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Toasters are commonly used kitchen appliances, and are typically not too expensive to purchase. They can be found at most department stores from anywhere between 6$ to 50$. They are small and compact, and are used to toast or heat slices of bread, and other bread-like foods. There are 3 kinds of toasters; pop-up, toaster oven, and toaster conveyer. For this design project, we will focus on the pop-up toaster.
The following study involves analysis of a $10.00 Chefmate toaster which was purchased from Target. In the course of this semester, we may analyze a more expensive toaster to compare what extra features are included for the price. Our hope is to find an innovative way to change current toaster designs, to address some common problems which users complain about most frequently.
Our analysis involves studying how the toaster is usually operated and how it works, combined with evaluating the toaster through Design for Manufacture and Assembly (DFMA), Failure Mode Evaluation and Analysis (FMEA), Design for the Environment (DFE), and a quantitative mechanical analysis.
Our first steps were to disassemble the toaster, to figure out what parts and components were involved in its operation. We were also able to understand the basics about how each of the parts of the toaster functioned. We categorized all the parts and assemblies, which can be seen in the assembly parts list.
After disassembly, we concluded that there were far too many parts in a toaster. We believe that this makes assembly extremely tedious, and increases the complexity of the product.
Our product to dissect and analyze was a toaster. For this part of the project, we purchased a $9.99 Chefmate toaster to dissect. We also purchased a $29.99 Hamilton Beach toaster that we hope to compare with our first toaster at a later date.
The purpose of a toaster is to assist in the making of food. Specifically, if the user wants toasted bread or bagels, warmed up waffles, etc. The toaster provides these functions.
To pictorially explain how a toaster is used, pictures from our usability study were taken to show customer product interaction. Following from that, we will delve into how a toaster actually works. From the outside, one might not think a toaster is a complex piece of equipment, but its internals are actually quite intricate. The inputs to the toaster are:
- 800 Watts of power
- food into the slot
- a toast setting
- a force to activate the latching and heating mechanisms.
With these inputs provided, the activating handle is depressed, which completes two separate circuits:
- First, a circuit is completed which provides power to an electromagnet and the timing circuit, holding the toastee inside the toaster where it can be heated.
- The second circuit that is completed is the heating circuit. A current is run through high resistance Nichrome wire, creating heat.
The timing system works as such:
- The variable resistor (regulator knob) controls current to a capacitor, regulating its charge rate. As soon as the capacitor reaches its maximum voltage, it discharges and breaks the power to the electromagnet.
- A spring then pulls the bread holder and toastee back up to where it can be safely removed from the toaster.
- As the spring pulls up the bread tray, the mechanism breaks the heating circuit.
Thus, the outputs of the toaster are heat and warmed food.
These stakeholders are very influential to the design process and the decisions made for different products. Each represent a different viewpoint and have various needs that are important to them.
- intuitive design and use
- quick toasting
- easy to use
- intuitive design and use
- easy to reach bread
- Commercial Customer
- high volume application
- intuitive and simple process
- quick toasting
- low part count
- least amount of tools necessary
- quick production time
- low volume packaging
- stacking packaging
Outer Case Assembly
Inner Front Assembly (Locking and trigger mechanism)
Inner Front Assembly (View 2) (Locking and trigger mechanism)
Side Assembly (Locking and trigger mechanism)
Side Assembly (2) (inner heating coils)
For our product evaluation, we looked at four different types of analysis. Design for Manufacture and Assembly (DFMA), Failure Mode Evaluation and Analysis (FMEA), Design for Environment (DFE), and a quantitative mechanical analysis.
The toaster is a fairly inexpensive kitchen appliance which is always in demand. Almost everyone owns a toaster or has access to it, meaning that it needs to be designed to allow mass production.
Design for Manufacture
The toaster is manufactured using several different processes. The main two process used are injection molding and stamping. Extrusion, deep draw methods, and an hydraulic press were also used.
Exterior of Toaster -
The case, bottom, and user interface of the toaster were all injection molded. Although injection molding is an expensive process due to its high price equipment and molds, with the toaster it is probably a good option. With the number of toasters being made and the consistency of the toaster design, the mold for a toaster is more than paid for. There are probably three different injection molds for this product: one mold for the case, one for the bottom, and one for the various buttons and knobs on the user interface. This combination of multiple parts in one mold is allowable due to the small size of the various parts and the lack of detail needed for each part. The shape of each of these parts were designed to be most efficient for injection molding. The bottom, which consists of many slots or vents, is the most complex of the shapes and is made up for this by having four locations that the plastic is injected.
