UWE Bristol Engineering showcase 2015

Liam Wakefield
BEng Mechanical Engineering
OPTIMISATION OF BLADDER MOULDING TECHNIQUES DURING
COMPOSITE MATERIAL MANUFACTURING
Project Supervisor
Dr David Richardson
Project summary
The purpose of this project was to investigate the
principles and practices of bladder moulding within
the field of composite material manufacturing.
Bladder fundamentals
Bladder moulding is often used for the production
of composite parts. Uniform consolidation is
provided via Pascal’s Law of Fluid Pressure
Transmission, making the method ideal for
producing hollow structures.
The most significant criteria governing bladder
moulding are:
• Mould geometry.
• Drape properties of reinforcing fabric.
• The extent to which the bladder fills the mould
cavity.
Moulds featuring sharp corners will often produce
poor results as surface friction at the mould walls
prevents the bladder from reaching into the
extremities of the mould. Whereas rounded
features will deviate less from the design shape,
creating a much smaller area in which air and
unsupported resin can accumulate.
When reinforcing fabrics with poor
drape/conforming properties are used in
combination with less than ideal mould shapes
weak, unsupported resin areas and pockets of
trapped air form as shown above. Here it is clear the
fabric is bridging the “V” rather than following it.
The solution to this problem is to either use a preforming
tool with a disposable polymer bladder or
create a bladder with a shape tailored to the mould
cavity.
Materials choice and Cost Analysis
Bladder moulding is traditionally performed with
disposable plastic films which are fabricated into
inflatable tubes. These bladders are so thin and
light that they are frequently left inside the part
after manufacture.
Even when removed after use, these bladders are
often irreversibly expanded or degraded by the
resin.
This necessitates making a new bladder for each
new part, greatly increasing the time and expense
involved producing multiple products.
Analysis showed that the accumulation of labour
and material costs makes polyurethane less cost
effective than Latex after 8 or more parts are
produced. Silicone becomes more cost effective
after 18 or more parts are produced.
Latex is one of the more common materials used to
produce reusable bladders, being easier to work
with than silicone and cheaper.
Latex bladders also tend to expand further than
the silicone equivalent. It is for these reasons that
Latex was chosen as the material from which the
prototype bladders would be made
Silicone bladders are not without their uses
however. Silicone is more resistant to chemical
attack from epoxy and vinyl ester resins. It is also
better able to the +180ᵒC temperatures often
employed while curing pre-impregnated materials.
Manufacturing method
Having established the underlying physical
principles governing bladder moulding and
selected a suitable material, the next step was to
ascertain a method of manufacture.
After some poor initial results applying Latex by
hand, the method of dipping a silicone core (a.k.a.
a master) was tried and found to have favourable
results.
It quickly became apparent that any damage or
flaws on the master were transferred in detail to
the bladder. The photograph on the right of a
bladder in testing clearly shows the start of a
“ballooning” fault originating at a point where
gouges in the master were located, near the first
“O” ring.
A second attempt to manufacture a bladder with a
more challenging shape proved much more
successful, owing largely to the master being in
much better condition and not over heating the
liquid Latex during application. This proved the
potential for large, high quality, reusable bladders
to be manufactured through this method.
Specific effort was placed in this project on the
determining a method of manufacture of bespoke
reusable bladders, which could be used to improve
the quality of laminates and the reliability of the
manufacture of said laminates at UWE through
bladder moulding.
Project Objectives
• Research existing bladder moulding techniques
and materials.
• Identify and record faults or problematic
regions/geometries of existing composite moulds.
• Investigate and determine how different design
features affect the effectiveness of bladder
moulding.
• Evaluate the cost benefits of using reusable
bladder materials.
• Investigate and determine a suitable method of
reusable bladder manufacturing for use in UWE’s
workshops.
• Produce, test and evaluate a prototype bladder
manufactured from reusable materials.
Project Conclusions
• Reusable bladders do offer significant cost benefits
over disposable bladders for the production of
multiple composite parts.
• Sharp corners will adversely affect the conformity
of a part produced by bladder moulding if steps
are not taken to form the reinforcing fabric before
the resin is applied or use a bladder specific to the
mould cavity.
• A method of manufacturing bespoke, reusable
bladders was found that is suitable for use in
UWE’s composites workshop