CNC Filament Winding Technology For Composite Moulding

27 Aug 08

Filament Winding

In a filament winding process, a band of continuous resin impregnated rovings or monofilaments is wrapped around a rotating mandrel and then cured either at room temperature or in an oven to produce the final product. The technique offers high speed and precise method for placing many composite layers. The mandrel can be cylindrical, round or any shape that does not have re-entrant curvature. Among the applications of filament winding are cylindrical and spherical pressure vessels, pipe lines, oxygen & other gas cylinders, rocket motor casings, helicopter blades, large underground storage tanks (for gasoline, oil, salts, acids, alkalies, water etc.). The process is not limited to axis-symmetric structures: prismatic shapes and more complex parts such as tee-joints, elbows may be wound on machines equipped with the appropriate number of degrees of freedom. Modern winding machines are numerically controlled with higher degrees of freedom for laying exact number of layers of reinforcement. Mechanical strength of the filament wound parts not only depends on composition of component material but also on process parameters like winding angle, fibre tension, resin chemistry and curing cycle.

Industrial Importance of Filament Winding Process

Since this fabrication technique allows production of strong, lightweight parts, it has proved particularly useful for components of aerospace, hydrospace and military applications and structures of commercial and industrial usefulness. Both the reinforcement and the matrix can be tailor- made to satisfy almost any property demand. This aids in widening the applicability of filament winding to the production of almost any commercial items wherein the strength to weight ratio is important. Apart from the strength-to-weight advantages and low cost of manufacturing, filament wound composite parts have better corrosion and electrical resistance properties.

Filament Winding: Process Technology

To begin with, a large number of fibre rovings is pulled from series of creels into bath containing liquid resin, catalyst and other ingredients such as pigments and UV retardants. Fibre tension is controlled by the guides or scissor bars located between each creel and resin bath. Just before entering the resin bath, the rovings are usually gathered into a band by passing them through a textile thread board or stainless steel comb.

At the end of the resin tank, the resin-impregnated rovings are pulled through a wiping device that removes the excess resin from the rovings and controls the resin coating thickness around each roving. The most commonly used wiping device is a set of squeeze rollers in which the position of the top roller is adjusted to control the resin content as well as the tension in fibre rovings. Another technique for wiping the resin-impregnated rovings is to pull each roving separately through an orifice. The latter method results in better control of resin content. Once the rovings have been thoroughly impregnated and wiped, they are gathered together in a flat band and positioned on the mandrel. Band formation can be achieved by passing through a stainless steel comb and later through the collecting eye. The transverse speed of the carriage and the winding speed of the mandrel are controlled to create the desired winding angle patterns.

After winding, the filament wound mandrel is subjected to curing and post curing operations during which the mandrel is continuously rotated to maintain uniformity of resin content around the circumference. After curing, product is removed from the mandrel, either by hydraulic or mechanical extractor.

Materials of Fabrication

Filament winding requires continuous fibre reinforcement and a resin system to bind things together. There are many types of materials that can be used in this process. The choice of materials for a particular product depends more upon the economics, the environmental resistance, corrosion resistance, the weight limitations and the strength performance requirements all play an important part in this decision :

(i) Reinforcement Type :

Continuous fibre reinforcement provides the structural performance required of the final part. The fibre is the primary contributor to the stiffness and strength of the composite. The dominant commercially available fibres are: E-glass, S-glass, aramid and carbon/graphite systems. To summarize these systems :

E-Glass ¾ good tensile strength (3450 MPa), low tensile modulus (70 GPa), lowest cost fibre, available in many forms, widely used in commercial and industrial products, most-used in filament winding;
S-Glass ¾ improved strength (4600 MPa), higher tensile modulus (85 GPa), higher cost fibre, used in aerospace and high performance pressure vessel applications;

Aramid ¾ good strength (2750 MPa), higher tensile modulus (130 GPa), higher cost fibre, very low density (one-half of glass fibre), excellent impact and damage tolerance properties, poor compression and shear strength.

Carbon/Graphite ¾ wide strength range (2050 to 5500 MPa) highest modulus (210-830 GPa), highest fibre cost, intermediate density (two-thirds of glass fibre), poor impact or damage tolerance, best tensile strength and stiffness properties.