Exterior Metal Slots-
The slots of the toaster was made through a deep draw process. This is a special type of stamping that gives radial stress to the flanges of the metal, and allows it to be stretched radially as well linearly. This gives the metal a rounder finish, making it look more complete and attractive. This process was used only for the exterior metal on the slots for the toaster. Its purpose was to make the toaster attractive. Although this process is more expensive than just stamping and bending metal, in mass quantities the cost is not unreasonable. This process in the toaster manufacturing could definitely be taken out, but it would also hinder the quality and attractiveness of the product.
Interior Metal Plates and strips-
The metal in the interior of the toaster was all stamped and bent. This is an easy process which uses sheet metal and then stamps out the shape of the metal needed. All unused sections of sheet metal can then be re-melted and used again. This is a very cost-effective method of manufacturing. You start of with the sheet metal, cut it in to shape by stamping it and then bend it to give it more strength and durability. This process is easy, low time, and easy to automate.
Metal Rods and Wires-
All of the metal rods and wires in the toaster were made through metal extrusion. Where metal is heated and then pushed through a die to its desired shape. This process is easy to do, and it is cost-efficient as while extruding the pieces can be easily cut to their desired length. This creates very little waste in material and makes the whole process extremely quick.
Mica is a great insulator and hard to burn. Thus is a great choice to contact the heating wires and insulate the various circuits and wires. In order to produce mica sheets, mica is ground fine and mixed with a colloid agent and water. A single sheet of uniform thickness is formed by pouring the mixture onto a mesh screen. Vacuum means and a hydraulic press are used to complete the formation of a sheet. Mica is not inexpensive, but its special properties make it a extremely valuable material, its used in a wide variance of products from dry wall to cosmetics for this reason. In this product's case, due to the restraints of needing an insulation, structure, and resistance to heat, mica was a perfect choice.
If you look at the toaster as a whole, you realize that the toaster was manufactured the way it was to try to optimize time, efficiency, and cost. They made some sacrifices in the manufacture for aesthetic appeal, by deep drawing the metal plate on top of the toaster. One problem with the design for manufacture is the number of parts needed to make a toaster. Since there are so many parts, many different processes are needed to be used to make them all. You have extrusion of metal and plastic (electrical wiring), stamping, press, injection molding, and all the electrical components. This gets expensive in the amount of machinery and tools needed, and how much time it takes. Even though there were so many parts, looking at all the different individual choices it is obvious that each part was made in the optimal way, looking at time and cost. Thus even though there are many parts, each was made as well as it possibly could.
Design for Assembly
The toaster is designed to be assembled by hand. It is made in China, where labor is cheap, and its parts were designed to allow easy and quick assembly.
Each of the metal plates in the entire were connected together with a series of tabs. The plates connected together when the tabs fit into small slots and were then folded over by hand. This allowed the plates to be easily and quickly adhered together. The one problem is that is it requires a lot of tedious work for the laborer. It was easy to see that different people worked on the tabs on our toaster, as different tabs were folded dramatically different for each plate.
There were also holes punched into the metal plates to allow for easy assembly of having either rods, wires, or metal pieces fit easily through a space into their correct location. This once again was optimized for hands-on assembly. As you can see from the picture below, this design was even incorporated for somewhat complex shapes and assembly to quicken the process and keep it accurate. This step, though most likely still quite challenging, has been helped a lot by this step. It allows the bread shelf to be assembled later on in the process (allowing other components to be placed first) while being able to reach its position easier than before.
The injection molded bottom had some built in slots that the circuit boards easily slipped into, making the circuit board addition a last minute process. All that needed to be done was to slide them in, connect the two boards together, and solder the wires to the leads. The stamped metal insides were also able to be slid into place, to secure the toasting cavity to the toaster base.
The hardest part of the assembly is probably trying to put in the mica sheets and wires into the stamped metal area. This requires sliding the mica sheets down while threading the wires at the same time. The toaster did a good job of organizing the assembly process so that there was little obstruction and everything was reachable.
Looking at the whole assembly process, the toaster has a lot of parts to put together. Automating more of this process would be beneficial for time and getting rid of labor. This would also increase costs due to machinery and maintenance. Although the toaster is made entirely by hand, the parts were designed to limit the laborer to easy tasks, improving time and reliability of the assembly.
