Hi, everyone. Carter and I worked on these LUCIA models for our midterm project.

We began by redesigning the molding box using dimensions from the LUCIA Box.LDPRT file. The selected four cervixes (CYSTS) were evenly distributed on the bottom surface to allow for post-processing. We also added a “XF CK Custom” marking for identification (Fig. 1). The LUCIA box and CYSTS models were 3D printed using a Bambu printer (Fig. 2). To achieve a smooth finish, tree supports were removed and the connections were polished. Tree supports proved advantageous as they were easier to remove, reducing sanding effort.

Fig. 1

Fig. 2

Once the positive mold was ready, we used Silicone Rubber from LET’S RESIN for the negative mold. Xavier’s experience emphasized the importance of degassing to remove bubbles formed during mixing. However, high-viscosity silicone could still trap bubbles even after degassing. A relatively elastic silicone was chosen to facilitate both polyurethane pouring and easy removal of casts (Fig. 3).

Fig. 3

Prior to molding, Mann EASE RELEASE 200 was applied to aid silicone release (Fig. 4). A light mist was sprayed from 30 cm above, followed by a brush application to ensure full coverage. Another mist was added and left to settle for 5 minutes.

Fig. 4

Silicone was mixed at a 1:1 ratio (by volume) of Parts A and B. To determine the required volume, we filled the mold with water and measured 210 mL, then prepared 240 g of silicone to account for losses (Figs. 5-7). Mixing lasted 3 minutes before degassing in a vacuum chamber. The mixture was poured at an angle to minimize trapped air (Fig. 8), followed by a second degassing. However, bubbles continued forming, likely due to the porous nature of the 3D print. We stopped degassing after several minutes and cured the mold in a fume hood for 20 hours (Fig. 9).

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

The cured silicone mold was successfully extracted (Fig. 10).

As instruction shown in Fig. 11, using Smooth-Cast 300, 80 mL of polyurethane was mixed (equal parts A and B). However, during the first attempt, Xavier missed the 3-minute pot life (Fig. 12), leading to premature solidification of the last poured mold (Fig. 13). Additionally, some air bubbles were trapped in lettered regions, likely due to narrow openings preventing air escape (Fig. 14).

Fig. 11

Fig. 12

Fig. 13

Fig. 14

To improve results, we shortened mixing time to 30 seconds and used two new techniques: (1) vibrating the mold with an angle grinder to release trapped air (Vid. 3) and (2) stretching letter openings before pouring polyurethane. A plastic cover was placed over the mold for a level top surface. These adjustments resulted in cleaner casts with well-preserved details (Figs. 15-16).

Vid 3

Fig. 15

Fig. 16

We opted for airbrushing due to its advantages: thin layers that preserve detail, fast drying, efficient paint use, and easier color correction. Airbrushing relies on the Venturi effect to propel tiny paint droplets, creating non-opaque layers that allow gradual color shifts.

Painting Plan:

Prime models with 3-4 coats of filler primer for smoothness.

Basecoat using a bright pink spray paint.

Fig. 17

Red color shift (1C & 2C): Airbrushed magenta-pink; 1C received more layers for a darker hue.

Fig. 18

Tinting 2C: A tinted version of 1C’s color was applied.

Orange shift (1D & 2D): Magenta, white, and yellow mix was applied, with more coats on 2D.

Fig. 19

Tinting 1D: A lighter version of the 2D mix was applied.

Detail airbrushing for dark red and yellow-tan areas.

Fig. 20

Final brushwork for deep reds, pink speckles, and yellow highlights.

Sealing with a light coat of gloss varnish.

Fig. 21

Comparing Figs. 17-18, the 3D-printed and casted 1C models are similar in features, though letter protrusions on the casts appear less distinct due to residual trapped air. This issue was observed across all models.

In Fig. 19, a comparison of the original reference, printed 1D, and casted 1D shows that airbrushing resulted in thin, even color layers that preserved details better than traditional brush painting. However, airbrushing deep red in the center created a blended transition instead of a sharp margin, which may or may not be desirable based on the actual appearance of diseased tissue.

Painting both printed and casted models simultaneously resulted in nearly identical finishes, making differentiation difficult without checking bottom marks. The airbrush method efficiently applied even layers, preventing paint buildup or dripping.

Cost analysis table is shown here:

Cost Type	Cost	Price	Source	Quantity	Total
Materials	PLA Filament (all iterations)	$16.99 / kg	Amazon	224.37 mL	$4.7
	LET’S RESIN SIlicone Rubber	$26.99 / 510g	LET’S RESIN	240 g	$12.7
	Smooth-Cast 300	$137.82 / 2 lbs	Smooth-Cast	160 mL	$29.17
	Gray Filler Primer Spray Paint	$8.99/can	RUST-OLEUM	¼ of a can	$2.25
	Cover Spray Paint	$5.98/can	Rust-Oleum	⅛ of a can	$0.75
	Acrylic Paint	$21.74/400 mL	Amazon	10 mL	$0.54
Labor	Prototyping Engineer (X2)	$25.91 / hour	indeed	5 hr	$129.55
Overhead	Utility Cost (considering both the cost of facility and electricity)	~$5.13/ hour	TexAgs	10 hr	$51.3
	Depreciation of Assets (3D printers and tools versus their typical life spans)	~$0.76/hour	ATO	4 hr	$3.04
Design	Engineering and Development	$44.34 / hour	ZipRecruiter	6 hr	$266.04
	Iterations (a continuation of the previous category)	$44.34 / hour	ZipRecruiter	2 hr	$88.68
Misc.	Waste and Scrap	~$49.5/L	UltiMaker FILAMENTIVE	0.3L	$14.85

Total=$603.57

In conclusion, our redesigned molding process, improved polyurethane casting techniques, and airbrush-based painting approach successfully preserved fine details and created realistic models. However, further refinements are needed to enhance letter clarity and control paint transitions for more defined color separation.

Molding polyurethane models with silicone negatives from 3D-printed master positives is quick, cheap, repeatable, and captures high-resolution details. While 3D printing is slower than 5-axis machining, it allows fast iteration with minimal setup. Silicone molding is straightforward—degassing helps, but bubbles can still be an issue. Pouring polyurethane smoothly, vibrating the mold, and flat-bottoming improve consistency. Airbrushing applies thin, even layers, preserving detail and making corrections easier. Mold shells could increase durability and reduce silicone use. Platinum-cured silicone costs more but lasts longer. Given these refinements, high-detail training models can be made efficiently without needing a separate low-detail process.

Here’s the cleaned table: