DIY PneuFlex Actuator Production: Step-by-Step Silicone Fabrication Guide

This section covers the complete process of mixing and applying silicone for mold making. Silicone liquid is sold with the correct amount of catalyst included, with a ratio of 3ml catalyst per 100ml silicone. A paper cup must be used for mixing (not plastic, which corrodes). Mix thoroughly in one direction to avoid bubbles. Apply silicone immediately after mixing using a soft brush to capture all details, then pour slowly into the mold base. The silicone begins curing immediately upon mixing. Allow approximately 24 hours for curing, though this may vary with climate. The mold is ready when it no longer leaves a mark when touched with a finger.

Understanding silicone types is fundamental to successful mold making. Tin cure silicone (condensation cure) requires 100A:10B mixing ratio, has high viscosity (~31,000 cps), and cures via ambient moisture absorption—drier environments slow curing while humid conditions accelerate it. Working time is 30-40 minutes, demold time 4-6 hours. Platinum cure silicone cures via addition reaction, offers faster demold times (1-1.5 hours), and eliminates cure inhibition risks. TC-5130F provides 25 Shore A hardness. Both require vacuum degassing, especially high-viscosity tin cure formulas. Mold release compatibility is critical—Zip 301 works with silicone without containing silicone oil that would cause bonding. Work surfaces must remain level for proper mold formation.

The Shore A scale measures silicone hardness from 10 (soft) to 90 (firm)—mechanical parts require firmer silicone (40A) to resist distortion under clamping pressure. 5140 Platinum Silicone has a 1:1 ratio, 6,800 cP viscosity, 30-minute working time, and 4-hour demold. Thorough mixing is critical—scrape container sides and bottom multiple times rather than timing alone. Colorless silicone benefits from added pigment for visual confirmation of thorough mixing. Vacuum degassing removes air bubbles, though some may initially appear before disappearing. Working in warm environments (85-90°F) significantly reduces cure times, so keeping a timer is essential.

5110 silicone is a 1:1 mix ratio material with Shore A hardness of 10, 2500 CPS viscosity, and 30-40 minute working time. It simulates firm human skin like finger tips. For skin-colored silicone, dispense Part A and Part B into labeled cups before adding pigment to ensure uniform color across batches. For fair flesh tones, use 2-3 grams of silicone pigment per cup and add flesh tone flocking with yellow, tan, and red flocking. Always match the lightest flesh tone first because silicone is translucent; starting lighter and adding darker colors preserves translucency and realism.

This segment explores the relationship between silicone hardness and its physical properties. Hardness 10 silicone is very flexible, stretches easily, and takes approximately 24 hours to cure. Hardness 30 silicone is stiffer, cures much faster (sometimes within one hour), and feels more like rubber without stickiness. The ideal hardness selection depends on application requirements—a middle-ground option like hardness 20 may provide optimal balance. The curing time directly affects workflow planning, with harder silicones offering faster turnaround. Surface texture and feel also vary with hardness, affecting how the mold interacts with cast materials.

This segment explains the vacuum degassing technique essential for proper polymer penetration. The process involves: (1) Placing wood pieces in a sealed container with the polymer solution, (2) Creating negative pressure in a vacuum chamber to remove air from the wood, (3) Gradually releasing air to prevent foaming and overflow, (4) Maintaining vacuum for approximately 30 minutes until bubble activity ceases, (5) Sealing the chamber completely overnight. The vacuum causes air trapped in the wood to rush out, creating space for the polymer to penetrate. After vacuuming, the wood absorbs the polymer solution, with approximately half a liter being absorbed by the wood pieces.

Air bubbles in silicone molds compromise surface quality and must be removed through vacuum degassing. The evacuation chamber removes air from the silicone mixture by creating negative pressure. As the vacuum pulls air out, bubbles expand and collapse, eventually being eliminated from the mixture. Running the vacuum for approximately five minutes achieves effective degassing. After stopping the pump, releasing pressure slowly prevents introducing new air bubbles.

A vacuum degassing chamber removes air bubbles from epoxy resin by creating negative pressure, which forces trapped air to escape from the liquid material, resulting in a clear, bubble-free finish instead of the cloudy appearance caused by residual air bubbles.

After mixing, thermal paste often contains entrained air that reduces thermal performance. A vacuum pump can remove this air by placing the material in a vacuum chamber and reducing pressure to a few microns for several hours (typically six hours). This degassing step noticeably improves thermal performance by eliminating the air pockets that conduct heat poorly. The vacuum removes dissolved gases and trapped air bubbles that were introduced during mixing, particularly during the mortar and pestle stage which folds air into the material. This final processing step is worth the time investment as it significantly enhances the final thermal paste performance.

Vacuum degassing removes air bubbles from mixed polyurethane to prevent voids and ensure part quality. A proper vacuum pump must pull 28-29 inches of mercury with a minimum flow rate of around 6 cubic feet per minute. Shop vacuums in coffee cans are insufficient. Time in the vacuum chamber varies based on mixture amount, viscosity, and container depth. Small mixtures take only 1-2 minutes. Allow material to degas beyond major bubble rise and fall, adjusting based on work time remaining. After releasing the air valve, small remaining bubbles dissipate naturally.

Structural mechanics is the science of determining internal forces and displacements in engineering structures under static and dynamic loads. Based on internal forces, strength and stability are verified; based on displacements, stiffness is evaluated. Key concepts include: (1) Displacement analysis - determining maximum deflection must not exceed allowable limits; (2) Stress calculations - normal stress (σ = M/W) and shear stress (τ = VQ/(It)) must not exceed material resistance; (3) Reinforcement design for concrete beams uses upper and lower moment values to determine reinforcement diameters. These principles apply to beams, frames, and other structural elements.

The instructor explains that if the expansion is restricted (prevented), compressive forces will develop in the materials. The instructor uses the analogy of an apple and orange on a support to explain that forces will develop to prevent the expansion.

Reinforcement positioning critically affects structural capacity—the moment capacity varies cubically with distance from the neutral axis (12cm vs 10cm yields over 70% difference). Minimum concrete cover (typically 3cm) protects reinforcement from corrosion and ensures bond development. Fiber reinforcement bridges micro-cracks, distributing stress throughout the matrix. Unlike discrete reinforcement, fibers provide uniform improvement in tensile strength and impact resistance. Concrete and glass are brittle materials that crack suddenly without warning, while steel offers ductility with significant pre-failure deformation. Cracking results from thermal stresses during curing and shrinkage from water evaporation. Fiber reinforcement controls these mechanisms by bridging cracks and distributing stress.

The thermal expansion coefficient (α) defines how much a material expands per unit temperature: ΔL/L = α × ΔT. When materials are constrained and cannot expand freely, mechanical stress develops. This occurs when actual expansion exceeds available space. The mechanical strain equals the difference between actual strain and free expansion strain. In fiber-matrix composites, fibers and matrix have different thermal expansion coefficients, causing differential expansion that generates internal stresses even without external loads.

Concrete undergoes two distinct shrinkage mechanisms causing cracking: plastic shrinkage (first 0-6 hours, rapid water evaporation exceeding absorption) and drying shrinkage (after hardening, moisture leaving cement paste). Microfibers (diameter <25μm) prevent plastic shrinkage by retarding water loss and bridging microcracks, controlling 80-90% of early-age shrinkage. Macrofibers (1-2 inch length, 0.3-1.0mm diameter) improve post-cracking performance through hooked ends providing anchorage. Tenacity (area under stress-strain curve) measures energy absorption capacity, enabling gradual failure modes with multiple cracks rather than catastrophic collapse.