Creating Thermoplastic Composite Parts Using Bladder Molding Process
Technical support experts from Composites One demonstrated at CAMX 2021 how to create a composite part with non-conformed shapes using the bladder molding composites manufacturing process. Expandable bladders are used in this process to make complex and hollow composite parts, for applications in aerospace and automotive.
How bladder molding works
The CAMX demonstration used a conformable braided commingled nylon carbon sleeve combined with carbon reinforcement. Technicians first inflate the silicone bladder with air pressure and use end fittings to seal the air pressure inside the silicone bladder. Bladder molding uses positive pressure inside the bladder and forces the sleeve to the inside surface of the tool. The sleeve material produces non-conforming shapes through a process that is repeatable and produces predictable results with each cycle.
Using precut sleeves, the technician stuffs the layers inside one other to create the pre-form, trimming excess material from the ends so the sleeve materials are the same length. The layered sleeve is then applied over the bladder for a quick and easy part layup. With the bladder and sleeves ready to be loaded into the two-piece aluminum tool, fittings are used to lock in the ends to hold the bladder in place between the two tool parts. Bolts are used to tension up the two parts to resist against the pressure from the bladder.
The mold is placed on a transfer table and an air-line is hooked up so air pressure can be applied slowly through pressure cycles. When the bladder slowly expands into the braided sleeve, the sleeves decrease in length. By pressure cycling, the bladder can be receded for more consolidation and a higher-quality part.
Application of heat
Technicians slide the mold with the inflated bladder between the heating plates and begin the cycle time. The thermoplastics process is driven by the application of heat, and with reinforced plastics, cycle times are relatively fast. For this demonstration, heat was applied for six minutes at 320 degrees Fahrenheit. Cycle times are determined by how quickly heat can be transferred into and out of the mold.
Heat management – the key to success
Heat management – the rate at which heat is melted and flowing – is the most important consideration with thermoplastics processing. Manufacturers reach two critical numbers during the process of being heated. Glass transition temperature of materials, or Tg, is the temperature at which material will soften, and can be reformed. Second, the melting temperature is when resin begins to flow.
Thermoplastics have varying Tg and melt temperatures. Composites One works with manufacturing customers to identify and achieve their ideal temperature based on materials used and other factors. For the specific process in the CAMX demonstration, which used commingled material of polymeric fibers blended with dry carbon fibers, the ideal temperature needed for the polymeric strands to melt and flow into dry carbon was 450 degrees Fahrenheit. Once the temperature is reached, the technician slides the mold onto cold plates to release some heat.
Another major factor to the overall cycle time is to identify the optimal demold temperature, or how quickly the thermoplastics can be released. For this demonstration, it was approximately 200 degrees Fahrenheit. Once the demold temperature is reached, air pressure is cut off to bleed pressure out of the bladder, and the finished part is demolded on the demold table. When ideal temperatures are reached and a proper release system in place, parts should release with ease on every cycle.
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