When every tenth of a millimeter counts
Midnight in the studio. Scale 1:200. Deadline at 8:00 AM. The facade relies on 0.5 mm ribs and precise shadow gaps. Plaster cracks. Epoxy warps. There's no time to waste. RTV-2 silicone brings order to the process: build the mold, demold, repeat. Every component is exactly the right size. The model appears calm, sharp, and convincing.
Requirements in architectural model making
- Detail accuracy down to sub-millimeters and negative radii.
- Process reliability from pot life to demolding.
- Compatible with Acrystal, Biothan, plaster, and concrete.
- Scalability from single pieces to small series.
- Clean handling in the studio, preferably solvent-free.
Addition silicone vs. polycondensation (PC)
| criterion | Addition-curing (e.g. PRO, MF types) | Polycondensation PC |
|---|---|---|
| shrinkage | very low, typically < 0.1% | System-related approx. 0.2–0.5% |
| mixture | often 1:1 according to weight | typically 100:5 to 100:10 |
| Curing | time-controlled, heat-accelerated | humidity-dependent, byproduct alcohols |
| sensitivity | sensitive to tin/sulfur contamination | Tolerant in handling |
| Mission | highest precision, delicate geometry | large, robust shapes, cost focus |
Temperature range (guideline): approx. -50 °C to +180 °C after curing.
The matching SILISIL series
MF – Molding & Form Making
All-round material for architectural model making. Hardness ranges approx. 20–40 Shore A. Balance of flexibility and stability.
PRO – Precision & Jewelry.
Addition-curing, minimal shrinkage, very high detail reproduction. Ideal for the finest contours.
PC – Polycondensation,
tin-catalyzed, easy handling. Typical mixing ratio 100:5 to 100:10. Shrinkage approx. 0.2–0.5%. For gypsum, concrete, and Acrystal in large molds. Generally not for food contact.
RP – Rapid Prototyping.
Fast-curing for tight timelines. Short pot life, early demolding. For rush orders and iterations.
hardness selection
- 10–20 Shore A: delicate, undercuts, complex negatives.
- 25–35 Shore A: All-round for facades, roofs and terrain.
- ≥ 40 Shore A: large-area, flat elements.
Parameter corridors and selection matrix
| parameter | PER | MF | PC | RP |
| Mixing ratio | mostly 1:1 | mostly 1:1 | 100:5 to 100:10 | 1:1 |
| Potting time | short to medium | short to long | medium to long | very short |
| demolding | fast to medium | medium | medium to long | very fast |
| shrinkage | very low | small amount | 0.2–0.5 % | small amount |
| Detailed image | very high | high | medium to high | high |
| fitness | intricate details | All-round | large, cost-efficient forms | Appointments are tight |
Note: Specific figures for each type can be found in the data sheet. Selection should always be based on geometry, cycle time, and casting material.
Overview of material combinations
- Acrystal: water-based, low-emission, mineral finish. Easy to apply with minimal bubbles.
- SILIRESIN Biothan: clear/translucent, polishable, suitable for light elements and LED encapsulations.
- Plaster/concrete: benefit from elastic demolding, clean edges.
Compatibility matrix (guidelines)
| Casting material | PER | MF | PC | RP |
| Crystal | ✓✓ | ✓✓ | ✓ | ✓✓ |
| Biothane (clear) | ✓✓ | ✓✓ | ✓ | ✓✓ |
| plaster | ✓✓ | ✓✓ | ✓✓ | ✓ |
| Concrete (fine) | ✓ | ✓✓ | ✓✓ | ✓ |
Legend: ✓✓ very suitable, ✓ suitable.
Process: how precision becomes reproducible
Calculation examples for planning and purchasing
A) Material requirements: Silicone mold
Mold volume [cm³] = (L × W × H of the mold box) − displacement volume of the master model.
Required amount [g] = Volume [cm³] × density of the chosen system [g/cm³] (see data sheet).
Tip: Allow 5–10% extra for sprues and losses.
B) Cycle time and throughput
Daily throughput = available time / (pot life + demolding time + setup time).
RP series increase the number of cycles per day, PC series are suitable for large molds with longer curing times.
C) Cost per casting
(mold material + labor + casting material) / number of usable cycles.
Durable additive types reduce unit costs.
Mold design: sprue, venting, parting lines
- Gate routing: long, smooth flow paths. Select cross-sections to prevent cold-running seams.
- Vents: at high points, fine and numerous, so that displaced air can escape safely.
- Separation cut: along natural contours to protect visible surfaces.
- Embedding: Stabilize the shape with a support plate or cassette if necessary, especially for ≥ 40 Shore A and large surfaces.
Practical example: Perforated high-rise facade (0.5 mm webs)
Soft Shore A alloy for the negative mold. Vacuum-vented, cast in two stages. Acrystal delivers reproducible mineral panels. Individual translucent segments are created with SILIRESIN Biothan . The result: a sharp-edged series, consistent fits, and harmonious lighting effects.
Troubleshooting
- Air inclusions: Check viscosity, extend vacuum, reduce pouring height, pre-flow film.
- Sticky zones (addition): often inhibited by contamination. Clean the surface, separate material sources.
- Distortion of large surfaces: Increase hardness or support shape.
- Cracks during demolding: optimize the cutting process using a softer Shore material.
Ensuring quality
- CTQ list (Critical to Quality): Dimensional accuracy, surface gloss, edge appearance, bubble content, demolding forces.
- Acceptance plan: Cast test specimens before series production, define tolerances, document release.
- process data: Mixing, pot life, room temperature, demolding time.
Project workflow checklist (studio practice)
- Seal and deburr the master model.
- Dimension the mold box, plan the sprue/vent.
- Homogenize the components and weigh them correctly.
- Release the vacuum and pour in two stages.
- Allow to harden, remove from the mold, and check the edges.
- Clean the mold, store it in a dust-free environment, and plan the next cycles.
FAQ
Here you will find the most frequently asked questions from model makers about mold making with RTV-2 silicone.
Not necessarily. A preliminary test with original materials is advisable.
Delicate objects → softer Shore (10 - 20). Flat objects → harder (25 - ≥ 40 as guidelines).
Avoid contact with tin/sulfur-containing media, separate workplaces.
Long, calm flow paths, vents at high points. Thin inlet, then volume.
