Table of Contents
Part A: Proposed Project Description
Introduction
Project Requirements
Evaluation Criteria
Technical Appendices
Theory of Electroplating
Quick Introduction to Motion
Introduction to Electricity
Safety Appendix
Part B: Proposed Project Solution
Part C: Educational Objectives
Introduction
In industry today, many materials are used because of the properties that they possess. A certain material may conduct heat well, be highly resistant to corrosion, or extremely durable under stress. However, some of the more desirable materials in industry are extremely expensive. To make a machine part out of a pure element is not economically feasible. Electroplating offers a solution to this problem. It is much cheaper and easier to build a desired part out of a more common material and then plate it with another substance. During the electroplating process, a thin layer of metal is plated onto another material. Because it is now coated by another metal, this material gains some of the properties of the plated substance at a fraction of the cost.
One of the most common applications of electroplating is for decorative purposes. Common examples include silverware and brass coated decorations. This experiment attempts to show the usefulness of decorative electroplating. Due to the nature of the solutions, however, the more common electroplating processes cannot be modelled. Copper will be plated onto zinc plates during this laboratory using a solution of copper sulphate and sulfuric acid. Other plating materials such as silver cannot be used because of the toxic chemicals involved. The copper bath provides an easy and safe method to demonstrate the principles of electroplating. Students will gain insight into the electroplating process, while becoming familiar with mechanical gear design, electrical concepts, and basic programming in the design of a mini-electroplating process.
Project Requirements
As part of a design team, you must design, build, and demonstrate the complete electroplating process.
Your company has the following resources:
1 LEGO/DACTA kit
2 600 mL beakers
2 400 mL beakers
Copper Sulphate and Sulfuric Acid solution
3/4" x 3/4" square zinc medallions
1 Macintosh computer
Hotplate
Eyewash Bottle
Copper anode
In addition, you may (at your own expense) provide additional material, such as your own personal stash of LEGO blocks.
The basic design of the project should follow the following process:
Your design should be fully automated. This means that, after the medallion is placed into a staging area, there should be no human interaction until the beautiful copper medallion is finished and setting in another staging area. In order to accomplish this, a computer program should be written to control the process. It is necessary to plate at least ten medallions.
Some of the chemicals used are toxic. Therefore, you must follow all safety procedures. For example, you should use a drip pan to collect random droplets and accidental spills. After the successful completion of your project, the chemicals must be disposed of properly. (See the Appendix for safety and disposal procedures.)
The larger of the two beakers is to be used to perform the electroplating operation. The smaller of the beakers is to be filled with water and will provide a bath to cleanse the electroplated object of the foul chemicals.
You must weigh the medallion before and after the process, so that you can determine the total mass plated onto the medallion.
You must also include a timer in your process. The timer should begin when the medallion is picked up from the staging area and should stop when the medallion is dropped off.
A lab report should be written detailing your entire electroplating experience. It should give a short summary of the problem, a description of your design (including the materials that you used), the results of the electroplating process, a hard copy of any computer program that you used, and a section commenting on any problems with the lab setup. In the "results" section, you should include the measurements of the masses added to the medallion as a percent increase in total mass, the time that the process took, and a description of the appearance of the plated medallions. Most importantly, you should explain the theory, in order to demonstrate that you know what is going on in the process.
One more thing to remember: LEGO blocks float in water and sulfuric acid!
Evaluation Criteria
The primary criteria for evaluation of this experiment will be the appearance of the lucky copper medallions that are being plated. Given the nature of the experiment, the plating will not be completely uniform. One criteria for the appearance will be the uniformity of plating on the medallion. More emphasis will be placed on the plating of the surfaces of the medallion and not the edges. Also, a minimum amount of plating will be required, measured by the percentage of weight gained by plating.
Related to the plating uniformity is the actual appearance of the medallion. The students are trying to produce a product that people will want to buy, and if it does not look good, people will not buy it. This factor would be based on the opinion of the laboratory evaluator. In order to make the criterion more objective, an estimate of the area plated on the surface of the medallion will be used.
Another major criteria is the economics of the experiment. The students are required to build an entire process from scratch. Materials need to be purchased in order to build the necessary equipment, along with the chemicals for the reaction itself. A scale will be set governing the cost of the construction of the apparatus. The cost of the chemicals supplied with the experiment will not be included, but any additional chemicals needed will be assigned a price. A cost per LEGO block beyond the blocks supplied in the kit will be set, along with the cost of operating the system. This operating cost will be a function of the power requirements for the process. Finally, any additional requirements will be added as costs, such as extra wires or broken beakers. These constants given by the lab administrator, in order to determine a grading curve for the class.
