Pouring resin into a mold is a precision-driven process for creating high-detail parts—from decorative crafts (e.g., epoxy resin coasters) to industrial components (e.g., polyurethane gaskets) and model replicas. Unlike die casting (high temperature/pressure), resin casting relies on low-viscosity liquid resin that cures (hardens) at room or moderate temperatures, making it accessible for both small-batch and industrial production.
How to Pour Resin Into a Mold: Core Overview
The success of resin pouring depends on three core factors: resin-mold compatibility, Defoaming control (the #1 cause of defects), and curing environment stability. The full process takes 2–48 hours (depending on resin type) and follows 7 technical stages:
  1. Preparatory Work (Mold/Resin Selection + Environment Control)
  1. Mold Pre-Treatment (Cleaning + Sealing + Release Agent Application)
  1. Resin Preparation (Mixing + Coloring + Additive Integration)
  1. Defoaming (Critical for Surface Quality)
  1. Resin Pouring (Flow Control + Cavity Filling)
  1. Curing (Temperature/Time Regulation)
  1. Demolding & Post-Processing
Below, we break down each stage with material specs, equipment parameters, and quality checks for consistent results.

Step 1: Preparatory Work (Lay the Groundwork for Success)

Before pouring, align your mold, resin, and environment to avoid common defects (e.g., bubbles, surface blemishes, incomplete curing).

A. Mold Selection (Match to Resin & Application)

Molds determine part detail, durability, and reusability—choose based on resin type and production volume:
Mold Material
Resin Compatibility
Detail Precision
Reusability
Best For
Silicone (Platinum-Cure)
Epoxy, Polyurethane, Unsaturated Polyester
High (captures 0.1mm fine details)
50–200 cycles
Small-batch crafts (coasters, jewelry), model replicas (1:24 car parts)
Silicone (Tin-Cure)
Polyurethane, Unsaturated Polyester
Medium (0.2–0.5mm details)
20–50 cycles
Low-cost prototypes, disposable parts
Metal (Aluminum/Steel)
High-temperature epoxy (≥80℃ curing)
High (0.05mm details)
1000+ cycles
Industrial parts (gaskets, electrical insulators), high-volume production
Plastic (ABS/PET)
Low-viscosity epoxy (<500 cP)
Medium (0.3–0.8mm details)
10–30 cycles
Simple parts (trinkets, keychains), one-off prototypes
Key Rule: Avoid using latex molds—resin chemicals break down latex, causing mold degradation and part contamination.

B. Resin Selection (Based on Strength & Use Case)

Resin type defines mechanical properties (hardness, flexibility) and curing requirements—common options include:
Resin Type
Viscosity (25℃)
Curing Time (25℃)
Final Hardness
Best For
Epoxy Resin
300–800 cP
24–48 hours
邵氏 D 80–90 (rigid)
High-strength parts (coasters, industrial insulators), transparent products (crystal crafts)
Polyurethane Resin
100–500 cP
4–24 hours
邵氏 A 50–90 (flexible to rigid)
Flexible parts (gaskets, toy components), rubber-like replicas
Unsaturated Polyester Resin
200–600 cP
1–6 hours
邵氏 D 70–85 (semi-rigid)
Low-cost crafts (fiberglass parts, figurines), large-volume production

C. Environment Control (Stability = Quality)

Resin curing is sensitive to temperature and humidity—maintain these conditions:
  • Temperature: 20–25℃ (±2℃) — too cold (<18℃) slows curing; too hot (>30℃) causes rapid exotherm (resin overheats, cracks).
  • Humidity: <60% RH — high humidity (>70%) leads to surface “blooming” (white, cloudy film) on epoxy resin.
  • Dust Control: Work in a cleanroom or cover the area with a dust sheet—dust particles (≥5μm) create visible blemishes on the part surface.

