Key Injection Molding Formulas for Efficient Production

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A major aspect in injection molding is its preciseness in calculation. Aside from helping in setting up and optimizing the injection molding process, these calculations facilitate the manufacture of high quality plastic parts.

Fully understanding the basic formulas allows you as a manufacturer to control factors like injection speed, pressure, and cooling time. Managing these factors ensures consistent results and minimizes defects.

In simple terms, accurate calculations are the core deliverables for efficient and reliable injection molding operations that lead to cost-effective production and quality superior products. Read on as we breakdown these formulas that can be used in injection molding projects. 

Essential Injection Molding Formulas

Let’s discuss the fundamental formulas that guide the setup and optimization of injection molding.

1. Clamping Force (F In TON)

In order to achieve consistent and high-quality parts, it’s important to apply an appropriate clamping Force (F). This ensures the intactness of the mold throughout the injection process resulting in precise and flawless products.

This force is calculated using the formula as follows: 

F = (Am x Pv) / 1000 

In this formula, the projected area of the mold cavity is represented by Am in cm^2

On the other hand, Pv denotes the filling pressure in kg/cm^2

This force makes sure that the mold is tightly closed against the force of the injected molten plastic thus preventing any deformations or defects in the final product. Generally, materials that require lower filling pressures have good flowability whereas those with poor flowability demand higher values.

For example, if the projected mold cavity area is 270 cm^2 and the filling pressure is 220 kg/cm^2, the clamping force is calculated as: 

F = (270 x 220) / 1000 = 59.4TON

2. Injection Pressure (Pi In Kg/Cm^2)

Calculating the injection pressure is key to the injection molding process. This pressure can be defined as the exerting force that pushes the molten plastic material into the mold cavity.

The formula to calculate injection pressure is:

Pi = (P x A) / Ao


 P corresponds to the pump pressure

A is the effective area of the injection cylinder

Ao is the cross-sectional area of the screw

Practically, based on factors like material viscosity, part geometry and mold design, injection pressure needs to be calibrated carefully. 

For example, with a pump pressure of 85 kg/cm^2, an effective injection cylinder area of 160cm^2, and a screw cross-sectional area of 17.9cm^2 (for a diameter of 45mm), the injection pressure is calculated as follows: 

Pi = (85 x 160) / 17.9 = 759.7 kg/cm^2

By accurately determining and controlling injection pressure, you can achieve optimal results in your injection molding operations, producing high-quality parts with consistency and efficiency.

3. Injection Volume (V In Cm^3)

This refers to the amount of molten plastic material that’s injected into the molding cavity during every cycle. It’s a crucial parameter that impacts the size, shape and quality of the final modeled parts directly.

The formula to calculate injection volume is:

V = π x (Do/2)^2 x ST

In the formula, 

Do represents the screw diameter ,

ST represents the injection stroke

You can ensure that the mold cavity is sufficiently filled with the required amount of material by accurately understanding and calculating the injection volume.This helps in achieving consistent part dimensions and minimizing defects such as voids or sink marks.. 


A screw with a diameter of 45mm and an injection stroke of 18.5mm, the injection volume can be calculated as:

V = π x (4.2/2)^2 x 18.5 = 228.6cm^3

Basically all this means is that during each injection cycle, 2286.^3 of molten plastic is injected into the mold cavity thus forming a desired part geometry.

The essence of controlling the volume is to allow you to adjust the processing parameters as required to meet specific requisites, such as filling complex mold geometries or producing parts with precise dimensional tolerances. 

4. Injection Weight (Vw In G)

Injection weight (Vw) in injection molding refers to the mass of the molten plastic material injected into the mold cavity during each cycle. Since it directly determines the outcome of the final molded parts, it is one of the crucial parameters here. 

The formula is as follows: 

Vw = V x η x δ


V represents the injection volume, 

η denotes the specific gravity of the material, and 

δ represents mechanical efficiency

Using this formula, one can ensure that the suitable amount of material is injected into the mold cavity further resulting in consistent part dimensions and mechanical properties.


Given an injection volume of 228.6cm^3, with a mechanical efficiency of 0.85, and a specific gravity of 0.92, the injection weight can be calculated as: 

Vw = 228.6 x 0.85 x 0.92 = 178.7g

This can be interpreted as approximately 178.7 grams of molten plastic material with a distinct gravity of 0.92 that is injected into the mold cavity during every injection cycle.

With this in mind, we therefore infer that injection is essential for achieving desired part characteristics like strength, durability and appearance.and appearance.

Injection Molding Formula 2

Injection Molding Parameters (Image Source: Zetamold)

5. Injection Speed (S In Cm/Sec)

Another crucial factor in injection molding is the injection speed, measured in Cm/sec. It dictates how fast molten plastic material is inserted into the mold cavity during manufacturing. 

Speed is also directly proportional to the filling time, flow characteristics and overall quality of the parts.

