Thermoforming ROHACELL® PMI Foam

Thermoforming ROHACELL® is a proven method for shaping PMI structural foam into accurate, lightweight curved forms used across aerospace, motorsport, defence, UAVs, medical technology and advanced industrial composites. ROHACELL® becomes formable only within a very specific temperature range, allowing designers to achieve precise 3D geometries while maintaining the exceptional stiffness-to-weight characteristics that make PMI foam the preferred core material in high-performance applications.

This page provides detailed information on how thermoforming works, the processing behaviour of ROHACELL®, and the engineering considerations that determine when thermoforming is the optimal manufacturing method.

How ROHACELL® Behaves During Thermoforming

Although ROHACELL® is a highly crosslinked PMI foam with thermoset-like stability, it becomes pliable for a short period when heated to approximately 195–210°C. Within this window, the foam softens evenly through its thickness and can be shaped, bent, draped or pressed into tooling. Once cooled, the material locks into its new geometry with excellent dimensional stability.

Because the material cools very quickly, particularly at its edges, forming must be executed with fast handling and correct thermal management. ROHACELL®’s closed-cell structure provides predictable behaviour during forming, but the window of time in which the material is “plastic” is extremely short.

Why Thermoforming is Used Instead of Machining

Machining ROHACELL® into thin, curved shapes is possible, but often inefficient. Thermoforming is typically chosen when:

• the part has thin walls or sweeping curves
• material waste from machining would be excessive
• production volume requires fast cycle times
• 5-axis registration or double-sided machining introduces tolerance risk
• the final shape benefits from smooth, continuous surfaces
• forming enables shapes that would be expensive or impractical to machine

Thermoforming uses near-net-shape sheet material rather than thick blocks, allowing the finished shape to be achieved with minimal machining, usually limited to trimming.

Important Processing Considerations

ROHACELL® requires correct preparation and controlled forming conditions to achieve consistent results. Key factors include:

Temperature Control

The forming window is narrow. Too low and the foam fractures; too high and cell damage or surface discolouration occurs. Even, controlled heating ensures the foam becomes uniformly pliable.

Moisture Content

ROHACELL® absorbs moisture naturally. If excess moisture is present during forming, steam pressure can cause swelling, surface defects or internal micro-damage. Ensuring the material is properly conditioned before forming significantly improves consistency.

Transfer Speed

Once removed from the heat source, ROHACELL® loses heat rapidly. Fast transfer to the forming tool—typically within one to two seconds—prevents edge-cooling, cracking or early stiffening.

Tool Temperature

Tools that draw too much heat from the foam cause premature stiffening. Non-metallic tools or temperature-controlled surfaces help maintain forming conditions.

Environmental Stability

Drafts, cold airflow, or inconsistent ambient temperatures can affect how ROHACELL® cools and therefore how accurately it forms.

Tooling Approaches for Thermoforming ROHACELL®

A range of tooling styles may be used depending on shape complexity and production volume:

Simple Forming Blocks

For straightforward curves or bends, the softened foam can be draped over a simple former and held in place while cooling.

Intensifiers

Local blocks or inserts help push the foam into tighter curves or local geometries without full matched tools.

Matched Tooling

For highly accurate, repeatable parts, matched male–female tools provide the greatest control, especially for deeper or more complex shapes.

Vacuum Forming with Membranes

A silicone membrane or vacuum system can draw the foam onto a tool surface, offering good definition with low mechanical stress.

Each method must account for foam thickness, draft angles, tooling clearance and the rapid cooling behaviour of PMI foam.

Typical Forming Challenges

Thermoforming ROHACELL® can introduce specific effects related to its cellular structure:

Springback

After forming, the material may relax slightly as internal stresses equalise. This requires forming the part beyond its final angle to compensate.

Cracking at Edges

Edges cool first and lose ductility sooner, making them more prone to cracking if stressed during forming.

Wrinkling in Deep Draws

Areas where the foam must stretch significantly may wrinkle if excess material cannot redistribute cleanly.

Variation in Cell Shape

Compression on the inside of bends and tension on the outside changes cell geometry. In most applications this is acceptable, but in highly loaded structural designs it must be considered.

These behaviours make tooling design and process trials an important part of successful thermoforming.

Post-Forming Operations

Thermoformed ROHACELL® parts usually require trimming or light machining to reach final dimensions. Because most of the net shape is achieved during forming, machining loads are low and tooling for dimensional finishing is straightforward.

Complex parts may require additional machining to create pockets, ledges, bonding surfaces or alignment features. Thermoforming typically reduces the overall machining time dramatically.

When Thermoforming is the Best Option

Thermoforming ROHACELL® is especially effective when:

• high production throughput is required
• the geometry is curved, tapered, or thin-walled
• near-net shapes reduce waste and cycle time
• composite layup benefits from smooth, continuous foam surfaces
• tight tolerances must be achieved consistently
• the part is unsuitable for CNC machining due to holding challenges

Production rates of many parts per hour are achievable when the forming process is optimised.

Why ROHACELL® Thermoforming Matters in Modern Composites

Thermoforming provides a route to high-performance shapes without sacrificing the mechanical advantage of PMI foam. By using controlled heat, correct conditioning, specialised tooling and fast handling, manufacturers can form ROHACELL® into complex structures while maintaining its signature stiffness, thermal stability and consistency.

For applications requiring a combination of lightness, precision and repeatability, thermoformed ROHACELL® offers a highly efficient, technically robust manufacturing option.

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