For previous article, please refer to PART 02: Basic Structure of an Injection Molding Machine: A Comprehensive Understanding of Three Maj.
In injection molding process, setting a reasonable clamping force is a prerequisite for producing finished products. Too low a force will result in flash and mold expansion; too high a force will damage mold, affect venting, and in severe cases, even cause mold deformation. This article will systematically explain knowledge and calculation methods related to clamping force.

 

Clamping force refers to maximum clamping force applied to mold by injection molding machine during injection and holding stages to resist enormous outward expansion force generated by molten plastic within mold cavity.
You can think of it as a tug-of-war: One side is molten plastic injected into mold cavity under high pressure, desperately trying to push mold open (creating flash).
The other side is injection molding machine's clamping system, which must provide sufficient pulling force to ensure mold remains perfectly still and tightly closed during process.
Outcome of this "race" directly determines whether product has flash, whether dimensions are stable, and whether mold is safe. Scientifically calculating clamping force is like devising a "just right winning strategy" for this race.

Required clamping force (tons, T) = Cavity projected area (cm²) * Average cavity pressure (kgf/cm²) ÷ 1000. The first key variable in formula is "cavity projected area," which is foundation of all calculations.
1. What is projected area? Projected area refers to the total area of mold cavity (including product, runner system, and all other spaces that need to be filled) projected onto a plane perpendicular to direction of clamping force (i.e., mold opening and closing direction).
Simply put, imagine a beam of parallel light shining from stationary mold platen of an injection molding machine onto moving mold platen; area of shadow cast by melt-filled area on moving mold platen is projected area.
2. How to accurately calculate projected area?
Calculation must include all parts that generate outward expansion forces during injection:
A. Projected area of product itself
For regular geometries: Rectangle area = length * width; Circle area = π * radius².
For complex parts: Measure directly using 2D drawings in CAD software, or approximately break it down into multiple regular shapes and sum measurements.
B. Projected area of runner system (gating system)
Cold runners (including main runners, branch runners, and cold slug wells): All must be included in calculation.
Hot runner system: Typically, only tiny area generated by sealing contact surface of hot runner tip is calculated. Runner plate portion is fixed by heating system and does not generate mold-supporting force. This is a common misconception; hot runners do not significantly increase projected area.
C. Area generated by side core-pulling (slider) mechanism
This is the most easily overlooked critical part!
When melt is injected into a cavity with a slider (slider), pressure acts on slider's inclined surface (locking surface), generating a force that causes slider to retract. This force is transmitted through slider to mold plate, generating a component force perpendicular to mold opening and closing direction, thus requiring additional clamping force to balance it.
Calculation method: Projected component of area of slider head forming portion in direction of its inclined surface normal needs to be calculated. Usually, mold design engineer will provide this data. Empirically, complex slider structures can increase required clamping force by 10%-25%.
D. Number of Cavities
Total Projected Area = (Projected Area of a Single Cavity + Corresponding Runner Area) * Number of Cavities
Note: Runners in multi-cavity molds are usually shared and need to be allocated reasonably.
【Practical Tips】 The most reliable method is to directly request the total projected area of mold from mold designer. If calculating it yourself, be sure to use 2D assembly drawing of mold and measure it on parting line view.

