Proper selection of the material and manufacturing process to be used for a given application is crucial to the success of the manufacturing process. Material selection should be based on material suitability and the possibility of modifying the material to make it suitable for the application while the selection of the manufacturing process should be based on the material selected, the shape required, dimensions of the item being manufactured and the cost of production. A product design specification (PDS) is a crucial element of design that outlines the design and operating requirements and, therefore, facilitates the selection of suitable materials. This paper seeks to select and justify the suitable material and manufacturing process for two electric kettle shapes, traditional style and jug kettle as shown in figure 1. The aesthetic aspect of the electric kettle should not be ignored considering that it is a kitchen item that out to be presentable in color and shape
Figure 1: Kettle shapes under consideration
Product design specification for the two kettle shapes
- The kettle body should hold 2 liters of water. This is about 2 kg of water (density of water is 1kg/liter)
- Maximum allowable mass of the kettle and water should be 3.5 kg. Subtracting the weight of water and other components, the weight of the kettle body is about 1.5 kg.
- The dimensions of the body of the jug kettle shall be 120 mm (Inner Diameter) by 200 mm high as shown in figure 1.
Figure 2: Dimensions of the body of the jug kettle
- The kettle body should withstand the temperature of boiling water (1000C for pure water and about 1020C for water with impurities). Therefore, the material should not deform under this temperature. The material should also avoid water seeping through at any temperature.
- The maximum allowed out of round distortion is 1 mm due to the weight of the water at any temperature.
- The kettle’s body should be a poor conductor of heat to avoid users getting burnt when they touch the kettle’s body containing hot water. The maximum allowable temperature on the outer surface of the body should be 370C (the body temperature of a normal human being) so that the kettle will not burn users or heat adjacent air leading to increase in room temperature.
- The body should not form rust due to interaction with water and air.
- It must resist to breakage under impact especially due to falls.
Material Selection and Justification for the Kettle Body
Possible materials for the manufacture of the kettle’s body
The main purpose of the kettle’s body is to hold water as it is being boiled. Therefore, any material that can hold water can be used to manufacture the kettle’s body. Such materials include:
- Plastics: nylons, polyesters, polycarbonates and polymers (thermosetting and thermosoftening polymers).
- Metals: common metals that can be used for the application include iron, steel (high carbon and low carbon steel), stainless steel, copper and aluminum.
Material selection and justification
Ceramics are heat and electrical insulators making them suitable for the application. When properly made, ceramics cannot allow water seepage thereby fulfilling one of the design considerations. Additionally, ceramics are corrosion resistant and easy to clean (Munz and Fett, 1999, p. 2). Crystalline ceramics, glasses, are transparent thereby easy to monitor the condition of the water inside the kettle including the capacity. However, ceramics fail on three crucial design specifications, the maximum weight allowable for the kettle’s body, the ability to withstand high temperatures and the ability of the kettle to resist breakage under impact. Firstly, ceramics are highly brittle meaning that they break rapidly without warning or undergoing elastic deformation (Munz and Fett, 1999, p. 2). The low strength of ceramics would require a thick-walled kettle, which would be extremely heavy. Finally, ceramics cannot withstand high temperature gradients especially occurring within a short time as they easily break due to internal pressure created since they take long to heat (Munz and Fett, 1999, p. 2). The application requires high temperature gradients (0-1000C) within a short time of about 5 minutes, which makes all ceramics unsuitable for this application.
All the listed metals can be used for the application particularly due to their high strength thereby making them resistant to breakage. Additionally, use of any of the listed metal would lead to a light weight vessel owing to the resulting thin wall. Consider, for example, stainless steel and aluminum whose modulus of elasticity are 200GPa at about 2930K and 65GPa respectively (Sakai, 2008, p. 255). According to figure 3, wall thickness of 0.6 mm and 0.8 mm, respectively, would be required to meet the design specification regarding maximum allowable distortion of 1 mm.
Figure 3: tensile modulus and suitable wall thickness for various out of round distortion values (Open University, n.d, p. 388)
The use of the listed metals would also ensure a water tight vessel. However, all the metals are electric and heat conductors. For instance, the thermal conductivities of aluminum alloy is about 180W/m-K implying that the outer temperature of the vessel will be almost equal to the inner temperature of the vessel (1000C in case of boiling water). Copper and steel also have high thermal conductivity values making them unsuitable for the application (Davis, 2000, p. 3). Such a temperature would be extremely dangerous for users. All the listed metals, particularly aluminum and copper, are good electric conductors being used in electrical appliances (Davis, 2000, p. 3). In case of fault with the heating element such that it passes electricity to the boiling water, the kettle would be extremely dangerous for users. Further, steel and iron are disqualified on the basis that they easily form rust when they interact with water and air, which is inevitable in this application. Rust would not only lead to rapid wear of the kettle’s body, but would be dangerous when consumed.