Another economic consideration in this project is the number of medallions that can be plated in a given time. The students will be evaluated on the number of medallions that can be produced in a specified time period.
All of the above criteria will add up to a total performance grade for the experiment by the following formula:
P = (N*App*Pp*C/T - TC)*B
where:
P = Performance
N = Number of medallions plated
App = Appearance of the medallions (average area plated over total area)
Pp = Weight gain of the medallions (average final weight over initial weight)
C = Market price of one medallion (set)
T = Total time spent electroplating
TC = Total cost of the process
B = An additional factor added for the creativity of the solution (defined below)
The minimum number of medallions to be plated is ten. The appearance of the medallions will range from 0 to 100 and will be estimated by comparison of the area plated to the total surface area. Each medallion produced will be given a percentage, and the average will be taken for all the medallions produced in one hour. This number determines how well the medallions would sell. The weight gain of the medallions will also be a percentage. However, its value will be greater than 100%, depending on the amount of plating. This does not have a real effect on the economy of the medallion, but represents the goal of actually getting the experiment to work properly. The cost of the medallion will be a set number, with a minimum value that would "break even" with the total cost of an average project. If the students' projects have a higher total cost, the cost of the medallion can be raised. In other words, the instructor will set the value of C based on the average project performance. The total cost of the process, TC, will be the sum of the cost of additional LEGO blocks, and the cost of power during the operating time to produce N medallions. The quantity TC is calculated by the following equation:
TC = (Cadd * Nadd) + (Cp * T)
Where Cadd is the cost per additional piece (LEGO or otherwise), Nadd is the number of additional pieces, Cp is the cost of electricity per unit time, and T is the time spent electroplating. Cadd and Cp are also constants given by the instructor. The quantity B will be calculated by the following equation:
B = [S+A]/150
In this equation, S is the safety factor, initially 100, ranging from 1 to 100 depending on the safety considerations of the design. Penalties are made when:
Operators do not wear proper clothing -15
Drops of chemicals escape -20
Either of the beakers break -60
The system fails while the plate is in the solution -40
Anyone nearby is harmed by any kind of chemicals -100
A is the percent to which the process is automated, ranging from 1 to 100. If no human interaction is necessary (i.e. the plate is picked up, plated, set down, and the system is reset without any alterations by humans), then A is 100. Penalties are made when:
Operator must load plate into plating rack -15
Operator must unload plate from plating rack -15
Movement of plate through the various stages is under huma n control -30
Process was stopped due to some kind of system error -20
In addition to the performance of the experiment, the students will also be required to provide a working demonstration of the experiment along with samples of the plated medallions.
Finally, the students will have to turn in a lab report describing the results of their experiment. This lab report should consist of a brief description of the theory behind the experiment. The students are not required to explain everything but only to demonstrate that they understand the principles of electrochemistry, gear reduction, and electricity. The students will also have to explain the results of their experiment. This should include the actual cost of the experiment and the number of medallions that can be produced. Students should also discuss the difficulties they faced in the experiment, including suggestions for improvements in future designs. The lab report should be 2 to 3 pages in length, not counting data tables or figures.
Electroplating Theory Appendix
An incoming freshmen will not have been exposed to the basics of electrochemistry or electroplating except for a brief overview in General Chemistry. The following information will provide a freshman with the background to successfully design and manipulate the electroplating process.
At the heart of the electroplating process is the electrochemical cell. In this cell, a chemical reaction takes place in which the anode is oxidized and the cathode is reduced. This type of chemical reaction occurs frequently and is given a special name, the redox reaction. There are two reactions taking place in the chemical cell, an oxidation reaction and a reduction reaction, and these reactions are always coupled. The two reactions are often called half reactions. At the anode, the general oxidation half reaction is:
M * Mn+ + ne-
In this half reaction, the metal M loses n valence electrons and becomes an n+ positively charged cation. The electrons that are lost react in the second half reaction, the reduction step.
N n+ + ne- * N
In general, a species that loses electrons is said to be oxidized, and a species that gains electrons is said to be reduced. The sum of the two half reactions gives the total redox reaction:
M + Nn+ * N + Mn+
A schematic of an electrochemical cell is given below:
In this figure, the anode loses its ions into solution, while the cathode is plated by its ions from solution. In an electrochemical cell, the two half reactions are separated by a membrane. A electrical connection is supplied between each electrode, resulting in a voltage drop between the two cell halves. This voltage drop determines the direction of the reaction. If the voltage drop is positive in a given direction, the reaction will proceed in that direction.
