Info

(1) Place Lay-up Between Diaphragms in Tool

Pressure

(1) Place Lay-up Between Diaphragms in Tool

Pressure

Vacuum

(3) Apply Vacuum and Slowly Apply Pressure to Conform Blank to Tool

Vacuum

(3) Apply Vacuum and Slowly Apply Pressure to Conform Blank to Tool

(4) Apply Full Pressure of 50-100 psi, Cool Tool, and Remove Part

Fig. 7.58. Diaphragm Forming Method for Carbon/PEEK Parts1

more difficult and expensive than that for thermosets. Resin transfer molding of thermoplastics is not a feasible process with the materials discussed so far. The viscosity is just too high for the long flow paths required for RTM, and total wet-out of a dry reinforced fiber bed is rarely achieved. However, a relatively new class of materials called "cyclics" offers great potential for RTM. These materials initially melt and flow like thermosets, and then undergo a ring opening mechanism to form a linear thermoplastic on further heating.43 The molecular weight increases during heating in the presence of an anionic catalyst.38 At the present time, the technology is applicable only to low temperature thermoplastics, such as nylon and polybutylene terephathalate (PBT). Currently, these materials show great promise for commercial industries such as the automotive industry. In the future, if this technology can be extended to high temperature thermoplastics, it could drastically alter the approach to thermoplastic composite processing and usage.

7.15.3 Thermoplastic Joining

Another unique advantage of thermoplastic composites is the rather extensive joining options available. While thermosets are restricted to either cocuring, adhesive bonding, or mechanical fastening, thermoplastic composites can be joined by melt fusion, dual resin bonding, resistance welding, ultrasonic welding, or induction welding, as well as by conventional adhesive bonding and mechanical fastening.

Adhesive Bonding. In general, structural bonds using thermoset (e.g., epoxy) adhesives produce lower bond strengths with thermoplastic composites than with thermoset composites. This is believed to be due primarily to the differences in surface chemistry between thermosets and thermoplastics. Thermoplastics contain rather inert, non-polar surfaces that impede the ability of the adhesive to wet the surface. A number of different surface preparations have been evaluated including the following:38 sodium hydroxide etch, grit blasting, acid etching, plasma treatments, silane coupling agents, corona discharge, and Kevlar (aramid) peel plies. While a number of these surface preparations, or combinations of them, give acceptable bond strengths, the long-term service durability of thermoplastic adhesively bonded joints has not been established.

Mechanical Fastening. Thermoplastic composites can be mechanically fastened in the same manner as thermoset composites. Initially, there was concern that thermoplastics would creep excessively, resulting in a loss of fastener preload and thus lower joint strengths. Extensive testing has shown that this was an unfounded fear and mechanically fastened thermoplastic composite joints behave very similar to thermoset composite joints.

Melt Fusion. Since thermoplastics can be processed multiple times by heating above their Tg for amorphous or Tm for semi-crystalline resins with minimal degradation, melt fusion essentially produces joints as strong as the parent resin. An extra layer of neat (unreinforced) resin film can be placed in the bondline for gap filling purposes and to insure that there is adequate resin to facilitate a good bond. However, if the joint is produced in a local area, adequate pressure must be provided over the heat affected zone (HAZ) to prevent the elasticity of the fiber bed from producing delaminations at the ply interfaces.

Dual Resin Bonding. In this method, a lower melting temperature thermoplastic film is placed at the interfaces of the joint to be bonded. As shown in Fig. 7.59, in a process called amorphous bonding or the Thermabond process, a layer of amorphous polyetherimide (PEI) is used to bond two PEEK composite laminates together. To provide the best bond strengths, a layer of PEI is fused to both PEEK laminate surfaces prior to bonding to enhance resin mixing. In addition, an extra layer of film may be used at the interfaces for gap filling purposes. Since the processing temperature for PEI is below the melt temperature of the PEEK laminates, the danger of ply delamination within the PEEK substrates is avoided. Like the melt fusion process, dual resin bonding would normally be used to join large sections together, such as bonding stringers to skins.

Carbon/PEEK Laminate

Resin Mixed Interlayer

PEI Film

Carbon/PEEK Laminate

Resin Mixed Interlayer

PEI Film

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