Forming Pressure Calculator
Quickly estimate vacuum or positive pressure required to form parts reliably.
Start CalculationThis calculator helps you determine the required vacuum or positive pressure for thermoforming based on sheet thickness, forming area, and material properties. Use it to optimize your forming process and ensure consistent part quality.
Positive pressure is typically used for forming thicker sheets or complex geometries, while vacuum forming is suitable for thinner materials and simpler shapes. The right pressure ensures proper material distribution and minimizes defects.
Input Parameters
Results
Empirical Method (Recommended for Production)
The empirical method uses industry-proven formulas that correlate well with actual production results. It's based on extensive testing and provides reliable pressure values for most common thermoforming applications.
The formula used is:
Preq_emp = (Kmat × tmm) / (√Am × η)
Where:
- Preq_emp = Required pressure (bar)
- Kmat = Material coefficient (bar·cm0.5/mm)
- tmm = Sheet thickness (mm)
- Am = Forming area (cm²)
- η = Machine efficiency (decimal, e.g. 0.85 for 85%)
Material Coefficients (Kmat)
| Material | Kmat (bar·cm0.5/mm) | Notes |
|---|---|---|
| PET | 0.12 | Standard for food packaging |
| PP | 0.10 | Lower stiffness than PET |
| PS | 0.09 | Easier to form |
| PLA | 0.11 | Biodegradable alternative |
| PVC | 0.13 | Higher stiffness |
Note: These coefficients are starting points and should be calibrated with your specific equipment and material batches.
Physics Method (For Engineering Verification)
The physics-based method provides a theoretical foundation for understanding the forces involved in thermoforming. While more complex, it's useful for validating empirical results and understanding edge cases.
The simplified physical model is:
Preq_phys = (σy × t) / (C × R)
Where:
- Preq_phys = Required pressure (bar)
- σy = Effective yield strength at forming temperature (N/mm²)
- t = Sheet thickness (mm)
- R = Characteristic length (e.g., √A or effective radius, mm)
- C = Constant accounting for friction and deformation mode
Material Properties at Forming Temperature
| Material | σy (N/mm²) | Typical Forming Temp (°C) |
|---|---|---|
| PET | 2.5 - 3.5 | 90 - 110 |
| PP | 1.8 - 2.5 | 140 - 160 |
| PS | 1.5 - 2.0 | 110 - 130 |
| PLA | 2.0 - 3.0 | 60 - 80 |
| PVC | 3.0 - 4.0 | 110 - 130 |
Note: The physics method typically gives a pressure range rather than a single value due to material property variations and assumptions in the model.
Example Calculations
Here are some typical forming scenarios with recommended pressure values:
Shallow Lunch Tray
Clamshell Container
Deep Dome
Frequently Asked Questions
Insufficient pressure can result in incomplete forming, poor detail reproduction, uneven wall thickness, and weak spots in the final product. The material may not properly conform to the mold, leading to defects like webbing or bridging between high points.
Excessive pressure can cause material thinning, especially in corners and deep draws. It may lead to tearing, increased cycle times (as the material needs more time to cool under pressure), and higher energy consumption. In vacuum forming, too much pressure can distort fine details or cause the mold to shift.
Stiffer materials (like PET and PVC) generally require higher forming pressures than more flexible materials (like PP and PS). The material's melt strength and elongation properties at forming temperature significantly influence the pressure needed for proper forming.
The safety factor accounts for variations in material properties, machine performance, and environmental conditions. It ensures reliable forming even with minor fluctuations in these parameters. A typical safety factor of 15-20% is recommended for most applications.
For large areas, consider segmenting the mold into multiple cavities, increasing sheet thickness, or using higher pressure machines. The pressure required increases with the square root of the forming area, so large areas can quickly require impractical pressure levels.
Start with the calculated pressure and perform test runs with your specific material batch and equipment. Measure part quality (wall thickness distribution, detail reproduction) and adjust the pressure as needed. Document successful settings for future reference.
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