D. Tools Preparation

  • Mixing Tools: Disposable plastic cups (100–500ml), wooden stir sticks (or electric mixers with low-speed setting: 300–500 RPM), electronic scale (precision ±0.1g—critical for resin proportioning).
  • Pouring Tools: Funnel (for narrow mold cavities), pipettes (for small-volume filling, e.g., <5ml), silicone spatula (to scrape residual resin from cups).
  • Defoaming Tools: Vacuum chamber (for industrial use, 5–10L capacity), heat gun (low-temperature grade: 50–80℃), toothpicks (for surface bubble popping).

Step 2: Mold Pre-Treatment (Prevent Leaks & Ensure Easy Demolding)

Even high-quality molds need pre-treatment to avoid resin leakage and ensure parts release without damage.

Key Tasks:

  1. Mold Cleaning:
    • Wipe silicone/metal molds with isopropyl alcohol (70–90% concentration) to remove dust, oil, or residual resin from previous use.
    • For deep-cavity molds (e.g., model car bodies), use compressed air (0.3–0.5 MPa) to blow out hidden debris.
    • Critical: Ensure molds are fully dry (alcohol evaporates in 5–10 minutes) — moisture causes resin bubbles.
  1. Sealing Leaks:
    • Inspect mold seams (especially silicone molds) for gaps (>0.1mm). Seal with high-temperature tape (e.g., 3M Kapton tape) or silicone sealant (cure for 24 hours before use).
    • For two-piece molds (e.g., metal molds), apply a thin layer of mold clamp grease to the mating surface—prevents resin from seeping between halves.
  1. Release Agent Application (When Needed):
    • Silicone Molds: Usually not required (silicone’s non-stick property), but for complex parts with undercuts, apply a light coat of silicone release spray (e.g., Mold Release 200) — let dry for 10–15 minutes.
    • Metal/Plastic Molds: Mandatory—use a wax-based release agent (for epoxy) or PTFE-based spray (for polyurethane). Apply 2–3 thin coats (allow 5 minutes between coats) to avoid resin sticking.

Common Mistake:

Overapplying release agent—excess agent causes resin surface defects (e.g., fish eyes, uneven coloring).

Step 3: Resin Preparation (Mixing Is Make-or-Break)

Resin curing relies on precise chemical ratios—even a 5% deviation from the manufacturer’s ratio causes incomplete curing (sticky parts) or brittleness.

Key Tasks:

  1. Resin & Hardener Ratio:
    • Use an electronic scale (not volume measurement) to weigh resin (Part A) and hardener (Part B). Common ratios:
      • Epoxy: 1:1 or 2:1 (by weight—check manufacturer specs, e.g., ArtResin 1:1, West System 2:1).
      • Polyurethane: 1:1 or 10:1 (e.g., Smooth-On Ecoflex 1:1, Castaldo polyurethane 10:1).
    • Example: For 100g of epoxy resin (1:1 ratio), weigh 50g Part A + 50g Part B.
  1. Mixing Process:
    • Pour Part A into a clean cup first, then add Part B (avoids hardener residue in the cup).
    • Stir slowly (300–500 RPM if using an electric mixer) for 3–5 minutes—scrape the cup bottom and sides to ensure no unmixed resin remains (unmixed resin causes soft spots in the cured part).
    • Avoid rapid stirring—this introduces air bubbles (we’ll fix this in Step 4, but minimizing bubbles here saves time).
  1. Adding Color/Additives:
    • Coloring: Add resin dye (0.5–2% of total resin weight, e.g., alcohol-based dye for epoxy) after mixing Part A and B. Stir for 1–2 minutes until uniform—too much dye weakens the resin.
    • Fillers: For strength (e.g., industrial parts), add glass fiber (5–15% by weight) or talc (10–20%)—mix at low speed to avoid clumping.
    • Glitter/Inlays: Add decorative elements (e.g., glitter, dried flowers) after defoaming (Step 4) to prevent them from sinking.

Critical Check:

Let mixed resin sit for 1–2 minutes—if it separates (Part A and B layer), remix for 1 more minute.

