Plastic injection molding is a widely used manufacturing process for producing high-quality, mass-produced plastic parts. Developing an accurate cost model is essential for determining part prices that cover production expenses while remaining competitive. In this article, we’ll walk through the key components of a cost model for plastic injection molded parts, explaining how to calculate cycle time, injection flow rate, ejection time, and other critical factors. We’ll also explore how material costs, machine rates, labor, tooling, and overheads influence the final price.

Key Components of a Plastic Injection Molding Cost Model

A comprehensive cost model involves understanding the full production process, from the initial injection of molten plastic to the ejection of the cooled part. The following sections break down each part of this process, providing formulas and methodologies to estimate costs accurately.

1. Cycle Time Estimation

Cycle time refers to the total time required to produce one part and consists of three main phases:

• Injection time: The time it takes to inject molten plastic into the mold cavity.

• Cooling time: The time required for the part to cool enough to be ejected from the mold.

• Ejection time: The time taken to eject the cooled part from the mold.

This formula captures the core components of the injection molding process and helps you estimate how long each part takes to produce. Let’s explore how to calculate each part of this formula.

a. Injection Time Calculation

The injection time is influenced by the volume of the part, the machine’s injection speed, and the material’s viscosity.

• Part Volume: The volume of the plastic part being produced, measured in cubic centimeters (cm³).

• Injection Flow Rate: The speed at which molten plastic is injected into the mold, determined by machine settings and material properties.

Factors affecting injection time:

• Machine capacity: Larger machines can inject more material at once, increasing the flow rate.

• Material viscosity: High-viscosity materials, like polycarbonate, are more resistant to flow, resulting in longer injection times.

• Part geometry: Thin-walled or complex geometries slow down the injection process, as they require more precise control to prevent defects.

b. Cooling Time Calculation

Cooling time is often the longest part of the cycle and is crucial to ensure the part solidifies properly without defects like warping.

Where:

• Wall thickness is the thickness of the part’s walls (in cm).

• Thermal diffusivity constant (a) is a material property indicating how fast heat spreads.

• T_mold, T_material melt, and T_ejection are the temperatures of the mold, the material in its melted state, and the material when it is ejected, respectively.

This formula is useful for estimating how long the part needs to stay in the mold before it’s solid enough to eject. The thicker the part, the longer the cooling time.

For a wall thickness of 0.2 cm, assuming the material is ABS (a = 0.00015 cm²/sec), with mold, material melt, and ejection temperatures of 50°C, 230°C, and 90°C, respectively, cooling time might result in approximately 15 seconds.

c. Ejection Time Calculation

Ejection time refers to the time required to remove the cooled part from the mold cavity. This includes the mechanical time taken by the ejector pins or air systems to push the part out of the mold.

• Ejection Delay Time: Time for the mold to open and prepare for part removal.

• Mechanical Ejection Time: Time for the ejector pins or other mechanical systems to push the part out.

Ejection time is generally short, usually between 1 and 5 seconds, depending on the complexity of the part and the number of ejector pins used.

Factors affecting ejection time:

• Part geometry: More complex shapes require more careful ejection to avoid damage.

• Mold design: The number and placement of ejector pins, as well as the presence of undercuts, influence ejection time.

• Material shrinkage: Materials with high shrinkage can contract around the mold, making ejection slower and more difficult.

For example, if ejection delay takes 1 second and mechanical ejection takes another second, the total ejection time would be 2 seconds.

2. Material Costs

Material cost is a straightforward calculation, based on the weight of the part and the cost per unit weight of the material.

Material costs vary depending on the type of plastic (e.g., ABS, polyethylene, polycarbonate) and the supplier. Material costs should also account for any scrap or waste generated during production.

3. Machine Rate

The machine rate is the cost associated with running the injection molding machine, calculated based on the machine’s hourly rate and the cycle time.

• Machine Hourly Rate: The operating cost of the machine per hour, which includes energy consumption, depreciation, maintenance, and overhead.

• Cycle Time: Total time (in seconds) to produce one part.

4. Labor Cost

Labor costs are associated with machine setup, operation, and quality control. These are usually calculated on a per-part basis by distributing the operator’s hourly wage across the cycle time.

Labor costs can vary depending on the level of automation. Highly automated processes will require less direct operator involvement, reducing labor costs.

5. Amortized Tooling Costs

Tooling refers to the mold used to shape the plastic part. Molds are a significant initial investment, so tooling costs are amortized over the total production volume.

6. Overhead Costs

Overhead costs cover expenses such as plant utilities, administrative costs, machine downtime, etc. These costs are typically spread across the entire production and calculated per part.

7. Profit Margin

Finally, the profit margin is added to cover the company’s profit expectations. This is usually a percentage markup on the total cost.

8. Final Cost Model Formula

The final price of the part can be calculated by summing all the cost components:

Conclusion

Developing a robust cost model for plastic injection molded parts involves understanding each phase of the injection molding process, from cycle time calculations to labor, material, and overhead costs. Accurate tracking of factors like injection flow rate, cooling time, and ejection time is crucial for estimating production time and costs. With a comprehensive model in place, manufacturers can ensure competitive pricing while covering production expenses and achieving desired profit margins.

However, many injection molders quote based on wrong assumptions, often adding hidden buffers or overestimating certain factors, which unnecessarily inflates the quote. Rely on Om Raj Tech and our manufacturer to provide a quote with precision—no unknown buffers—giving you accurate, transparent, and precise quotes every time.

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