1. INTRODUCTION:

Of some 100 elements that exist in the world, metal is a simple substance with ductility to be stretched into a wire, malleability to deform under compressive stress, conductivity to deliver electricity and heat, and metallic luster when it gets solid. For product design, metal materials play an important role to not only make products be luxurious but increase customers' reliability of products due to materials.

The usage of metal materials in the industry varies much according to the characteristics of materials. The utilization of the properties and inherent nature of materials enhance performance and functions to help improve appearance quality and functions of the product. In addition, it is rational and future-oriented. This, however, is a stereotype created due to metal's great contribution to human civilization. Metal attracts high expectation to lead the unknown future civilization as it has led well the current civilization. Therefore, many spaces and props of SF movies which depict the future are metallic. In all ages, the window to see the future is mostly metallic. The development of alloy technology has greatly contributed to the future-oriented image of metal materials. The alloy technology which is developing day by day allows to dream of new materials, new products and new design. If you are a designer who dreams of the future, you should learn the properties of metal materials for better design in the future. It is because design which understands well the nature of materials is good design. Metal contains elements necessary for sustainable life. It has played an important role in human life since early times. Metal's natural antibacterial effect has been widely known from long time ago. That's why metal has been used to eradicate diseases. This effect shows, however, that metal contains toxicity and pollutants. In the design field, metal is applied to structural bridges or decorative packages. Perhaps, there are many metals on the globe that people have not discovered yet. If they are 'discovered', a new concept of design will be 'invented'.

2. UNDERSTANDING OF METAL MATERIALS:

2.1. Stainless Steel:

Stainless steel contains more than 11% of Cr, by which a very thin and continuous oxide film is created to have corrosion resistance by protecting surface from chemical reaction. Stainless steel is a steel alloy with chromium contents whose corrosion and rust are less than ordinary steel. In addition, until the life ends, stainless steel can make appearance suited for environments and recently tends to increasingly use as not only non-structural member but structural member¹. It is widely used in construction materials, vehicle materials, various instruments and components for kitchen Fig. 1 which need corrosion resistance and metallic luster.

Figure 1. Stainles Steel_Bird Kettle / Alessi / 1985 / Stainless steel

2.2. Aluminum alloys:

Specific gravity of aluminum alloys is only 30%. Aluminum alloys are excellent in corrosion resistance, toughness and cold shock so that they are very advantageous in terms of weight lightening. Therefore, they are widely used as main structure materials of light structures such as high speed ships, airplanes, vehicles and high-speed trains. In addition, as they are excellent in castability, processability, mechanical characteristics and recyclability, their application scope as substitute goods of existing steel materials is increasingly expanding². But in the case of aluminum alloy structure, it has many difficulties to apply due to low material hardness and strength of vulnerable welded joint. Therefore, to design and manufacture a light structure using aluminum alloys, it is extremely important to identify the welded joint and the basic material of various aluminum alloys. Aluminum alloys that we frequently see are added by metal including copper and magnesium to have great strength and excellent corrosion resistance. In addition, aluminum alloys are used in various products like Fig.2 by blending other metal according to heat proof and brilliance.

Figure 2. Aluminum alloys_Beosound 3200 / Bang and Olufsen / 2003

2.3. Magnesium alloys:

Magnesium is the lightest structure metal. Though it has many advantages such as high specific strength, 16 times strength more than steel of equal weight, dust proof and excellent castability, it is mainly used at room temperature due to extremely poor high temperature oxidation resistance. To meet increasing demands for weight lightening, it is urgent to expand the application scope of magnesium alloys by developing heat-resistant magnesium which is usable at high temperature³. As magnesium is the greatest specific strength (strength-to-weight ratio) among structural metal materials, it is very suitable for design of structure aiming for weight lightening is shown in Fig. 3. Therefore, expansion of usage and development of light components attracts people's attention in improving fuel efficiency to reduce CO2 emission of vehicles including camera, cell phone and computer.

