Debulking and Autoclave Processing for Tooling Materials

This article deals with debulking and autoclave processing for tooling materials.

Debulking

It is required to debulk the prepreg at least at the stages stated:

• After ply 1
• Approximately every subsequent 3 plies
• After the final ply has been completed

This will ensure that there is even consolidation and air is removed from the laminate before final curing. Complicated shapes can be easier to laminate if more frequent debulks are used, but in these cases, the time factor must be taken into consideration.

In case a laminate must take more that one day to lay-up then it must be de-bulked overnight to ensure that it stays in place. This is shown in Figure 1.

Figure 1. The debulking process.

The steps followed are:

• Cover the entire laminate surface with a perforated release film type P3, extending beyond the lay-up by approximately 25 mm.
• Apply a breather coat of around 350 gsm (10.3 oz/yd²) in total to the surface.
• Tailor to fit to avoid bridging. At this stage, the breather can be omitted from the tightest corners if not practical.
• Cover the laminate/assembly with a vacuum bag ensuring that enough slack has been provided to pull into all corners without any bridging.
• Apply full vacuum and leave for 20 minutes.
• Care must be taken when materials are removed from the surface afterward, be careful not to lift up the previously laminated plies.

Preparation for Autoclave

Preparation for autoclave is done as follows:

• A thermocouple must be fitted underneath the first material ply on a non-critical mold surface area.
• Strips of glass rovings are laid every 600 mm (24 inches) around the edge of the laminate continuing to the area on the periphery about to be covered with breather
• The entire laminate is then covered with a non-perforated release film and edges extended by around 25 mm (1 inch).
• A breather coat of around 700 gsm total weight is applied, typically 2 plies of 350 gsm (10.3 oz/yd²) Tailor to fit and ensure all areas are interlinked.
• Extra 2 plies are fitted around the periphery between the edge of the laminate and the inside of the vacuum seal. It is not advisable to miss a breather from any surface at this stage.
• Cover with a vacuum bag ensuring that enough slack has been provided to pull into all corners without bridging. At this stage, the vacuum pack will appear bulky and care must be taken to make sure all materials remain in position as the vacuum bag pulls down.
• Full vacuum pressure is applied and left for 25 minutes before autoclave processing. Check for vacuum integrity and position of tucks in the bag.

This is shown in Figure 2.

Figure 2. Preparation for Autoclave.

Autoclave Cure

The highly reactive nature of the resin system makes it necessary for curing to be done under the strictest control possible, to avoid deviation and hence possible exotherm during cure.

The steps followed are:

• Apply vacuum pressure and hold at 100F for 30 minutes
• Apply 21 psi with vacuum
• Vent to atmosphere
• Raise pressure to between 40 psi and 90 psi
• Increase air temperature at 1 deg F per minute to required curing temperature and cure for the stated minimum time.

Curing time temperature is shown in Table 1.

Table 1. Curing time/temperature.

For TC40 and TC44Si Cure/Postcure

For TC40 and TC44Si cure/post-cure the following steps are followed:

• The laminate is held under a minimum 28”-Hg vacuum at R.T. for a minimum of 24 h before cure. The connection is maintained to the vacuum during transfer to the autoclave system
• Maintain initial 28”-Hg minimum vacuum to part bag and increase vessel pressure to 95 ±5 psig at max 1.5psig/minute while ramping tool to 265 ±10 °F at max 2 °F/minute.
• Hold at 265 ±10 °F for 65 ±5 minutes based on lagging TC except proceed to next step if lead TC has been at 265 ±10 °F for more than 90minutes.
• The part temperature is then ramped to 360 ±10 °F at a maximum 4 °F/minute and the part is held at 360 ±10 °F for 120 to 360 minutes based on the lagging part TC.
• The part temperature is then increased to postcure temperature of 410 ±10 °F at max 2 °F/minute. Maintain application of system vacuum and/or minimum of 15 psig vessel pressure during postcure.
• Based on the lagging part TC, the part is held at 410 ±10 °F for 240 to 360 minutes
• The part is then cooled to below 160 °F at max 5 °F/minute.
• Vent system vacuum and/or pressure and remove part from the vessel.

For out-of-autoclave curing or if the master used is thin-walled, such as epoxy/wet lay-up splash, an alternative cure should be used to include a dwell at low temperature. To satisfy this requirement introduces a dwell at 40 °C (104 °F) or for 2h, then continue with the standard cure cycle.

Removal from Master

In case the tool needs a backing structure such as to prevent a large tool from distorting under its own weight, it should be fitted at this stage prior to release from the master.

Care should be taken not to induce stresses on removing the tool from the master, since it will be mechanically weak at this stage. The mold should be gently eased off the master and lifted evenly all round.

This information has been sourced, reviewed and adapted from materials provided by Toray Advanced Composites.

For more information on this source, please visit Toray Advanced Composites.