Modeling and drop test analysis of the built-in dish washer

In white appliances sector, the process of storage and transportation of products, impact and the falling damage to products lead to significant cost. The products may become unusable or may need a costly repair as a result of the possible dropping and impact cases. After the products are out of the production line, there is always a risk that the product may be subjected to drops and impacts. Therefore, to avoid this possible damage during transportation, white appliances are packaged using protective materials. Polymer foam materials that have high energy absorbing capability under compression are preferred in the white appliances industry. In order to guarentee that the packaged products have sufficient level of protection againist possible dropping or impacts, these products are subjected to so-called dropping tests. Such tests may become quite expensive if the tests are to be repeated many times in case the prototype does not meet the requirements of the standards. Thus, engineers who work in the industry are searching for alternative methods to minimize such costs. Nowadays, the importance of computer aided engineering tools is becoming more important as computer technology is advancing. Finite elements software, which is one of main computer aided engineering tools, is commonly used in the industry. This means that complex engineering problems can be modeled and solved numerically by using computer aided engineering tools. Finite element software are being used during design process, helping the engineers optimise their design in the shortest time. Projects are being completed faster and more effective by using advanced enginnering tools in industry. However, accuracy and reliability of the numerical simulations depend on many factors including the material properties of parts comparing a product. Therefore, mechanical properties of the packaging materials used in the white appliances industry must be modelled accurately for drop test simulations. Expanded polystrene is mostly preferred as a packaging material in household industry. Expanded polystrene, also named as styropor or polymer foam, is obtained from petroleum and has a high energy absorbing property under compression. Mechanical properties of styropor materials must be known for numerical simulations aiming to optimise the performance of the packaging. Optimized protection of goods against impact during transportation can save very significant amount of cost to the industry. In this study, physical drop tests and drop tests analysis of built-in dish washer are carried out. Associated drop test standarts required that built-in dish washer should have no damage when product is dropped from 25 cm height. As specified in the standard, drop tests should be carried out as free falling tests with 0o and 10o inclination angles with respect to the edges of the product. The built-in dish washer machine should not be damaged during these tests to get. On the other hand, drop tests from higher levels are also carried in practise to assess the impact resistance of products. For instance, built-in dish washer was dropped from 40 cm height to evaluate the impact resistance. In this thesis, firstly, mechanical properties of expanded polystrene materials at high strain rates are invastigated in the literature for drop test analysis. However, when studies available in the literature are rewieved, it is not possible to find the properties of packaging foam materials used in Arçelik. Therefore, determination of the mechanical properties of the foam material became one of the main tasks in this thesis. It was found in the literature that mechanical properties of polymer materials mainly depend on density, strain rate and ambient temperature. It was therefore necessary to obtain the material properties at appropriate strain rates for drop tests described in the standards. There are available research centers and universities having test equipments and devices suitable for the determination of the mechanical properties of polymer materials at various strain rates. However, available test equipments were found to be not capable of carrying out the tests at the desired strain rates. Therefore, a drop test rig was designed and developed in this thesis to determine the mechanical properties of polymer foam material used in white appliances industry at desired strain rates. After free falling mass is dropped from 25 cm, 40 cm and 70 cm heights on a styropor specimen, the appropriate data are collected using appropriate sensors of the drop test rig. Force sensor, laser displacement sensor and accelerometer are utilised in the drop test rig. A cubic specimens with dimensions 50 x 50 x 50 mm3 with variying densities (20, 24, 26 and 30 kg/m3) were tested at high strain rates using the test rig developed in the thesis. Also, the same specimens were tested using Zwick standart tensile test machine under uniaxial compression at low strain rates so as to compare strain rate effects on mechanical properties of foam materials. After all tests were carried out, mechanical properties of the foam materials were compared at low and high various strain rates. 20 kg/m3 density styropor is used as a packaging material for packaging the built-in dish washer in this study. Hyperworks finite element software was used to model and simulate the drop test rig. Hexahedral elements are commonly used in literature to model foam materials. However, tetrahedral elements were used to model packaging materials in built-in dish washer because of complex geometry of packaging materials. Therefore, finite element model of polymer foam must be verified not only hexahedral elements but also tetrahedral elements to obtain accurately results from drop test simulations of the built-in dish washer. On the other hand, various material models were researched in Hyperworks finite element software to model accurately for polymer foam materials and visco-elasto-plastic model was chosen to model styropor. Radioss explicit solver in Hyperworks was utilised for the numerical simulation of the drop test rig. Then, the results obtained from the transient simulations were compared with the drop test results. It is found that the results obtained from measurements and simulations agree very well both hexahedral and tetrahedral elements. After material model of styropor was verified, a finite element model suitable for transient non-lineer analysis for the built-in dish washer was created. Built-in dish washer components can be divided into four groups that are sheet metals, plastic components, auxillary components and packaging materials. After all components of the machine were created, material properties of individual components were defined. For sheet metal components of machine, Johnson-Cook material model was used. Johnson-Cook material model is commonly used in literature for isotropic materials such as sheet metals. On the other hand, elasto-plastic material model was selected to define properties of plastic components such as chassis, air channel, salt cellar. Finally, auxillary components such as motor, compressor and evaporator whose inertia properties are very important during impact are defined using elastic material model. After the finite element model of built-in dish washer machine was ready, drop test simulations were carried out for 25 cm and 40 cm height cases. The results of the drop test simulations showed that there was no risk of any damage to the machine when it was dropped from 25 cm height that is specified in standard. However, drop test simulations for 40 cm height revealed that some plastic deformations could occur, at the plastic bottom chassis where the compressor is assembled to the machine. Physical drop tests were also carried out in Ankara Dish Washer Plant using a prototype built-in dish washer. Only one prototype was available for physical drop tests. Therefore, the same built-in dish washer prototype was used in tests with varying drop tests heights. After each drop test, packaging material was removed from built-in dish washer and the possible damage to the machine was inspected. Also, before each test, built-in dish washer was packed using unused packaging material. As predicted using numerical simulation, physical drop test from 25 cm height also showed no sign of any damage on machine, confirming that the product has sufficient resistance to drop damage. However, some damage occurred when the machine was dropped from 40 cm height. This result is also in agreement with numerical simulations.