How To Calculate The Clamping Force In Injection Molding?

Before we start to understand the clamping force in injection molding, let me share a story.

Company X received an order for mold injection business from their European customer, Company Y. Company Y sent three employees to visit Company X’s site. Mr. B, Company X’s General Manager, and Mr. C, the head of mold engineering, accompanied them on tour, along with the mold designer and the supervisor of injection molding production, Mr. D. While Mr. A from Company Y was observing the carefully finished injection molded products near the injection molding machine, Mr. B, the business leader of Company X, approached him.

A: Dear Mr. B, do you have any concerns?

B: Why is there a burr on the edge of this product? I am not satisfied with it.

Mr. C, the head of mold engineering, quickly approached, picked up the product, and examined it, stating, “Perhaps it was caused by the injection production operator setting the clamping force improperly.”

After understanding Mr. C’s job responsibilities, Mr. A turned to him. He asked, “Wasn’t the critical clamping force indicated in the mold instruction manual when you delivered the mold for injection molding production?”

Simultaneously, Mr. A also questioned Mr. D, the supervisor of injection molding production, saying, “Did the production operator not follow the parameter table in the mold manual? Wasn’t the best clamping force determined?”

Both Mr. C and Mr. D shook their heads.

Again, Mr. A turned to Mr. B and remarked, “This is unfortunate. Your colleagues seem to lack care for my mold and my product.”

Mr. C and Mr. D were left speechless.

Mr. B, the GM, appeared helpless.

My dear friends, do you understand what Mr. A means?

Tips in this story

When a mold factory delivers a mold, it is standard practice to provide an instruction manual that indicates the optimal clamping force for that particular mold. This information is essential for ensuring the proper operation and longevity of the mold.

During injection production, it is crucial to confirm and set the optimal clamping force based on the specifications provided in the mold manual. This involves inputting the appropriate machine parameters to achieve the desired clamping force without causing any damage to the mold. Adhering to the recommended clamping force helps ensure the production process runs smoothly and maintains the quality of the molded products.

Understanding the clamping force

The clamping force in injection molding keeps the mold closed during the injection and cooling process. It is generated by a hydraulic press, and in the injection molding mechanism, it is classified into Hydraulic Clamping Force and Mechanical Clamping Force. As a plastic product engineering staff, understanding and controlling all of these is crucial, especially for products without exposure or testing.

Factors affect the clamping force of the mold

There are several factors that can affect it in injection molding, including:

Part geometry: The shape, size, and complexity of the part being molded

Material properties: The type and characteristics of the plastic material

Wall thickness: Thicker walls generally require higher clamping forces to withstand the injection pressure.

Mold design: The design of the mold, including the number and complexity of cavities, gating system, and cooling channels

Injection pressure: The injection pressure applied during molding

Mold temperature: The mold’s operating temperature

The CAE analysis, such as mold flow analysis, typically includes a section that evaluates the clamping force. This section can provide valuable information for a specific mold and part. However, there may be instances where the clamping force parameter is not explicitly specified or available in the analysis, requiring an approximate estimation based on other factors and engineering knowledge.

How to calculate the clamping force?

Method 1: Established at the mold development stage.

When calculating the expansion force of a mold, it is typically advisable to consider the maximum value. This calculated expansion force represents the minimum critical clamping force required to produce the product without any flashes, and it is often referred to as the best clamping force.

The calculation formula for the critical clamping force is as follows:

F (critical clamping force) = P (cavity average pressure)(bar) × S (the projected area of the product and the runner) (c㎡)

To accurately determine the cavity pressure, several factors come into play, including the viscosity of the polymer material, the size and location of the runner and gate, the size and thickness of the product, the injection speed, the mold temperature, the barrel temperature, and the mold venting, among others. These factors collectively contribute to the complexity of the pressure inside the cavity during the molding process.

For instance, consider a product made of ABS material with the following specifications: a main runner length of 50mm, a square gate measuring 1.5mm, and a wall thickness of 2.0mm. The image below illustrates the shape of the product.

Before you start calculating, please familiarize yourself with these two tables

1. Table of flow coefficients of common thermoplastic materials.

2. Diagram of cavity pressure versus wall thickness and flow path to thickness ratio.

Step 1: Calculate the flow length ratio first

The material’s longest flow path is approximately 200+30/2+50=265mm, and the thinnest wall thickness is 1.5mm at the gate.

Flow path to wall thickness ratio = longest flow of material/thinnest wall thickness

= 265/1.5

= 177:1

Step 2: Calculate the average pressure P in the cavity by using the relationship diagram

For a thin wall of 1.5 mm and a flow path to thickness ratio of 177, the cross corresponding curve point is P1 = 250 (bar).

P cavity average pressure = P1 * K flow coefficient = 250 * 1.55 = 387.5 (bar).

Step 3: Calculate the projected area

This projected area can be calculated in the mold design software when the mold is finished and must be marked clearly on the mold specification and nameplate.

S = product projection area + runner projection area

S = 20*15*2+3*1

S = 603 c㎡

Step 4: Calculate the optimal clamping force

F = P average cavity pressure （bar） × S projected area of product and runner （c㎡）

F =387.5bar*603 （c㎡）

F =233662.5kg

F =234Ton.

We have calculated the critical clamping force for the ABS product, considering the maximum value of the coefficient. In this case, it is unnecessary to multiply it by a safety factor, as we have already considered the maximum value. This calculated value represents the theoretical optimum clamping force for the specific mold and product.

To ensure clarity and reference for the injection molding production personnel, it is important to clearly mark this critical clamping force value in the mold manual and on the mold nameplate. By doing so, the production personnel will have a standard reference for setting and maintaining the appropriate clamping force during production.

Method 2: Calculate By Production Test

This method can be quickly tested on any machine and mold using a kilo electronic scale and adjusting the clamping force settings. The following steps outline the process:

Step 1: Set the clamping force to 90% of the maximum pressure and use medium pressure (around 60%~70%) and medium speed (30%~60%) for injection. Set the holding position and pressure, and ensure that the product has no appearance defects. Inject the product 3 times and record the weight and appearance condition in a table.

Step 2: Decrease the clamping force by 10 tons sequentially and record the weight while confirming the presence of any appearance defects. Continue decreasing the clamping force until the product weight increases by approximately 5% and flashes start to occur.

Based on the data collected in the table, the best clamping force parameter for this specific product on this machine can be determined to be between 80 tons and 90 tons.

During injection molding production, if there are no specific requirements for the mold products, the PMC (Production, Material, and Control) staff typically schedule production based on the mold size relative to the size of the machine. The adjusting technician can set the value at around 70%~80% of the maximum clamping force of the machine. This approach is considered fast and effective in achieving optimal results.

The maximum clamping force of common injection molding machine models in the market

If there are any errors in the table below, it is recommended to contact the relevant sources or verify the information with me. The table is intended for reference purposes.

Please note:

1. To determine the clamping force required for a specific injection molding application, we should consider the specific requirements of the product being manufactured.

2. A higher clamping force does not necessarily indicate a better machine. Instead, we should select a proper one within the appropriate range for the specific application.