Calculation Method of Injection Volume in Injection Molding

In injection molding production, injection volume is a core parameter for matching injection molding machines, optimizing molding processes, and ensuring product quality. It directly determines the maximum volume of molten plastic that can be melted and injected by the injection molding machine in a single cycle, and controls common product defects such as insufficient filling, shrinkage, flash, and material burning. Accurate calculation of injection volume is a fundamental task for mold design, machine model selection, and process debugging. Combining universal industrial standards and on-site practical experience, this paper systematically introduces the definition, calculation formulas, parameter values, correction methods, and practical application key points of injection volume.

1. Core Definition of Injection Volume

The rated injection volume of an injection molding machine refers to the volume of molten plastic discharged in a single injection cycle under the maximum effective injection stroke of the screw, with the common unit of cubic centimeter (cm³). In the industry, it is divided into theoretical injection volume and actual injection volume, which serve as the core basis for molding calculation. The differences are as follows:

• Theoretical Injection Volume: An ideal maximum injection volume calculated based on screw structural parameters, ignoring melt compression, screw clearance, pressure loss and other factors. It is the benchmark data for equipment parameter labeling and mold selection.

• Actual Injection Volume: The effective injection volume corrected according to actual working conditions such as the compression characteristics of molten plastic, molding pressure and equipment loss, representing the real available volume in production practice.

2. Standard Calculation Formula for Theoretical Injection Volume

The theoretical injection volume is only related to the screw diameter and maximum effective injection stroke of the injection molding machine. As a universal basic formula applicable to all screw-type injection molding machines, it is defined as follows:

V = π × (D/2)² × S

Parameter Explanations:

- V: Theoretical injection volume, unit: cm³;

- π: Pi, conventionally valued at 3.1416;

- D: Screw diameter, unit: cm, subject to the nameplate parameters of the injection molding machine;

- S: Maximum effective injection stroke, unit: cm, referring to the maximum effective moving distance of the screw.

Example: For an injection molding machine with a screw diameter of 30mm (3cm) and a maximum injection stroke of 120mm (12cm), the calculation is as follows: V=3.1416×(3/2)²×12≈84.82cm³, which means the theoretical maximum injection volume of the equipment is 84.82cm³.

3. Calculation of Actual Injection Volume (Practical Production Formula)

In actual injection molding production, molten plastic is compressible under high temperature and high pressure. In addition, tiny clearance leakage exists between the screw and barrel, resulting in melt conveying loss. Therefore, the actual available injection volume is always lower than the theoretical value. The universal practical calculation formula in the industry is:

V = V × η

Parameter Explanations:

- V: Actual effective injection volume, unit: cm³;

- η: Injection efficiency coefficient, ranging from 0.80 to 0.95 based on plastic material type, molding pressure and equipment aging condition.

Precise Value Rules of Efficiency Coefficient η:

• General plastics (PP, PE, ABS), new equipment, conventional molding pressure (80~120MPa): η = 0.90~0.95;

• Engineering plastics (PA, PC, POM), moderately aged equipment, medium and high pressure molding (120~160MPa): η = 0.85~0.90;

• High-viscosity and high-compressibility plastics, severely aged equipment, high-speed and high-pressure molding: η = 0.80~0.85.

4. Calculation of Required Product Injection Volume (Core of Machine Matching)

In the mold selection stage, it is necessary to calculate the total injection volume required for single-cycle product molding first, and then match it with the actual injection volume of the injection molding machine, which is the key to avoid inappropriate machine model selection (over-sized or under-sized). The total single-cycle injection volume includes the weight of molded products, the weight of runner and gate sprue, and a safety margin. The calculation formula is as follows:

V = (W + W) / ρ × (1 + 10%~20%)

Parameter Explanations:

- V: Injection volume required for product molding, unit: cm³;

- W: Total weight of all products in a single mold, unit: g; sum the weight of all cavity products for multi-cavity molds;

- W: Total weight of condensed materials such as gates, runners and sprue bushes, unit: g;

- ρ: Melt density of plastic, unit: g/cm³; solid-state density at room temperature can be used for approximate calculation, while melt density is required for high-precision calculation;

- 10%~20%: Safety margin coefficient; 20% for complex-structured and thin-walled products, 10% for simple thick-walled products.

5. Industrial Standards for Injection Volume Matching (Core Prohibitions)

After calculating the actual injection volume of the injection molding machine and the required product injection volume, strict industrial matching specifications must be followed to avoid molding defects and equipment loss. The standards are as follows:

1. Minimum Usage Limit: The required product injection volume shall not be less than 20% of the actual injection volume of the machine. Excessively small usage will lead to long melt residence time in the barrel, causing material decomposition, yellowing and burning, and reducing product performance.

2. Maximum Usage Limit: The required product injection volume shall not exceed 80% of the actual injection volume of the machine. A 20% margin ensures stable melt filling and balanced pressure, and prevents flash, overflow and equipment overload caused by high-speed and high-pressure injection.

3. Optimal Molding Range: The product injection volume is controlled within 40%~70% of the actual machine capacity. This range features the best molding stability, the largest process adjustable range and the highest product qualification rate.

6. Common Calculation Errors and Correction Key Points

1. Matching production directly with theoretical volume: Ignoring melt compression and equipment loss will result in insufficient actual injection volume and product short shot. Corrected actual volume must be adopted for verification.

2. Ignoring the weight of runner condensed materials: Calculating only the weight of products will cause insufficient injection volume, and the error is particularly prominent for multi-cavity molds and thin-runner molds.

3. Fixed safety margin value: Thin-walled, deep-cavity and complex-textured products have high melt flow resistance and require an increased safety margin to avoid insufficient filling and sink marks.

4. Universal parameters for new and old equipment: Aged injection molding machines have worn screws and increased clearance, leading to reduced injection efficiency. The efficiency coefficient η needs to be reduced by 0.03~0.05 to ensure calculation accuracy.

7. Conclusion

The calculation of injection volume follows three core logics: first, calculate the theoretical volume through screw diameter and stroke; second, correct it into the actual available volume according to material characteristics and equipment working conditions; finally, calculate the required molding volume based on product and runner weight to complete machine matching. Accurate volume calculation can avoid production waste caused by mismatched machine and mold sizes, and fundamentally prevent problems such as insufficient filling, material decomposition and unstable molding. It is an essential basic skill for mold design, machine selection and standardized injection production. In actual production, parameters should be adjusted flexibly according to plastic characteristics, equipment status and product structure to achieve efficient and high-quality molding.