Step 4: Defoaming (Eliminate Bubbles for Perfect Surfaces)

Bubbles are the most common defect in resin casting—even small bubbles (0.1mm) ruin transparent parts or structural integrity. Use these methods based on your needs:

A. Static Defoaming (For Small Batches/Crafts)

  • Process: Let mixed resin sit in a cup for 5–10 minutes at 20–25℃. Bubbles rise to the surface due to buoyancy.
  • Tips: Tap the cup gently (2–3 times) on a hard surface—releases trapped air from the cup bottom. Pop surface bubbles with a toothpick or heat gun (low 档，15–20cm distance).

B. Vacuum Defoaming (For Industrial/High-Precision Parts)

  • Equipment: Tabletop vacuum chamber (5–10L, e.g., BestEquip 5L Vacuum Degassing Chamber) with a vacuum pump (0.5 HP, capable of -0.095 MPa).
  • Process:
    1. Pour mixed resin into a vacuum-safe cup (fill to 1/3 capacity—resin expands when vacuumed).
    1. Place the cup in the chamber, close the lid, and turn on the pump.
    1. Maintain -0.095 MPa for 2–3 minutes—bubbles expand and burst (resin may foam, but it settles as air is removed).
    1. Release the vacuum slowly (over 30 seconds)—prevents resin from splashing.

C. Heat-Assisted Defoaming (For Viscous Resins)

  • Use Case: Resins with high viscosity (>800 cP, e.g., some epoxy blends) that trap bubbles.
  • Process: Heat the mixed resin to 30–35℃ (using a water bath or heat mat) before defoaming—lowers viscosity, making bubbles easier to release.

Common Mistake:

Vacuuming for too long (>5 minutes)—resin may start curing prematurely due to exotherm.

Step 5: Resin Pouring (Control Flow to Avoid Air Traps)

Pouring technique directly impacts bubble formation and cavity filling—focus on slow, steady flow to ensure resin reaches all areas of the mold.

Key Tasks:

  1. Mold Positioning:
    • Place the mold on a level surface (use a spirit level to confirm—±0.5° deviation causes uneven resin distribution).
    • For complex molds with deep cavities (e.g., model engine parts), tilt the mold at 15–30°—lets resin flow into narrow gaps first.
  1. Pouring Method:
    • Slow Pour: Tilt the resin cup and pour along the mold’s edge (not directly into the cavity) at a speed of 5–10ml/s. This avoids splashing (which introduces bubbles) and lets air escape ahead of the resin.
    • flame diversion troughUse: For molds with small openings (e.g., jewelry molds), insert a small funnel or plastic tube into the cavity—guides resin to the bottom, preventing air traps.
    • Layered Pouring (For Thick Parts): If the part is >10mm thick (e.g., a resin tabletop), pour in 2–3 layers (each 3–5mm thick) with 1–2 hours of curing between layers. This prevents excessive exotherm (thick resin overheats and cracks).
  1. Cavity Filling:
    • Fill the mold to 1–2mm above the desired part height—resin shrinks 1–3% during curing (check manufacturer specs: epoxy shrinks ~1.5%, polyurethane ~2.5%).
    • After pouring, tap the mold gently (2–3 times) on the surface—releases air trapped in corners or undercuts.

Critical Check:

Inspect the mold from all angles—ensure no air pockets remain (look for clear, bubble-free resin in all cavities).

Step 6: Curing (Regulate Temperature & Time for Full Hardness)

Curing is the final chemical reaction that transforms liquid resin into a solid part—rushing this stage causes soft, brittle, or deformed parts.

Key Parameters by Resin Type:

Resin Type
Curing Temperature
Full Cure Time
Post-Curing (Optional)
Epoxy
20–25℃ (room temp)
24–48 hours
60–80℃ for 2–4 hours (boosts hardness by 10–15%)
Polyurethane
20–25℃
4–24 hours
Not recommended (may cause discoloration)
Unsaturated Polyester
20–25℃
1–6 hours
40–50℃ for 1 hour (reduces residual odor)

Curing Best Practices:

  • Avoid Disturbance: Do not move or touch the mold during curing—even slight movement causes surface ripples.
  • Exotherm Control: For large parts (>500ml resin volume), place the mold in a water bath (20–22℃) to dissipate heat—prevents overheating (epoxy can reach 80℃+ in large batches, causing cracking).
  • Cure Check: After the recommended time, press the resin surface gently with a gloved finger—no indentation or stickiness means full curing.