Stainless steel and aluminum are good candidates for use since they are corrosion resistant. Stainless steel is widely used on various kitchen utensils including plates, spoons, cooking pots, cooking pans and tea pots among others although it is a good heat and electrical conductor. Aluminum is also used for a wide range of kitchen applications including cooking pots although stainless steel is gaining wider application than aluminum. The aspect of aesthetic is crucial for the kettle because it is a house appliance that needs to be appealing like other kitchen utensils. The shinny nature of stainless steel makes it more applicable that aluminum since aluminum is usually has a dull appearance. Further, aluminum can conduct heat 10 times more than stainless steel (16 W/m-K for stainless steel against 180W/m-K foe aluminum) making stainless steel better than aluminum for this application (Davis, 2000, p. 3). Therefore, it will not pose a heat risk to users as aluminum would do. Stainless steel will be used for the traditional electric kettle body because it is resistant to corrosion, can be cleaned easily and cannot break easily when dropped.
Wood cannot be used because it will allow water to pass through it and it cannot be easily cleaned.
All nylons are eliminated because they react with water especially at high temperatures leading to the formation of hydrogen bonds between the plastic matrix (polymer chains) and water molecules. The impact of such an interaction is dimensional change when water is added to the kettle and heated to high temperatures (Woishnis and Ebnesajjad, 2012). Polyesters and polycarbonates are also eliminated because they react with water compromising mechanical and physical properties (Chung, 2000, p. 158). Thermosetting plastics, such as Bakelite and silicones, are also eliminated for use because they are considered not food grade as they sometimes contain reactive metals that can be dangerous upon consumption (Open University, n.d, p. 386).
The kettle’s body should retain its shape (insignificant shape distortion) at high temperatures. Polypropylene (PP) is particularly an excellent thermoplastic polymer that is widely used in making kitchen items, such as plastic containers, and for food packaging because it is considered a food grade material. The melting point of PP is far beyond the operating conditions of the electric kettle because PP melts at about 1710C (Tripathi, 2002). Therefore, this material will be used for the jar kettle because it can be colored to achieve the desired aesthetic value. Further, it is easy to mould it to the desired shape considering the complex shape of the jar kettle. Other candidates include Polyetheretherketone (PEEK), Polyethersulphone (PES), Polyphenylene (PPO) and Polyacetal (POM) (Open University, n.d, p. 391).
The selection of the most appropriate manufacturing process requires the determination of the thickness of wall of the kettle body. The maximum thickness of the kettle body is dependent on the design mass, density of the material selected and the surface area of the body (the assumption is that the kettle body is a perfect cylinder)
Thickness=masssurface area x massdensity
Density of stainless steel = 8.0g/cm3 (Fritz and Hazarian, 1999, p. 20-6)
Density of polypropylene is 0.90/cm3 (Tripathi, 2002, p. 24)
Surface area for the electric kettle body = 0.087 m2
Thickness=10.087 x 1density
For stainless steel, maximum thickness is 1.45 mm and 12.6 mm for polypropylene.
There are various ways through which the items can be made particularly cutting, joining, forming and casting.
Cutting and Joining
Cutting and joining are not viable manufacturing processes for these two items because the two processes would be unrealistic. In the case of cutting, too much time and resources would be used removing a lot of material only to remain with a small percentage of the original material. This is also associated with material wastage thereby making the process unnecessarily costly. The small size and simplicity of the two components does not warrant joining
Stainless steel is a dense metal with high modulus value that makes casting an unsuitable manufacturing process for this material. Casting on such materials, such as stainless steel can only be done on exceptionally thin walled items. On the other hand, polypropylene and other polymers deemed suitable for the application have low density and modulus values thereby increasing the range of wall thickness that can be casted. Therefore, casting will be used in manufacturing the jug kettle body made of thermoplastic polymers. PP seems to be the best in terms of casting costs compared to PPO, PES and PEEK based on the materials’ physical properties particularly flow and solidification properties (Open University, n.d, p. 394).
There are no wall thickness restriction when forming polymers implying that the jug kettle (made of thermoplastic polymers) can be formed to shape (Open University, n.d, p. 395). These polymers can be formed through blow molding or injection molding. On the contrary, metals differ in formability, which requires the selection of the most appropriate forming process for the traditional kettle made of stainless steel in order to achieve the required shape (Open University, n.d, p. 395). Drawing is one forming process that can be used for the stainless steel kettle. During drawing, the material may fail (crack) at two points, the shoulder of the die or at the shoulder of the punch, which restricts metal thickness and the forming diameter defined by the drawing ratio (ratio of the diameter of the largest blank that can be made (DO) to the punch diameter DP). An attempt to draw a thick plate to form a sharp edge results to failure scenario shown in figure 4.
Figure 4: Failure mode
Figure 6: Ironing process
Figure 5: Drawing and redrawing
Proper selection of the material and manufacturing process to be used for a given application is crucial to the success of the manufacturing process. The PDS, functional requirements and aesthetic requirements of the electric kettle have facilitated the selection of the suitable materials, which include thermoplastic polymers and stainless steel. The various polymers deemed suitable for the application can be casted or formed to shape through blow molding or injection molding. Stainless steel can be formed to the required shape through forming and ironing.
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