Coremat Guide: What It Is, Types, and How to Use Core Material in FRP Laminates

Coremat guide for FRP laminates. Learn what coremat is, how it increases stiffness, installation steps and when to use it instead of foam or balsa cores.

Coremat Guide: What It Is, Types, and How to Use Core Material in FRP Laminates

Coremat is a non-woven, synthetic core material used to increase the thickness and stiffness of fiberglass laminates without the weight of solid glass. It is made from polyester fibers that compress and absorb resin during lamination, forming a low-density core between fiberglass skins. This core material is popular in boat decks, hatches, cabin tops, and industrial panels where bending stiffness is important but high shear loads are not. It is sometimes called "bulking core" because its primary job is to add thickness rather than carry heavy structural loads.

This guide explains what coremat is, how it works, how it compares to other core materials, and how to install it correctly. At PrimeMat, we stock coremat and custom combo mat products for sandwich laminates used in marine, industrial, and transportation applications. It is written for boat builders, FRP fabricators, and procurement teams evaluating core materials for sandwich laminates.

Key Takeaways

When Coremat Makes Sense

• Coremat is a non-woven polyester core material that absorbs resin and forms a low-density core between fiberglass skins.

• It increases laminate stiffness and thickness at lower weight and cost than solid fiberglass.

• This bulking core is not a structural core like balsa or high-density foam. It should not be used in high-shear or high-load joints.

• Common thicknesses range from 2 mm to 10 mm. Thicker cores provide more stiffness but require careful resin control.

• It is compatible with polyester, vinyl ester, and epoxy resin systems.

• Proper rolling is essential. Too much resin causes the core to compress excessively; too little causes voids and weak bonding.

What Is Coremat?

Structure and Typical Properties

Coremat , sometimes called core mat or synthetic core, is a needle-punched or bonded non-woven mat made from polyester fibers. Unlike rigid foam or balsa cores, this material is soft and flexible before lamination. When resin is applied, the polyester fibers absorb resin and swell slightly, creating a low-density core after cure.

Typical properties:

This core material is often used as a bulking core rather than a primary structural core. It adds stiffness by separating the fiberglass skins, increasing the moment of inertia of the laminate.

Why It Is Called a Bulking Core

The term "bulking core" describes the main benefit of the material: it adds thickness and stiffness without adding much weight or cost. In many FRP panels, stiffness is limited by part thickness, not by the absolute strength of the skins. A 5 mm coremat core can turn a thin, flexible panel into a rigid deck or sidewall.

Types of Core Materials for FRP

When to Choose Coremat Over Foam or Balsa

This core material is one of several core options. The right choice depends on stiffness, weight, cost, and load requirements.

Balsa and PVC foam are structural cores. They carry higher shear and compressive loads than coremat but cost more and require more precise cutting and bonding. This bulking core is the lowest-cost and easiest-to-handle option, but it provides the lowest shear strength and compressive strength of the group.

How Coremat Works in a Laminate

Resin Absorption and Compression

A coremat sandwich laminate consists of:

[Outer fiberglass skin] ↓ [Coremat core] ↓ [Inner fiberglass skin] During lamination:

• The inner skin (usually CSM or cloth) is laid and wet out.

• The core material is placed on the wet skin.

• Resin is applied to the core, which absorbs resin and begins to compress.

• The outer skin is placed over the core and rolled firmly.

• The laminate cures with the core trapped between the skins.

The polyester fibers bond mechanically with the resin and provide a low-density separator. The final core density is controlled by how much resin is added and how hard the laminate is rolled.

Because this material absorbs resin, the resin-to-glass ratio for the overall sandwich is higher than for solid laminate. A typical coremat sandwich may use 2.5–3.5 kg of resin per kilogram of core, depending on thickness and grade. Always confirm the exact absorption with the manufacturer data sheet, because the value varies by product.

Coremat in Contact Molding

Contact molding includes hand lay-up and spray-up processes where pressure is limited to rolling. This core material works well in contact molding because it conforms to the mold surface and absorbs resin from the surrounding laminate. In hand lay-up, the laminator applies resin gradually and rolls the core firmly before adding the outer skin. In spray-up, the core is often placed over a wet chopped strand mat layer, then resin is sprayed onto the core before the outer skin is applied. In both cases, the key is to avoid excess resin that collapses the core.

Coremat vs Solid Fiberglass Laminate

Stiffness-to-Weight Trade-Off

The sandwich principle is the same as in structural engineering: separating two strong skins with a lightweight core creates a panel with high bending stiffness. However, the core must be thick enough and the skins well-bonded for the benefit to be realized.

When to Use Coremat

Marine Applications and Boundaries

This core material is a good choice when:

• The part needs improved stiffness without significant weight increase

• The surface area is large and flat or moderately curved

• Shear loads are low to moderate

• Cost is a primary concern

• Fast, simple installation is preferred over maximum performance

Typical applications include:

• Boat decks and cabin tops

• Hatches and locker lids

• Shower pans and bathroom modules

• Industrial covers and panels

• RV and caravan sidewalls

• Door panels and interior liners

• Non-walkway areas of large molded parts

In boat building, this material is often used in decks and cabin structures where people walk and gear is stored, but not in hull bottoms or transoms where high loads occur. The core stiffens the panel enough to prevent oil-canning and flexing without the cost of a structural foam or balsa core.

