# Continuous manufacturing and performance of 3D reinforced sandwich structures

Guy Leroy, Christophe Binetruy, Patricia Krawczak

► To cite this version:

Guy Leroy, Christophe Binetruy, Patricia Krawczak. Continuous manufacturing and performance of 3D reinforced sandwich structures. 7th International Conference on Sandwich Structures (ICSS-7), Aug 2005, Aalborg, Denmark. pp.693-702, 10.1007/1-4020-3848-8_70 . hal-01781143

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Submitted on 5 Jan 2026

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
CONTINUOUS MANUFACTURING AND
PERFORMANCE OF 3D REINFORCED
SANDWICH STRUCTURES

G. Le Roy¹, C. Binetruy² and P. Krawczak²

¹NAPCO Technologies, 14 rue de Beauce, 75003 Paris, France,
E mail: le roy napco@wanadoo.fr

²Ecole des Mines de Douai, Polymers and Composites Technology Department, 941 Rue Charles Bourseul, 59508 Douai, France

**Abstract** The patented Napco® technology presented in this paper is designed to create in a continuous way 3D tailored sandwich structures while maintaining the production efficiency. The through thickness reinforcement is obtained from regular fabrics. This process allows the production of complex preforms that can be post formed and impregnated with liquid resin using a closed molding production method or thermoformed in the case of thermoplastic composites.

**Keywords:** sandwich, manufacturing, 3D structure.

# 1. INTRODUCTION

The high strength, high specific weight and durability of sandwich composites make these materials highly attractive for aerospace marine and ground transportation applications. In this latter case, sandwich components have been successfully introduced to several applications such as roof panels in train and in bus structures, front cabins of high-speed locomotives, interior panels etc. Minimizing the weight of a structure is becoming a common key design objective as it allows many options such as higher speed, longer range, larger pay loads, less engine power and better operating economy. The sandwich material must be light whilst remaining economically viable. Moreover high shear strength and modulus are also major considerations and the core should have good load bearing capability, sufficient compressive
strength to withstand local indentation loads. Finally good thermal stability as well as good fatigue resistance is usually required.

Consequently, in order to increase the load carrying capacity of the sandwich construction without penalizing its lightness, it is desirable to maximize the through-thickness stiffness and strength of the core. One strategy of achieving this is to add through-thickness reinforcement to the core, with the ends of the reinforcement material embedded in composite face sheets. The best option is to have a continuous 3D reinforcement to increase the delamination strength and toughness between face sheets and core. In recent years several investigators have considered a number of innovative designs to improve the strength of foam cores such as 3D weaving [1], 3D Z-pins embedded in foam [2], stitch bonding [3] and hollow integrated core sandwich [4]. Usually those solutions lead to the decrease of the production rate and the increase of the part cost.

The patented Napco® technology presented in this paper is designed to create in a continuous way 3D tailored sandwich structures while maintaining the production efficiency [5]. The main features of this technology are presented in this paper.

## 2. NAPCO TECHNOLOGY

The Napco® technology is a manufacturing process of 3D fibrous structures and 3D sandwich composites in which the face fabrics and core are integrated together in one construction. A machinery called “3D Web Linker” has been specially designed and produced by the company Laroche S.A in partnership with the inventor (Figure 1). The sandwich construction is based on the needle punching. The through-thickness reinforcement is obtained from regular fabrics such as chopped strand mats, continuous fiber mats and fabrics made of pre-oriented fiber yarns. A multi-needle arrangement set at desired through-thickness pattern and density, penetrate the assembly of glass fabric layers and the foam core on both sides. During this process, the needles take glass yarns from faces and carry them through the rigid foam. As the needles are withdrawn, fibers remain inside the core (Figure 2). Then the sandwich panel assembly is advanced by a desired spacing to produce the next set of bounds.

This process allows manufacturing of single sandwich from 3 mm up to 60 mm in thickness, or multiple sandwich using an additional fiberglass mats inserted between foam cores. The maximum fiber density is currently of 18 fiber bonds / cm² with regular needles. Optimized needles are being developed to increase the amount of fibers carried through the sandwich thickness. The maximum production speed depends on the fiber bounds
density, a value of 3 up to 10 m/min is achievable with an industrial
machinery. The width of sandwich components ranges from 1m up to 4m
with commercially available machines.

