|
Introduction
In this section, those
manufacturing processes typically used to make products found in
construction/civil infrastructure market are covered. Unique to the
composites industry is the ability to create a product from many
different manufacturing processes. There are a wide variety of
processes available to the composites manufacturer to produce cost
efficient products. Each of the fabrication processes has
characteristics that define the type of products to be produced.
This is advantageous because this expertise allows the manufacturer
to provide the best solution for the customer. In order to select
the most efficient manufacturing process, the manufacturing team
considers several factors such as:
 |
User needs
|
|
|
Performance requirements
|
|
|
Size of the product
|
|
|
Surface
complexity
|
|
|
Appearance
|
|
|
Production
rate |
|
|
|
Total
production volume
|
|
|
Economic
targets/limitations
|
|
|
Labor
|
|
|
Materials
|
|
|
Tooling/assembly
|
|
|
Equipment |
|
Pultrusion
Pultrusion is a
continuous molding process that combines fiber reinforcements and
thermosetting
resin. The pultrusion process is used in the fabrication of
composite parts that have a constant cross-section profile. Typical
examples include various rods and bar section, ladder side rails,
tool handles, and electrical cable tray components and now bridge
beams and decks. Most pultruded
laminates are
formed using
rovings
aligned down the major axis of the part. Various continuous strand
mats, fabrics (braided, woven and knitted), and texturized or bulked
rovings are used to obtain strength in the cross axis or transverse
direction.
The process is
normally continuous and highly automated. Reinforcement materials,
such as roving,
mat or
fabrics,
positioned in a specific location using preforming shapers or guides
to form the profile. The reinforcements are drawn through a resin
bath or wet-out where the material is thoroughly coated or
impregnated
with a liquid thermosetting resin. The resin-saturated
reinforcements enter a heated metal pultrusion die. The dimensions
and shape of the die will define the finished part being fabricated.
Inside the metal die, heat is transferred initiated by precise
temperature control to the reinforcements and liquid resin. The heat
energy activates the curing or
polymerization
of the thermoset resin changing it from a liquid to a solid. The
solid laminate emerges from the pultrusion die to the exact shape of
the die cavity. The laminate solidifies when cooled and it is
continuously pulled through the pultrusion machine and cut to the
desired length. The process is driven by a system of caterpillar or
tandem pullers located between the die exit and the cut-off
mechanism.
The initial capital investment for pultrusion is generally higher
than open mold or hand layup processes. The primary expense for
pultrusion manufacturers is the material handling guides and die
fabrication costs. The net result is a low-cost process for high
volume.
The process provides flexibility in the design of pultruded
profiles. Currently, profiles up to 72 inches wide and 21 inches
high are possible. Pultrusion can manufacture both simple and
complex profiles, eliminating the need for extensive post-production
assembly of components. Since the process is continuous, length
variations are limited to shipping capabilities. This process allows
for optimized fiber architectures with uniform color eliminating the
need for many painting requirements.
Resin Transfer
Molding (RTM)
Resin Transfer
Molding or RTM as it is commonly referred to is a “Closed Mold
Process” in which reinforcement material is placed between two
matching
mold surfaces
– one being male and one being female.
The matching mold set is then closed and clamped and a low-viscosity
thermoset
resin is injected under moderate pressures
(50 – 100 psi typical) into the mold cavity through a port or
series of ports within the mold. The resin is injected to fill all
voids within the mold set and thus penetrates and wets out all
surfaces of the reinforcing materials. The reinforcements may
include a variety of fiber types, in various forms such as
continuous fibers, mat or woven type construction as well as a
hybrid of more that one fiber type. Vacuum is sometimes used to
enhance the resin flow and reduce void formation. The part is
typically cured with heat. In some applications, the
exothermic
reaction of the resin may be sufficient for proper cure.
RTM as a process, is multi-compatible with a variety of resin
systems including
polyester,
vinyl ester,
epoxy,
phenolic,
modified acrylic and hybrid resins such as polyester and urethane.
Typically, it requires a resin viscosity of 200 to 600 centerpoise
to penetrate all surfaces of the mold cavity.
Advantages of the RTM process include:
|
|
As a closed mold
process, emissions are lower than open mold processes such as
spray up or hand lay up
|
|
|
The mold surface
can produce a high quality finish (like those on an automobile)
|
|
|
This process can
produce parts faster – as much as 5 –20 times faster than open
molding techniques. |
|
|
RTM produces tighter dimensional tolerances to within
±
.005inch.
|
|
|
Complex mold shapes
can be achieved. Cabling and other fittings can be incorporated into
the mold designs. |
Disadvantages of the
RTM process are:
|
|
High production
volumes required to offset high tooling costs compared to open
molding techniques.
|
|
|
Reinforcement
materials are limited due to the flow and resin saturation of
the fibers.
|
|
|
Size of the part
is limited by the mold.
|
Vacuum Assisted
Resin Transfer Molding (VARTM)
In the traditional RTM
process, a matched set of molds or “closed mold” is used. The fiber
reinforcements are usually preformed off line to enhance the
production cycle time of the molds to perform at a respectable
production rate. Resin is injected at high pressures and the process
is sometimes assisted with vacuum.
