There are many advantages to vacuum bag molding, a few of which include:

• Improved resin/glass ratio
• More consistency across the laminate and part -to -part as compared to open layup
• Containment of air emissions from the resins

As compared to hand layup and chop layup, there are a few disadvantages, including

• Higher consumable material cost
• Higher capital equipment cost
• Difficulty with superior surface finish

Some parts are more suitable for vacuum bag molding than others.  It also depends upon which process it is being compared with.

Vacuum bag molding requires an extremely tight seal between the mold and the bag.  Molds with multiple pieces or holes for inserts can be difficult to complete a seal.

Parts that are overly large and complex can present challenges with placing resin and reinforcement before the cure cycle starts.  The bag must be completely sealed and under full vacuum before the curing cycle of the resin begins.

The basic premise of vacuum bag molding is that the air is removed from the bag, allowing the atmosphere (air on the outside of the bag) to push the bag onto the part on the mold, compressing the layers of resin and reinforcement.  Many misinterpret the process as “sucking the extra resin out.”  We are merely allowing the laminate to be compressed by the weight of the air above us in the atmosphere to consolidate it before cure.  The excess resin is usually absorbed by extra layers of sacrificial material inside the bag.

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Fender corner damaged by cracks

I began the work by preparing the area by grinding away the outside layer of existing material.  This helps us get a good mechanical bond and a clean surface.  It also allows for a feather edge transition between the repair area and the existing composite.

Fender getting epoxy and glass

The ground-out area is within the black outline created by a primer layer, and the underlying SMC is white in color.  This area was ground with 40 grit sandpaper and followed by a healthy dose of compressed air to remove the dust from the process.  I then wet-coated the areas with epoxy resin before turning to the glass and wetting that out.  The soaked glass was applied to the back side of the fender to add strength back to the cracked laminate.  A thickened epoxy paste was added to the front to fill the cracks that were ground out, as well as filling the holes drilled in the end of them.

Driver's fender from the backside

The glass applied to the area was applied down with a strong bristle brush to get maximum surface contact and remove air bubbles between the layers.  The strength of the glass and epoxy will restore this area of the hood back to its original specification as long as a good bond is made to the composite laminate.

Trimmed after cure

As the resin and glass cure, there is a time window when the resin is in the solid phase, but not rock hard.  This is when it is appropriate to take a utility knife and trim the edge of the loose glass.  If it is tried too soon, it will move the glass on the laminate repair, and if it is too late, the knife will not cut through.  If this time frame is passed, it still can be cut with a saw blade or grinder, but is much messier.  This is also a good time to wipe up any resin that is draining from the repair.

The repair area can now be sanded and body finished before paint and primer!  This resin will need to be covered to prevent UV damage in the future.

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One of the considerations that needs to be taken into account is that the ratios are different and the glass and resin are more compacted by the process. Using the same layup schedule would result in a thinner laminate that is lighter weight and uses less resin. One drawback to this is that the cross sectional area is less, usually resulting in a loss of stiffness. This can be regained by increasing the core thickness to compensate for that loss and to restore overall panel thickness.

Infusion processing does require specialized equipment, consumables, and materials. The resins need to have much lower viscosity(flow like water). The core and glass need to have holes and channels for the resin to flow. A bag needs to be created to cover and seal off the laminate to the mold with out ANY air leaks. The mold needs to be able to be sealed to the bag and be airtight. A high vacuum needs to be able to be drawn on the bagged mold to move the resin. The vacuum pump or venturi needs to be able to achieve a minimum of 28 inches of mercury.

The basic process is that the mold is gelcoated abd skinned with a good, low-shrink resin and chopped strand mat. Then layers of dry glass are added. Coring is placed nice and tight and glued if needed. And the final layers of glass are placed. During the process, any gaps in coring or glass will “bridge” where the vacuum doesnt pull them down, and solid resin will fill them. So good fit is very important. The the bag is placed and sealed after auxililary runner strips are added and resin feed hoses are placed. The perimeter vacuum channel is plumbed to the vacuum pump.

Air is removed from the laminate for a good amount of time before the resin lines are opened up and resin flows into the part (do not forget to catalyze it). This is part of the art of flowing resin to fill the whole laminate with resin before an area gets cutoff from the vacuum or the resin begins to harden. There are some tricks and practice is key.

Resin is stopped just before fill and hopefully resin doesnt run into the vacuum line too far, but that is what resin traps are for. The vacuum is left running as the part hardens and gets to full cure in a couple of hours.

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