This core material is not suitable for:

• High-load structural joints

• Areas with concentrated fastener loads

• Parts exposed to high impact or abrasion

• Applications requiring insulation or buoyancy

Reducing Print-Through with Coremat

Print-through is the visible pattern of reinforcement weave or chopped strand mat texture that shows on the surface of a laminate. A layer of this core material between the structural reinforcement and the outer skin can mask the weave pattern because the compressible, resin-rich core absorbs surface irregularities. For show surfaces, a common approach is:

Gel coat → surfacing veil → thin CSM → coremat → structural laminate The core layer creates a smoother, more uniform surface than heavy structural fabric alone. This is why coremat is often used on boat cabin tops and automotive panels where appearance matters.

Installation Steps with Coremat

Step 1: Prepare the Inner Skin

Lay the inner skin using CSM, cloth, or a combination. Wet out the reinforcement fully and roll out air before adding the core. The inner skin must be wet enough to bond to the core but not flooded with excess resin.

Step 2: Cut and Place the Coremat

Cut the core material to size, allowing for overlaps at joints. Overlap pieces by 25–50 mm to maintain core continuity. Butt joints without overlap create weak lines in the sandwich.

Step 3: Apply Resin

Apply resin to the core surface. The material will absorb resin quickly. Add resin gradually and watch for saturation. The goal is full wet-out without excess resin pooling underneath or on top.

Step 4: Roll Firmly

Use a wide roller to press the core into the inner skin and remove air. Rolling also controls the final thickness. Over-rolling squeezes out too much resin and compresses the core; under-rolling leaves voids.

Step 5: Apply the Outer Skin

Place the outer fiberglass skin over the wet core. Wet out and roll the outer skin thoroughly. The outer skin is usually CSM or cloth, depending on finish and strength requirements.

For visible surfaces, a fiberglass cloth outer skin provides a smoother finish than CSM. For industrial panels, CSM is usually sufficient and more economical.

Step 6: Cure and Finish

Let the laminate cure before sanding, drilling, or attaching hardware. Cure time depends on resin chemistry, catalyst level, and temperature. Post-cure may be needed for high-temperature service.

Coremat in Combo Mat Products

Factory-Stitched Coremat Combinations

Coremat is also used as the core layer in stitched combo mats, where fiberglass skins are factory-stitched to the core. These products speed up production by reducing the number of separate layers a laminator must handle.

For buyers interested in multi-layer products, the combo mat guide explains sandwich constructions with foam, balsa, and coremat cores. Custom combo mats can be specified with this core material for specific part geometries and production volumes.

Coremat Thickness Selection

Matching Thickness to Panel Span

The right core thickness depends on panel span, load, and skin thickness.

Thicker cores provide more stiffness but absorb more resin. For any given application, increasing skin thickness often improves strength more than increasing core thickness alone.

Calculating Weight and Cost of a Coremat Sandwich

A simple estimate helps compare this core material against solid laminate or other core options.

Example: 1 m² Panel with 5 mm Coremat

Inner skin: 450 g/m² CSM = 0.45 kg Coremat: 5 mm × 150 kg/m³ = 0.75 kg dry Resin absorbed by coremat: 0.75 kg × 2.5 = 1.88 kg Outer skin: 450 g/m² CSM = 0.45 kg Resin for skins: 0.90 kg × 2.2 = 1.98 kg Total weight ≈ 5.51 kg/m² A solid 5 mm laminate of CSM and resin would weigh roughly 7–8 kg/m² and be much stiffer but heavier. The sandwich saves weight while maintaining adequate stiffness for many applications.

Cost Considerations

This core material is inexpensive per square meter, but resin absorption is the hidden cost. When budgeting, include:

• Core material cost

• Additional resin cost

• Labor for rolling and handling

• Potential rework from over- or under-saturation

In many cases, the total cost per square meter of a coremat sandwich is still lower than solid laminate of the same thickness, making it economical for large panels.

What You Should Do Now

Next Steps

Evaluate whether your part needs this core material or a higher-performance core. For large, lightly loaded panels where stiffness and cost matter, coremat is a practical choice. For structural areas or high-load points, specify balsa, PVC foam, or a higher-density core instead.

Review the combo mat guide for multi-layer sandwich options and the reinforcement comparison guide for a broader view of CSM, cloth, and core materials. If you are sourcing coremat or custom combo mat products for a production program, contact PrimeMat through the quote request page .

Data Sources and Verification

Standards and Verification Methods

• Coremat composition and density: based on manufacturer data for non-woven polyester core materials.

• Core density reference: ISO 845 (Cellular Plastics and Elastomers — Determination of Apparent Density) or ASTM C271 (Standard Test Method for Density of Sandwich Core Materials).

• Sandwich laminate stiffness principle: based on standard composite mechanics (moment of inertia increases with core thickness).

• Resin absorption and compression behavior: based on industry handling guidelines for coremat installation; confirm with manufacturer data sheet.

• Core material comparison: based on typical densities and properties for marine and industrial sandwich constructions.

• Applications and limitations: based on common use in boat building and panel manufacturing.

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