Hands-on testing in the lab of 3D printed samples is at the essence of this project. Through isolating specific variables a great deal of information regarding the structural integrity of 3D printed products will be found.
While a lot of information is available on the physical properties of ABS plastic (the material chosen for this project), the rendering of the specimen tested is generally injection molding; it is unsound to believe the same characteristics apply to an identical test specimen produced using a 3D printer. It is thus the first task of this team to identify variables produced in the rendering of a 3D printed object, and obtain data that will provide the team with applicable scenarios in which a hammer prototype can be designed to maximize structural integrity.
All testing will take place in UB’s material labs under the supervision of MAE faculty and according to ASTM standards for testing plastics. While ASTM does not have fused deposition modeling 3D printing testing standards, the group plans to use ASTM’s standards of plastics in tandem with identifying structural differences that arise during the 3D printing process.
3D manufacturing Variables
Important factors considered by the group are as follows:
Infill scheme:
In the 3D production of a specimen the group has control over infill pattern, infill density, and wall thickness. Many infill scheme possibilities exist and through testing the group must identify the optimal combination of these three factors. At the outset of these tests, there is no data to make inferences upon and only through testing the gambit of combinations will the strongest be determined. It is also important to note that the infill direction will be taken into account. It is believed that depending upon how a force is applied, different characteristics of the infill will be tested. For example if the infill is idealized as pipes of the pattern stacked upon each other; and if an axial force is applied the strength of the infill is being tested as a column as oppose to perhaps the same force being applied in shear thus testing the actual cross sectional infill pattern.
Material laying direction:
Another unique 3D printing attribute is the introduction of material laying direction. Material direction is just the direction in which the filament was laid. For example if a single 3D layer was printed, on a small enough scale the specimen would look rather like tubes stuck to each other in single file. It is imperative that material direction is accounted for as depending on which direction a force is applied testes different material phenomena. For example, if the force is applied orthogonal to the direction of the material, then the strength being tested is the bonding at the boundary between each of the filament threads laid. However, if the force is applied in the direction of the material, then the test is gauging the axial strength of filament itself.
Batch consistency:
Due to the 3D printing process many pieces can be made in one “batch”. Very little data is available as to how parts differ from batch to batch, and even between specimen made in a single batch. We hope to observe trends from the testing data.
Thermal testing:
While it is the goal of the team to produce an operating range for the use of the prototype, at this time the team has no plans on thermal testing.
Lab Tests
Currently three tests will be run according to ASTM to determine characteristics that the team has decided are important.
D256 – Izod Pendulum
The Izod impact test has been decided as an important test due to its data regarding impact energy that an object will absorb before it factures and fails. Izod (unlike Charpy) is also pertinent to this project as the cantilever aspect of Izod is more applicable to the scenario of the head of hammer vs. its handle during impact.
During the Izod test, Test C will be conducted; while the additional step will take more time in the lab, ASTM suggests Test C should be conducted on materials that have an impact energy less than 27 J/m. Conducting Test C will yield more precise results as more energies are accounted for.
As for the grain direction the group has decided that since it is assumed that the boundary layer between each printed layer is very weak, hence, the structurally strongest material direction is orthogonal to the direction of the impact. It should be noted however that the strength of the boundary layer could be found using D732 if needed. However as the Izod test hopes to emulate the intersection of the hammer’s head and handle, the design would never have boundary layers parallel to the force due to impact.
Notching, according to ASTM, the prism must be notches to decrease material cross section at its designed fracture location. According to ASTM the specimen is supposed to be manufactured as a 3D prism and then material is to be cut away; however the ability to manufacture the specimen with the notch missing should be considered. After confirming with the Mark Lukowski (Lab manager for MAE UB), he says printing with the notching is acceptable, and what he has done in the past.
Infill direction, depending on which way the infill is facing perhaps we will have different structural properties. Since only two infill directions make sense we could easily test the impact direction has on strength during stage two; however a single direction should be agreed upon for testing stage one for consistency. The group has decided to have the infill direction orthogonal to the direction of impact.
Machine available to us.
D695 – Compression
Compressive strength is a standard material property and will thus be obtained through testing.
Material laying direction, two options exist for the material laying direction of this testing and that is applying the compressive load axially, or applying the compressive load orthogonally to the grain pattern. Each of these tests will determine a different property of the specimen; the axial load tests the direct compressive strength of the plastic whereas orthogonal loading would test the compressive strength of the plastic and the boundary layer interaction between each layer. It is unclear which layout will provide the maximum strength and both should be tested. The group has decided to have two solid control groups, one in each direction, then conduct the infill pattern groups so the load is applied axially to the material laying direction.
Infill direction, the orientation of the infill direction will probably have some impact upon compressional strength; for example if the load is applied orthogonally to the in plane infill design the patterns geometry will be carrying the load distribution and will vary from pattern to patter, however if the load is applied to the columns of designs then perhaps the difference in patterns will have a lesser effect on the structural strength. While this will have a large impact on strength, under the assumption of strength varying more between loads applied in plane to infill pattern. The group has decided to have the infill direction face orthogonal to the load applied.
Machine is available to us.
D790 – Flexural properties
In lieu of both D638 (tensile properties) and D732 (shear strength), Mark has suggested that the group conduct ASTM’s three point bending test. This test will allow the group to gain insight towards tensile strength and the specimens flexibility.
For this test the same specimen as Izod will be used (minus the notch), and the same assumptions about material laying and infill direction.
Testing Stages
The testing plans thus far are divided into three stages of testing.
- The goal of the first stage of testing is to reduce the field of three infill patterns down to the best one, and to gain insight as to manufacturer quality and consistency.
- The goal of the second stage is to determine the optimal infill density and wall thickness.
- Confirm analytical prediction of hammers strength, and ability.
Stage One
In stage one the all three of the outlined tests will be ran on the array of the infill patterns. The key to testing stage one is that a control groups are carefully conducted to obtain a baseline to compare future results to. From the data collected, the team believes the top infill patterns will be found.
Stage Two
In stage two the best infill pattern will be tested again using the three tests outlined above. However for this testing gate, a variety of combinations of infill density, wall thickness, and perhaps pattern direction will be tested to fully optimize and understand as much as possible about the specimen.
Stage three
Stage three will be the final stage of testing. After tabulating the strengths and weaknesses of all of the possible infill designs a prototype hammer will be unconventionally tested. Obviously the hammer will be used in its standard role, however other tests using strain gauges, force censors, and fatigue style testing will highlight the hammers longevity and parameter. Stage three will also confirm prior analysis conducted upon the strength of the hammer once it has been fully designed.