If projected area is "size of target," then cavity pressure is "power of arrow." This is the most variable and experience-dependent part of calculation.
Cavity pressure refers to average pressure exerted by plastic melt on the surface of mold cavity during filling and holding. It is not a fixed value, but a dynamic result influenced by seven factors:
Material Flowability:
High flowability materials (such as LDPE, PP) have low filling resistance and require low pressure.
Low flowability materials (such as PC, high viscosity PA) are difficult to fill and require extremely high pressure.
Experience-based reference ranges: Easy-flow materials (PE, PP, PS): 200 ~ 350 kgf/cm²
Medium-flow materials (ABS, SAN): 300 ~ 450 kgf/cm²
Difficult-flow materials (PC, PMMA, unplasticized PVC): 400 ~ 550 kgf/cm² or higher
Engineering plastics (PA, POM, PBT): 350 ~ 500 kgf/cm²
Product wall thickness: Thinner walls result in greater flow resistance, requiring higher injection pressure for filling, thus potentially higher cavity pressure.
Thicker walls, while easier to fill, require higher holding pressure and longer holding time to prevent shrinkage, resulting in a relatively high overall cavity pressure.
Flow length ratio (flow length/wall thickness): A larger flow length ratio results in a longer and more tortuous melt flow path, greater pressure loss, higher inlet pressure required to fill cavity.
Gate type and size: Point gate, needle valve gate: High pressure loss, lower cavity pressure. Direct gate and fan gate: Offer better pressure transmission, resulting in cavity pressure closer to injection pressure.
An excessively small gate size can become a bottleneck, leading to extremely high localized pressure losses.
Mold temperature: Higher mold temperatures enhance melt flowability, reducing filling pressure.
Injection speed: High-speed injection generates higher viscous shear heat and reduces melt viscosity, potentially benefiting filling, but may also cause sudden increases in localized pressure due to poor venting.
Holding pressure and time: Holding pressure stage compensates for shrinkage; its pressure acts directly and continuously on cavity, forming a major component of "average pressure." High holding pressure significantly increases average cavity pressure.
Theoretical estimation: Simulation using mold flow analysis software (such as Moldflow) can directly obtain accurate cavity pressure distribution and peak values.
Empirical Formula (Most Commonly Used): Average Cavity Pressure (P) = Injection Pressure (P_inj) * Pressure Loss Coefficient (C)
(Where pressure loss coefficient C is typically between 0.3 and 0.6. For products with good flowability, moderate wall thickness, and good gates, 0.3-0.4 can be used; for products with poor flowability, thin walls, and long flow paths, 0.5-0.6 or even higher can be used.)
Experimental Measurement Method: Installing a cavity pressure sensor inside mold is the most accurate method, allowing real-time monitoring of pressure curve, but it is more expensive.

 

Given Conditions:
1. Material: ABS (medium flowability)
2. Product Dimensions: 100mm * 60mm, average wall thickness 2mm
3. Mold: 1-out-2-outlet, cold runner, no slider
4. Projected area of a single product (including small holes within product area): 100mm * 60mm = 60 cm²
5. Estimated projected area of runner in a single cavity: 8 cm²
6. Number of cavities: 2
Calculation Steps: Calculate the total projected area (A)
A = (Area of a single product + Area of runner in a single cavity) * Number of cavities = (60 + 8) * 2 = 136 cm²
Estimate average cavity pressure (P): ABS is a medium flowability material with moderate wall thickness. Select a pressure loss coefficient C = 0.4.
Assume injection pressure set on injection molding machine is 1200 kgf/cm² (approximately 118MPa). Therefore: P = 1200 * 0.4 = 480 kgf/cm². Calculation of theoretically required clamping force (F)
F = A * P / 1000 = 136 * 480 / 1000 ≈ 65.3 T (tons)
Considering a safety factor. To prevent fluctuations, a safety margin of 10%-20% is usually added.
65.3 T * 1.15 ≈ 75 T
Conclusion: To ensure stable production of this product, an injection molding machine with a clamping force of not less than 80 tons should be selected, and actual clamping force should be set at around 75 tons as starting point for process debugging.

Insufficient clamping force (too small)
Product defects: Burrs (flash) are generated, especially at parting surface, slide block mating area, and around ejector pin holes.
Mold damage: Long-term low-pressure operation may cause parting surface to be micro-eroded by high-pressure melt, resulting in "cratering" and permanent damage. Dimensional Instability: Slight mold expansion causes fluctuations in product dimensions (especially thickness).
Excessive Clamping Force
Energy Waste: Clamping force is one of main sources of energy consumption in injection molding machines; excessive clamping force directly leads to soaring electricity costs.
Machine and Mold Damage:
1. Causes excessive stretching of tie rods, leading to long-term plastic deformation or fatigue fracture.
2. Causes concave deformation of moving and fixed mold plates, affecting parallelism.
3. Accelerates wear of mold guide pillars and bushings, even damaging parting surface.
4. Affects mold opening: Excessive clamping force may result in huge tension during initial mold opening, potentially damaging or sticking product to mold.

 

Start with Theoretical Value: Calculate a value using above method as a starting point.
Gradual Reduction Method (Finding Critical Point): While ensuring product quality (no flash, dimensions within specifications), gradually and slowly reduce clamping force.
When extremely slight flash begins to appear on product, record clamping force value F_min at this point.
Set actual clamping force to F_min * (1.1 ~ 1.15), leaving a safety margin of 10%-15%. This is optimal production point for energy saving and equipment protection.
Monitoring and Recording: Record optimized clamping force for each mold in process card to achieve standardization.