Chapter-07 Selection of Feasible Alternative Materials for Manual Compost Maker using Weighted Average Method 7.1 Introduction Material selection is a step in the process of designing any physical object. In the context of product design, the main goal of material selection is to minimize cost while meeting product performance goals. It is a fundamental step for the proper and efficient functioning of our product and also for valuing its environmental impact. Material is needed to execute any physical design, thought or concept. Not every material is suitable for every product. It defers from product to product and the situation the product will be used. So, finding suitable material is very important because at the end of the day it’ll make the difference from an average product to a quality product. That’s why we need to undergo a material selection process to find out the perfect material for our product. From the perspective of engineering design, it is a multiple attribute decision-making problem. To ensure a simple, systematic and logical methods to make right decisions for material selection, we use weighted average method which considers the objective weights of importance of the attributes as well as the subjective preferences of the decisions maker to decide the integrated weights of importance of the attributes. 7.2 Structural parts of Manual Compost Maker 1. 2. 3. 4. 5. 6. 7. Hopper Hand Wheel Chopping Plate Shaft Base Tray Bin 7.3 Grouping of Components Based on our assumption of using same material, components had been assigned in three different group. Group -1: Hand Wheel, Shaft, Base Group -2: Hopper, Chopping Plate, Tray Group -3: Bin 7.4.1 Material selection for Hand wheel, Base and Shaft 1 2 3 4 5 6 7 8 9 Cost 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 1 1 1 0 0 0 1 Availability 0 Compressive Strength Modulus of Elasticity Rust Resistance 4 0.143 3 0.107 1 0.0365 3 0.107 4 0.143 4 0.143 0 5 0.179 0 1 4 0.143 28 ∑α=1 1 1 0 1 0 0 0 0 0 1 Hardness 0 0 0 1 0 0 1 1 1 Machinability 1 1 0 0 1 0 1 Mass Density 1 1 0 0 0 0 1 0 0 1 1 1 1 0 1 1 1 1 0 Relative Emphasis coefficien t Number of positive decision, N =n (n-1)/2=8(8-1)/2=28 Selection Criteria Positive decision Table 7.1: determination of relative importance of goals using digital logic method Total number of positive decisions Table 7.2 Calculation of the Performance Index: Selection Criteria Weighting Factor(α) Cost Availability Mild steel Stainless steel Aluminum Scaled Property(β) Weighted Score(αβ) Scaled Property(β) Weighted Score(αβ) Scaled Property(β) Weighted Score(αβ) 0.143 100 14.3 60 8.58 100 14.3 0.107 100 10.7 75 8.03 75 8.03 0.0365 100 3.5 41.70 1.46 68.68 2.40 0.107 69.08 7.4 100 10.7 22.70 2.43 0.143 100 14.3 75 10.72 50 7.15 Hardness 0.143 70.27 10.05 100 14.3 1.51 0.22 Machinability 0.179 100 17.9 50 8.95 75 13.43 Mass Density 0.143 34.65 4.95 33.88 4.84 100 14.3 Compressive Strength Modulus of Elasticity Rust Resistance Material performance index, γ ∑αβ=83.1 ∑αβ=67.58 ∑αβ=62.26 Result Material Performance Index (γ) is greater for Mild steel (83.1). So, we will use mild steel for Hand wheel, Base and Shaft. Table 7.3: Material properties of Mild steel, Stainless steel and Aluminum Property Mild steel Stainless steel Aluminum Density 7820 8000 2710 Compressive Strength 407.7×106 170×106 280×106 Modulus of elasticity 210 304 69 Hardness 130 185 2.8 Table 7.4: Numerical Values for Cost, Availability, Corrosion resistant and machinability Very high 5 High 4 Medium 3 Low 2 Poor 1 Selection criteria Mild steel Stainless steel Aluminum Cost 3 5 3 Availability 4 3 3 Rust Resistance 4 3 2 Machinability 4 2 3 Formula Used: 1. For goals: Availability Compressive Strength Modulus of Elasticity Rust Resistance Hardness MachinabilityNumerical value of property Scaled Property, β = × 100% Maximum value in list 2. For goals: Cost & Mass density Minimum value in list Scaled Property, β = × 100% Numerical value of property 7.4.2 Material selection for Hopper, Chopping plate, Tray Table 7.5: determination of relative importance of goals using digital logic method 1 2 3 4 5 6 Cost 1 0 0 0 1 0 Availability 0 Shear Modulus Density Wear Resistance Hardness Rust Resistance 7 0 1 0 0 1 0 0 1 0 1 0 1 2 0.095 3 0.142 3 0.142 5 0.238 0 1 0.047 1 5 0.238 21 ∑α=1 0 1 1 0 0.095 1 1 1 2 8 9 10 11 12 13 14 15 16 17 18 19 20 21 1 1 Relative Emphasis coefficient Selection Criteria Positive decision Number of positive decision, N =n (n-1)/2=7(7-1)/2=21 1 0 0 Total number of positive decisions 0 0 1 0 1 1 0 1 1 Table 7.6 Calculation of Performance index for Hopper, Chopping plate, Tray Mild steel Stainless steel Alloy steel Scaled Scaled Scaled Weighted Weighted Weighted Property(β Property(β Property(β) Score(αβ) Score(αβ) Score(αβ) ) ) 100 9.5 75 7.125 60 5.7 Selection Criteria Weighting Factor(α) Cost 0.095 Availability 0.095 100 9.5 100 9.5 50 4.75 Shear Modulus 0.142 87.5 12.425 32.5 4.615 100 14.2 Density 0.142 98 13.916 100 14.2 95 13.49 Wear Resistance 0.238 60 14.28 100 23.8 40 9.52 Hardness 0.047 50.71 2.383 100 4.7 45.71 2.148 Rust Resistance 0.238 60 14.28 100 23.8 80 19.04 Material performance index, γ ∑αβ= 76.28 ∑αβ= 87.74 ∑αβ= 68.84 Result Material Performance Index (γ) is greater for Stainless steel (87.74). So, we will use mild steel for Hopper, Chopping plate, Tray. Table 7.7: Material properties of mild steel, stainless steel and alloy steel Property Mild steel Stainless steel Alloy steel Shear Modulus 70 74 80 Density 7820 7690 8020 Hardness 71 140 64 Table 7.8: numerical values for cost, availability, wear resistance, rust resistance Very high 5 High 4 Medium 3 Low 2 Poor 1 Selection criteria Mild steel Stainless steel Alloy Steel Cost 5 4 5 Availability 4 4 2 Wear resistance 3 5 2 5 4 Rust resistance 3 Formula Used: 1. For goals: Availability Compressive Strength Modulus of Elasticity Rust Resistance Hardness Machinability- Scaled Property, β = Numerical value of property Maximum value in list × 100% 2. For goals: Cost & Mass density Scaled Property, β = Minimum value in list Numerical value of property × 100% 7.5 Conclusion Applying weighted average method between those alternatives we found the decision for performing material for our Manual Compost Maker in different group. We always have considered all criteria and required properties of the materials while applying weighted average method that gave us a clear vision and helped us to make a decision to choose one between of those candidates. So, we can say weighted average method was reliable and effective in our case and quite easy. Now we can move to the next section of product design where suitable manufacturing criterion will be selected by implying weighted average method for different parts and processes.