Different Bagging Films and Their Benefits – Part 4 – Silicone & Reusable Vacuum Bagging Films

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 Background

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.

Advantages

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.

Types

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)