Reusable Bag Molding
Reusable Bag Molding is often described as a hybrid process that borrows desirable features from both LRTM and VIP. Reusable Bag Molding is especially useful in applications where part geometry is complex and where high glass-to-resin ratios are desired, but where disposables and slow cycle times are not acceptable.
In the last five years, Reusable Bag Molding has emerged as an economically viable process for a wide range of composites molding applications. These bags are constructed from a variety of different processes. The majority of manufacturers using Reusable Bag Molding are working with one of two different methods:
Closed Cavity Bag Molding (CCBM), a patented process that is simple to adopt and features tooling with one rigid mold half and one very flexible mold half made by applying silicone material with a brush; and Silicone Bag Molding (SBM), which is also a process that features one rigid mold half with one flexible mold half, only the flexible mold half is typically made by splattering silicone through specially designed equipment rather than solely with a brush on method. With Silicone Bag Molding there are also several controls that can be added to the process that assist with injection, flow and cure. These controls can also be applied to VIP and Light RTM as well. These controls are often referred to as Flex Molding.
Closed Cavity Bag Molding (CCBM)
With the exception of a wide perimeter flange, the rigid mold half can be constructed in exactly the same manner as an open mold. The CCBM process uses vacuum to clamp the mold halves and to help pull the resin through the reinforcement. However, the resin is injected, rather than infused as in VIP, but under relatively low pressure (the use of vacuum and injection pressure simultaneously is often referred to as a push/pull system).
CCBM is popular because it works well in many applications and because it has truly fulfilled the promise of a simple low-cost entry into the closed molding process. Don’t misunderstand; there is a learning curve, as there is with any newly adopted process, and the tooling will cost several times that of an open mold. But the process, the tooling, and the startup costs are well within the financial means and capabilities of most open and closed molders.
Each of the popular low-cost molding processes has advantages and disadvantages. A typical molder should learn all three processes, choosing the process that is most suitable for each application, based on part geometry, customer specifications, and desired production volume. If an open molder has concerns related to styrene emissions, shop working conditions, worker retention and training, or part-to-part consistency, CCBM may provide a viable alternative to open molding.
Silicone Bag Molding (SBM)
Basically SBM is not that much different than CCBM…just an easier process to work with if you are making multiple parts. It also does not use a clamping ring like CCBM and relies solely on vacuum pressure to secure the mold in place prior to resin injections. In some cases, with SBM less than one day is needed to convert a mold to closed molding. An existing part, rather than sheet wax, may be sufficient for creating the part thickness. And, no release agents are required. Silicone bags if cared for properly are good for hundreds of part cycles. And, unlike VIP, very few disposables are required for SBM.
In addition, the many advantages that CCBM offers are also experienced by the silicone bag user. However, the silicone used for the SBM process is a more environmentally pleasing product with little to no smell.
It is important to note that both CCBM and SBM eliminate virtually all styrene emissions when the process is used properly.