Figure 3. Magnesium alloys_D3S Magnesium alloys body / Nikon / 2009

2.4. Titanium alloys:

Titanium and titanium alloys are widely used in the fields of automobile industry, aircraft industry and biomaterials by transforming mechanical characteristics through various process technologies⁴. The USA financial crisis in late 2008 reduced the demand for titanium in the short term but the industrial efforts to use its excellent mechanical, physical and chemical properties are expected to expand in the mid- and long-term. Titanium has been used in artificial dental implants for long time due to economic strength compared to jewelry alloys, excellent corrosion resistance, biocompatibility, high strength and microhardness. Guggenheim Museum Bilbao located in the Basque city of Bilbao in northern Spain Fig. 4 is called 'Metal Flower' due to its superb beauty. The structure which petals are swirling about toward sky cannot be identical with any other architectures in the world. Hundred thousands of titanium panels attached to surface like fish scales give out soft lights for better beauty. Like the above-mentioned examples, use of metal varies according to its characteristics and expression methods are diverse according to its processing methods.

Figure 4. Titanium alloys_Guggenheim Museum in bilbao, Spain / Frank Gehry / 1997

3. Various Processing Methods of Aluminum:

3.1. Extrusion molding:

Extrusion molding is a process to create continuous objects with a specific shape by pressing a material through a die. Extrusion was used to make macaroni in Italy in the mid-19th Century and is originated from which the sheath of a submarine cable was made by gutta-percha's ram type extruder. In the recent extrusion industry, the method to reduce the cost and post-processing attracts attention. Extrusion molding is a process that a unit operation such as mixture, grinding, heating and molding happens in a short time. It is a relatively economic and efficient process compared to other thermal treatment processing process⁵. Extrusion molding has the low cost of equipment per unit production compared to injection molding, does not to need special technique and has advantages that need a few workers. Meanwhile, there are limits on complex structure and post processing is needed for cross sections. In cases of other products applied by extrusion molding technology, it is mainly used at appearance of the products such as a pipe, a straw and a frame like Fig. 5.

Figure 5. Extrusion molding_iPod Nano 4th generation

3.2. Laser Processing:

Laser processing is the technology to remove substances in the desired locations by irradiating high-density laser beam on a surface of an object, and melting and evaporating the surface on which a temperature rapidly rises. Various lasers such as CO2, ruby and YAG lasers are used according to the characteristics of materials and processing process. These lasers are used in processing such as cutting of materials, drilling, welding, thermal treatment, carving a surface, etc. The scope of application is expanding increasingly. As laser processing does not have abrasion of and damage to tools unlike conventional mechanical working and does not need a strong worktable, it is used in various processing such as cutting, welding, surface processing, scribing, making and drilling of diverse materials including metal, ceramic, glass, diamond, composite materials and plastic⁶.

3.3. Aluminum Anodizing:

Aluminum anodizing is post processing of an aluminum surface. It is technology to build up an oxide film on an aluminum surface by artificially using an electrochemical reaction, or oxidation process occurred when aluminum and oxygen meet. Anodizing process should not remain pollutants or oxides before film process. In addition, if it takes long between processes, pollution or oxidation can occur⁷. Hard anodizing process which applies to appearance of Apple' desktop or laptop computer is technology that converts aluminum surface into alumina ceramic by using an electrochemical method. Aluminum which has low corrosiveness, is excellent in thermal and electric conduction and has high processability is widely used in aircraft, vehicle, electronic, construction industries⁸. In addition, domestic aluminum surface treatment companies are increasingly pursued by overseas companies due to China's low wage structure including domestic workers' avoidance of 3D (difficult, dirty and dangerous) jobs. Therefore, if the domestic aluminum surface treatment industry has international competitiveness, the industry's structural productivity should increase through standardization⁹.

4. High Quality Refrigerator Design and Cases of Aluminum Materials Being Applied to Refrigerator:

4.1. High Quality Refrigerator Components:

Due to consumers' increase of income and material affluence, luxurious appearance of products stimulates consumers' emotion rather than home appliances' adaptability to neighboring space. Therefore, artistic design which colors and patterns are applied to surface design of home appliances stimulates consumers' emotion. In the recent 10 years, home appliances have changed from white to various colors. Representative double door refrigerator in the country is colorful and its artistic home appliances have become a trend called the innovation of home appliances. High quality design of refrigerator interior component has also become popular¹⁰. Refrigerator internal parts which plastic was consistently used to increase productivity in the manufacturers' position are increasingly expanding the application of metal materials and increase decorative elements such as LED lighting. Especially metal materials were mainly used for appearance in the past but are expanding their application to refrigerator interior like Fig. 6 to emphasize luxury and functions ¹¹.

