between 61 to 67ο°C. This means that this is the highest temperature the material can be used for during an extended period without crucial complications such as excessive creep, oxidation, loss of
strength etc. The minimum service temperature indicates the lowest temperature the material can be used without becoming too brittle comparing to the materials brittleness in room temperature. The minimum service temperature for flame retardant PLA is between -20 to -12ο°C [51]. The costumer requirement specification states that the Smart Plug must be able to operate during temperatures between 0-40ο°C, making PLA a suited material to use. However, PLA is brittle under tensile and bending loads and develops serious physical aging under application [50].
PLA is a plant-based hydro-biodegradable polymer and can decompose into water and carbon dioxide in 47-90 days when under specific and controlled composting conditions [52]. The decomposition of PLA is most effective in commercial composting facilities at high temperatures. However, PLAβs good biocompatibility applies under optimal conditions. When for example buried in a landfill, a product will take several hundred years to decompose [53].
Without the inclusion of FRs in PLA, PLA is relatively flammable and has a high ignitability.
To produce the flame retardancy of PLA with improved properties, the FRs should be incorporated into the polymer by using conventional thermoplastic processing [47].
According to Chow et al. [49], NEC Corporation has developed a flame-retardant biodegradable PLA resin, which contains no toxic halogen or phosphorus FR. This resin is rated UL-94 V-0 when specimen thickness 1.6 mm is used. This results in that the thickness of the Smart Plug must be 1.6 mm [49].
4.6 FE-analysis
Analysis corresponding to drop simulations were performed on two separate bases; a rigid- and a wood basis. The wooden basis was simulated with properties corresponding to red oak.
The simulations were performed on a wooden surface due to the common use of wooden floors in households. Other material is also common in households, e.g. clinkers which are rigid, and a second analysis were therefore performed for this material type. The amplitude the Smart Plug will obtain after impact will be at its maximum after impact on the rigid surface, while the amplitude will be longer after impact on the wooden surface.
Impact simulations for the Smart Plug was created from four different angles per surface; top-, bottom-, front- and back drop, see figure 4.46 for clarifications;
Figure 4.46. Definitions of drop angles.
Impact simulations performed on a rigid surface and a wooden substrate
On all simulations below, the maximum stress in the scale was reduced to 65 MPa. This was done since the terminal pins and the ground springs hade a yield stress of about 390 MPa, resulting in very small visible impact results on the casing due to the much lower stresses. The maximum stress obtained during impact can be seen on the top of the scale were generally very high. These stresses were mostly absorbed by either the female- or male ground spring, or the terminal spring and not by the designed casing.
4.6.1.1 Top drop impact simulations
The impact simulation on the rigid surface revealed that the protective ribbon on the socket was designed too close to the switch on the digital PCB since they collided during the top drop simulation performed on the rigid surface, see figure 4.47d. In the simulation on the wooden surface, the protective ribbon and the switch did not collide but was still in close proximities, see figure 4.48e. The protective ribbon should therefore be reduced in length.
There were three locations on the designed plug during impact on the rigid surface, see figure 4.47, and one location during the impact on the wooden surface, see figure 4.48, where the absorption of stresses exceeded the yield stress of PLA, causing plastic deformation. Since the simulations were a linear elastic analysis, the analysis assumes that the structure is subjected to elastic deformation and not plastic. The design of the plug should therefore be closer examined.
Front
(Side with button) Back
Top
Bottom
Figure 4.47. Top-drop impact simulation on rigid surface.
a) b) c) d)
e) f)
a) b) c)
d) e)
4.6.1.2 Bottom drop impact simulations
On the bottom drop impact simulation on the rigid surface, the casing did not reach, or come close proximity, to the materials yield stress, see figure 4.49. However, when bottom drop impact simulations were performed on the wooden surface, the designed plug would, at certain locations around the terminal pins, reach and even exceed the yield stress, meaning the casing will plastically deform, see figure 4.50. The high stresses occurred by the ribbons around the terminal spring, meaning they should be further examined and modified.
Figure 4.49. Bottom-drop simulation on rigid surface.
Figure 4.50. Bottom-drop simulation on wooden substrate.
a) b) c)
a) b) c)
d) e)
4.6.1.3 Front drop impact simulation
Front drop simulations resulted in high stress absorption for the designed plug and button. These stresses surpassed the yield stress for PLA on several locations, resulting in required further examination for both the design of the plug and the button, see figure 4.51 and 4.52
Figure 4.51. Front-drop simulation on rigid surface.
a) b) c)
d) e) f)
Figure 4.52. Front-drop simulation on wooden substrate.
4.6.1.4 Back drop impact simulation
The back drop simulations also resulted in high absorption of stresses for the designed plug when impact on both the rigid- and wooden surface occurred, see figure 4.53 and 4.54. Once again, the design of the Smart Plug should be further examined and modified.
Figure 4.53. Back-drop simulation on rigid surface.
a) b)
c)
d)
e)
f)
a) b) c)
d) e)
Figure 4.54. Back-drop simulation on wooden substrate.
4.6.1.5 Optimization
The plug and button absorbed very high stresses during impact, resulting in that these designs should be improved. Impact simulations should then be performed on the improved designs and so on until satisfaction in design is reached. This iterative process would result in an optimized product design. An iterative process was however not performed due to time limitations.
a) b) c)
d) e) f) g)
5 Discussion and conclusions
In this section a discussion is made over the product development method used to develop the Smart Plug, the product design results, the material selection process and the finite element analysis.