Mastering Calculation of Shot Size in Injection Molding

In the world of injection molding, accurate shot size calculation is crucial for ensuring that every part is produced with precision and consistency. This article provides a detailed guide on how to calculate the shot size, including key factors like part volume, melt density, and safety factors. Additionally, we explore the critical role of shrinkage allowances in molding design to achieve the desired product dimensions.

Understanding Shot Size Calculation in Injection Molding

Shot size, a critical parameter in injection molding, is the volume of molten plastic material required to fill a mold cavity in a single injection cycle. Accurate shot size calculations help in selecting the right injection molding machine and setting the appropriate injection parameters. This article outlines the steps and formulas to calculate shot size, ensuring a higher success rate in the manufacturing process.

Key Factors in Shot Size Calculation

1. Part Volume

The first step in calculating shot size is determining the part volume. This involves measuring the volume of the part being molded, including any additional volume required for runners, gates, and sprues. Runners, gates, and sprues are integral parts of the injection molding process, as they direct the flow of molten plastic to the mold cavity.

2. Melt Density

Melt density plays a significant role in shot size calculations. It refers to the density of the molten plastic at the injection temperature. This information can be found in the material data sheet or obtained from the material supplier. Understanding the melt density helps in ensuring accurate calculations and, consequently, consistent part sizes.

3. Safety Factor

A safety factor is included to account for various variables, such as material shrinkage during cooling, packing pressure, and other molding process variables. Safety factors typically range between 1.1 to 1.2, thereby ensuring a margin of error in the shot size calculation.

The Formula for Shot Size Calculation

Using the above factors, the formula to calculate the shot size is:

Shot Size (Part Volume Runner/Gate/Sprue Volume) × Safety Factor / Melt Density

Example Calculation

Let's use an example to illustrate the calculation. If the part volume is 50 cm3, the runner/gate/sprue volume is 10 cm3, the safety factor is 1.2, and the melt density is 1.2 g/cm3. The shot size would be calculated as follows:

Shot Size (50 cm3 10 cm3) × 1.2 / 1.2 g/cm3 72 cm3

This calculated shot size represents the volume of molten plastic required to fill the mold cavity and its associated channels. It is crucial for selecting the appropriate injection molding machine and setting the injection parameters correctly during the molding process.

Incorporating Shrinkage Allowance into Molding Design

Another critical aspect of injection molding is accounting for shrinkage allowance. Shrinkage occurs during the cooling process, and it's necessary to consider this when designing the mold to ensure that the final product meets the required dimensions.

To incorporate shrinkage allowance, use the following formula:

A B × 1 S

Where:

A -- Straight line size of the mold cavity at room temperature (mm) B -- Straight line dimensions of the product at room temperature (mm) S -- Shrinkage value, which varies depending on the material. Refer to the table below for specific shrinkage values for different materials.

Shrinkage Values for Different Materials

Material Shrinkage (%) ABS 0.3 - 0.5 Polyethylene (PE) 0.6 - 1.0 Polypropylene (PP) 0.5 - 1.5 Polystyrene (PS) 0.5 - 1.5 Polycarbonate (PC) 0.7 - 1.2

Practical Application: Ensuring Adequate Mold Capacity

After calculating the shot size, the next step is to determine the appropriate mold size. First, confirm the weight of the product. For instance, if the product weighs 200 grams, then selecting a 300-gram injection molding machine is a good starting point. This ensures that the machine can handle the weight of the product comfortably.

Measure the length and width of the product and add 100 mm to each side to determine the mold size. Compare this size with the available injection molding machines to ensure that the weight of the machine can accommodate the product. For example, if the mold can hold 90 products and 10 are exceptions, this machine would be a viable choice.

Conclusion: Accurate shot size calculation and proper shrinkage allowance consideration are vital for successful injection molding. By following the steps and formulas outlined in this article, molders can ensure consistent and high-quality product outputs. This not only enhances production efficiency but also reduces waste and rework, leading to improved profitability.

Keywords: Injection Molding, Shot Size Calculation, Melt Density