Casting resin embeds and protects a wide range of materials, but some inclusions ruin the casting or fail over time. Knowing what to keep out of a mold prevents wasted material, scrapped parts, and safety problems. The main issues are moisture, chemical reactivity, trapped air, and hazardous content.

Moisture Is the Primary Problem

Epoxy and polyurethane casting resins are sensitive to water during cure. Even small amounts cause real defects. Water reacts with the hardener and blocks cross-linking, leaving a sticky or gummy surface that never fully hardens. It can also generate gas that produces bubbles, cloudiness, or a milky cast, and it creates dimples and craters on the surface.

Materials to keep out because of moisture content include fresh flowers, leaves, and greenery, which also rot inside the casting; fresh fruit, vegetables, and undried wood, especially green wood; and unsealed paper or cardboard that absorbs ambient humidity. Wood and concrete can be cast successfully, but only after thorough drying and sealing with a thin coat of resin or a compatible sealer.

Materials That React or Degrade

Some inclusions react with the resin or break down inside it over time:

• Low-surface-energy plastics. Polyethylene, polypropylene, silicone, and PTFE resist adhesion. Resin will not bond to them, which is useful for molds but poor for embedding. Some vinyls react with resin and turn soft or sticky.
• Unsealed paper and prints. Standard paper and inkjet prints hold moisture, can go translucent, and can fade. If a print must be embedded, dry and seal it fully first.
• Volatile or reactive items. Materials that off-gas during the exothermic cure produce large bubbles or cracks. Water-based paints and dyes cause cloudiness and can inhibit cure; use only pigments made for resin. Strongly acidic or alkaline substances interfere with resin chemistry.
• Food items. Even dried foods can retain moisture or degrade, and are generally not stable long-term without specialized preparation.
• Sealed air-filled items. Hollow beads, unsealed pinecones, and similar items trap air that expands during the exothermic cure, cracking the item or releasing bubbles. Drill a vent hole or pre-fill with resin.

Differential Expansion of Embedded Objects

A rigid metal or glass object cast into resin expands at a different rate than the resin around it. Every temperature change strains that interface, and over enough cycles it can craze or delaminate. Our guide to how CTE mismatch causes adhesive and bond failure explains the mechanism, which matters for any functional encapsulation exposed to thermal cycling. Choosing a slightly tougher, lower-modulus resin reduces the stress.

Preparing Marginal Inclusions

Some materials sit on the boundary and can be cast successfully with preparation. Wood must be kiln-dried or oven-dried to below about 10 percent moisture and then sealed with a thin resin coat or a spray fixative to stop grain outgassing. Dried botanicals should be pressed and fully dehydrated, then sealed, and are best placed in a thin flood layer rather than a deep pour so any residual moisture has less resin to cloud. Porous stone and concrete need drying and a penetrating seal. Metal and glass should be degreased with isopropyl alcohol immediately before the pour so no oil film interferes with the resin contacting the surface.

Signs of an Incompatible Inclusion

Catch problems on a test pour rather than a production casting. A halo of tiny bubbles forming around an object minutes after the pour indicates trapped moisture or air escaping. A cloudy or milky zone next to an inclusion points to a chemical reaction with the surface or a water-based coating on it. A soft, sticky region around an item signals cure inhibition, often from sulfur-containing rubbers, certain modeling clays, or amine-bearing residues. Any of these means the inclusion needs sealing, substitution, or exclusion.

Safety Considerations

Avoid embedding highly flammable materials, since the liquid components and the exothermic cure carry fire risk. Do not embed toxic substances that could leach into the resin or become exposed if the casting breaks.

Practical Guidance

• Dry organic material completely before embedding, using a dehydrator or low oven.
• Seal porous items with a thin resin coat or spray fixative before the main pour.
• Test unknown materials with a small casting before a production run.
• Use only resin-specific colorants, not water-based paints or dyes.
• Research unusual inclusions and consult your resin supplier when unsure.

Incure casting resins are formulated for predictable behavior with compatible inclusions, and each data sheet states the ideal conditions and known sensitivities. For clarity-critical encapsulation, the cure-chemistry trade-offs are covered in our comparison of UV adhesive versus epoxy for transparent bonding.

If you are unsure whether a specific material can be safely cast, Email Us with the details and intended service conditions.

The wrong inclusion turns a clear casting into a cloudy, cracked, or tacky reject. Dry and seal porous items, keep out reactive plastics and water-based colorants, vent hollow objects, and test anything unfamiliar. Contact Our Team to discuss your casting application.

Visit www.incurelab.com for more information.