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Integrated approach aids move to closed moulding
As published in Reinforced Plastics Magazine November 2003
(www.reinforcedplastics.com)
The composites industry is moving inexorably away from its humble
origins towards more mechanised and efficient manufacturing processes. Stephen
Leonard-Williams and Richard Bland of the UK’s Composite Integration Ltd explain
how their company helps moulders to set up successful closed moulding
operations.
Introduction
Simple contact moulding (hand or spray lay-up) is, inevitably, becoming less and
less tenable. Environmental concerns about emissions within and from the
workplace have led to ever tighter legislation. Moreover, composites are
increasingly competing with more ‘conventional’ materials in high specification
applications, which demands more sophisticated, efficient and better controlled
manufacturing processes. The group of processes broadly described as ‘closed
moulding’ (resin transfer moulding (RTM), RTM Light, vacuum assisted RTM (VARTM),
for example) go a long way towards resolving the emissions issues usually
associated with contact moulding and lend themselves admirably to automation,
opening the way to high volume production of sophisticated composite structures
with precisely controlled laminate thicknesses and fibre placement.
Composites polymers is currently one of the most innovative and rapidly
expanding fields of engineering worldwide. Indeed, the 'Infrastructure
Composites Report' 2001 published by Composites Worldwide predicts that
globally, the use of composites will grow by more than 525% between 2000 and
2010. However, the rapid and rather haphazard step-up from very 'low-tech'
contact moulding methods to more sophisticated techniques is creating real
confusion for many composites fabricators. Successful management of this
transition will be a key issue in the future survival of the composites
industry.
Process implementation
One major cause of confusion for those moving into closed moulding is a lack of
relevant education and training in composites-related subjects. Moulders are
increasingly encouraged to improve their skill and knowledge base, but specific
process-related training is often left to the suppliers of processing equipment
and raw materials. To successfully move into new and more complex composites
manufacturing processes, it is vital to ensure an efficient transfer of
information based on sound practical knowledge.
The implementation of a new production process can be broken down into several
specific stages. Each stage brings its own diverse challenges and the boundaries
between them can be quite 'grey'. A typical sequence might be:
• feasibility — is the design of the product suited to the manufacturing
process?;
• materials selection;
• equipment selection;
• tooling design;
• tooling construction;
• production set-up and installation;
• training; and
• technical support.
Within the composites industry, the management of this process is often poorly
supported. Moulders wanting to implement a new process have two possible sources
of help: technical support from specialist equipment/material suppliers; or
formal composites training from an academic body.
Support from suppliers will usually be given enthusiastically and has the added
advantage of being free. However, there is an inevitable bias favouring that
company's wares and few manufacturers have the resources to offer more than
'just enough help to get started'. Often, the help supplied is limited in its
presentation and scope but, being part of a selling process, there is usually no
cost to the moulder. This, in itself, has made the industry reluctant to invest
properly in this vital stage.
The alternative, academic training, can provide a good insight into theoretical
issues but is inevitably rather limited when it comes to transferring theory
into a production environment. Whilst a sound academic knowledge is an excellent
basis from which to start, there is no substitute for practical hands-on
experience and assistance.
Integrated approach
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Comparison of closed moulding techniques
Vacuum moulding/RTM Light - usually composite tooling
For:
• Often used as an 'entry level' process but possible to scale up to
higher volume
production (>2000 per annum).
• Tooling significantly cheaper than RTM (approx 40% saving) with the
possibility of using existing 'open' tooling as starting point
• Tooling can be manufactured in-house with the appropriate training
• Short tooling lead times
• Light weight tooling poses less handling problems
• Very 'scalable' with large structures (>20 m²) being achievable
Against:
• Laminate thickness control less precise than RTM
• relatively labour intensive and difficult to automate
RTM - composite tooling
For:
• Relatively short lead times and tooling
can be manufactured in house with the appropriate training
• Automation is possible, with good control over process variables
• Rigid tooling allows faster injection and thus shorter cycle times
• Tooling can be produced from a
conventional 'pattern' without the need
for CAD data.
