How to Fix Shrinkage, Air Bubbles and Warpage in Resin Parts

Alpha Model Tech Share Series · 2026 · All Scales
During mould making and casting, resin parts can suffer shrinkage, air bubbles and warpage. Here is how to diagnose each defect and repair it to a clean, level finish.

01 · Understanding Resin Casting Defects

• Shrinkage — the part shrinks in volume after cooling, causing undersized dimensions, sink marks on edges and increased fitting gaps.
• Air bubbles — air not expelled during pouring forms voids on the surface or inside the part.
• Warpage — after demolding, parts warp or twist from internal stress release and no longer sit flat.

02 · Fixing Shrinkage Issues

Common Signs of Shrinkage

• Sink marks appearing on part edges.
• Noticeably increased gaps at fitting surfaces.
• Screw holes shifted or reduced in diameter.

Repair Methods

Minor sink marks:

1. Lightly sand the area with 800-grit paper.
2. Fill the depression with a paste of 502 glue (cyanoacrylate) mixed with baby powder.
3. When fully dry, flatten with 800-grit, then finish with 2000-grit for a smooth surface.

Larger dimensional deviations:

1. Measure the original design dimensions and the actual deviation.
2. Apply thin coats of epoxy putty where buildup is needed, and sand to shape after curing.
3. For oversized fitting gaps, insert 0.1–0.2mm shims (plastic or paper) as compensation.

03 · Fixing Air Bubble Issues

Common Locations of Air Bubbles

Small pits on the part surface (pop bubbles) · internal voids in thick sections · fine detail areas and corners.

Repair Methods

Surface micro-bubbles:

1. Poke open the bubble hole with a toothpick or fine needle.
2. Fill with the 502 glue + baby powder mixture.
3. Sand smooth after drying.

Deeper voids:

1. Thin down the surrounding area with 800-grit to expose a clean surface.
2. Spray a thin coat of surface primer to reveal hidden bubbles underneath.
3. If found, continue filling and sanding; if not, fill flat with epoxy putty, then sand.

Internal large bubbles (affecting structural strength):

1. Saw or grind open the part to expose the bubble.
2. Fill with two-part epoxy resin (5-minute epoxy).
3. Clamp tight, allow to cure, then sand and polish to restore shape.

04 · Fixing Warpage Issues

Common Causes of Warpage

• Demolding too early, before the resin is fully cured.
• Uneven cooling at thick-thin junctions.
• External force during curing, or the mould not placed level.

Correction Methods

Minor warpage (slight edge lifting):

1. Soak the part in hot water (60–70°C) for 1–2 minutes to soften the resin.
2. While hot, press the part flat on a glass plate or flat board.
3. Place a weight on top and let it cool to room temperature before removing.

Moderate warpage (obvious twist but recoverable):

1. Prepare a flat board matching the part size.
2. Sand the concave side rough to improve bonding.
3. Apply a thin layer of epoxy or 502 glue and attach a 0.5–1mm reinforcement plate (plastic or carbon fiber).
4. Clamp tight and allow to fully cure.

Severe warpage (beyond correction): recommend re-casting — it cannot be restored to acceptable quality through repair.

05 · Chassis and Body Shell Reaming

Body shell chassis mounting holes and screw holes usually need reaming before installation. Alpha Model body shells have small original holes; directly driving screws may not fit or tighten properly.

1. Ream holes to 2.1mm so screws pass through smoothly and secure firmly.
2. First, use a small drill bit matching the original hole to remove burrs and chamfers.
3. Then step up gradually (1.5mm, 1.8mm, 2.1mm), reaming step by step.
4. Keep the drill perpendicular to the chassis plane to avoid tilted holes.
5. Test fit with a 2.1mm screw to confirm smooth insertion.

Note: confirm screw specifications before reaming. 2.1mm is the standard hole size for Alpha Model chassis.

06 · Tools and Materials Checklist

07 · Practical Tips

• Never fill too thick in one pass — apply in thin layers, letting each dry fully to avoid creating new bubbles and shrinkage.
• Always test sand after filling — hold the part sideways under a light to see high and low spots clearly.
• Internal bubbles are hardest to fix — tap lightly and listen; hollow areas sound duller.
• Warpage correction must be done while hot — once the part softens, press flat immediately; it will not work once cooled.
• When reaming chassis holes, do not skip steps — ream gradually to avoid the bit binding or hole-wall chipping.

Build. Detail. Inspire. — Alpha Model Technical Team