Are Silicone Bags Right For Your Closed Molding Application?

Since COMPOSITES 2004, there have been more than 100 customers that we know of who have focused on making silicone vacuum bags part of their closed molding strategy. That includes a serious spike in interest developing in Canada and Australia. It can certainly be considered a growing trend on the FRP side of the composites business, while aerospace continues to strongly embrace silicone bags for compacting and closed molding.

And a lot has been learned in that year about where silicone vacuum bags actually fit in the industrial marketplace, and what features and benefits they offer. We've also figured out a lot about how to manage the learning curve and define problems and limitations. Like any part of the closed molding technology tool kit, a molder must match the application and required product specs with the closed molding process choice.

Having a couple hundred closed molding demos under our belt, we have seen every technology fail and it is clear you can make every closed molding technology fail, including the use of silicone bags. Application, process choice, and expectations must be coordinated or there will be a lot of finger pointing, wasted time, and money.

Our theory on why closed molding in all forms is moving at a slower than expected pace is because it is less predictable than open molding, and while you are learning the technique it can be slower and more expensive to produce a part. Plus that pile of wasted parts out back creates an eye sore. This is not a very good recipe for technology change. Still the march to closed molding is obvious and silicone vacuum bags, as a part of the vacuum infusion tool kit, are going to play a significant roll as will enhanced training on how to make and use silicone vacuum bags.

Advantages - Features and benefits of silicone bags are the:

  • Ability to draw silicone elastomer sheet into/over complex and fairly dead part surfaces;
  • Relative ease of building a custom fit silicone bag using combinations of silicone sheet and elastomer caulk;
  • Transparency;
  • Availability of a variety of silicone sealing systems with and without frames;
  • Design flexibility/adaptability;
  • Ability to adjust lay-up schedule and placement of reinforcements without a need to rebuild a silicone bag;
  • Demonstration of bag toughness and useful life,
  • Surface quality achieved in the part; and
  • Case studies of economic benefits and labor savings.

Limitations and Challenges

There are limitations and challenges as well. Silicone sheet stock has over 1000 percent elasticity. This is only true, however, if every inch is being stretched uniformly and there is no resin backpressure under the bag. This has led to dissatisfaction about resin rich corners and sharp parts radius.

The obvious answer is to build a custom fit bag and if it fits well, the problems can be eliminated much like they are when using a hard counter tool. However, it takes practice to build a custom fit bag that really meets requirements. While moving up the learning curve there can be considerable frustration, regression, and time wasted. One repeated lesson seems to be that a custom fit bag must be made on a real part plug or it will generally be too big, and that if the bag is not made on a real part plug that this will lead to fit issues and wrinkles under vacuum.

There also have been issues with some seal systems, particularly zipper seals. The most popular way to mount and handle silicone bags is with a fiberglass or metal frame, attaching the bulb seal system and bag directly to the underside of the frame or plenum. Seals must be mounted with the frame off the mold and upside down. Attaching the seals with silicone adhesive is relatively easy, but seal alignment is very important. We have experienced problems where the outer seal is too close to the edge of the mold flange to hold a good vacuum, seal buckling then occurs on the corners. The importance of keeping the sealing surface free from adhesive residue is critical. Any of these issues can lead to vacuum leaks and air in the part.

And we have learned there is nothing magic about silicone bags, or for that matter nylon bags, in making the vacuum infusion process work properly. Yes we can get a quick and effective seal, but that must be combined with the right infusion grade glass and resin to get a completed and filled part. There is nothing more disappointing than getting about 90 percent filled after several hours of setup and having a failure. Blame goes everywhere, including the silicone bag for pressing to tight into the glass profile and closing off resin flow.

What Works?

Since COMPOSITES 2004, there are at least 50 new vacuum infusion customers who employ silicone bags and who have navigated the learning curve well, learning what to do and what not to do when processing small parts (less than 100 square foot surface area). Some like FIBERTECH are making custom fit bags using nothing but silicone caulk. Others have successfully developed the technique for using flat silicone elastomer on the flat part surfaces and sealing it together with caulk and usually Dacron reinforcement.

Some customers are working with very large silicone sheet stock bags and having great success. S2 TIARA continues to have great success with its 800 square foot silicone bag used to mold multiple cruiser floors. Production with this bag is well over 250 parts. NEMATO has used several 500 square foot frameless flat bags to vacuum infuse large sewage treatment covers, resulting in hundreds of hours of labor savings and better finished surfaces versus using a disposable nylon bag. Company officials estimate parts are 30 percent lighter than open molding.

Why Not Composites has promoted its CARTM (Channel Assisted Resin Transfer Molding) infusion process using a double flat sheet silicone bag system with a spacer between them in a variety of applications, some very large parts included. In all cases, this system is attached to a frame with appropriate lifting manipulators in place. CARTM is being used in vacuum infusion of 50-foot (400 square foot) transportation structures. CARTM is proving to be a very viable infusion technology for large and small product applications including a new marine dock system where parts are 50-150 square feet. CARTM adds considerable control to the process and can greatly increase productivity with accelerated part filling speed.

Vacuum lock zipper seals that fix to the edge of the tool flange have proven very effective with silicone bags in vacuum infusion. Even "mechanical only" zipper seals have performed effectively on envelope bags where a flat sheet of silicone elastomer is folded on itself and the zipper is used to seal the open three sides. This concept is being used for compacting flat parts under vacuum and also for vacuum infusion of automotive aftermarket parts. This application has proven to be very time efficient, and has improved part weight and quality. This has proven effective with fiberglass and carbon parts.

The Look Ahead

If you are thinking about converting to closed molding and vacuum infusion, silicone bags certainly should be part of the equation. Designing the part with the bag in mind gives you lots of flexibility. As we are all learning, sometimes the hard way, these are all process decisions with several components in play in that decision-making process.

There is an economic incentive to a silicone bag versus light RTM and even nylon bags, that includes materials for construction and time to construct and seal. But silicone does weigh about 1/3 pound per square foot, so if you want to build a big shaped bag for a boat mold, it is going to have to be mounted to a frame to be handled effectively. Boat building using vacuum infusion and silicone bags represents a great opportunity with the right mounting and handling systems combined with the right vacuum infusion grade glass and resin. If you are building 200-300 units of a particular model, vacuum infusion with an automated silicone bagging process, could provide a real productivity payback.

There are equally exciting opportunities for fabricating all shapes and sizes of panels, decking, and siding. These applications span the marine, transportation, and construction industries. Large silicone bags have proven to be a vacuum infusion process asset for several applications mentioned above and more are under development. For instance, another version of a large panel is a wind blade. With more and more parts being fabricated via vacuum infusion, silicone bags and the CARTM process could prove to be very attractive option for innovative parts manufacture.

Each day brings new questions, new ideas, and new applications. We are growing smarter about how to answer in the affirmative and also to point out issues and limitations concerning silicone bags. We hope COMPOSITES 2005 generates another 100 (or more) serious inquires for silicone bags as part of the vacuum infusion process.

Brad Lienhart is the manufacturer’s representative for infusion processes: 919.322.0722; lienhartb@yahoo.com.

Keith Charles is director of sales: 817-335-3451; kcharles@mositesrubber.com.

Kyle Hearne is director of research and development: 817-335-3451;

khearne@mositesrubber.com. All three work for the Mosites Rubber Co., in Fort Worth, Texas.