The voltage of an electrochemical cell is determined by the half cell potentials of each cell. Each half cell reaction will have a different voltage associated with it. The standard half cell voltages have been measured in the past, usually in reference to a standard hydrogen electrode. The standard half cell reaction for hydrogen is as follows:
2H+(aq) + 2e- * H2(g)
This half cell reaction is assigned a potential of 0 V by convention. The standard potential of all other half reactions is generated by coupling the reaction with the hydrogen half cell and measuring the voltage drop. These standard potential are tabulated in many references. The potentials are normally written as reduction reactions, so for an oxidation reaction, the sign of the voltage switches.
When coupling two reactions together, the total potential drop is found by the following equation:
*V0 = V02 - V01
in which V02 is the reduction reaction and -V01 is the oxidation reaction. If this total is positive the reaction is spontaneous and the cell is termed a galvanic cell. Otherwise, an external voltage drop must be supplied in order to force the reaction in the desired direction, and this is called an electrolytic cell.
Electroplating works slightly different than the basic electrochemical cell described above. In this lab, copper is plated onto zinc. In electroplating, the two electrodes are not separated from each other by a membrane. Both the anode and cathode are in the same solution. A power is supplied to the cell, resulting in a small voltage drop across the solution. This power supply also supplies electrons to the solution at the cathode. This results in a negatively charged cathode. The cations within the solution are attracted to this negative charge at the cathode. Upon reaching the cathode, these cations gain the electrons necessary to become a neutral species. As a result, the species is plated onto the cathode. While this is happening, more cations from the anode enter solution by losing electrons, replacing the cations plated on the cathode. These lost electrons travel back to the power source, thus completing the circuit.
The thickness of the plating on the cathode depends on a couple of factors, normally given in Faraday's Laws. The first law gives the weight of an element liberated from the anode as a function of the current, time and a constant:
W = Z i t
where W is the weight of the element, i is the current, t is the time, and Z is a proportionality constat. The weight of the element liberated is also proportional to its chemical equivalent, given by the following equation:
W = i t A / n F
where A is the atomic weight of the element, n is the number of electrons lost, and F is the Faraday constant, 96500 Coulombs.
Data for electroplating reactions are normally given in current densities and not current. The current density is defined as the amount of current supplied to the cell per area of the electrode. Given a particular area of an electrode, the necessary current can be found.
Also, in practice, the efficiency of the cathode and anode will not be 100%. A C.C.E, or cathode current efficiency is defined as the # of coulombs depositing metal divided by the total number of coulombs passing times 100%. Tabulated values usually take into account the efficiency of a given cathode or electrode.
In this experiment, copper is being plated onto zinc. A solution of copper sulphate and sulfuric acid will provide the ions needed for the plating process. The exact composition of the bath is not set, and can vary within certain bounds. The following bath compositions should be followed:
CuSO4 * 5H2O: 195-248 g/L
Sulfuric Acid: 30-75 g/L
The current density should be maintained between 2-10 A/dm2, and the areas of the anode and cathode should be approximately equal. The temperature of the bath can vary between 20 - 50 *C. If these conditions are followed, the efficiency of the cathode will be above 95%. Varying these conditions will determine the how well the zinc is plated, and it is the student's job to find the optimum conditions.
Quick introduction to motion
The direct output of most motors is the rotation of a shaft. Often, other types of motion are desired by the designer. For example, a car must change the rotation of a shaft into the translation of the whole system. How can the engineer use the available components to change the motor output into something useful?
Changing the speed of the shaft rotation
Sometimes, the designer needs a shaft that rotates slower or faster than that of the motor. One main reason for this is that the power delivered by the motor is constant. Remembering freshman physics, the power of a rotating system is equal to the torque times the rotational velocity. By reducing the speed, the designer can get higher torques out of the motor. How can this be accomplished?
Consider two rotating gears. If connected properly, the gear teeth do not slip. This means that the linear velocity at the point of contact is the same for both gears.
The velocity at the point of contact for each gear is thus:
V1 = (R1)(*1)
V2 = (R2)(*2)
and V1 = V2 , so:
*1 / *2 = R2 / R1
(Note that the number of teeth on a gear is proportional to the radius of the gear.)
This means that the angular speed of a shaft can be shifted by means of gears. For example, suppose gear 1 was on the motor shaft. If the larger gear 2 was placed into contact with gear 1, the speed of gear 2's shaft would be less than that of gear 1's. The same principle works with pulleys and belt drives. Worm gears can be considered to be normal (spur) gears with very large radii.
Changing shaft rotation to translation
Sometimes, the designer needs to move a system back and forth. Since a motor's output is the rotation of a shaft, it may present a problem to change this into translation.