Against:
• Tool life must be carefully balanced against cost
• Tool manufacture must be carefully controlled at all stages
• Tooling must be meticulously maintained to achieve maximum life
RTM- metal tooling
For:
• Tool life- very high production numbers possible (>10,000)
• Balance between tool cost and tool life
can render metal significantly cheaper than composite if production numbers
are
suitable.
• Tooling can be cut directly from CAD data without the need for a
'pattern'
• Very fine detail/surface finishes are achievable
• High temperature, pressure and abrasion resistance are inherent
features
• Certain shapes can actually be cheaper
to produce in metal than as a composite structure
Against:
• Larger structures can be prohibitively
expensive either to machine or simply in
material cost
• Specialist expertise needed to avoid costly mistakes
• Weight. tool handling infrastructure can
add significant cost |
Composite Integration was formed in 2002 with the express aim of
assisting composite manufacturers wishing to move into closed moulding processes
by providing an integrated and systematic approach. This is normal, production
engineering practice when installing a new manufacturing solution. Processes are
selected, materials identified, equipment is procured, personnel are trained and
a production environment is designed and implemented.
The company is based on a combined experience of 30 years manufacturing
equipment and tooling for RTM type processes, and is able to offer this
experience at any stage. The company aims to avoid the label of 'consultancy'
but provides practical assistance in all key areas.
This help often starts with input at the component design stage. The tendency to
simply copy components that were originally designed for a completely different
process or indeed, material, has often led to manufacturing problems. For
instance, parts designed for compression moulding processes will typically have
details (ribs, webs etc) that are not only impractical to mould in an RTM/VM
process, but are also often unnecessary, due to the scope for more controlled
fibre management. Help to 'design for process' can pay real dividends in
production.
The next stage would be to identify the best tooling option, and to specify
peripheral equipment (injection machinery, vacuum plant, tool manipulation and
clamping devices and materials handling). Finally, assistance can be provided to
ensure that the process continues to run smoothly in production and that
personnel are suitably trained.
Demand for this service, since the launch of the company, has been substantial.
There is little doubt that the adoption of this approach can have a significant
influence on the successful outcome of projects of this nature.
Tool building
As well as technical assistance and training, the company also builds tools and
teaches others how to do so. This extends to a complete training service for RTM
and vacuum processes. Tooling is a recognised 'weak link' in newly implemented
moulding processes, failure often being caused by a lack of understanding of the
basic principles of the moulding process and the materials involved.
Consequences can range from premature mould failure to inaccurate, inconsistent
or wasteful production.
Much of our industry relies on tooling produced 'in-house', with traditional
tried and tested techniques. Many of these systems work adequately, but, tool
construction is often perceived as a 'black art' with many conflicting views as
to the best route to take.
Tooling for closed mould applications is invariably more costly to manufacture
than the simple tools used in contact moulding techniques. Consequently, issues
of longevity and efficiency are of prime concern. The choice of tooling
materials, and the detailed design of the structure, is vitally important in
achieving a workable solution. However, the number of completely different
tooling 'systems' currently marketed makes this choice even more confusing.
Tooling options can range from relatively simple and inexpensive composite
structures to fully machined metal, and the decision as to which direction to go
is not always obvious. The
table above compares the different
options available. The information is by no means exhaustive but covers some of
the issues involved.
Case studies
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Moulded
composite door skin |
Composite
Integration is employed in a diverse range of areas, ranging from highly
structural carbon/epoxy aerospace components to vehicle body panels. Two current
projects, with very different end products, give an insight into some of the
activities needed when introducing a closed mould process.
The first is a project to manufacture a seemingly simple product but in high
volumes. This is coupled with a requirement for extremely high cosmetic
quality. The customer, New World Developments Ltd of Northern Ireland,
manufactures composite door skins and asked Composite Integration to design, specify and install an RTM manufacturing plant with the capability to produce
more than 20 000 door skins per annum in a number of designs.
