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Team Vision for Preliminary Detailed Design Phase
Going into the preliminary detailed design phase, some goals were to narrow down the design concepts and to answer some feasibility questions. The first feasibility question was to see if the mixing system we had in mind could even mix concrete and water. The second question was focused on the concrete mixture used, to narrow down the field of potential candidates for concrete mixes. These two feasibility questions were in regards to their respective subsystems which we intended to prototype and set test plans for. Since the previous design review, the team drafted test plans and carried out testing. The tests didn't go perfectly but were extremely helpful in narrowing the field of our design concepts and concrete mixture options.
Concrete Recipe
Using the risk metrics established previously in the project it was deemed that the concrete recipe had a high impact on decisions make for sub-systems in the 3D printer. This section discusses measures to mitigate and eliminate the range of possible of our cement recipe to aid it sub-system development.
Test Plans
Feasibility: Prototyping, Analysis, Simulation
The purpose of the mixture test was to assess the various cement bags the team had inherited from the previous team. This was chosen due to its high impact to the sub-systems of the printer. Three brands where in consideration ranging from Sakrete, Quikrete, and DAP. The goal of this test was to narrow down the possible options of a concrete recipe. This would aid the team in moving forward with sub-systems with the clearer assumptions on the concrete.
This test resulted in the identification of a few types of cement that has been deemed unstable and unpredictable for the printer. Additionally, provided two promising cement powders that will continue with testing. Some design drivers aided in the elimination of several options. During testing it was noted that some of the cement mixes were far more rocky that initially thought. These posed risks of damage or malfunction to the other sub-systems. These were noted, but continued to be tested carefully. When all material were set to dry a few issues arose. The plaster dried up too quick for it to properly fit the mold of the container it was placed in. This posed a great risk to clogs of the system. This eliminated the Quikretes and the DAP options.
With this new information, the team will continue testing of the Sakrete mixes and move forward with the sub-system development.
Volumetric Concrete Mixing and Dosing System
Prototyping, Engineering Analysis, Simulation
Iterative activities to demonstrate feasibility, including assumptions you made in your analyses or simulations. Have you completed sufficient analysis to ensure that your design will satisfy requirements? Have you included all usage scenarios in your modeling?
Test Plans
Feasibility Prototyping
Purpose
The purpose of the test was to determine the feasibility of a concrete mixing system of this design and scale. Vertically mounter augers are common in large scale, on demand, concrete mixing systems. This testing will inform the team whether this solution is worth pursuing for on-demand concrete mixing. The team developed, CAD modeled, and constructed a concrete mixer design that utilizes an consumer-off-the-shelf (COTS) auger, PVC tubing components, and 3D printed parts. This concrete mixture was used in the mixing system tests and will hopefully become the basis of the final mixing design.
Inputs and Source
- Design needs to use COTS parts that are affordable and easily replaceable.
- Design needs to be simple, yet representative of a solution that is feasible for the final product. Design should utilize information from commercially available solutions.
- Design should recognize and attempt to address the following Engineering Requirements (refer to documentation for detailed descriptions): ER5, ER6, ER8, ER9, and ER29. All of these engineering requirements are directly linked to the uniformity the system can produce and the rate at which it can produce concrete.
- Uniformity of the concrete mixture produced will be compared to traditionally mixed samples.
Outputs and Destination
- Primary output of this feasibility study is to determine the capability of a system of this design and scale to produce concrete at an appropriate rate and uniformity. This will simply be answered as a "yes" or "no". It should be clear whether continuing down this path seems reasonable based on the concrete samples that are produced. This will refine our concept selection and help us determine future testing parameters and quantifiable targets.
- A preliminary prototype like the one produced for this feasibility study should be the basis of a future design. Evaluating the performance of this design should inform future iterations of the design and highlight areas of improvement.
Design Images
Figure 1: The full assembly of the Concrete Mixing Tester. Cross sectional view shows the internal structure.
Figure 2: The full assembly of the Concrete Mixing Tester. Top View shows how the concrete and water is put into the mixing chamber.
Figure 3: The internal assembly of the Concrete Mixing Tester. This view shows the 3D printed parts and how they hold the auger in the appropriate radial location while still allowing rotation of the auger and flow of concrete.
The design was tested with Type S Concrete at varying hydration levels to see if it was capable of mixing the concrete thoroughly. The concrete that was mixed by the system was then placed into cups to dry, akin to the concrete testing being done in parallel. This concrete was dried and then evaluated on the basis of strength and uniformity. More information on this can be found in the test plans for the Concrete Mixing System and the Concrete Mixture.
Drawings, Schematics, Flow Charts, Simulations
Based from the findings of the Concrete Mixing Process test we have found that an auger has a high confidence of being able to mix concrete thoroughly, if enough material is introduced into the system at a rate such that the space between each flight is filled by some amount material. From these conclusions we have found that a smaller auger and mixing chamber may be beneficial to reduce the amount of material that needs to be mixed. This reduction will help us more closely match the concrete production rate to the concrete extrusion rate. In addition, we plan to look into the mixing system at an angle such that the inlet is lower than outlet to help ensure the water and concrete dry mixing to be more thoroughly incorporated.
Figure 4: Mixing System, the inlet for the unmixed parts of the concrete recipe is found in the side of the body of the system and the outlet for the mixed concrete mixture.
Figure 5: Mixing System Exploded View
Design and Flowcharts
The two designs previously considered were either mixed automatically or manually. During this phase, we have decided to select the automatic mix design. After doing some feasibility analysis on the manual design, we concluded that concrete had the potential to set in the mixer. Our feasibility analysis also showed that the automatic mixing system was capable of mixing. Due to these reasons, we selected the automatic mix design to move forward with, instead of the manual mix design.
Risk Assessment
Design Review Materials
Plans for next phase
The tests conducted this phase perfectly segue into more testing and analysis on the highest risk subsystems. Both the concrete mixture and mixing system require more specific testing in order to verify that our designs meet the requirements. In addition, the extruder and Z-axis subsystems will be prototyped and tested for feasibility before the next design review. By the Detailed Design review, we hope to have a single concrete mixture, a feasible mixing/extruding system design, and the software subsystem outlined.
Next phase will also include the development, design and testing of a pumping system to transport the mixed concrete from the mixing system to the extrusion system and a dosing system to insert a measured amount of dry mix and water into the mixing system to ensure the correct concrete recipe that will be determined is met. These two systems have seen preliminary design work. For the pumping system a peristaltic pump will likely be pursued for its ability to pump material that is not completely fluid as well as it will not be damaged if air is in the system. For the dosing system a rotating disk with a known volume of empty space is filled with concrete dry mix and then dropped into the mixing system. If the rotational velocity of the disk is known a volumetric flow rate of dry mix can be calculated and the correct concrete recipe can be achieved if an adequate amount of accuracy and precision can be met.
Figure 6: Peristaltic Pump Sketch
Figure 7: Dry Mix Dosing System Sketch