The simplest way to solve this problem is to, again, rely on a no-slip condition. Since the ground does not move, the velocity of (for example) a tire must be zero at the ground. Since the tire is rotating, the geometric center of the tire must be translating.
Tires are a decent way of achieving a translation. Another good idea is to spin a gear on a rack (this is called a rack-and-pinion setup). Advantages of this setup include the fact that there definitely is no slip at the point of contact and that the motion of the gear is completely controlled. Drawbacks include the fact that the rack pieces must cover the entire range of motion
Changing the direction of rotation
Sometimes, a motor can only be orientated in one direction, but it must spin a shaft that points in a different direction. The LEGO sets include several bevel gears, which can be used to drive a shaft at 90* to the original shaft.
Remember, the shafts must be at right angles in order for the bevel gears to work. This may raise some concerns about the proper clearances between the two shafts.
The principle behind bevel gears is the same as that behind normal gears. There is no slip at the point of contact, so the velocity is constant at that point, so angular velocity is transmitted from one shaft to another.
Pulleys
Pulleys work much like gears. When functioning properly, the belt will have the same velocity at all points along itself. This is equivalent to a no-slip condition, as in gears. Again, the relative sizes of the pulleys determine the angular speed reduction.
Pulleys are useful in several situations. In LEGO systems, shafts can only be set at discrete intervals. Given the small variety of gear sizes, gears may not fit between two set shafts. Pulleys offer an alternative, because the distance between the two pulleys is not as important, as long as the belt has sufficient tension. Pulleys can also work as a "fuse." If the gear train become locked up somehow, one of the pulleys will not be able to rotate. However, assuming that the belt is connected properly, the other pulley can slip inside the belt. The motor would still be able to turn. It is better, from a wear point of view, for the motor to be able to turn when it is turned on.
Summary
Gears can be used in a wide variety of situations, and the above suggestions are only a few of the possibilities. More often than not, you will use gears to simply transmit power between parallel shafts. But is usually a good idea to start your gear train with a pulley (in order to provide slippage in case your system gets locked up). And almost every situation requires the angular speed of the motor to be stepped down.
You should also be forewarned that not every LEGO gear will fit anywhere you place it. In the real world, you can probably place a shaft anywhere there is clearance. But in the LEGO world, you can only place shafts at discrete intervals along the beam.
Safety Appendix
When working with the chemicals involved in this experiment, students should follow general laboratory procedures. Safety glasses or goggles should be worn at all times. Lab coats are also preferable, as the copper sulphate solution stains easily. Closed shoes are required, and long hair should be tied back. When working with the sulfuric acid, gloves should be worn. Wash rags and a wash bottle should be kept at hand to clean up any spills. In case any of the chemicals should come in contact with someone, the following measures should be taken. For copper sulphate, if it is swallowed, large amounts of water should be taken and vomiting induced. If inhaled, remove the person to fresh air and give artificial respiration if necessary. If it comes in contact with the eyes or skin, flush the area with water for at least 15 minutes. In the case of sulfuric acid, the following measures should be taken. If ingested, give at least 2-4 cups of water or milk, but do not induce vomiting. Contact a doctor immediately. If it is contacted with the skin, wash with soap and water for at least 15 minutes. The eyes should be flushed for 15 minutes with water also. If inhaled, remove to fresh air. In all cases, a doctor should be contacted as soon as possible. Any solutions that are waste should be collected in the waste bottle for disposal by the laboratory instructor.
Introduction to Ohm's Law
The primary concern relating to electrical engineering for this project is determining how to supply enough voltage and current to the reaction in order for it to work as desired. This project will utilize a power supply which plugs into the wall outlet and is capable of delivering 5V at 0-12 Amps. This power supply will supply the necessary voltage and current needed to drive the reaction with the +5 Volts terminal connected to the cathode and ground (0 Volts) terminal connected to the anode.
The fundamental relationship governing voltage and current is Ohm's Law:
V=IR (Voltage=Current*Resistance)
{Where V is in Volts, I is in Amps, and R is in Ohms}
Using only this mathematical equation it is possible to fully analyze our electrical system.
In order to maintain the forward chemical reaction needed to electroplate the zinc, a voltage must be maintained between the cathode and the anode. However, at the same time there is a desired current which will optimize the rate of copper deposition. From Ohm's Law it can be seen then that the resistance of the system is of monumental importance. By varying the resistance with a constant voltage, the current can be increased and decreased as desired. But how is the resistance of the system altered?
The resistance of the system can be easily varied by these simple relationships:
1) When two resistors are in series, their resistances add.
