This project allowed us all to explore and learn several skills in fabrication and prototyping. Even within each process, there were varied application methods to implement, which I will share throughout the post.

The purpose of this project was to replicate models of cysts and polyps on the cervix utilizing two distinct methodologies: three-dimensional (3D) printed models and polyurethane models.

Step 1: 3D Preparation

The SLDPRT files were downloaded and converted into STL files using Autodesk Fusion360 for the 3D-printed method. We also prepared for the polyurethane models by printing a positive mold of all four cervix models together in one box, which was also an SLDPRT file. A positive mold replicates an object’s exact shape and details, whereas a negative mold creates a cavity to form the shape and details of an object.1 Each of the four models was grounded to the bottom of the box to allow preparation for the silicone pour, which will make the negative mold that is to be used in the second method. This approach enables us to create multiple replicas from a single negative mold.

Step 2: 3D Prints

The next step involved printing five items: four cervix models and one positive mold. We used the Bambu slicer to prepare the STL files for the Bambu 3D printers. The first iteration of the 3D prints for the first method (3D prints) resulted in noticeable lines on each cervix model with a layer height of 0.1. To improve the smoothness, we conducted a second iteration with a layer height of 0.08 (extra-fine), the smallest possible setting for our Bambu machine. The total print time for this print was one hour and fifty minutes, which included tree support.

We printed the positive mold for the polyurethane method using the same settings as the second iteration 3D print mentioned above. The positive mold printed in four hours and nine minutes.

 

 

 

 

 

 

This section taught us about the impact of layer height and how the sizes of objects can affect detailing. I believe that due to the size and cone-shaped top, this illustrates the effect of layer height more than larger objects or objects without curves.

Step 3: Create the Silicone Mold

Step three involved creating a silicone mold to facilitate the production of polyurethane models of the cervixes. The box containing the four grounded molds measured a total volume of 248 mL, which meant we needed to prepare equal weights for part A and part B of the BJB Platinum-based silicone formula. The measurements were initially determined by filling the box with water, and 124mL of part A was first poured and weighed to match the weight of part B.

We first sprayed the inside of the box with a mold release agent to ensure easy removal of the silicone mold once it was cured. Next, we combined parts A and B into a mixing cup, meticulously scraping the bottom and sides to ensure thorough mixing. Given silicone takes approximately 24 hours to set and does not settle immediately, this allowed our group time to mix enough silicone to fill up the entire box to the 248 mL. 

To minimize air bubbles during the pouring process, we poured the silicone slowly and steadily from one corner of the box. It was crucial for the silicone to fill in beyond the height of the four models to accurately capture the intricate details, particularly at the tops, where the models depict cysts and polyps. After the pour, we gently tapped the container on the table to remove any trapped air bubbles for a smooth mold finish. For added precaution, we allowed our silicone mold to cure for 48 hours.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This section taught us the importance of mixing and pouring silicon, as the air bubbles are tough to remove afterward by gently tapping the bubbles out.

Step 4: Making the Polyurethane Molds

Step four was to make the polyurethane molds. We used Smooth Cast 300. Smooth Cast 300 requires equal volumes of part A and part B. We estimated the cavity volume of the four models within the silicone mold to be approximately 60 mL. Before pouring, the silicone mold was sprayed with a mold release to facilitate easier removal.

It is important to note that Smooth Cast 300 has a limited pot life of 3 minutes, during which the mixture must be thoroughly combined and poured into the negative molds. We mixed the components thoroughly for about a minute and poured the mixture into each of the four sections. The curing time was 10 minutes from the time the 3-minute pot life ended. Due to the short pot life, we used 40 mL of parts A and B, totaling 80 mL, in case we needed more than estimated.

Video 1: Timelapse of the polyurethane (mixing to molds

 

 

 

 

 

 

 

 

We attempted to redo our models 1C and 2D in polyurethane to improve the clarity of the lettering; however, both iterations yielded similar results. This was most likely due to how the lettering was set in the silicone model, despite our efforts to minimize the bubbles during the silicone pour in step three.

Video 2: Timelapse of the polyurethane redo for 1C and 2D (mixing to mold curing)

This section allowed us to explore different pouring techniques into the cavities. Our second trial of the 1C and 2D used a method to pour directly into the letters first and then fill out the rest of the cavity in the negative mold. This process also taught us the importance of not trying to have air bubbles, as it can impact details (in our case, the letters).

Step Five: Final Processing of 3D-Printed and Polyurethane Models

The final step involved processing the 3D-printed and polyurethane models using sandpaper and paint. We used 400-grit to smooth the surfaces of the models, focusing on the sides and tops, while 80-grit was used for leveling the bottoms.

Due to the light color of 1C in the reference photo, we spray-painted both versions of the 1C models a candy pink. We created a custom mixture of pink and red spray paint for models 1D and 2C, which were slightly darker colors, and applied it with a sponge brush. This approach provided a glossy finish; however, minor white spots appeared after drying. To address this, we used acrylic paint to cover these spots and added another coat for consistency. 

All eight models were painted using a combination of pink, white, and red acrylic paints in an attempt to match our reference pictures. After allowing sufficient time for the paint to dry, we applied a gloss spray to enhance the overall finish of the models.

 

 

 

 

 

 

 

 

We did our best to create the cysts despite the confusing connection of the raised parts on the 3D model vs. the parts shown in the reference photo. Unfortunately, we did not see the LUCIA model after the first day in class; therefore, this is the photo we used to do the final detailing.

 

 

 

 

 

 

In this section, we learned the different looks spray paint and acrylic paint give on PLA filament and polyurethane. Spray paint for both methods gave a glossy look, whereas acrylic paint gave a dull look, requiring glossy spray.