Figure 6. High Quality Refrigerator Components

4.2. Eco-System to Increase Energy Efficiency:

By adapting independent refrigerating system at each space, a temperature at each space varies to reduce energy and have suitable receipt space. In addition, cold air does not escape by adapting vacuum insulators with excellent heat insulation property. By adapting improved cooling cycle, it has also reduced more than 15% of energy compared to existing products.

4.3. Additional and Convenience Functions Besides Refrigerating Function:

Hygienic food preservation function is applied by sterilizing airborne bacteria and periphytic bacteria in refrigerator and being equipped with sterilization, deodorization and cleaning functions.

5. RESEARCH OF APPLYING REFRIGERATOR POCKET OF ALUMINUM MATERIALS:

5.1. Idea Sketch:

Ideas centered on structural problems of aluminum extrusion and plastic injection molding are developed through idea sketch as in Fig. 7.

Figure 7. Idea Sketch

5.2. 3D Rendering:

Based on the idea sketch drawn from 5.1, 3D rendering is performed as in <Figure 8>. By inserting plastic structures into cross-section of aluminum extrusion, it increases compatibility to be jointly applied to each pocket of refrigerator's freezing/cooling rooms. In addition, refrigerator interior is made luxurious by performing various post processing such as 'Anodizing' and 'Hairline' on an aluminum surface through shape change of a pocket.

Figure 8. 3D Rendering

5.3. Working Mock-up Fabrication

Based on design data drawn from 5.2, working mock-up to check thermal conductivity of aluminum pockets is fabricated like Fig. 9 Aluminum extrusion is applied to the parts of the working mock-up with direct contact with food. Refrigerator's assembling parts are finished by plastic materials (ABS) which are applied to refrigerator materials.

Figure 9. Working mock-up

5.4. Thermal Conductivity Test:

Through working mock-up, a thermal conductivity test is performed under two test conditions. Test items are tested by test methods in 5.4.1.

5.4.1. Detailed Test Method

Comparing Materials: Styrene/Acrylonitrile (SAN), Aluminum (Al)

Thermal Equivalent: Check unique thermal capacity of original materials

In the case of storage of food, cooling speed under the same condition

Evaluation Method: Check the time to lower the temperature of 130g substance from 30℃ to 5℃

Each freezing/cooling room

Test 1. Existing Injection Type Cooling Speed Test

In the result of measuring cooling speed of existing pockets, it took 90 minutes and 30 seconds to lower the temperature of 130g substance from 30℃ to 5℃ as in Fig. 10.

Figure 10. Cooling of existing injection mold Speed Test

Test 2. Developed Extruded Aluminum Pocket's Cooling Speed Test

In the case of aluminum pockets developed in this study, they lowered about 0.3℃ at room temperature compared to existing injection-molded pockets In the result to measure cooling speed under the same condition as existing pockets, it took 32 minutes and 30 seconds to lower from 30℃ to 5℃. Therefore, aluminum material pockets are 58 minutes less than existing pockets as in Fig. 11.

Figure 11. Cooling Speed Test

Test Results:

In the result to measure cooling speed under the same condition as existing pockets, it took 32 minutes and 30 seconds to lower from 30℃ to 5℃. Therefore, aluminum material pockets are 58 minutes less than existing pockets which means refrigerator's cooling efficiency increases in case of applying aluminum pockets rather than existing plastic pockets.

6. CONCLUSION:

The study showed various usage is possible according to various metal materials and materials can increase functions and appearance quality of products. Especially it can confirm the effect that energy efficiency increases by applying metal materials to home appliances that temperature maintenance is important like refrigerator. However, manufacturers should fully review that there are the limits to expansion of the application of aluminum as the aluminum material cost is higher than the plastic one. The future study will research and discover materials with various strengths besides aluminum and aim to expand their application scope.

7. ACKNOWLEDGMENT:

This Study was conducted by research funds from Gwangju University in 2016, Korea

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Received on 12.10.2018 Modified on 17.11.2018

Research J. Pharm. and Tech. 2019; 12(9):4327-4331.

DOI: 10.5958/0974-360X.2019.00744.3