Casted Large Play Bricks

Urethane Casted Large Play Bricks and Vacuum Formed Base Plate

Final Product & Results

Individual Parts 

The final design consists of three components: 

• Slope Brick (Cast): A 3×1 LEGO-style slope brick with two cylindrical studs on top and three hollow circular cavities on the underside for stud-connection. Cast from SmoothCast 300 urethane in a two-part MoldStar 30 silicone mold. The angled top surface rises from ~0.3" to ~0.75" across the length of the brick, creating a recognizable slope tile geometry. 

• Square Brick (Cast): A 2×2 LEGO-style brick with four cylindrical studs on top and one hollow circular cavities on the underside for stud-connection. Cast from SmoothCast 300 urethane in a two-part MoldStar 30 silicone mold. 

• Base Plate (Vacuum-Formed): A 8×8 stud LEGO-compatible base plate vacuum-formed from 0.03" clear PETG. The mold was 3D-printed in Stratasys ABS on a Fortus printer and features an 8×8 grid of stud cylinders with vacuum holes drilled between studs. The finished plate is transparent, allowing the stud detail to read clearly. 

Assembly 

The square and slope bricks seat directly onto the base plate studs via the underside cavities, allowing them to be positioned anywhere on the 8x8 grid. The assembly demonstrates LEGO compatibility and the product platform concept: the same base plate tooling could accept any cast brick variant using the same stud pitch. 

Evaluation Against Objectives 

The design met the core constraints: both parts fit comfortably within the 10" × 10" × 10" box, part volumes were well within the 7 in³ limit, and no 3D-printed geometry appears in the final product. Draft angles were incorporated throughout. The PETG base plate successfully captures stud detail, and the cast bricks shows clean surface finish with only minor post-processing needed at the parting line. Areas for improvement include dimensional consistency of the underside cavities (which showed slight shrinkage variation between pours) and reducing flash at the mold parting line. 

Manufacturing

Silicon Mold Making 

A two-part MoldStar 30 silicone mold was created to cast the bricks. The 3D-printed PLA master was placed in a modified food container, and the two mold halves were poured in sequence. Keys were incorporated into the parting plane to ensure repeatable alignment between mold halves. Rubber bands held the mold closed during casting. 

Casting the Bricks 

SmoothCast 300 (low-viscosity urethane) was mixed in a 1A:1B ratio by volume, poured through the sprues, and allowed to cure for approximately 8 hours before demold. The low viscosity of SC300 was chosen to help fill the narrow cylindrical stud features and underside cavities. A short working time (~10 minutes) required rapid mixing and pour. There was some flashing due to the mold being held together by just rubber bands but it was easily removed.

Vacuum Forming 

The ABS base plate mold was printed on a Stratasys Fortus machine. Vacuum holes were drilled between each stud position to allow air evacuation during forming. PETG sheets (0.03" clear) were used for the final parts due to their flexibility and detail capture. The process involved heating the sheet until sag was observed, then dropping the mold and activating the vacuum. Colored HIPS sheets (red and blue) were also tested during development. 

Challenges

Air Pockets in the Cast Brick Studs and Cavities 

Problem 

The slope brick geometry includes narrow cylindrical studs on the top surface (OD ~4.8 mm) and corresponding hollow sockets on the underside. During initial cast pours, air became trapped inside these features; particularly the underside cavities, which point downward relative to the pour direction. This resulted in voids at the base of the studs and incomplete socket walls, both of which are critical to the brick's connection function. 

Analysis 

The root cause was a combination of geometry (small-diameter features that trap air bubbles) and fill direction. With the sprue at the top, the underside sockets faced downward during the pour; preventing bubbles from naturally rising and escaping. The low viscosity of SmoothCast 300 helped penetrate the features, but surface tension was sufficient to hold air pockets in the 4.8 mm cylinders. 

DFM Solution 

Three corrective actions were implemented: (1) the pour direction was changed so that the underside of the brick faced upward during fill, allowing bubbles to rise toward the open sprue; (2) the mold was gently tapped on the table after pouring to dislodge trapped bubbles; and (3) the cylindrical features were given a slight taper (1°–2° draft) to reduce the surface area in contact with the air pocket. The combination reduced visible voids significantly. For higher-volume production, a vacuum degassing chamber for the mixed resin would be the next improvement. 