Common Mistake:

Demolding too early (e.g., epoxy before 24 hours)—parts deform or stick to the mold.

Step 7: Demolding & Post-Processing (Finish to Specification)

Proper demolding preserves part detail, while post-processing fixes minor defects and improves aesthetics.

A. Demolding:

  • Silicone Molds:
    1. Gently stretch the mold from the edges (avoid pulling the part directly).
    1. For complex parts with undercuts, peel the mold in small sections (e.g., start with the part’s least detailed area) to avoid tearing.
  • Metal/Plastic Molds:
    1. Remove clamps (if using two-piece molds) and separate the mold halves using a plastic pry tool (avoids scratching the part).
    1. For tight-fitting parts, tap the mold lightly with a rubber mallet—loosens the part without damage.

B. Post-Processing:

  1. Trimming: Use a utility knife (for soft resin) or a CNC router (for rigid epoxy) to remove excess resin (flash) from mold seams.
  1. Sanding: Sand the part with abrasive paper (400–2000 grit) to smooth rough edges—start with 400 grit for flash removal, then 1200–2000 grit for a polished finish.
  1. Polishing: For transparent parts (e.g., epoxy coasters), use a polishing compound (e.g., Turtle Wax) and a buffing wheel (1500 RPM) to achieve a mirror finish.
  1. Repair: Fill small scratches or bubbles with a thin layer of resin (mixed at 1:1 ratio) and cure for 2–4 hours.

Critical Check:

Inspect the part for defects (bubbles, cracks, incomplete filling) — if defects are >5%, repeat the process with mold adjustments (e.g., better 消泡，slower pouring).

Key Quality Control (QC) Nodes (Must Not Skip)

To ensure consistent part quality, perform QC at these 4 stages:
  1. After Mixing: Check resin for uniformity (no streaks) — unmixed resin = soft spots.
  1. After Defoaming: Inspect resin under bright light—no visible bubbles (>0.1mm) for high-precision parts.
  1. After Curing: Test hardness with a Shore hardness tester (epoxy ≥80D, polyurethane ≥70A).
  1. After Post-Processing: Use a caliper to check dimensions—tolerance should be ±0.1mm for industrial parts, ±0.5mm for crafts.

Application-Specific Tips

  • Transparent Parts (e.g., Resin Art):
    • Use high-transparency epoxy (e.g., Clear Cast Epoxy) and avoid dust during curing—cover the mold with a glass plate.
    • Polish with 2000–5000 grit sandpaper before buffing for maximum clarity.
  • Industrial Parts (e.g., Electrical Insulators):
    • Use flame-retardant epoxy (UL94 V0 rated) and post-cure at 80℃ for 4 hours to boost heat resistance.
    • Test for electrical insulation (use a megohmmeter—resistance ≥10¹² Ω).
  • Flexible Parts (e.g., Toy Wheels):
    • Choose polyurethane resin with 邵氏 A 50–70 hardness and avoid post-curing (preserves flexibility).

Final Takeaway

Pouring resin into a mold is a blend of precision and patience—success depends on:
  • Matching mold/resin to your application (silicone for crafts, metal for industrial parts).
  • Controlling bubbles (vacuum defoaming for high requirement，static defoaming for small batches).
  • Following curing times/temperatures (never rush demolding).
For beginners, start with small, simple molds (e.g., coaster molds) and epoxy resin (most forgiving). For industrial production, invest in vacuum defoaming equipment and metal molds—they reduce defects and lower long-term costs.
Got a specific resin project (e.g., “epoxy resin model car parts” or “polyurethane gaskets”)? Share your specs in the comments, and we’ll help you tailor the process!

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