Off the shelf, this product should function just fine. Most failures should be found before the product leaves the factory. The major failure modes mostly involve mechanical fatiguing of parts and mechanisms accelerated by thermal loading. Therefore, this product should be put through Hyper Accelerated Life Testing (HALT) to determine an average lifespan and address major failures. This testing should include average and heavy use under normal and extreme conditions.
The risk may also be lowered if the properties of the metals used are already known. If the components will be able to function properly after 4 years of use, then the product will have reached an acceptable lifespan. This will cut down on reliability testing costs and keep the overall product costs low.
The following table outlines the basic Failure Modes and Effects Analysis (FMEA) for the toaster. The first nine parts on the list were analyzed at the component level while the rest were at assembly level. Since most failures for the assembly level parts had similar failures and effects, they were analyzed together. The scale used can be found in the Engineering Design textbook by Dieter and Schmidt.
|Part #||Name & Function||Failure Mode||Effects of Failure||S||Causes of Failure||O||Design Controls||D||RPN||Recommended Actions||Responsibility|
|001|| Plastic Casing
||Warped||Improper fit, Unintended contact with heating element||6||Repeated heating and cooling||4||Material Selection||8||192||HALT for Temperature Variations||Reliability|
|Improper Manufacturing||1||Material Selection, Mold Design||1||6||None||Manufacturing|
|Fractured||Exposure of internal components||8||Improper handling||5||Material Selection, Packaging||4||160||Drop Test with and without packaging||Reliability|
|Wrong color / surface finish||User dissatisfaction||1||Incorrect material / process||1||Material Selection, Mold Specs||1||1||None||Manufacturing|
|002|| Crumb Tray
||Warped, Bent||Will no longer fit in bottom of toaster, may get stuck in toaster||3||Improper manufacturing||4||Assign process and tolerance||1||12||none||Manufacturing|
|Cleaning, bent while in use||7||Material selection||4||84||HALT for typical washing cycles||Reliability|
|003|| Crumb Tray Handle
||Warped||Improper fit onto crumb tray||4||Incorrect manufacturing||1||Assign process and tolerance||1||4||None||Assembly|
|Improper fit when slid into assembly, may stick out too far||3||Incorrect manufacturing||1||Assign process and tolerance||1||3||None||Assembly|
|Broken attachment clip||Handle may disconnect from tray||4||Incorrect manufacturing or assembly||2||Assign process||1||8||None||Assembly|
|004|| Toaster Top
||Warped, Bent||Misaligned components||8||Improper Manufacturing||2||Assign process, Material selection||1||16||None||Manufacturing|
|Corroded||Deterioration of part||2||Heavy use, poor upkeep||2||Material selection||1||4||None||Design|
|005|| Toaster Base
||Warped||Misalignment of components||8||Improper manufacturing||1||Specify process, Material selection||1||8||None||Manufacturing|
|Fractured||Exposure / Misalignment of components||8||Improper handling||3||Material Selection, Packaging||2||48||Drop test with and without packaging||Reliability|
|006|| Regulator Knob
||Warped||Improper fit onto variable resistor or into casing||6||Improper manufacturing||2||Material selection||1||12||None||Manufacturing|
|Fractured||May detach from variable resistor||5||Poor handling, Impact||4||Material Selection, Packaging||1||24||None||Manufacturing, Assembly|
|007|| Press Handle
||Warped||Detaches from ejection system too easily||4||Improper Manufacturing||2||Material selection, mold design||5||40||None||Assembly|
|Incorrect fit into casing channel, toaster may stick||8||Improper Manufacturing||1||Material selection, mold design||2||16||None||Assembly|
|008|| Cancel Button
||Warped||Detaches from toaster||4||Improper Manufacturing||1||Material selection, injection mold design||5||20||None||Manufacturing, Assembly|
|Does not fit into casing or gets stuck||8||Improper Manufacturing||1||Material selection||1||8||None||Assembly|
|009|| AC Power Cord
||Short circuit||Unit cannot function||10||Cord pinching||2||Set assembly process||5||100||Test completed units before distribution||Assembly|
|Internal cord problem||1||Choose reliable supplier||5||50||Test completed units before distribution||Assembly|
|010 - 019|| Ejection Assembly
||Mechanism fails to activate circuit||Unit is inoperable||8||Warped switch||1||Material selection||1||8||None||Assembly|
|Switch fatigue||4||Material selection||5||160||HALT for normal use||Reliability|
|Mechanism fails to release||Heating elements stay on||9||Internal circuit failure||1||Circuit test||1||9||None||Assembly|
|Warped internal components||2||Material selection, Set manufacturing process specifications||1||18||None||Assembly|
|Fatigue||5||Material selection||5||225||HALT for normal use||Reliability|
|Finished product does not eject||6||Warped internal components||2||Material selection, Set manufacturing process specifications||1||12||None||Assembly|
|Fatigue||5||Material selection||5||150||HALT for normal use||Reliability|
|020 - 034|| Heating and alignment assembly
||Heating elements turn on partially||Uneven heating||7||Short circuit||1||Ensure electrical elements have sufficient clearance||1||7||None||Manufacturing and Assembly|
|Fatigue||5||Material selection||5||175||HALT for normal use||Reliability|
|Heating elements do not turn on||No heating of product||8||Short circuit||1||Ensure electrical components have sufficient clearance||1||8||None||Manufacturing and Assembly|
|Fatigue||5||Material selection||5||200||HALT for normal use||Reliability|
|Improperly aligned product||Uneven heating||7||Wrong dimensions||1||Material selection, set tolerances||1||7||None||Assembly|
Putting a slice of bread in the toaster, one does not often think about the environmental implications this act may have: “What steps were taken to enable this toaster to arrive on my countertop, how much power is it using while toasting, and what will happen to it after it I throw it away?” Delving into the life cycle of a toaster, we see that there is much room for improvement.
