What are the Causes of Thickness Changes in Thermoformed Products?

Thermoforming is a versatile manufacturing process used across industries like packaging, automotive, and medical devices to create complex shapes from plastic sheets. However, one persistent challenge in thermoforming is managing thickness changes¹—variations in the wall thickness of the final product that can impact its strength, appearance, and functionality. This blog post explores the root causes of these thickness variations and offers insights into how they can be controlled to optimize product quality.

Thickness changes in thermoformed products are primarily caused by stretching during the forming process, influenced by mold design, forming methods, process parameters, and material properties.

By delving into these factors, manufacturers can better understand and mitigate thickness variations, ensuring consistent and high-performing products.

What is Thermoforming and Why Do Thickness Changes Occur?

Thermoforming involves heating a plastic sheet to a pliable state, shaping it over a mold, and trimming it into a final product. Thickness changes arise as the plastic stretches during forming, leading to thinner walls in some areas and thicker ones in others. These variations are influenced by multiple factors, including mold design, forming techniques, process conditions, and the properties of the plastic used.

Thermoforming heats a plastic sheet and molds it, with thickness changes resulting from stretching, driven by mold complexity, material behavior, and process controls.

Factor	Impact on Thickness
Mold Design2	Sharp angles and deep draws increase stretching
Material Properties3	Stretch resistance varies by plastic type
Process Parameters	Temperature and pressure affect material flow

Mold Design and Complexity

Molds with deep cavities or intricate shapes require the plastic to stretch more in certain areas, thinning the walls. For example, a deep tray will have thinner sides and bases compared to flatter sections.

Forming Method

Techniques like vacuum forming, pressure forming, and plug-assisted forming influence how evenly the plastic stretches. Vacuum forming⁴, for instance, may thin out deep sections more than pressure forming.

Process Parameters

Temperature, heating time, and pressure settings dictate how the plastic flows. Uneven heating can cause inconsistent stretching, while improper pressure may exaggerate thinning.

Material Properties

Thermoplastics like PET, PVC, and polystyrene⁵ stretch differently. PET may thin more in deep draws, while PVC’s rigidity can resist excessive stretching if heated properly.

How Do Different Forming Methods Affect Thickness Changes?

The method used to shape the plastic sheet plays a critical role in thickness distribution. Each technique applies force differently, affecting how the material stretches and conforms to the mold.

Vacuum forming, pressure forming, and plug-assisted forming⁶ each impact thickness differently, with plug-assisted forming often yielding the most uniform results.

Vacuum Forming

This method uses a vacuum to pull the heated sheet onto the mold. It’s cost-effective but can result in uneven thickness, especially in deep parts where the base thins significantly.

Pressure Forming

By applying positive air pressure, this technique pushes the sheet onto the mold, offering better control and more uniform thickness, particularly for detailed designs.

Plug-Assisted Forming

A plug pre-stretches the sheet before it contacts the mold, distributing the material more evenly. This reduces thinning in complex or deep parts.

What Role Do Process Parameters Play in Thickness Changes?

Process parameters—such as temperature, heating duration, and pressure—directly affect how the plastic behaves during forming, influencing thickness uniformity.

Forming temperature, heating time, and pressure⁷ control the plastic’s flow and stretch, impacting thickness distribution.

Forming Temperature

Too low a temperature restricts even stretching, while excessive heat can cause thinning or tearing. The ideal range depends on the material.

Heating Time

Uneven or insufficient heating creates hot spots, leading to inconsistent stretching and thickness variations across the part.

Pressure Levels

In pressure forming, higher pressure enhances mold detail but can stretch certain areas more, while low pressure may leave thickness uneven.

How Do Material Properties Influence Thickness Changes?

The choice of thermoplastic affects how the sheet stretches and, thus, the thickness of the final product. Each material has unique characteristics that influence its forming behavior.

Thermoplastics like PET, PVC, and polystyrene exhibit different stretching properties, affecting thickness outcomes in thermoforming.

PET (Polyethylene Terephthalate)

Widely used in packaging, PET offers clarity and strength but thins more in deep draws, requiring careful mold planning.

PVC (Polyvinyl Chloride)

PVC’s rigidity resists excessive stretching, but uneven heating can still cause thickness inconsistencies.

Polystyrene

Common in disposable items, polystyrene stretches relatively evenly but demands precise temperature control to avoid over-thinning.

What Are Practical Ways to Manage Thickness Changes?

Controlling thickness variations requires a holistic approach, combining smart design, precise process control, and informed material choices.

Minimize thickness changes through optimized mold design⁸, uniform heating, suitable forming methods, and tailored material selection.

Design Checklist for Managing Thickness

• Mold Design: Use gradual transitions and avoid sharp corners to limit excessive stretching.

• Uniform Heating⁹: Ensure consistent sheet temperature to prevent uneven flow.

• Forming Method: Opt for plug-assisted forming for complex parts needing even thickness.

• Material Selection¹⁰: Pick plastics with stretching properties suited to the design.

Process Selection Decision-Making

Consider thermoforming when:

• Cost-Effectiveness: It’s ideal for low-to-medium runs and prototyping.

• Complexity: It handles intricate shapes, though thickness needs management.

• Material Use: It excels with thin-walled parts but lacks injection molding’s precision for high volumes.

Conclusion

Thickness changes in thermoformed products stem from the stretching of the plastic sheet during forming, driven by mold design, forming methods, process parameters, and material properties. While these variations can’t be entirely eliminated, they can be effectively managed through thoughtful design, precise process adjustments, and strategic material choices. Understanding these causes empowers manufacturers to enhance product quality, performance, and cost-efficiency.