Plastic Injection Mold Dimension Calculation Methods
Injection mold dimension calculation is a core process of mold design, which directly determines the dimensional accuracy and mass production qualification rate of plastic products. Plastic products shrink after cooling and forming. Therefore, mold dimensions cannot be directly copied from product drawings. Scientific formulas are required to compensate for shrinkage, combined with tolerance, structure and production conditions for correction. This article briefly introduces the standard calculation logic, core formulas and practical key points of injection mold dimensions, providing references for the design and manufacturing of various injection molds.
1. Core Principle of Dimension Calculation
Plastic melt undergoes volume shrinkage during high-temperature injection molding and normal-temperature cooling. If the mold is manufactured according to the nominal product size, the final product size will be generally smaller. The core logic of mold dimension calculation is to enlarge the mold forming size in advance based on the plastic shrinkage rate to offset the cooling shrinkage of products, and make corrections in combination with product tolerance, machining error and mold wear to ensure the finished product dimensions are within the qualified range.
2. Core Parameter: Plastic Shrinkage Rate
The calculated shrinkage rate is uniformly adopted in mold design and serves as the basic basis for dimension verification. The formula is as follows:
Calculated Shrinkage Rate (S) = (Mold Dimension at Room Temperature – Product Dimension at Room Temperature) / Product Dimension at Room Temperature × 100%
Reference shrinkage rates of common plastics:
- Low shrinkage (precision parts): ABS, PC, PMMA, 0.3%~0.8%
- Medium shrinkage (general parts): PP, PE, PS, 1.0%~2.0%
- High shrinkage (structural plastics): POM, PA6, PA66, 2.0%~3.5%
- Glass fiber reinforced modified materials: 0.2%~1.0%
Value Selection Principles: Select a slightly lower value for precision appearance parts to reserve fine-tuning space; select a slightly higher value for thick-walled and large-size parts. For modified materials and special working conditions, prioritize the parameters provided by material manufacturers or test mold data.
3. Standard Calculation Formulas for Core Mold Dimensions
Mold dimensions are divided into three categories: cavity dimension, core dimension and center distance dimension, with corresponding independent calculation formulas. The mold manufacturing tolerance (δ) is uniformly set to 1/3~1/4 of the product tolerance.
3.1 Cavity Dimension (Product External Dimension)
Applied to external dimensions such as product shells, outer contours, length, width and outer diameters, adapting to the negative tolerance rule of product external sizes.
Formula: A = (A0 + A0×S - 3/4Δ) ± δ
Parameters: A = Mold cavity dimension; A0 = Nominal product dimension; S = Plastic shrinkage rate; Δ = Product dimensional tolerance; δ = Mold manufacturing tolerance.
3.2 Core Dimension (Product Inner Hole/Inner Groove Dimension)
Used for internal dimensions such as product holes, clamping grooves and inner cavities, adapting to the positive tolerance rule of product internal sizes to compensate for the shrinkage and reduction of inner holes after molding.
Formula: B = (B0 + B0×S + 3/4Δ) ± δ
The parameter definitions are consistent with those of cavity dimensions.
3.3 Center Distance Dimension (Hole and Pillar Spacing)
The center distance features uniform shrinkage and symmetrical tolerance, mostly used for the design of screw hole spacing and positioning pillar spacing.
Formula: C = (C0 + C0×S) ± δ/2
4. Practical Dimension Correction Key Points
- Structural Correction: Appropriately increase the shrinkage rate for thick-walled products, and finely adjust local areas such as thin walls and rib glue accumulation positions to improve shrinkage and deformation.
- Wear Correction: Reserve a wear allowance of 0.02~0.05mm for molds with an annual output of more than 100,000 pieces to extend mold service life.
- Process Correction: Products formed under high pressure holding and high mold temperature have lower shrinkage, allowing a slight reduction of the shrinkage rate value.
- Precision Part Correction: Avoid applying standard shrinkage rates directly for high-precision products; revise mold dimensions reversely based on actual shrinkage data from test molding.
5. Common Calculation Errors
- Adopting a single fixed shrinkage rate for the entire mold without zoning adjustment according to wall thickness and structure;
- Only calculating theoretical dimensions while ignoring mold machining tolerance with no fine-tuning allowance;
- Mixing up cavity and core calculation formulas, resulting in unqualified internal and external dimensions and assembly failure;
- Applying shrinkage rates of raw materials directly without adapting to glass fiber reinforced, flame retardant and other modified plastics.
6. Conclusion
The core of injection mold dimension calculation is taking the shrinkage rate as the basis, adopting differentiated calculations for cavity, core and center distance dimensions, and making corrections combined with product structure, tolerance and mass production conditions. Accurate dimension calculation is the key to ensuring product precision, reducing defect rates and realizing stable mass production. In design practice, it is necessary to combine theoretical formulas with actual test molding effects to continuously optimize dimensional parameters.
