Understanding How PMI Foam Interacts with Resin in Composite Manufacturing

Resin uptake is one of the most critical factors in determining the final weight, stiffness and surface quality of a composite part. ROHACELL® PMI foam is widely valued for its extremely low and highly predictable resin absorption, making it one of the most efficient core materials for lightweight structural composites. Its closed-cell structure, fine microcell morphology and consistent density allow manufacturers to achieve tight laminate weights and excellent surface finishes across a wide range of processing methods.

This page explains how resin uptake works, why ROHACELL® keeps absorption minimal, and how this influences composite performance in aerospace, motorsport, UAVs, medical and industrial applications.

What Is Resin Uptake?

Resin uptake describes how much resin penetrates the surface of the foam core during layup and cure. Every foam core absorbs some amount of resin, but the degree of absorption varies significantly depending on:

• the foam’s cell structure
• cell size and density
• open-cell vs closed-cell nature
• surface finish
• laminate pressure and temperature
• resin viscosity

In lightweight composite structures, excess resin uptake adds unnecessary mass and may cause unpredictable variations in part stiffness, local density and bond quality.

ROHACELL® is engineered to minimise this.

Why ROHACELL® Has Naturally Low Resin Uptake

ROHACELL®’s very low resin absorption is the result of its unique PMI chemistry and exceptionally consistent manufacturing. Key factors include:

Closed-Cell Structure

The foam consists of tightly sealed, uniformly distributed closed cells. This prevents resin from seeping deep into the core — unlike open-cell foams, which can absorb resin like a sponge.

Fine Microcell Morphology

ROHACELL®’s cell diameters are extremely small and consistent. Fewer open or fractured cells means fewer resin pathways.

Low Surface Porosity

The skin of ROHACELL® sheets is naturally smooth and dense, reducing the amount of resin that wets into the surface during layup.

High Thermal Stability During Cure

ROHACELL® maintains dimensional and structural stability during high-temperature prepreg curing, preventing microcracks that could otherwise increase resin penetration.

No Collapse Under Pressure

Many foams deform under autoclave pressure, creating open pathways for resin. ROHACELL® remains rigid and maintains its closed-cell integrity.

The combination of these characteristics means resin uptake is both minimal and predictable, which is one of the reasons ROHACELL® remains the go-to PMI foam for precision composites.

How Resin Uptake Affects Composite Performance

1. Part Weight

Lower resin uptake means a lighter component. For aerospace and motorsport, every gram matters. Consistent uptake leads to predictable laminate weights and easier compliance with weight targets.

2. Surface Quality

Because the surface does not soak resin deeply, ROHACELL® delivers exceptionally smooth surfaces under thin skins — especially in fine-cell grades such as IG-F and WF.

3. Bond Quality

ROHACELL® allows resin to wet the immediate surface enough to form a strong adhesive interface while preventing excess penetration that weakens core–skin bonding efficiency.

4. Stiffness and Structural Predictability

Variations in resin absorption can change local core density and bond-line thickness. ROHACELL® avoids this, giving reliable mechanical performance across the panel.

5. Cure Behaviour

Stable cell walls prevent resin-rich pockets during pressure cycles, improving uniformity across large or complex parts.

Resin Uptake Across Different ROHACELL® Grades

Different grades of ROHACELL® are engineered for specific applications, which influences resin interaction:

ROHACELL® IG & IG-F

General-purpose grades with very low resin uptake suitable for aerospace interiors, fairings, housings, motorsport structures and medical components.

ROHACELL® WF

Very fine cells and thin skins minimise resin absorption even further, making WF ideal for ultra-light laminates and thin composite skins.

ROHACELL® HERO / HT

High-temperature grades maintain closed-cell structure under elevated cure cycles, preventing resin flow into cell surfaces during high-pressure autoclave processing.

ROHACRYL®

With its ultra-fine microstructure, ROHACRYL® exhibits some of the lowest resin uptake behaviour of any PMI foam, particularly in thin-skin aerospace or motorsport applications.

How Process Variables Influence Resin Uptake

Although ROHACELL® inherently minimises absorption, processing still plays a role:

Laminate Pressure

High autoclave pressure produces thin, consistent bond lines and reduces resin pooling.

Resin Rheology

Low-viscosity resins may penetrate slightly deeper than high-viscosity resins, though still minimal with ROHACELL®.

Temperature Cycles

Stable cell walls prevent microcracking, maintaining consistent resin behaviour even in fast cure cycles.

Surface Preparation

Light abrasion or surface shaping (where required) can open a small number of cells; controlling surface prep maintains low uptake.

Prepreg vs Wet Layup

Prepregs deliver extremely consistent resin films. Wet layup systems may require careful monitoring to avoid resin-rich areas.

Benefits of Low Resin Uptake in ROHACELL®-Based Composites

• lighter components with predictable mass
• excellent surface finish under thin skins
• strong and consistent adhesive bonding
• reduced resin waste
• improved process repeatability
• superior fatigue and impact performance
• consistent mechanical behaviour across large structures
• easier quality control for aerospace and motorsport parts

Low and predictable resin uptake contributes significantly to the reliability and repeatability of ROHACELL®-based structures.

Why Resin Uptake Matters in High-Performance Industries

In sectors where grams, tolerances and repeatability determine performance — such as aerospace, F1, electric motorsport, drones and defence — the consistency of resin interaction is directly tied to:

• weight targets
• stiffness envelopes
• vibration behaviour
• fuel efficiency
• aerodynamic precision
• crashworthiness
• certification requirements

Minimising resin uptake is not simply a material advantage — it affects the entire structural design philosophy.

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