MakerBot PLA Material/Design

Problem Statement

Our group aims to discover the material properties of the PLA product used with the makerbot. We intend to construct several test to quantify these properties.

The Design Process

Requirements for each element or component derived from system level goals and requirements

The tests that pass to the implement phase must:

1. be feasible.
2. be economically viable.
3. have reproducible results.
4. be general enough to test multiple variables.
5. not have over 10% error.
6. identify essential material properties in discovering the limitations of PLA.

Alternatives in design

Many tests were initially conceived, but many rejected due to their ability to meet all requirements.

1. The Tension Test - a cylindrical test specimen with a smaller central diameter is clamped to a surface. Weight is slowly added and the amount of deformation is measured at each data point. These values are then plotted on a stress/strain diagram and the elastic modulus is found using a graphical method. This test failed the first requirement, it was not a feasible test. The amount of force needed to produce a very small amount of deformation was far too high.

2. The Heating and Cooling Test - a rectangular test specimen is first measured using vernier calipers. The specimen is then subjected to hot temperatures in order to thermally expand. The expanded dimensions are measured, recorded and the coefficient of thermal expansion is calculated. The test is preformed a second time in cold temperatures in order to obtain multiple values to average. This test did not pass into to implement phase due to the lack of importance the group found for CTE. Most printed parts would be operating slightly above or below room temperature causing CTE to not be a necessary factor in determining PLA's limitations.

3. The Sound Test - a small cylindrical test specimen is first used to calculate the density of PLA using the water displacement method. Then a very thin and long cylindrical specimen is laid out with two people at both sides. One person sends a wave through the PLA and the other times how long the disturbance takes to reach them, calculating the wave's velocity. The bulk modulus, also known as the elastic modulus, is then calculated using these experimental values. This test was thrown out due to technical limitations, feasibility, and accuracy. The wave would travel too fast to be accurately recorded and no technology was available to measure the propagating wave's wavelength or frequency to then calculate velocity.

The initial design

The first test that met all requirements was the 3 point bending test and later the 2 point bending test. A prototype of the 3 pt test was design and tested, seen below.

Experimental prototypes and testing conducted during design

Prototype

The first prototype of the 3 point bend test can be seen to the right. The setup consists of two raised triangular blocks supporting the test specimen and a level. A clamp is applied to the center point of the beam and a string is tied to the clamp. Weights are measured with a bathroom scale and tied to string. Deflection of the beam is measured by hand with a small ruler for each weight and recorded. These values were then used to calculate the elastic modulus of each data point and percent error was recorded. The percent error of the wood specimen recorded was around 40%, breaking one of our initial requirements. In order to track the source of error and reduce it a second material was tested, Cpvc pipe. After the second trial percent error was reduced to around 23%. While this value was still outside of our range it was still plausible to improve the experiment and meet requirements with the next design.

Appropriate optimization in the presence of constraints

The most present constraint during the design phase was finances. The incredibly high cost of tension test machines caused the group to think more creatively and to design very simple, yet practical tests. However, the simplistic design of our 3 point prototype incorporated a significant amount of error. Some sources of error from the prototype design that could be approved upon included different weight hanging and weight measuring systems, different deflection measuring system, and a shorter string to prevent swaying.

Iteration until convergence

Not applicable.

The final design

Final Design

The final design, seen to the right, is very similar to our initial prototype. The process was maintained and improved upon significantly reducing sources of error. To begin with, labeled hanging masses were used in the experiment leading to a calculated value of force incorporating very little error. Next, the clamp was removed and simply replaced by a loop at the end of the string, this assures the load will be concentrated at the direct middle of the beam. Lastly, and most importantly, the method of measuring was altered. Before beginning the test, paper is mounted behind the beam and the initial position is marked, this becomes the base line for measuring deflection. Hanging mass is then added and a deflection line is drawn. This is repeated for several data points in order to incorporate an average value. Once all data points are drawn the paper is removed. The distance between the base line and each deflection point is measured with vernier calipers and recorded. After conducting a dry run of this design, the calculated percent error was around 6%, well within our requirements.

Utilization of Knowledge in Design

Technical and scientific knowledge

Being that our goal was to design a scientific method in order to calculated the material properties of PLA, formulas were an essential part of our research.

Equations used in the 3 point bending test:

E = (FL^3)/(4wh^3d)

E = Elastic Modulus
F = Applied Load
L = Distance between the outer supports
w = width of beam
h = height of beam
d = deflection of beam

The applied load, F, was calculated using hanging masses. F = mass(kg)*acceleration due to gravity (9.81 m/s^2)

Equations used in 2 point bending test:

σult = (FLc)/(12wh^3)

σult = Ultimate Stress
F = Applies load , calculated as above
L = Distance between the hanging mass and the clamp
c = .5h
w = width of the beam
h = height of the beam

Creativity, problem solving, and group decision-making

As referenced before, most of our initial ideas required extremely expensive equipment. This forced the group to look outside the box, away from the most common source of a material's elastic modulus, the tension test. At first a simplified version of the tension test was thought to be an excellent test, but ended up being not feasible, too much weight was required for too little deformation. Our group had to then think creatively and discover a new test to accomplish our goal drawing inspiration from our Professor.

Prior work in the field, standardization and reuse of designs (including reverse engineering and redesign)

Most of our ideas on the 3 point bending test were formed by studying other more complex bending test machines. We used these designs and simplified them in order to meet our financial constraints.

Modeling and/or Simulation

Basic tests designs were drawn in engineering notebooks, but went straight to the prototype stage due to their simplistic nature. The only modeling done was for the test specimen in makerware, a simple 220 x 10 x 5 (mm) rectangle.

Multi-Objective Design (DFX)

As long as a test meets all requirements it may be passed to the implement phase.

Performance, life cycle cost and value

Our final design meets all initial requirements. It has a relatively low percent error and is incredibly affordable, totaling around $10 (not counting supplies available through the engineering lab). However the test is still financially feasible to others who wish to test other materials outside the engineering lab. For those with no access to a lab the total cost is around $40 plus the cost of a material specimen. Due to the simplistic nature of the test it requires no maintenance and is likely to last a significant amount of time.

Aesthetics and human factors

Not applicable. Our goal was to design a simplistic and accurate test for the calculations of material properties, not to appeal to consumers.

Implementation, verification, test and environmental sustainability

Below is the finalized experimental procedure we are passing on the implement phase. It is our hope they will be able to accurately measure the elastic modulus of PLA and determine any factors that affect it, such as color, fill direction, and temperature.
3 Point Experimental Procedure

When testing this procedure, the data and calculations recorded can be seen in the example below. This will hopefully give the implement group a place to start and a base to compare their values.
3 pt Example

Experimental Procedure 2 Point Bending Test that we have design to pass on to the next group to the implement phase. 2 Point Bending Test

Maintainability, reliability, and safety

Our very simple design incorporates no moving parts. This greatly reduces maintenance to practically zero and makes our test very reliable. Even though the design still has some sources of error, results are reproducible and very similar.

Robustness, evolution, product improvement and retirement

Even though our test met all our initial requirement and goals, there is always room for improvement. Look forward to the implement stage, any ideas that would continue to reduce error in the test would be welcome. While conducting the dry run test some notes for the implement group, the PLA beam tends to shift during loads finding a way to secure the beam while maintaining concentrated supports would improve accuracy. Also, while measuring accuracy has greatly improved it is still one of the main sources of error due to the dependence of human accuracy.