The injection speed formula:

 S = Q / A

Q represents the pump discharge volume per revolution

A denotes the effective area of the injection cylinder

An explicit calculation of injection speed helps you optimize operations for desired outcomes and efficiency.. 


Given a motor speed of 1000 RPM, a pump discharge volume of 85 cc/RPM, and an effective injection cylinder area of 140 cm^2, injection speed can be calculated as:

S = (85 x 1000) / (60 x 140) = 10.1 cm/sec 

From this, we can deduce that in approximately 10.1cm/sec during each injection cycle, the molten plastic material flows into the mold cavity thus minimizing defects and ensuring uniform filling.

6. Injection Rate (Sv In G/Sec)

The amount of material injected into the mold cavity per unit of time is called Injection Rate and is measured in grams per second(g/sec). It is pivotal in the contribution of an efficient and quality molding process.

The injection rate formula:

Sv = S x Ao


S is the injection speed

Ao is cross-sectional area of the screw (Ao)

By determining the injection rate, one can achieve precise control over the flow of the molten plastic material during the injection molding process.


Given an injection speed of 10 cm/sec and a screw diameter of 42 mm, the injection rate can be calculated as:

Sv = 16.85 x 10 = 168.5 g/sec

The results above indicate that 168.5grams of material are injected in the mold cavity every second..

Advanced Injection Molding Calculations

There are more formulas and parameters used in injection molding calculations. They include:

1. Theoretical Injection Volume

Theoretical injection Volume (cm³) is  calculated using the formula: 

Theoretical Injection Volume (cm³) = Screw Diameter² x 0.785 x Injection Stroke

This calculation is based on the constant π/4, which is approximately 0.785. The Injection Stroke (cm) is derived from the Theoretical Injection Volume divided by (0.785 x Screw Diameter).

2. Injection Weight 

Another important calculation in injection molding is the injection weight which is calculated by multiplying the Theoretical Injection Volume by the Plastic Specific Gravity and a constant of 0.95.

Simply, the formula is as follows: 

Injection Weight (gr) = Theoretical Injection Volume x Plastic Specific Gravity x Injection Constant (0.95).

3. Injection Pressure

This parameter is computed using various formulas depending on the specific requirements of the injection molding process. 

One method involves calculating the ratio of the Injection Cylinder Area² to the Screw Area², multiplied by the square of the System Maximum Pressure (140kg/cm²). 

Injection Pressure (kg/cm²) = (Injection Cylinder Area² / Screw Area²) x System Maximum Pressure (140kg/cm²)²

The other approach  is to directly compute the ratio of the Injection Cylinder Diameter² to the Screw Diameter², multiplied by the System Maximum Pressure. 

Injection Pressure (kg/cm²) = (Injection Cylinder Diameter² / Screw Diameter²) x System Maximum Pressure (140kg/cm²)

Furthermore , the Injection Pressure can be determined by multiplying the Maximum Injection Pressure of the Barrel Assembly by the Actual Usage Pressure (kg/cm²), then dividing by the System Maximum Pressure.

Injection Pressure (kg/cm²) = Maximum Injection Pressure of the Barrel Assembly x Actual Usage Pressure (kg/cm²) / System Maximum Pressure (140kg/cm²)

Practical Applications and Case Studies

In real-world scenarios, accurate calculations are key for optimizing injection molding processes. Let’s discuss some of the industries that you can apply the formulas above. 

Automotive Parts Industry

Manufacturers in the automotive industry e.g Mercedes or Toyota need to precisely calculate the clamping force to ensure mold integrity during high-pressure injection.. 

By using the formula F = (Am x Pv) / 1000, these manufacturers can determine that a clamping force of 59.4TON is required for a mold cavity projected area of 270cm^2 and a filling pressure of 220kg/cm^2. This prevents deformations and ensures quality car parts.

Electronics Industry

Market leaders in the electronics industry like Apple or Sony, utilize injection pressure calculations during production of intricate components with accurate dimensions.

They can make use of the formula Pi = (P x A) / Ao, to determine the injection pressure required to be 707 kg/cm^2 for a pump pressure of 75 kg/cm^2, an effective injection cylinder area of 150cm^2, and a screw cross-sectional area of 15.9cm^2. 

This is usually crucial for efficient filling of complex mold geometries without deformations. Such precise calculations help to streamline production processes and thus reducing defects and enhancing efficiency which in turn leads to cost savings and manufacture of quality products.

Injection Molding Formula 1

Injection Molding Process (Image Source: MagneticPlaten)


Being an expert in injection molding techniques requires a solid understanding and application of mathematical formulas. These formulas together with their variables, are very important tools for ensuring precise control over the injection molding process.

Embracing the power of mathematical precision to unlock the full potential of injection molding for manufacturers is paramount. Since technology is in constant gradual growth, learning and applying these formulas gives manufacturers an upper edge to stay competitive in the industry.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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