Thick HIPS Sheet Not Fully Conforming to Stud Features 

Problem 

Early vacuum-forming trials used 0.06" HIPS (colored red and blue sheets) as the forming material. While the sheets heated and sagged correctly, they did not fully conform to the cylindrical stud tops of the base plate mold; the sheet bridged across the top of adjacent studs rather than wrapping the full stud height. This created a 'webbed' appearance and reduced stud definition, which is critical for LEGO compatibility. 

Analysis 

Thicker sheets require more heat to become fully pliable and have higher resistance to stretching over deep features relative to their thickness. The stud height (nominally 1.8 mm above the plate surface) represents a relatively high aspect ratio compared to the 0.06" (1.52 mm) sheet thickness. Additionally, the colored HIPS material is stiffer than clear PETG at the same temperature. The combination of material stiffness and insufficient local draw meant the sheet could not fully wrap the stud cylinders.  

DFM Solution 

The primary fix was to switch to 0.03" clear PETG for final production parts. PETG heats more uniformly, is more compliant at forming temperatures, and at half the gauge of the HIPS, conforms more readily to small raised features. As a secondary improvement, the stud draft angle on the mold was increased from 1° to 3° to reduce resistance to sheet sliding along the stud wall. The combination produced clean stud definition on all final PETG pulls. 

Insufficient Vacuum Through the 3D-Printed ABS Mold 

Problem 

The 3D-printed ABS base plate mold relies on small vacuum holes drilled between the stud positions to pull the heated sheet down onto the mold surface. During initial trials, the vacuum was insufficient to fully seat the sheet on the flat field between studs; the sheet did not pull tightly to the base and left a slightly domed surface in the field areas, reducing the flatness of the finished plate. 

Analysis 

Two contributing factors were identified. First, the FDM-printed ABS surface has micro-porosity between printed roads, which can allow air to sneak back under the sheet between vacuum holes and reduce the net pressure differential. Second, the spacing between vacuum holes was too large relative to the stud pitch; the holes were placed at every other stud position, leaving some flat field areas with no nearby evacuation point. Air at those locations could not be fully evacuated before the sheet cooled and set. 

DFM Solution 

Vacuum hole density was increased by drilling additional holes at intermediate positions on the mold plate. This changes resulted in noticeably better field flatness on subsequent pulls. For future molds, a solid (non-FDM) mold material such as tooling board or a machined aluminum plate would eliminate the porosity issue entirely; though at higher tooling cost. 

Bill of Materials & Cost Analysis

Cost vs. Quantity 

At low quantities (1–2 sets), fixed tooling costs dominate and cost per set is very high. At higher volumes, variable material costs per set drop significantly: 

• 2 sets (1 mold set): ~$275/set (as presented in final presentation) 

• 100 sets (same mold set): ~$19.79/set (variable materials only) 

• The ABS mold can realistically produce 50–200+ vacuum-formed pulls before degradation, making the per-unit tooling cost negligible at mid-scale production 

External Service Comparison 

If the same parts were ordered through a service like Hubs : 

• Cast brick (via urethane casting service): ~$50–$80/part at 1-off quantity; drops to ~$15–$25/part at 50+ units 

• Base plate (via SLS or vacuum forming service): ~$40–$60/part at 1-off; ~$8–$15/part at 50+ units 

• Comparison: In-house tooling is cost-competitive beyond ~25–50 sets, and provides faster iteration capability 

The rate-limiting step in production is silicone mold cure (8 hours) for the first mold set, and urethane cure (8 hours per cast) during production runs. With two molds operating in parallel, effective cast cycle time can be halved. Vacuum forming is the fastest operation and is not a production bottleneck. 

Skills

• Two-part silicone mold design and fabrication (MoldStar 30) 

• Urethane casting process control: mixing, pouring, demold timing 

• Vacuum thermoforming: mold design constraints, draft angles, vacuum hole placement 

• 3D printing material selection for manufacturing tooling (ABS/ASA vs. PLA) 

• Design for manufacturability (DFM): draft angles, wall thickness, gate/vent placement 

• Cost analysis and break-even analysis across production quantities 

• Tolerancing and fit analysis for a stud-and-socket connection 

• SolidWorks CAD and parametric modeling of mold geometry