However, Vacuum Assisted
Resin Transfer Molding (VARTM) is different for many reasons. First,
the fabrication of parts can be accomplished on a single open mold.
Second, the process uses the injection of resin in combination with
a vacuum and captured under a bag to thoroughly
impregnate
the fiber reinforcement. In the late 1980’s, Bill Seemann invented
and patented a variation to the VARTM process called SCRIMPTM,
which is Seemann Composite Resin Infusion Molding Process. This
process has been used in many new and large applications ranging
from turbine blades and boats to rail cars and bridge decks. Unique
to this process is the manufacturing method that allows the
efficient processing of VARTM to produce large structural shapes
that are virtually void-free. This process has been used to make
both thin and very thick laminates. In addition, complex shapes with
unique fiber architectures allow the fabrication of large parts that
have a high structural performance.
Parts using VARTM are made by placing dry fiber reinforcing fabrics
into a mold, applying a vacuum bag to the open surface and pulling a
vacuum while at the same time infusing a resin to saturate the
fibers until the part is fully cured. This process allows for easy
visual monitoring of the resin to ensure complete coverage to
produce good parts without defects.
Lamination technology is based
on the joining or bonding of two or more laminae to form a laminate.
The materials can vary in type and mechanical properties in addition
to property specific orientation particularly pertaining to wood and
composites.
Laminate Materials
There are an
infinite number of laminate types that can be developed. These
materials can be categorized into three basic areas,
core
materials, high strength and stiffness skins and outer protective
layers. Core materials typically serve the function of connecting
and spacing of the skins to develop stiffness and strength in a
sandwich arrangement. The key property of core materials is shear
strength to insure shear conductivity between the skins, thus the
ability to sustain loads and bending. Core materials are normally
wood,
honeycomb and
structural foams. The outer structural layer or skins traditionally
are either metal or composite, either in combination with a core
material or a multitude of high strength and stiffness layers.
Composite materials offer the widest range of high strength skins
with the ability to change fiber type (fiberglass,
carbon and
aramid) in
addition to the fiber volume and orientation. Composites are well
suited for large deflection applications where high strain
capability and fatigue is required. Typically they are more
corrosion and environmentally resistant than metals. Composite
materials in a lamina form are applied in the form of precured,
prepreg or
“B” stage and
wet layup configurations. The final group of laminae is made up of
thermoplastic
and thermoset materials, which act as a covering to the laminate
structure. In some applications a plastic film or coating is
incorporated into the laminate structure to protect the structure
from impact and environmental effects.
Hand lay up is the
oldest and simplest method used for producing reinforced plastic
laminates. Capital investment for hand lay up processes is
relatively low. The most expensive piece of equipment typically is a
spray gun for resin and gel coat application. Some fabricators pour
or brush the resin into the molds so that a spray gun is not
required for this step. There is virtually no limit to the size of
the part that can be made. The molds can be made of wood, sheet
metal, plaster, and FRP composites.
In a particular
hand lay up process (otherwise known as wet lay up), high solubility
resin is sprayed, poured, or brushed into a mold. The reinforcement
is then
wet out with
resin. The reinforcement is placed in the mold. Depending upon the
thickness or density of the reinforcement, it may receive additional
resin to improve wet out and allow better drapeability into the mold
surface. The reinforcement is then rolled, brushed, or applied using
a squeegee to remove entrapped air and to compact it against the
mold surface.
Chopped strand mat
is the lowest cost form of reinforcement used in wet lay up. It also
provides equal reinforcing strength in all directions due to the
random orientation of the fibers that form the mat. Woven roving is
especially suitable for thick laminates requiring greater strength.
Woven fabric and
braid can
also provide a low cost reinforcement. Once the reinforcement is
thoroughly wet out with resin, it can be easily formed into complex
shapes.
Surface Preparation
and Bonding
A key component
to a successful lamination is the bonding process of the layers.
There are three basic components, which make up the bonding process.
First is the surface preparation of the laminae, which improves the
substrate’s
ability to accept and adhere to an
adhesive.
Surface preparation varies depending on material type. Composites
use sanding and grinding, surface texturing, or solvent cleaning.
The second component is the adhesive itself, including epoxies,
urethanes, phenolics, polyesters, solvents, acrylics and others.