R(total) = Ra + Rb
2) When two resistors are in parallel, the inverse of their resistances are added and then inverted:
R(total) = (Ra-1+Rb-1)-1
{Hence if Ra=Rb, R(total)=Ra/2}
In this manner it is possible to increase or decrease the resistance of the system such that the desired current will be delivered.
Note that there is a practical limitation to the current. If Ra=0, an infinite current would try to be delivered (ignoring the resistance of the wire); obviously this is not possible and a fuse will be blown (at about 15 Amps).
For this laboratory project then, if the measured current is too low, the resistance of the system should be lowered by putting the appropriate resistor in parallel with the anode and cathode. Likewise, if the current is too high, then a resistor should be placed in series with the system. The desired components and connectors needed to implement any of the above circuits will be supplied if necessary.
Note also that before the power supply is initially turned on, the resistance of the cathode and anode should be measured with a multimeter so that the current can be calculated and be insured not to blow a fuse (for safety, keep the current below 2 amps! - Remember that it takes only .2 Amps to stop the human heart!)
Proposed Solution
To date our solution will work like this:
The initial staging area, the two beakers, and the drop-off area will be placed in a straight line. Above these areas, a track made of LEGOs will be built. This track is supported by pieces of wood in order to raise it above the hot plate. On the track, a car will run. This car has two motors: one to drive the car forward and backward, and one to spin a spool. A cable will be attached to the spool, so that it winds up or down. The cable will raise and lower a rack. This rack will connect the power source to the object to be plated, as well as raise and lower the object.
The object to be plated, a small zinc plate, will be placed into position in a holding area. The holding area itself is a conveyor belt running to the rack. New plates must be placed manually on the conveyor belt for each new run. The car will move above the holding area and the cable will lower the rack. The conveyor belt will then move the plate onto the rack. Next, the cable will raise and the car will move over the first beaker containing the plating solution. The cable will submerge the plate into the bath. In the bath, a copper anode is permanently mounted and connected to the power source. When the rack is placed in the beaker, the power source is turned on by a switch. This switch is operated by the program. The voltage source will cause a current to flow between the plate and the anode, through the solution. This will cause copper ions to collect on the plate. After a certain time has elapsed, the cable will raise. The plate will be suspended above the beaker for a short period of time to allow some of the chemical solution to drip off. Next the car will move to the second beaker full of water. Again, the plate will be submerged. This bath will clean the plate and rack of some of the dangerous chemicals. After a set time, the plate will be raised, allowed to drip off, and the car will move to the terminus of the track. There, the plate will be lowered onto the final holding area. In this final holding area, the plate will be removed, either by tipping the rack with a LEGO structure or manually taking the plate off. For now, an automatic method has not been perfected for for this procedure.
A flow chart of the program used to control the process is given below.
ELECTROPLATING CONTROL SYSTEM PROGRAM FLOWCHART
The most difficult problem faced was the design of the rack. It is difficult to raise and lower the plate evenly and without losing hold of the plate. The wire to the power supply made the design even more cumbersome. Other problems included the removal of the plate from the rack after it has been plated and the fact that LEGO bricks are quite buoyant. Another major problem was that the necessary clearances between the track, the beakers, and the hotplate were rather small. If attempted again, the designers might place the baths in a circle and use a robotic arm to move the plate from station to station.
Assessment of Educational Objectives:
This project, very simply, attempts to integrate a wide range of engineering disciplines and technologies. The chemical engineering aspect is perhaps the most prevalent; upon completion of this project the student will have a solid introduction to the fascinating topic of electrochemistry, understanding the theory and practical applications of electroplating. Another heavily stressed topic will be the design and implementation of the mechanical system which must autonomously carry the object to be electroplated from station to station; this will serve as a sound introduction to gear design. The student will also become experienced in the basic understanding of electricity as it relates to power. Since there is a required power needed to drive the electroplating, the student will discover the basic relationships of voltage and current. A final aspect of engineering in which the student will become enlightened is in the programming of the system controller. A program must be written which will determine the system's movements and figure in a correction factor for error; hence, the student will learn the basics of programming and come to understand several of the concerns with which must be dealt when controlling a system.
In order to assess what the students have learned upon completion of this project we expect the students to write a laboratory report explicitly stating the results of the lab, reviewing the theory which was applied, and explaining any difficulties which they encountered along with possible solutions for these difficulties. In return the project mentors should return an analysis of the project implementation according to the guidelines outlined in the previous section "Evaluation Criteria." The goal of this project is not necessarily a clever solution to the electroplating problem, but rather the introduction of several useful engineering ideas (design of automated systems, electrochemistry, programming, and mechanical systems). The students should be graded according to this goal.