Rapid Prototyping For The Masses

Introduction

Commercial rapid prototyping machines are expensive to buy, main-
tain, and use. Do it yourself rapid prototypers, such as the Darwin
developed by the RepRap group, have dramatically reduced the entry
level cost of producing and printing. Commercial rapid prototypers
can cost $ 20,000, but the RepRap Darwin and the commercialized
variant called the CupCake cost less than a thousand dollars. Addi-
tionally, they can print out an object for about $0.02 per cc whereas-
commercial machines cost about $2.00 per cc. To put this into per-
spective consumer laser printers were about that much several years
ago. However, with the price reduction comes a reduction in printing
quality. Cheaper alternatives can not compete with the resolution of
commercial printers but they still can produce useful items especially
for the hobbyist or producing rough prototypes. Feasible improve-
ments can increase the resolution.
Commercial machines use expensive hard to .nd feedstock ma-
terial, but cheap plastics such as ABS rods used for plastic welding
can also used as feedstock for the RepRap. ABS is not the only plas-
tic that can be used, HDPE and PLA plastic can also be adopted
for extrusion. The cost of the ABS rod is about $5/lb making it
economically superior to material used in commercial rapid prototyp- 

ing. As a comparison, the 109 plastic parts required to construct the
RepRap can cost $1500 when ordered from Shapeways, a commercial




3D printing service, but only about $20 when you do it yourself with
a RepRap.
Plastics replaced building materials such as wood, glass, and even
metal because they have many advantages. They are cheaper, more
durable, and more malleable. Also, they can be recycled. Plastic
bottles are made of PLA, milk containers are made of HDPE, and
Playstation chassis are made of ABS. Surely a do it your self recycling
station would bring the cost of rapid prototyping even lower. This
would add the ability to recycle plastics and print new objects from
them. Instead of incurring the cost of transportation to a recycling
center it may be possible to re-purpose and reuse the plastics on site.


 



CAD Design of PCB Holder
 

Physical Copy of the same Design for less than $5


Building a CupCake


The full assembly of a CupCake consist of Cartesian robot, extruder tool head, and electronics. The Cartesian robot is made of mostly laser cut plywood parts. It also includes all the X-Y-Z axis manipulators, a compartment for the power supply and the mount for the electronic PCBs on the side. The extruder tool head is made of heater barrel, DC motor for moving the plastic into the barrel, and electronics to control the speed and the temperature of the extrusion. The electronics curently used are known as the the Generation 3 electronics as they are the third iteration of the Darwin electronics. Unfortunately, the design integrated all the components into the three boards: the motherboard, the extender controller board, and the stepper motor driver boards. Combining several of the boards into one reduced the modularity and the electronics while also reducing the cost of construction.