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The different
stages of the project break down as follows:
1. Liase with product designers to ensure suitability for the
chosen manufacturing process. A high level of detail is required on both A &
B surfaces which must be both practical to machine (as tooling) and to mould
in composites.
2. Identify suitable materials (glass, resin, release agents etc).
A highly reactive modified acrylic resin system is chosen that will allow a
six minute cycle time.
3. Design tooling, injection gating, venting and seal profiles. An
essential requirement is a feature which allows the tools to be changed
quickly to accommodate eight different designs.
4. Identify tooling suppliers able to produce the moulds within
the time scale and budget. The tooling is rendered more complex by the
requirement for a grained surface finish which is achieved by acid etching
as a post-machining operation.
5. Specify tooling materials. In this case tooling grade aluminium
is chosen for its toughness and suitability for the various machining
processes.
6. Specify the surface treatment of the tooling blocks to
withstand the abrasion of the glass fibre reinforcement.
7. Specify the necessary hydraulic press and identify supplier.
The rapid cycle time requires very high speed injection of the resin. The
clamping press must potentially provide 1600 kN of force to resist injection
pressure and maintain thickness accuracy.
8. Specify injection equipment and identify supplier.
9. Integrate equipment for automation.
10. Specify materials handling equipment and identify suppliers.
Bulk resin mixing must allow the addition of accelerators, fillers and
pigments.
11. Liase with all suppliers throughout production to identify any
problems and ensure key targets are met.
12. Design layout of the production area. The production area must
provide a smooth process flow and be suitable for future expansion.
13. Oversee installation and commissioning of the equipment.
Testing of various raw materials provides prototype panels for testing.
14. Train operators in use of the equipment and materials and
provide on-going assistance once full production is underway. |
The second example is in distinct contrast. The project is to develop a high
fibre volume composite structure as a direct replacement for an existing
non-composite component.
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Fibre
preparation of a composite strut |
Customer Vortok
International (part of the Pandrol Group, a specialist in the design of products
for the maintenance of railway track and infrastructure) wanted to produce a
composite strut which could withstand high tensile and impact loads. This
resulted in a glass/epoxy moulding with complex geometry which includes metallic
and polymer inserts encapsulated within the laminate.
Composite Integration’s project aim was to produce pre-production tooling and
prototype mouldings for product evaluation and testing based on laminate design
provided by the Advanced Composites Manufacturing Centre, Plymouth University,
UK. The final design is to be developed into a viable production process.
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The project activities are as follows:
1. Assist during design to ensure practicality at the moulding
stage.
2. Design and build heated composite RTM moulds for the various
components. The tooling must be able to accurately locate the various
inserts and allow injection under vacuum to gain full wet-out of the tightly
packed fibre.
3. Assess the fibre lay-up and develop a practical way of
producing the required fibre preform. The design requires 50% (volume) of
almost entirely unidirectional (UD) glass fibre with a requirement for
transferring maximum tensile loads to the metallic inserts at either end.
4. Identify suppliers for the various UD glass tapes, multiaxial
cloth and rovings.
5. Identify a suitable epoxy resin system. The resin must allow
rapid and relatively low temperature cure (<60ºC) without compromising the
physical characteristics.
6. Manufacture prototype components whilst developing a viable
production process.
7. Work with the customer to make any modifications to the
design/tooling based on test results. |
The key to
success
Closed moulding is now an accepted route to producing high quality and
potentially complex structures. When making the transition, however, more people
fail than succeed, with too many projects resulting in disillusionment and
wasted investment. Why does this happen when there is sufficient evidence of
very successful operations to indicate that it need not be so? One answer is a
lack of professional guidance.
The key to most successful operations has been the use of sound engineering
principles, combining theoretical knowledge with practical and relevant
experience. Investing in this expertise is probably the most cost-effective
decision that any company can make if considering the closed mould option.
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