Molds are used extensively to lay-up large scale composite structures, such as wings, fuselages, propellers, antennas, wind turbine blades, boat hulls and vehicles (Figure 1). These molds are then placed into autoclaves, which subject the composite lay-ups to elevated temperatures and pressures until they are cured, after which the molds are taken apart and the finished composite parts are removed.

Figure 1. Large-scale composite lay-up. Source: Saint-GobainFigure 1. Large-scale composite lay-up. Source: Saint-Gobain

In order to facilitate the removal of the parts from the molds, silicone-based coatings have typically been sprayed, painted or dipped onto the surface of the mold prior to the lay-up of each part. This process could be labor-intensive and prone to error if the mold surface is not properly coated, leading to undesirable surface defects in the composite parts. Furthermore, certain materials in the composite lay-up may not be compatible with silicone, leading to undesirable chemical reactions, poor surface finish or sticking of the part to the mold.

An alternative to silicone coatings, composite molding tapes may offer certain advantages, such as reduced labor costs, longer tool life, greater production throughput and better part quality.

This article describes some of the important properties and characteristics of composite molding tapes and how Saint-Gobain provides products and services for composite manufacturers and OEM suppliers.

Composite molding tape applications and advantages

Composite molding tapes are being widely used instead of silicone based coatings in three important applications: (1) mold release liners are applied to the inside of molds to facilitate the removal of the composite parts (Figure 2); (2) protection and seaming tapes are used to protect mold or tool surfaces, which may extend their life or increase time between preventive maintenance intervals; and (3) flash masking and hold down tapes that are applied to mold surfaces to ensure smooth bond lines and to remove or minimize flash.

Figure 2. Mold release liner. Source: Saint-GobainFigure 2. Mold release liner. Source: Saint-Gobain

One of the major advantages of composite molding tapes is that they can be used and reused several times, eliminating the need to apply silicone coatings to the molds for each part. While material costs of composite molding tapes may be greater than silicone coatings, they offer a net labor savings since it is not necessary to clean the tooling and re-apply the silicone coatings for each part fabricated.

Furthermore, in some composite molding processes, silicone release products are not compatible with the binder of the composite material. These incompatibilities can create defects such as cracks, pinholes, surface defects and spots. Non-silicone composite tapes can solve this problem by providing a silicone-free solution when silicones become troublesome.

Composite molding tapes can have a lower coefficient of friction (COF) than silicone. Because of this lower COF, the part will remove from the mold much more easily. This can be especially important for large parts and large molds. The tapes can also provide a visual indicator showing which portions of the mold are covered, reducing the likelihood of the composite part sticking to the mold. These and other advantages make composite molding tapes an attractive alternative to silicone coatings.

Composite molding tape characteristics

Composite molding tapes consist of a substrate, which is the surface that comes into contact with the composite, and an adhesive, typically silicone, that is used to bond the tape rigidly to the mold or tooling. During the autoclaving process, the tapes are subjected to high temperatures and pressures, so they must be dimensionally stable, and have a high tensile and yield strength. The substrates must be smooth, have low friction and low surface energy to prevent the composite materials from sticking to them over a wide temperature range, enabling their easy removal from the mold. The adhesives must have a high peel strength so that the tape does not de-bond from the mold during the release process after the parts have been cured.

Certain properties or characteristics of the tapes make them appropriate for use as a mold release liner, protection and seaming tape, or a flash masking and hold down tape, as follows.

Mold release liners have a substrate made of polytetrafluoroethylene (PTFE), which is commonly known by the DuPont brand name Teflon, for its superior non-stick properties and temperature resistance. The PTFE material is typically configured as a glass cloth, laminate or film. Saint-Gobain has a unique process that ensures the PTFE has a smooth surface finish that is free of cracks and pinholes. This defect-free surface is the key to the PTFE tapes lasting longer in a composite molding process.

Tool protection and seaming tapes consist of extruded, high modulus and skived tapes, which have better conformability than PTFE glass cloth tapes. Conformability is important for applying the tape to complex geometries without wrinkles and without the tape lifting prematurely.

Flash masking and hold down tapes use polyester substrate materials to enable operation at high temperatures without leaving residue. These tapes may use a silicone adhesive, or if contamination is an issue, may use a rubber adhesive.

Saint-Gobain composite molding tapes

Saint-Gobain is a leading manufacturer of fluoropolymers that offers a complete range of tapes that can be used for mold release, tool protection and flash masking. Besides having expertise in silicone adhesives, they have developed the highest temperature non-silicone masking tapes in the industry. These tapes can be custom-engineered and tailored to meet the performance, cost and manufacturing requirements for specific composite molding applications.

For example, Saint-Gobain produces some of the widest tapes in the industry, with film-based tapes in widths up to 1219 mm, and PTFE/glass cloth tapes in widths up to 1524 mm. The Saint-Gobain CHR M851, M852, M783 and M855 are the highest temperature non-silicone masking tapes available. They can perform at curing temperatures of 200° C, which is very close to the temperature performance of a silicone adhesive. Furthermore, their CHEMLAM® fabrics have longer lives than dip-coated competitive products.

For more information on composite molding tapes used for mold release, tool protection and flash molding applications, visit the Saint-Gobain Tape Solutions website.