Each adhesive has its attributes depending on substrate type, in use
requirements and process constraints. As a general rule, a maximum
bond is achieved for a given substrate type when the material itself
fails during an ultimate strength test. The maximum lap
shear strength
of an adhesive is achieved when the adhesive exhibits a cohesive
failure in the bond line. The third component of lamination is the
process by which the materials are bonded together. This involves a
host of parameters primarily time, heat pressure, mixture, moisture
and
catalysts (initiators).
It is important that the three basic components of bonding are
properly employed to achieve a successful lamination.
Laminate
Construction
There are three types of
laminated construction. These include sandwich lamination consisting
of at least two high stiffness and strength outer layers connected
by a core, all laminated construction consisting of relatively high
stiffness and strength layers and a third type consisting of a
structural member that is reinforced on the tensile or compression
or both sides of a flexural beam.
Sandwich
lamination constructions are found in many applications from
satellite structures to snow skis. Although both applications may
utilize a sandwich approach, satellite applications generally
require stiffness, strength and extreme lightweight, while the snow
ski requires the laminated beam or composite structure to withstand
large deflections and dynamic performance requirements. In addition
the ski structure is integrated with
thermoplastic
surfaces and metal components. Typically metal and composite
materials are applied to these sandwich structures. Even complex
shapes can be achieved by using composite prepregs and wet lay-ups.
All laminate
constructions utilize relatively high strength/stiffness materials.
An array of laminate configurations is possible. In aerospace
applications, multiple
plys are
orientated in various directions and to provide customized
structural strength and stiffness. These same principles, although
less complex are used in automotive, industrial and recreational
products in the form of structural members, springs, archery limbs
and bicycles.
Multiply Construction
The third type
of laminated construction is utilized most often in construction and
infrastructure applications. To date, reinforcements have been
integrated into Glue-Laminated wood beams (Glu-Lams) and
infrastructure components for fabricating columns, beams and walls.
In many of these applications, a reinforcement, most typically a
composite, is used via lamination to the
tensile side
of a beam or as a wrap on a column. Precured, prepreg or wet layup
composite materials have all been utilized in these applications.
These types of reinforcements improve the strength of laminated wood
beams and or reduce the use of E-Rated lumber with less costly new
growth wood. Lamination in bridges and buildings provide a lower
cost simplified method to revitalize existing structures, increase
load-carrying capability and increase resistance to seismic events.
In the automotive
industry composites are being combined with metals for performance,
weight reduction and cost advantages. The construction industry is
applying composites to wood laminated beams and I-Joists.
The possibilities and
advantages of laminated materials are significant and provide
solutions to product requirements generally not achievable by using
a single material. The advantages in structural performance, reduced
weight and oriented structural properties are just some the
advantages of this approach. In many cases, the result is a
simplified and less costly solution to many engineering structural
problems.
Compression Molding
Compression
molding is the most common method of molding thermosetting materials
such as
SMC (sheet molding compound)
and BMC (bulk molding compound). This molding technique involves
compressing materials containing a temperature-activated
catalyst in a
heated matched metal die using a vertical press.
The molding process begins with the delivery
of high
viscosity
uncured composite material to the mold. Mold temperatures typically
are in the range of 350°
- 400°
F. As the mold closes, composite viscosity is reduced under the heat
and pressure approximating 1000 psi. The resin and the
isotropically
distributed reinforcements flow to fill the mold cavity.
While the mold
remains closed, the
thermoset
material undergoes a chemical change (cure) that permanently hardens
it into the shape of the mold cavity. Mold closure times vary from
30 seconds up to several minutes depending on part design and
material formulation.
When the mold opens, parts are
ready for finishing operations such as deflashing, painting,
bonding, and installation of inserts for fasteners. By varying the
formulation of the thermoset material and the reinforcements, parts
can be molded to meet applications ranging from automotive class ‘A’
exterior body panels to structural members such as automobile bumper
beams.
Filament Winding
The filament winding
process is used in the fabrication of tubular composite parts.
Typical examples are composite pipe, electrical conduit, and
composite tanks. Fiberglass roving strands are
impregnated
with a liquid thermosetting resin and wrapped onto a rotating
mandrel in a specific pattern. When the winding operation is
completed, the resin is cured or polymerized and the composite part
is removed from the
mandrel.
Capital investment is
relatively higher compared to open mold processes. The primary
expense for an existing filament winder would be the cost of the
winding mandrel for a specific application.
References
Shook, Gerald, 1986,
Reinforced Plastics for Commercial Composites Source Book, ASM,
Metals Park, OH.
U.S. Department of Agriculture Forest Service, 1987, Wood
Handbook: Wood as an Engineering Material, Agricultural Handbook
No. 72, Washington, DC.
Introduction to
Composites, 4th Edition, Composites Institute,
Society of the Plastics Industry, New York, NY, 1998.
Rosato, Dominick V.,
Designing with Reinforced Plastics, Hanser/Gardner, Cincinnati,
Ohio, 1997
á
Top |
Print
Entire Section 
