Table of Contents
The final presentation can be found here.
Team Vision for Detailed Design PhaseSummary of Phase:
- Decided to continue with half-sized model
- Went to home depot and confirmed all fittings and interactions between casing, instrumentation, and lungs
- Altered idea for casing construction
- Tested magnetism for lung and ribs interaction
- Altered idea for diaphragm construction
- Attempted lung pressure calculations
- Ordered rubber for lungs
- Updated Engineering Requirements to accommodate for half-size model
- Tolerancing for piston manufacturing
- Mechanical rib attachment strategy
- Thermoforming of acrylic, transition to PETG
Prototyping, Engineering Analysis, Simulation
CasingThe casing will fabricated from sheets of PETG.The resulting construction will be thermoformed in order to have more control over shape. This material has a higher impact resistance than acrylic and will not form air pockets during thermoforming. This method is being used instead of the purchased cylinder.
A new flour/tin foil model was constructed in order to accommodate for the half-size model. Each lung will have the capacity for 1.5L of air maximum. Preliminary calculations for a mathematical model to test the accuracy of our physical lung model were made and checked by Dr. Bailey. These calculations were incorrect. As of this point, the calculations have not been finalized. This does not hinder project progression in any way. Since this mathematical model is to be used as a means of testing ,it can be developed during the summer and MSD 2.
The pressure sensor on a blood pressure cuff was utilized to validate that the rubber lung would produce pressures that the Pasco sensor would read. The pressure sensor needed to read at least 20 mmhg. When the test was performed, the rubber lung recorded pressure values between 30 and 40 mmhg.
A stencil was created for tracing on the rubber sheets. The half-sized model was utilize for tracing on cloth. The tracing was than traced onto the cardboard stencil shown below.
The attempt at calculating a mathematical model for the lung pressures is below.
Rib and Lung MovementMagnets were used to aid in rib and lung interaction. The current model was utilized for testing purposes. Images are shown below.
The following table provides information such as number and type of magnets, magnet location, distance of interaction, and notes on interaction.
It was found that the magnets need to be stacked and an attractive force is more efficient and easier to manage than a repulsive force in terms of movement of the ribs. The magnets need to be separated at least 2.59 inches to prevent latching. This distance was measured using the 4 large magnets, to be stacked in the ribs, and 3 small magnets, to be mounted inside each lung.
DiaphragmThe diaphragm will be 3-D printed out of a flexible material that will act as a plunger to inflate and deflate the lungs. The material that will be used here is Ninjaflex, and has been selected to enhance the recognizability of the diaphragm as well as create a sealed piston in the chest cavity. The diaphragm is designed similar to current rubber pistons that are sold commercially, and customized to fit within the base of the formed chest cavity. The tolerancing has been adjusted to minimize slipping and improper sealing of the diaphragm. Additionally, there are redundancies present in the form of additional ridges along the side of the piston to ensure a better seal in case of misprints in the 3-D model.
In order to validate this model, a one-fifth scaled model was printed and attached to the plunger of a simple syringe. The plunger was then replaced and used to check for leaks that occurred. The tolerancing range was determined using multiple prints and checked for sealing again. This will be the final tolerancing recommended for the diaphragm model.
A handle has been provided at the case of the diaphragm to serve as an attachment point for the load cell or for the student to easily facilitate the movement of the piston. This feature has been designed such that minimal material is removed for the cavity created by the handle while preserving the structural integrity of the diaphragm itself.
Test PlansMuch of this project hinges on the interdependency of subsystems. For this reason the lungs will be tested first, for proper sealing. The casing will undergo a primary test phase immediately after thermoforming it, again, to check for proper sealing. The ribs and diaphragm will be mechanically linked, so their test must occur after both are constructed. Once the rib/diaphragm subsystem has been fabricated and tested, it can be attached to the casing and proper sealing of the diaphragm can be assessed as described below. After the entire system has been constructed we will perform the lab associated with the current lung model to ensure that all major objectives in the protocol can be easily and accurately performed.
Casing and DiaphragmThe casing will be tested after the shell of the piston is built. All outlets will be closed off and water will be sprayed around any fittings. The casing will then be pumped with air to test for any air leaks. Additionally, a weigh boat of dry ice will be sprayed with water and placed inside a fully sealed model. It will subsequently be visually assessed for vapor leakage. After assembly to the casing, the diaphragm will be tested for it's ability to hold a seal in the same way. After all subsystems have been tested for quality the casing can be fitted for all of its nylon attachments and tested for its ability to hold pressure.
LungsAfter the lungs are constructed, air will be blown into the lung and checked for any leakage. If any leakage is noted, the section will be re-sealed. The pressures will be checked when the entire model is made with the Pasco instrumentation. Those will values will be checked against the mathematical model and all benchmarking data available.
RibsDifferent infill percentages will be tested for rib strength. Current 1:5 model was constructed with standard 10% infill. Infill percentage can be increased up to 100% if necessary, print time will increase with increased infill percentage.
Tensile test for true Young’s Modulus for 3D printed Ninjaflex. Depending on availability of a test apparatus a hardness tester can also be used.
Bill of Material (BOM)
The image above is a snapshot of our Bill of Materials. The live document can be found here. Due to our switch from a full size to half size model we were able to cut down on costs. We will potentially be able to provide up to 3 lung models by the end of MSD II.
The image above is a snapshot of our Risk Assessment. The live document can be found here.
Plans for next phaseAlice
- Thermoforming of chest cavity and associated CAD
- Achieving acceptable volumes and flow rates within chest
- Diaphragm piston tolerancing and design and associated CAD
- + Dakota : Rib installation and magnet system to move
- Work with Meaghan to create and test lungs
- Spearhead/manage progress of our technical paper
- Work with Meaghan to build and test stand for model
- Create user manual for assembly,use, and part replacement
- Create lungs with rubber sheet, stencil, micro filler beads, and glue
- Order and create back-up lungs
- Finalize lung pressure calculations
- Work with Jade to test lung pressure when new model is created
- Work with Jade to procure materials for stand, finalize stand design, and create since our design will be dependent upon materials obtained
- Print and assemble ABS rib components
- Print and assemble Ninjaxfex ligaments
- Print Diaphragm
- + Alice : Rib installation and magnet system to move
A preliminary Gantt Chart for MSD 2 is pictured below.