  1. Preparation should include review of the tools required and methods used in the building process. Early Cupcakes were sold with SMD electronics which required more skill than traditional soldering and equipment like a hot plate, special soldering paste used in SMD, and fine tipped soldering iron. After the complete assembly, the microcontrollers on the motherboard and extruder controller need to be flashed with a bootloader and finally uploaded with firmware. As of today the Cupcake kit is being sold with mostly assembled electronics and flashed chips.
  2. For the complete assembly of the laster version of the Cupcake kit a small number of tools are required. Those include imperial and metric hex keys, small wrenches, sand paper, super glue, and clamps. Additionally, a pair of calipers is needed for proper calibration and measurement of the metal guide rails and threaded rods used to move the stage and tool head.
  3. For the complete assembly of the later version of the Cupcake kit a small number of tools are required. Those include imperial and metric hex keys, small wrenches, sand paper, super glue, and clamps. Additionally, a pair of calipers is needed for proper calibration and measurement of the metal guide rails and threaded rods used to move the stage and toolhead.
  4. The building process is made of optional painting of the wooden pieces, assembly of the Cartesian bot, assembly of the extruder tool head, and assembly and programing of the electronics components. After the compete assembly, all components should be tested to verify that they are are working properly. If however the extrusion quality is no satisfactory, it is necessary to realign and recalibrate the components that caused the build extrusion the be faulty.
  5. Cartesian Bot - Painting.The Cupcake Cartesian bot is made of laser cut plywood so painting the wood with coats of Polyurethane is suggested in order to make the chassis resistant to oil, moisture and staining. Before painting all surfaces, all the laser cut pieces should be sanded. Special attention should be given to the surfaces with the burn marks from the laser cutting process. The coats of Polyurethane should then be applied in several layers lightly sanding between layers. For a better looking surface finish, gentle sanding of the whole surface is necessary.
  6. Cartesian Bot - Assembly.The body of the Cartesian bot is held together with a T joints which makes it an easy
    assembly. The T joint is made of a screw and a nut. The screw is put through the drill hole of one piece while the nut is places into the pre cut shape that looks like t on the other piece. All the wooden pieces have their name written on them with laser, so proper orientation is self explanatory. More detail instructions can be found on the MakerBot website (click here). There are seven major panels that that must be assembled in sequence. Care must be taken to prevent the chipping or braking of the wooden pieces during the assembly. Also the assembly require to tighten all the bolts before proceeding to the next step. Another important component is the placement of the ball bearing on the threaded rods for the Z axis. The ball bearings are position on the rods and hold securely with nut above and below. However the each rod need to have two ball bearings on it and they have to specific distance from each other otherwise the Z axis will wobble and be inaccurate. It is possible the realign them later but it will require dis-assembly of some other components. 
  7. The extruder tool head is the most delicate part which requires construction. The head body is made out of sandwiched layers of acrylic. A DC motor with a pinch wheel pushes the plastic ABS rod down to a heated barrel. The cable needs to be pressed on to the pinch wheel by ball bearing on the opposite side. The barrel is heated by 9 ohm nichrome wire and contorted by thermistor. The nichrome wire has to be winded as close as possible to the nozzle so that melting occurs at the end of the barrel. The position of the thermistor is crucial for proper heating of the brass barrel and the nozzle. If it is places too close to the nichrome wire it will read the temperature of the heating element and not the temperature of the barrel. The best position is on the tip of the nozzle since the temperature at the nozzle is the extrusion temperature. 
  8. The assembly of the electronics is tedious but not that difficult when it is made with hot plate method. The PCB boards are cleaned and then small amount of soldering paste in put onto each small surface that will hold SMD part. With tweezers and precise hand each piece is placed in proper place and orientation. If the part is not properly align then small pushes can reorient the part. After putting all the parts on to the board and checking the correct placement, the board can be placed on to the turn off hot plate. Then tuning the hot plate on and setting it on low and waiting for the solder to melt by turning into silver liquid. If however low setting does not provided enough heat then slowly increase the power. The whole melting should occur within couple of minutes. If too much temperature is set up too high it can damage the electronics. 
  9. Flashing and programing the electronics. The chips on the motherboard and extruder needed to be flashed. Usually that is done using an AVR programmer but there is a way around it. Having only the TTL cable it is possible to manually flash the firmware using a "bit-banging" approach that take advantage of the FT232R IC in the cable.
    More information is available at Burning AVR boot loader with USB TTL Serial Cable[Burning_AVR]. A simple adapter was needed in order to make the necessary ICSP (In Circuit Serial Programing) connection.
  10. Test extrusion build. The first extrusion test it the proof that all the components are working. Not necessary working properly. Test object should be carefully analyzed to diagnose for any calibration and alignments. For example, the first extrusion printed in the picture below shown that the X-stage is skipping steps. Also the plastic filaments does not stick to each other very strongly is a prof that the temperature sensor is to close to the heating element and overshoot the temperature range set by the user.
  11. After test alignments and calibration. After the diagnosis, the Cartesian bot and the extruder controller should be taken apart and realign and recalibrate. From the example above the thermal sensor has been places closer to the tip of the nozzle so that it would read proper temperate. Also, the X-Y stage have been taken apart and look for any misalignment. In the picture below it is made sure that the smooth rod has difficulty moving through the two slider that had been placed into the both X and Y stages. The measurements are taken with caliper and it shown that the two rods are in parallel to each other within 0.1 mm. Also a wooden spacer, Popsicle, is added to keep the perfect geometry when the belt is puling onto the stage. of the tools required and methods used in the building process. Early Cupcakes were sold with SMD electronics which required more skill than traditional soldering and equipment like a hot plate, special soldering paste used in SMD, and fine tipped soldering iron. After the complete assembly, the micro-controllers on the motherboard and extruder controller need to be flashed with a bootloader and finally uploaded with firmware. As of today the Cupcake kit is being sold with mostly assembled electronics and flashed chips.









My Cupcake 3D Printer

                                                  


Materials used for extrusion

  • Acrylonitrile Butadiene Styrene (ABS) with chemical formula C8H8· C4H6·C3H3 is the most favorable material used for extrusion on Darwin and CupCake. It is very strong, light, and durable type of polymer that has many types of applications such as automotive body parts, toys, and home appliances. The ABS can be manufactured with different types of ratios of its components. The butadiene is want gives it the resilience and the other two compounds creates links between it. The temperature that ic can be used in is between -25 and 60 degree Celsius. It melting temperature between 200 to 240 degree Celsius, but as the viscosity of the plastic decreases with higher temperatures it is easier for the DC motor to push the filament through the nozzle. The reason for being most favorable is that is is the same type of material that Lego bricks are made of. Therefore it is very tough material. During the extrusion it binds well with itself and most material that it comes in contact with. The shrinkage factor is minimal and therefore can make objects that are water thigh.
  • High-Density Polyethylene (HDPE) is a class of polyethylen with higher density which results in stronger and has more opaque structure. The HDPE is very abundant and used in many products that can be recycled afterwords. When used for extrusion, the HDPE has very noticeable factor if shrinkage resulting in very light build objects. Also the coefficient of friction is higher then that of ABS making. However the problem with the HDPE is that it is harder to bond to the build base.
  • Polylactic acid (PLA) is made from renewable resources such as corn and sugarcane plants. It is a biodegradable material unlike ABS or HDPE. Unlike other materials used for extrusion, PLA does not require raft for build. It also does not wrap and peal off the build base. It extrusion temperature can range from 210 to 180 degree Celsius lowering the viscosity, but does not seem to have impact on the build quality.
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