<figure><img src="image_2.png" /><figcaption>Figure 1. “3D WEB LINKER®” machine installed in horizontal feed in position for insertion of rigid cores and panels.</figcaption></figure>

This process allows the production of complex preforms that can be post-
formed (radius of curvature of 0.5 m or less) and impregnated with liquid
resin using a closed molding production method or thermoformed in the case
of thermoplastic composites. Another processing route is being investigated
that would allow one to infuse on-line the 3D reinforced sandwich structure
including injection of grinded materials.

<figure><img src="image_3.png" /><figcaption>Figure 2. Schematic illustration of the Napco technology and the 3D sandwich construction.</figcaption></figure>
## 3. APPLICATIONS

The applications of the Napco® technology cover a wide range from traditional textile to advanced polymer composites. Some of them are detailed hereafter. A few auxiliary actions are implemented in the Napco® technology to enable a wide range of capabilities. Table 1 reports actions currently available.

<table><caption>Table 1. Auxiliary actions of the Napco® technology.</caption><thead><tr><th rowspan="2">Auxiliary actions</th><th colspan="2">Working position</th></tr><tr><th>Horizontal</th><th>Vertical</th></tr></thead><tbody><tr><td>Guiding of solid inserts</td><td>X</td><td></td></tr><tr><td>Distribution of grinded materials</td><td></td><td>X</td></tr><tr><td>Injection of viscous material</td><td>X</td><td>X</td></tr><tr><td>Spraying</td><td>X</td><td></td></tr><tr><td>Blowing</td><td>X</td><td></td></tr></tbody></table>

### 3.1 Manufacturing of 3D textile preforms

The Napco® technology offers the potential of creating integrally textile sandwich structures. Compared to other process designed to produce distance fabrics, this technology is a highly automated process that leads to significant cost benefits. Figure 3 shows a variety of 3D textile preforms manufactured by means of this technology.

<figure><img src="image_4.png" /><figcaption>Figure 3. Example of textile preforms produced with the Napco® technology.</figcaption></figure>
## 3.2 Manufacturing of 3D sandwich preforms

Once the 3D sandwich preform is produced, it can be impregnated by a liquid resin means of any composite molding technique. Closed mold methods (Liquid Composite Molding) are found to be appropriate. For those technologies, it has been noticed that the transverse fibrous bounds are properly impregnated. In addition, they help to balance resin flow on both sides of the sandwich.

Another processing route is being investigated that would allow one to infuse on-line the 3D reinforced sandwich structure. This one-step consolidation process is expected to lead to further cost benefits.

The manufacture of double-curved sandwich parts is still carried by hand lay up, except for thermoplastic composite. The Napco® technology offers potential improvements in manufacturability as it facilitates the post-forming of initially flat sandwich performs into simple or double-curved shapes. To achieve this, a dedicated strategy for needle placement is necessary to break the foam core and create disconnected blocks of foam separated by the pile yarns. It is not necessary to use grooved foams to achieve this operation.

A few examples of flat and curved 3D reinforced sandwiches are presented in Figure 4.

<figure><img src="image_5.png" /><figcaption>Figure 4. Illustration and example of sandwich preforms and panels produced with the Napco® technology.</figcaption></figure>
Figure 5 shows an example of complex automotive part made of sandwich preforms manufactured with the Napco® technology.

<figure><img src="image_6.png" /><figcaption>Figure 5. Example of a resin transfer molded automotive part with a sandwich preform produced with the Napco® technology.</figcaption></figure>

## 3.3 Recycling of materials

Another interesting feature of this technology is the ability to recycle scraped materials used in the form of crushed or grinded elements. Figure 6 shows the “3D Web Linker” machine installed in vertical feed position to allow grinded materials to be inserted simultaneously inside the sandwich preform.

<figure><img src="image_7.png" /><figcaption>Figure 6. Example of sandwich panels made of recycled materials and produced with the Napco® technology.</figcaption></figure>
Examples of application are presented in Figure 7. The line of fiber bounds creates channels for storage of powders, pellets, particles, hollow spheres, etc. They are inserted while creating the hollow cells or channels with the Napco® machinery working in vertical position.