While nearly all the materials in the toaster are recyclable, due to the complex nature of a toaster, it is costly to fully recycle one. Thus, toasters generally end up in landfills at the end of their lives.
However, the key element in the life cycle of this appliance is energy. In both the production and use phases of a toaster, power generation and supply is the overwhelming contributor of greenhouse gas emissions. This seems fairly straightforward for the use end – in use, toasters produce little more than heat, from electrical power, and (hopefully) golden brown toast – but this may come as a surprise for the manufacturing end. Indeed, power generation and supply produces nearly four times as much CO2-equivalent than the nearest sectors, truck transportation and steel mills, during the manufacturing of such appliances.
Using the EIO-LCA software from Carnegie Mellon University’s Green Design Institute (results pictured below), we see that every $1,000,000 of production in the ‘Electric housewares and household fans’ sector produces 693 MTCO2E. Given our $10 toaster, this shows us that approximately 0.007 MTCO2E caused by the manufacturing of one toaster. To compare that to the use end, a simple calculation is done:
From the EIO-LCA, we see that for every $1,000,000 of production in the ‘Power generation and supply’ sector, 10500 MTCO2E are emitted.
Assuming: $0.10/kWh electricity cost, power consumption is 800W, usage is 3 min (0.05hr) per day, used 300 days out of the year, over the course of a 4 year use-life.
We see that it would cost approximately $4.80 to run this toaster over the course of its life. Given this information, it is seen that the use phase of a toaster’s life emits approximately 0.050 MTCO2E. This is an entire order of magnitude higher than the emissions production phase. Clearly, one of our assumptions, above, needs to be modified. The only option which may not impact the functionality and longevity of the toaster is the power consumption. Furthermore, looking at other toasters, we see that this 800W usage is on the low end of the spectrum. All residential toasters use the same heating method – running current through a filament, which generates heat.
The results for conventional air pollutants, such as SO2, NOx, and CO, show the same result: power generation, both for the creation and use of a toaster, is the largest contributor to harmful emissions.
The numerical results of GHG emissions, above, come with a good level of confidence. Though there are many sub-sectors of the ‘Electric housewares and household fans’ segment, the ‘Small electric household cooking appliances’ sub-sector represents nearly 1/3 of the overall sector.
From our analysis, it appears the best way to approach the DFE is to reduce the power consumption of a toaster. Perhaps an alternative heating method should be considered. A more realistic and attainable possibility is to better insulate the toasting area, as to reduce the amount of heat lost during the toasting process. Also, reducing the complexity of a toaster would increase its potential to be recycled, further reducing its environmental footprint.
|EIO-LCA GHG Results for Electric Housewares and Household Fans|
|EIO-LCA GHG Results for Power Generation and Supply|
The toaster bread shelf is a very important part of the toaster. Not only does it pop the toast back up after it has completed the heating cycle, but it keeps the bread at the appropriate position between the heating coils. If the shelf happens to fail, the toaster becomes a potentially dangerous appliance. This analysis is intended to find the probability of failure for the shelf.
