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How to Use Prepregs
First, prepare your mold as you usually would. To obtain a finished part, it is necessary to have a flawless mold which has been waxed and coated with PVA. The prepreg is supplied between two backing sheets which need to away before laying the fabric into the mold. Because the prepreg surface is very tacky, it is necessary to place the material carefully into the mold. Unlike a dry fabric in a hand lamination, you can not reposition the prepreg without fixing your mold release. Repositioning will pull up . You can continue to place layers on top of each other up to a total thickness of one-quarter inch at a single time.
(Note: You can use water-based mold releases which tend not to pull up when repositioning but as always, these mold releases tend to result in a duller and potentially swirled surface.)
The prepreg is partially cured which means that it is in B-Stage. It can be trimmed, pleated, and formed using a Knife or a pair of scissors. The prepreg handles very easily and because of its tackiness, it will lay tightly into your mold. You might still have relief cuts or pleats to contend with just like you would with a dry fabric.
After placing the layers in the mold, the prepreg must be compressed to allow bonding, to remove any trapped air between the layers and to squeeze out any excess resin. Although there will be very little excess, the resin will naturally become thinner as the temperature is raised and there will be resin flow prior to a full cure of the resin.
The needed compression can be accomplished using a press but unless the part is perfectly flat, a press is not an option. Consequently, the part is generally vacuum bagged.
With prepreg fabric, there is a benefit in having plenty of time to build your bag and get the part ready for the oven. You are not fighting the natural pot life of the resin. However, the more layers you have laid, the more likely that there will be air trapped between the layers. You should really have perfected your vacuum bagging technique before laying multiple layers of for a single curing.
A big advantage of prepregs is that after curing and cooling, additional layers can be laid onto the part without scuffing or additional surface preparation. Again, up to one-quarter inch can be laid at a single time with the same consideration of not trapping air between layers.
After the layup is complete, the vacuum bag is built and sealed, and you are ready to pull and begin the heat cycle. There are three recommended cure cycles for Fibre Carbon prepregs and all will produce similar properties.
All curing cycles begin with a temperature ramp up and end with a ramp down. The difference is the target temperature and the amount of time required for a complete cure. Always ramp up at a rate of no more than 5°F per minute until the target temperature is attained. Maintain the target temperature throughout the cure cycle and then ramp down at a rate of less than 5°F per minute to at least 150°F before removing from the oven.
Target Temperature Hold For 310°F (154°C) 1-2 Hours 290°F (143°C) 2-4 Hours 270°F (132°C) 4-6 Hours
Let the part cool to room temperature, remove the vacuum bagging materials, and release the part from the mold. Your part should be fully cured and can be put into immediate service.
There are several considerations when determining the amount of time and heat to use. First, if you are not in a hurry, plan to go low on temperature and long on time. You can leave the part in the heat up to 24 hours without any risk of damage to the part. You cannot “overcook” your prepreg. You can, however, “undercook” it.
Second, when determining your temperature and cure cycle, consider the mass and conductivity of your mold. If it is thin metal (low mass and conductive), it will heat up quickly and not interfere with the curing cycle. If it is thick Carbon Fiber (high mass and not conductive), it will take a while to heat up and you will want to be on the conservative side of time and temperature.
Third, you also must consider the quality of your mold and the temperature constraints of the mold material. If your mold is thin and constructed out of typical polyester/fiberglass materials, it has a possibility of distorting when subjected to any heat. If your mold is the recommended thickness of no less than five times the part and is properly constructed out of Carbon and polyester resin, it should not distort or have problems at 270°F. However, this is the higher this type of material.