Large-size plastic components such as automobile bumpers, large home appliance housings and bulk turnover boxes feature long melt flow path, uneven cooling and high demolding resistance, putting higher requirements on mold rigidity, gating layout, cooling design and ejection system than small plastic parts. Improper mold type selection easily triggers incomplete filling, severe weld line, irregular warpage and excessive flash during molding. Combining abundant practical experience of China’s injection molding industry, this article sorts mold classification and targeted selection plans based on product batch output, appearance requirement and raw material characteristics.
1. Core General Selection Principles for Large-Size Product MoldsFour core design standards guide mold configuration: enough structural rigidity against high clamping pressure, balanced multi-point feeding for uniform melt filling, zoned independent cooling to shrink uneven deformation, and composite ejection structure for smooth demolding. Oversized projection area leads to high mold opening pressure; thus reinforced frame and support pillar are essential to prevent mold expansion and flash. Long melt flow distance requires distributed gate layout to reduce pressure loss during filling, while inconsistent wall thickness demands partition cooling design for synchronous shrinkage. Integrated single-block cavity is abandoned for large mold due to high heat-treatment deformation risk, replaced by split insert combined structure.
2. Two-Plate and Three-Plate Mold Selection by Appearance Standard
Two-plate large-sprue mold owns simple structure and low manufacturing cost, fit for low-volume trial production and non-appearance internal structural parts, but suffers big leftover gate mark and limited feeding position resulting in far-end filling shortage. Three-plate mold equipped with pinpoint gate realizes automatic gate breaking during mold opening without manual trimming, and multi-point gating shortens effective melt flow length to improve filling balance, becoming mainstream choice for mass-produced high-standard appearance large plastic parts. Production cost gap between two structures is offset by later automatic production efficiency improvement of three-plate mold for long-term bulk orders.
3. Hot Runner and Cold Runner Configuration Based on Production BatchCold runner structure is economical for small-batch trial production to cut initial mold investment despite more sprue waste and unbalanced feeding pressure. Mass production over 30,000 shots with high flatness requirement adopts valve-gate hot runner mold, which keeps molten plastic constant temperature inside manifold without cold waste and controls sequential gate opening to adjust melt flow direction, greatly improving product warpage and prominent weld line defects. Open-type hot runner is selected for ordinary large-size products to balance cost and molding performance compared with expensive needle valve hot runner system.
4. Split Insert Cavity and Reinforced Mold Frame DesignLarge mold fully adopts split splicing cavity instead of monoblock structure. Dividing whole cavity into independent inserts reduces overall thermal deformation during heat treatment and facilitates single-piece replacement of locally worn position instead of entire mold scrapping, lowering later maintenance expense drastically. Mold frame uses thickened heavy-duty standard plate plus distributed support posts and compression blocks to enhance whole mold rigidity and resist expansion under long-term high clamping force.
5. Partition Cooling and Composite Ejection System Matching
Conformal dense cooling channel and inclined drilling waterway replace ordinary straight hole design; high heat accumulation position adds copper insert for accelerated heat conduction, and ultra-large products adopt separated temperature-controlled cooling circuits to adjust shrinkage rate in different zones independently. Composite ejection combining ejector pin, ejector sleeve and stripper plate is used to disperse demolding stress; flat large-area surface uses whole stripper ejection to avoid ejection mark, and deep-cavity oversized parts add auxiliary air ejection to prevent pulling crack and surface scratch during product stripping.
