Jul 20th 2021
To finish up our series on Bagging Films we’ll hit on Silicone & Reusable Bagging Films. There’s a lot to cover in this one so grab a beverage and buckle in.
Silicone & Reusable Vacuum Bagging Films
Many composite parts are made from the bagging selection above. However, there is no restriction in using other types of materials that can be used for bagging processes. There are many options in silicone type bagging films that offer unique advantages over conventional bagging films. These types of bags have an array of applications and are often reusable in composite bagging schemes. These advancements in bagging films have been described as game changing to the composite industry.
These bagging films can be used with resin infusion and traditional layup methods and autoclave cures dependent of the service temp of the materials used for the bagging film. These is most chemically compatible with Polyester, Vinylester and Epoxy layups. Phenolic resins may pose a challenge; however, Teflon release or barrier films may be used to mitigate issues related to phenolic resin’s corrosive nature.
Silicone is one of the most widely used rubber like substances on earth. When Silicone is used as a bagging film, it’s versatility, flexibility, strength, and durability characteristics provide for a high-quality reusable bagging film in the even the most high-performance composite application. The main advantage of silicone bags lies in manufacturing parts with little degrees of variation in the bagging process. Ending with a high degree of repeatability throughout the cure process.
Silicone bagging may sound like more than it’s worth for one off parts or tools that may not see more than 5-10 cycles, but as for repeatability, Silicone bags are revolutionizing composite manufacturing. In addition, this process provides reduced production costs over time as fewer “expendable” materials are being used and/or wasted. The ease of bagging reduces labor in time of bag application as it simply goes over the uncured part with relative ease.
Disadvantages / Limitations
Some silicone bags can be complicated, some even more complicated than the layup itself. The cost of the material is generally much higher than that of a single use bag. Experience plays a key role with getting a custom fit bag for reusable bagging films. The learning curves that go along with formation of these bagging types are comparable to composite parts themselves. One key factor in mitigating these growing pains are using a real, acceptable part or plug for fabrication of the bags.
Silicone bags may be made from many different fabrication methods and the design process of some bagging types can be just as complicated. The time spent exploring the options for design in silicone bagging processes could eliminate many problems that arise from using conventional bagging films when industrializing processes for manufacture. There is not always a right or wrong answer when selecting and designing for specific fabrication processes. Below is an overview of some silicone bagging types and fabrication methods. While all the fabrication steps are not listed, the following will give an overview for selection of the possible type of silicone bags that could be used or designed for a given project.
Spray-On Silicone Bag Applications
“Spray-On” Silicone bagging films are made by dispensing silicone material through a nozzle using a spray gun and applying it to a tool with a plug or cured part usually already on the tool surface. Because silicone doesn’t usually stick to anything other than itself, it can be applied to a clean mold / part surface. One of the most important pieces of the spray on process is proper mixing of the silicone during the application.
Basically, one would start spraying onto the surface with multiple layers until the desired thickness is reached for the parts, usually being thicker for larger parts but using (~1/4”-1/2”) as a baseline thickness. Vacuum port inserts, reinforcement fabric, and breathable airways may be added as part of the fabrication process.
The advantage of the spray application lies in its ability to conform to an already cured part and mold surface. What one sprays onto the surface will be consistent and intimate to with the tool / part used. Consolidation of plys through the layup procedures are then standardized by the bag through the cure process. This eventually leads to higher control of final part thicknesses and eliminates variation in bagging processes.
Disadvantages & Limitations
The main disadvantages to the spray on type of bag are the variables that go into fabricating the bag. On one hand, once a good silicone bag is achieved, part consistency and quality are higher. On the other, getting a good bag from the spray method can be messy and great care must be taken through the process. Cleanliness is key and ensuring correct setup is a must for success. Tools and spraying mechanisms must be cleaned and inspected before each use. It is common practice to test the process successfully a few times before actually making a vacuum bag. Once experience is gained in proper mixing and cure, this process can be just as easy as making a single bag in a conventional method.
Initial startup costs of some systems within sprayable bagging can be as much as $10k for a industrial setup, which may seem astronomical to some applications. Other limitations to this method are brought upon spraying onto vertical / transitional surfaces on larger tools. Often it can be very tough to apply the sprayed liquid silicone evenly and adequately over complicated surfaces along the tool or part. While it is not impossible to apply this method to vertical surfaces, experience with a given system are needed and additional processing may be required dependent of the types used.
Silicone Rubber Sheets & Formable Fabrication
Silicone Rubber sheets may also be used for providing a vacuum bagging film surface. Variations of widths and thicknesses and fabrication methods are available for almost any application need. Multiple layers are placed upon the tool and mold surface (as the spray method) It can be applied surfaces on its own to provide a reusable film for vacuum pressure.
Silicone bagging options include the ability to form to custom shapes and sizes. The options for this bagging type are nearly endless. When parts have multiple contour changes and vertical bagging surfaces, parts may be bagged quicker and easier by using this method over all others. Preform sheets may be used that are purchased as uncured. Coming off a roll, these soft and workable rubberlike sheets are laid up onto a cured part to a desired thickness and are cured by vulcanization (heat) to form an elastic and reusable bagging surface. These bags may be designed thicker in high stress areas and may be fabric reinforced (usually by twill or woven fiberglass). Formable silicone bags have proven to provide ease in bagging & demold processes in comparison to conventional bagging methods. These methods eliminate the messy nature of sprayable bagging and mitigate variation during the bag fabrication process. The only main disadvantage to using this type of bagging process is the need for a high heat cure and post cure. Temperatures needed for cure usually consist of 300 to 400 ºF.
As an option, sheets cured of silicone may be used with tools that have vacuum application applied inside of the tool and draw vacuum down differently than conventional methods of vacuum port placement. In the case illustrated below, instead of a vacuum port being on the bagging film surface, it is integrated into the tool. A lip is added onto the layup tool surface for initial pressure buildup for the silicone sheet (bagging film) to be sucked down to the part and underlying tool surface. These types of bags hold higher standards than even Airtech’s Stretchlon line in terms of flexibility and elongation and come in differing thicknesses and temperature selections based upon application need. Heated cures are not needed when using the precured sheet method.
These types of sheets are also made to be used as intensifiers or caulplates under conventional bagging films as part of some overall bagging schemes. Though it may look complicated, in application these sheets have been proven to provide higher laminate quality in male and female radii and along laminates that are prone to wrinkling. (As shown below)