One of the most visually damaging failures in embedding projects is epoxy that pulls away from the edges of an embedded object, leaving a visible gap or void. It defeats the entire point of encapsulation and usually points to a mismatch between how the epoxy and the embedded item behave during cure — not contamination.

The Root Causes of Epoxy Gapping

Epoxy, like most thermoset plastics, shrinks slightly as it converts from liquid to solid during polymerization. When two materials with very different shrinkage rates are bonded together, the more rigid one — almost always the epoxy — pulls away from the less compatible surface.

Differential Curing Shrinkage

Standard laminating or coating epoxies typically shrink 2% to 3% during cure. In shallow coatings this is negligible, but across the volume of a deep encapsulation pour, that same percentage is magnified into real tension that pulls the epoxy inward. When the embedded object — glass, a polished stone, or metal — is completely rigid and non-porous, the epoxy cannot maintain a tight bond to the side walls under that tension, producing a visible gap, most often right at the top surface.

Poor Wetting on Slick Surfaces

If the embedded object has a very smooth or polished surface, liquid epoxy may not properly wet out or cling to it during the pour. Even a microscopic layer of wax, fingerprint oil, or release spray used to position the object can prevent the tight initial contact needed to resist the inevitable shrinkage that follows.

Outgassing from Porous Objects

Porous embeds — unsealed wood, bone, or natural stone — can outgas trapped air or moisture as the exothermic heat of the cure warms them. That outgassing forces a thin layer of air between the object and the liquid epoxy, creating a void line around the perimeter that cannot be filled once the epoxy gels.

Genuine Solutions for Seamless Encapsulation

Eliminating gaps requires a low-shrinkage resin, correct preparation of the embedded object, and a deliberate pouring sequence.

Choose a Low-Shrinkage Casting Resin

For encapsulation, use a specialty deep-pour or casting resin rather than a standard coating epoxy. These formulations are engineered for cure shrinkage often under 1%, which directly reduces the pulling force on the embedded object. Even with a low-shrinkage resin, pouring in thinner stages within the manufacturer’s recommended depth helps dissipate heat and further limits total shrinkage stress in any single batch.

Prepare the Embedded Object

For non-porous, slick objects like glass or polished stone, lightly etch the contact area with 80- to 120-grit sandpaper or a diamond wheel. This creates a mechanical “tooth” the epoxy can key into, which meaningfully improves resistance to shrinkage forces. Clean meticulously with acetone or isopropyl alcohol to remove fingerprint oils and polish residue, and avoid touching the cleaned surface with bare hands. Any object suspected of holding air or moisture — wood, concrete, porous stone — should receive a thin epoxy seal coat before the final pour, locking in trapped gas before the thicker layer heats up.

Application Technique

Before placing the object in the mold, mix a small, fresh batch and brush a thin pre-coat onto the object, particularly around edges that will remain visible. This ensures the epoxy wets out every micron of the surface before submersion, eliminating the air pockets that later show up as gaps. When placing the object into the final pour, watch closely for bubbles clinging underneath and gently agitate around the edges with a clean utensil to release trapped air.

Reading the Gap to Diagnose the Real Cause

Not every gap has the same origin, and the shape of the void is a useful diagnostic. A thin, uniform gap that traces the entire perimeter of a rigid object almost always points to shrinkage stress rather than contamination — the fix is a lower-shrinkage resin and staged pours, not more aggressive cleaning. A gap that appears only on one side, or that widens near the top of a porous object, is more consistent with localized outgassing and calls for a seal coat rather than a resin change. If cleaning and re-pouring the same combination keeps producing the identical failure pattern, the substrate-resin pairing itself is likely the problem rather than technique. Incure’s technical team reviews substrate compatibility questions like these regularly; Email Us with the object material and current resin to get a faster answer than another round of trial pours.

Encapsulation work that also spans dissimilar substrates — say, a metal insert embedded alongside a plastic housing — should also account for how CTE mismatch causes adhesive bond failure, since thermal cycling after the pour can reopen a gap that looked sealed at room temperature. Where the embedded object is glass or metal and precise, low-shrinkage bonding matters more than mass encapsulation, Uni-Weld UV Glass & Metal Bonder grades offer an alternative worth comparing for edge-bonding applications rather than full encapsulation.

For encapsulation projects with tight tolerances or unusual embedded materials, Contact Our Team before the pour to confirm resin selection and preparation steps.

Visit www.incurelab.com for more information.