This part is made from steel sheets, stamped and bent into the above shape. The properties of the material are as follows:
- Elastic modulus = 30.5e6 psi
- Poisson's ratio = .28
- Tensile streght = 58000 psi
- Yield strength = 32000 psi
The bent tabs are intended to support the bread while fitting in between the side guide wires of the toaster. A long indent along the side of the shelf helps improve lateral stiffness.
The loading conditions are designed to reflect the same conditions the shelf would experience within the toaster. The two rear tabs are fixed to the rest of the ejector assembly, so they are held fixed for this analysis. A normal piece of bread is about 4 inches across the bottom edge, where the bread would be contacting the shelf. The bread is simulated using this dimension and assuming a maximum weight of this bread is 1 pound. This is assuming it is slightly shoved into the toaster.
The first image shows the stress distribution while the second shows the displacement.
As seen above, even with the heaviest anticipated load of one pound, the shelf only deflects 0.0008" at the furthest point. The probability of shelf deformation under these conditions is low, since the heavily stressed parts and displacements are very small. These small sections occur at the sharp corners, which alter the results and produce stress concentrations. Accordingly, this failure mode, while potentially serious, is extremely unlikely. The shelf was obviously designed to withstand more than it should ever encounter in this way.
The next test demonstrates the effect of toasting a bagel or other product that would only contact one side of the shelf. One side will have the same one pound loading as before.
The first image shows the stress distribution while the second shows the displacement.
Now this loading presents more of a problem. While the max displacement, which is at the tips of the shelf "wings" is only 0.0012", this loading comes closer to the yield stress of the material in more and larger places. A typical bagel is more dense than bread, and the user would be more able to shove the bagel into the toaster, since it is stiffer as well. These considerations combined make it very possible for the shelf to experience forces higher than one pound on the outer edges. If the toaster is used to toast bagels facing the same direction repeatedly, the shelf will slowly twist toward one side.
A twisted shelf means that a percentage of the force exerted by the ejection system is lost into the side of the toaster, since the shelf will push it to the side as well. This is one way the ejection system could fail. If the shelf twists so far that it cannot retract enough the break the heating circuit, the toaster could catch fire. This could potentially be very dangerous and should be addressed. The indent along the side of the shelf seems to be in an effort to combat this twisting.
The first image shows the stress distribution while the second shows the displacement.
The indent actually does not improve the shelf in this way. The shelf without the indent would perform better than the indented shelf. The max displacement here is .0010", less than the indented.
All analysis and pictures generated using SolidWorks Office Premium 2007 SP0.0
Dieter; Schmidt. Engineering Design, Fourth Edition. New York: McGraw-Hill, 2009
Degentesh, Drew. "Metal Processing." 24-443: Design for Manufacture. Carnegie Mellon University. 11 Sept. 2008.
Degentesh, Drew. "Injection Molding." 24-443: Design for Manufacture. Carnegie Mellon University. 4 Sept. 2008.
Degentesh, Drew. "Plastics Processing." 24-443: Design for Manufacture. Carnegie Mellon University. 2 Sept. 2008.
Thompson, Rob. Manufacturing Processes for Design Professionals. London: Thames & Hudson, 2007.
Carnegie Mellon University Green Design Institute. (2008) Economic Input-Output Life Cycle Assessment (EIO-LCA) model
Team Members & Process
|Leigh Fortenberry||Jon Goettler||Akshay Jayaram||Ian Price|
|DFMA, Usability study, and Stakeholders||FMEA and Mechanical Analysis||Component List, Photography, and Executive Summary||DFE, Product and System Function|
After choosing the toaster for our dissection, we went to Target and looked at the selection of toasters in stock there. There were toasters ranging from $6.00 all the way up to $70.00. We opted to buy two: a $10.00 toaster and a $30.00 toaster. This decision was based on our estimation of toaster purchasing: the $10.00 toaster represents the consumer who just wants something that will get the job done, or an infrequent toaster user, and the $30.00 toaster represents the consumer who wants a quality toaster with a good range of features. The pricier toasters seemed excessive -- we could not see justification for purchasing a toaster of that price.
Having these toasters, we dissected the less expensive one. We intended on dissecting the pricier one as well, in order to compare materials, assembly, and manufacturing; however, time did not allow for this. We will likely use the $30.00 toaster as a source for parts for our prototypes.
Next, we created a brief user survey and obtained approximately 20 responses. This data was used to begin the process of rethinking the toaster, as well as contributing to the stakeholder needs. Following that, we split up the remainder of the work, as is noted above.
- Dissect second toaster to continue analysis
- Brainstorm Innovations and Improvements
- Visit our next report: Toaster redesign