I. Causes of Cracks

Cracks occurring in furan resin self-set sand during the self-hardening process result from the combined effects of resin curing characteristics, sand properties, environmental conditions and process operations. Only comprehensive analysis and precise optimization of process parameters can effectively eliminate cracking defects.

1. Influence of Reaction Characteristics between Resin and Hardener

   When furan self-set resin, hardener and sand are mixed, an exothermic crosslinking reaction takes place to harden the mould. If heat release from the reaction is intense and concentrated, a significant temperature gradient forms inside the mould. The outer layer hardens rapidly to form a rigid shell, while the inner section continues reacting and expands due to heat accumulation. Cracks emerge once the internal expansion stress exceeds the tensile strength limit of the outer rigid shell.

   For large moulds used for heavy-section castings, higher dosages of resin and hardener lead to accumulated reaction heat and a substantial temperature difference between inner and outer layers. The outer layer bears tensile stress and is prone to cracking.

2. Influence of Sand Properties and Gradation

   Sand particle size distribution, grain shape, clay content and fines content greatly affect crack tendency. Improper gradation with excessive fine sand increases the bonding area for resin, resulting in higher curing shrinkage and stress concentration points that initiate cracks.

   Compared with rounded sand grains, angular sand grains create greater frictional resistance during resin shrinkage, restrict free deformation of the sand mass and raise cracking risks. Excessive clay and fines absorb part of the resin, reducing effective bonding strength between sand grains. Additional volume changes during dehydration and phase transformation, coupled with resin shrinkage stress, further induce cracks.

3. Influence of Process Operations

• Insufficient mixing time prevents uniform distribution of resin and hardener around sand grains. Areas with concentrated resin feature large shrinkage, while regions with insufficient resin have low strength. Uneven internal stress distribution triggers cracks.

• Uneven compaction is another adverse factor. Highly compacted zones possess high rigidity and cannot relieve internal stress through minor deformation, whereas loosely compacted areas tend to deform. Interaction between these zones creates conditions for crack formation.

• Improper stripping time is critical. Early stripping takes place before the mould develops adequate strength; microcracks form under external stripping force and self-weight, then propagate in subsequent processes.

• When applying fast-drying coatings, inappropriate coating Baumé value and mismatched solvents prolong coating application time. Excessive heating after ignition causes stress concentration and cracking.

  4. Influence of Environmental Conditions

      Ambient temperature and humidity act as regulators for the self-setting reaction.

• High temperature accelerates curing, shortens the working time of sand mixture, and hardens the mould rapidly. Internal stress cannot dissipate sufficiently, leading to cracking.

• Low temperature slows down the reaction and results in incomplete curing. The mould gains strength slowly, and slight external force during handling may cause cracks.

• High humidity enables sand to absorb free moisture. During curing, water vaporizes and generates steam pressure that breaks resin bonding bridges. Stress concentration around pores further develops into cracks.

II. Preventive Process Measures for Cracks

1. Control of Curing Rate

   Avoid excessively fast curing. Determine reasonable working time and stripping time of furan resin sand based on actual production conditions. At low ambient temperature, preheat metal patterns and chills for mould and core making to achieve consistent curing speed across all sections and reduce temperature differences.

2. Optimization of Mould Structural Design

   Adopt hollow structures for heavy-section moulds to lower material consumption and prevent deformation and cracking. Optimize the structure and rigidity of core frames; cast iron frames are preferred. If steel frames are adopted, sufficient rigidity must be guaranteed to avoid elastic stress.

   Place large and complex moulds on dedicated pallets to prevent deformation under self-weight.

3. Optimization of Moulding Materials

   Adopt low-shrinkage thermoplastic furan self-set resin. Control resin addition rate, generally no more than 1% of sand mass. Select resins with low coking tendency at elevated temperatures.

   Deploy high-precision continuous mixing equipment with accurate dosing. Follow specified mixing duration to achieve uniform blending of all components. Use qualified sulfonic acid hardener with stable free acid content.

4.  Standardization of Process Operations

   Complete sand filling and compaction within the working time of mixed sand, and maintain uniform mould compaction.

   Carry out coating application only after sufficient mould hardening. For alcohol-based coatings, apply quickly and ignite promptly to shorten thermal impact from flame on the mould. Use manufacturer-specified solvents to adjust coating Baumé value and avoid overheating and excessive shrinkage.

   Turn on used sand dedusting system during shakeout. Monitor fines content and acid demand value of reclaimed sand to sustain stable sand quality.

XINDA furan self-setting resin is a thermosetting resin containing furan rings, which hardens at room temperature via acid catalysis. It features energy saving, low carbon emission and low environmental pollution during application. Widely used for manufacturing moulds and cores for single-piece, small-batch, medium, large and extra-large castings. The resin delivers stable curing performance, ensures reliable mould quality and helps reduce casting defects.