Dimensional instability of plastic molded parts mainly manifests as mold shrinkage, post-molding warpage, thermal expansion deformation, and moisture-induced size drift. Proper selection of calcium carbonate (CaCO₃) can effectively reduce shrinkage rate, lower thermal expansion coefficient, relieve internal stress and minimize moisture absorption, so as to stabilize part size and improve batch consistency. Below is a systematic selection standard.
1. Select by CaCO₃ Crystal Type
Ground Calcium Carbonate (GCC) – Preferred Choice
- Rhombohedral crystal structure, compact particle morphology
- Low oil absorption, low specific surface area, low moisture adsorption
- Uniform shrinkage compensation, minimal warpage, excellent dimensional stability
- Suitable for most injection molding and extrusion precision parts
Precipitated Calcium Carbonate (PCC) – Restricted Use
- Needle/chain-like crystal shape, large specific surface area and high oil absorption
- Easy to cause uneven resin shrinkage and internal stress
- Prone to warpage and dimensional deviation; only applicable for low-precision non-structural parts
Rule: Prioritize modified GCC for high dimensional stability requirements; avoid ordinary needle-shaped PCC.
2. Select by Particle Size & Particle Size Distribution
- 1250–3000 mesh ultra-fine GCC: Uniform filling in resin, greatly reduces molding shrinkage and thermal deformation, ideal for precision electronic and thin-wall molded parts.
- 800–1000 mesh medium-fine GCC: Balanced cost and dimensional performance, suitable for general automotive and household molded parts.
- Below 600 mesh coarse GCC: Poor dispersion, uneven local shrinkage, obvious dimensional fluctuation; not recommended for precision products.
- Key requirement: Narrow particle size distribution, no oversize coarse particles, to ensure consistent melt flow and uniform shrinkage.
3. Select by Surface Modification (Core Factor)
Stearic Acid Coated CaCO₃
- Forms hydrophobic surface, greatly reduces moisture absorption
- Improves compatibility with polyolefin, PP, PE, ABS; reduces internal stress
- Effectively inhibits post-molding size change caused by moisture and stress relaxation
Titanate / Aluminate Coupling Agent Coated CaCO₃
- Higher interfacial bonding with engineering plastics (PA, PBT, PC)
- Lower linear thermal expansion coefficient, stable size under high temperature molding and service conditions
- Best for high-temperature resistant precision structural parts
Uncoated Raw CaCO₃ – Not Recommended
- Strong hydrophilicity, easy to absorb ambient moisture
- Poor resin compatibility, severe internal stress, obvious warpage and dimensional drift over time
4. Purity & Impurity Control
- CaCO₃ purity ≥ 98%: Low content of MgO, SiO₂, Fe₂O₃ impurities; stable thermal performance and consistent shrinkage behavior batch by batch.
- Low acid-insoluble substances and low heavy metal content: Avoid molecular chain degradation during molding, prevent abnormal shrinkage and size deviation.
- Ignore excessive high whiteness; prioritize purity and particle consistency for dimensional stability.
5. Strict Moisture Content Standard
- Target moisture content: ≤0.1% for precision molded parts.
- High residual moisture will cause micro-voids, hydrolysis and uneven post-molding shrinkage, leading to slow size deformation.
- Select factory pre-dried, vacuum sealed moisture-proof packaged CaCO₃.
6. Low Oil Absorption & Dispersion Performance
- Choose low oil absorption grade CaCO₃: Ensures good melt fluidity, complete mold filling, no local under-filling or over-filling.
- No agglomeration: Uniform dispersion in resin avoids local density difference which causes shrinkage inconsistency and dimensional error.
7. Application-Based Grade Matching
| Molded Part Type | Recommended CaCO₃ Grade |
|---|---|
| Precision electronic thin-wall parts | 2000–3000 mesh stearic acid coated high-purity GCC |
| Automotive structural & interior parts | 1500–2000 mesh modified GCC |
| General household plastic parts | 1000–1250 mesh standard coated GCC |
| Moisture-sensitive engineering plastic parts | Coupling agent coated GCC, moisture <0.08% |
8. Common Selection Mistakes to Avoid
- Choosing cheap uncoated CaCO₃ → serious moisture absorption and long-term warpage.
- Blindly using ultra-fine PCC → high oil absorption, large internal stress, poor dimensional stability.
- Ignoring moisture index → slow size drift after molding.
- Neglecting batch consistency → unstable dimensional repeatability in mass production.
Core Selection Summary
To improve dimensional stability of molded parts:
High-purity coated GCC + 1250–3000 mesh narrow particle size + low moisture ≤0.1% + low oil absorption is the optimal matching standard.