<figure><img src="image_8.png" /><figcaption>*Figure 7. Example of sandwich panels made of recycled materials and produced with the Napco® technology.*</figcaption></figure>

**3.4 Hollow integrated core sandwich**

This construction has several advantages as the upper and lower skins are integrated with the hollow core, thus preventing the weak interface between the core and skin seen in traditional sandwich construction and the interstitial space provide multi-functionality in terms of storage, routing of electronic wires, fire retardancy, space for embedding miniature components etc (figure 8). Solid inserts as metal, wood or plastic profiles can be embedded in cavities.

<figure><img src="image_9.png" /><figcaption>*Figure 8. Example of hollow core panel produced with the Napco® technology.*</figcaption></figure>
## 3.5 Manufacturing of multiple sandwich

This process allows manufacturing of multiple sandwich up to 60 mm in thickness using an additional fiberglass mat inserted between foam cores. A example of such a panel is presented in Figure 9.

<figure><img src="image_10.png" /><figcaption>Figure 9. Example of a double sandwich produced with the Napco® technology. It is made with two foam cores, one vegetal non woven fabric in the center and fiberglass mats in composite skins.</figcaption></figure>

## 4. MECHANICAL PROPERTIES

Flexural tests were carried out according to ASTM C 393 standard. Tested specimens were cut out from flat panels constituted of two composite skins made from polyester resin reinforced by one layer of fiberglass mat (350 g/m², Saertex), a 40 kg/m³ PUR foam core and various fiber bounds densities and steps. The total thickness is 23 mm; 20 mm core thickness and skin thickness of 1.5 mm. Figure 10 shows the fiber bound density in the length (highest) and width direction (lowest). Figure 11 presents a typical 3-point bending test in the direction of the lowest fiber bound density. For sake of comparison, a non-reinforced sandwich made with the same materials has a bending strength of 10 MPa. Figure 12 presents the core shear strength measured with the ASTM C 393 standard. Although ASTM C 273 should have been preferred, testing results showed that the core shear strength can be increased by a factor two as the fiber bound density is about twice as high.

The vertical pile yarns look like a multiple series of miniature I-beam columns, distributing loads and providing better mechanical properties compared with the non-reinforced sandwich. The strength of the reinforced sandwich is improved by increasing the fiber bound density. Current works are addressing the effect of the fiber bound size on mechanical properties. The fiber bound size depends on the needles used to drive fibers through the core.
<figure><img src="image_11.png" /><figcaption>Figure 10. Fiber bound steps and density.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Figure 11. 3 point bending test (fiber bounds steps and density: 16 x 6.3, 496).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 12. Core shear test (fiber bounds steps and density: 16 x 6.3, 496).</figcaption></figure>
**5. CONCLUSION**

The patented Napco® technology presented in this paper is designed to create in a continuous way 3D tailored sandwich structures while maintaining the production efficiency. It is a reliable, industrial, reproducible and cost effective process for manufacturing of sandwich structures. Single or multiple sandwiches with thickness varying from 3mm up to 60mm can be produced. A flat sandwich preform can be post-formed in a doubly curved shape using a dedicated strategy for needle placement to break the foam core and create disconnected blocks of foam separated by the pile yarns. Dry sandwich preforms produced with this technology can be easily infused using any Liquid Composite Molding process.

The mechanical performance can be tailored to actual needs by adjusting the number and pattern of through-the-thickness fiber bounds. Additional functions can be integrated to the sandwich panel thanks to the insertion of extra components in spaces created by this technology.

**REFERENCES**

1. M. K. Bannister, R. Braemar and P. J. Crothers, "The mechanical performance of 3D woven sandwich composites" Composite Structures, Vol. 47, No. 1-4, pp. 687-690, 1999.

2. U. K. Vaidya, S. Nelson, B. Sinn and B. Mathew, "Processing and high strain rate impact response of multi functional sandwich composites", Composite Structures, Vol. 52, No. 3 4, pp. 429 440, 2001.

3. P. Potluri, E. Kusak and T. Y. Reddy, "Novel stitch bonded sandwich composite structures", Composite Structures, Vol. 59, No.2, pp. 251 259, 2003.

4. M. V. Hosur, M. Abdullah and S. Jeelani, "Manufacturing and low velocity impact characterization of hollow integrated core sandwich composites with hybrid face sheets Novel stitch bonded sandwich composite structures", Composite Structures, Vol. 65, No.1, pp. 117 127, 2004.

5. European patent No. EP 0 594 700 B1.