(ii) Resins :

The resin matrix that holds everything together, provides the load transfer mechanism between the fibres that are wound onto the structure. In addition to binding the composite structure together, the resin matrix serves to provide the corrosion resistance, protects the fibres from external damage, and contribute to the overall composite toughness from surface impacts, cuts, abrasion, and rough handling. Resin systems come in a variety of chemical families, each designed to provide certain structural performance, cost, environmental, and/or environmental resistance. (Note: Only the thermoset family is covered in this article.) A few major resin matrix families of interest to filament winders are:

General Purpose Polyester ¾ classified as orthophthalic polyesters, lowest cost systems, widely used in FRP industry, moderate strength and corrosion resistance, room temperature curing.

Improved Polyester ¾ classified as isophthalic polyesters, slightly higher cost, good strength and corrosion resistance, widely used in FRP corrosion applications, room temperature curing

Epoxy ¾ wide range of resins available, best strength properties, curing at elevated temperature, good chemical resistance, higher viscosity systems, higher material cost, applications across broad market segment range.

Vinyl Ester ¾ chemical combination of epoxy and polyester technology, excellent corrosion resistance, higher cost, excellent strength and toughness properties, widely used as corrosion liner in FRP products.
Bisphenol-A Fumarate, Chlorendic ¾ more exotic systems for improved corrosion resistance in harsh environments, higher cost resins, higher temperature capability, applications in paper & pulp industry; and
Phenolic ¾ possess excellent flammability properties (e.g. flame retardance, low smoke emissivity), higher cost systems, lower elongation, moderate strength, applications involve fire resistant systems structures

iii) Additives:

By using various additives liquid resin systems can be made suitable to provide specific performance. Fillers constitute the greatest proportion of a formulation, second to the base resin. The most commonly used fillers are calcium carbonate, alumina silicate (clay) and alumina trihydrate. Calcium carbonate is primarily used as a volume extender to provide the lowest-cost-resin formulation in areas in which performance is not critical. Alumina trihydrate is an additive that is used for its ability to suppress flame and smoke generation. Fillers can be incorporated into the resins in quantities up to 50% of the total resin formulation by weight (100 parts filler per 100 parts resin). The usual volume limitation is based on the development of usable viscosity, which depends on the particle size and the characteristics of the resin.

Special purpose additives include ultraviolet radiation screens for improved weatherability, antimony oxide for flame retardance, pigments for coloration and low-profile agents for surface smoothness and crack suppression characteristics. Mould release agents (metallic stearates, silicon gel or organic phosphate esters etc.) are important for adequate release from the mandrel to provide smooth surfaces and low processing friction.

Winding Methods

There are two different winding methods : (i) wet winding, in which the fibres are passed through a resin bath and wound onto a rotating mandrel (ii) prepreg winding, in which the preimpregnated fibre tows are placed on the rotating mandrel. Among these winding methods, wet winding is more commonly used for manufacturing fibre reinforced thermosetting matrix composite cylinders. Compared with prepreg winding, wet winding has several advantages: low material cost; short winding time; and the resin formulation can be easily varied to meet specific requirements. The reviews covered in this article are limited to wet filament winding process and the term "filament winding" is thus referred to the wet winding process hereafter.

Winding Patterns

In filament winding, one can vary winding tension, winding angle and/or resin content in each layer of reinforcement until desired thickness and strength of the composite are achieved. The properties of the finished composite can be varied by the type of winding pattern selected. Three basic filament winding patterns are:

i) Hoop Winding:

It is known as girth or circumferential winding. Strictly speaking, hoop winding is a high angle helical winding that approaches an angle of 90 degrees. Each full rotation of the mandrel advances the band delivery by one full bandwidth.

ii) Helical Winding:

In helical winding, mandrel rotates at a constant speed while the fibre feed carriage transverses back and forth at a speed regulated to generate the desired helical angles.

iii) Polar Winding:

In polar winding, the fibre passes tangentially to the polar opening at one end of the chamber, reverses direction, and passes tangentially to the opposite side of the polar opening at the other end. In other words, fibres are wrapped from pole to pole, as the mandrel arm rotates about the longitudinal axis. It is used to wind almost axial fibres on domed end type of pressure vessels. On vessels with parallel sides, a subsequent circumferential winding would be done

In the above three, helical winding has great versatility. Almost any combination of diameter and length may be wound by trading off wind angle and circuits to close the patterns. Usually, all composite tubes and pressure vessels are produced by means of helical winding.

Recent advances in Filament Winding Technology

Now a days, most of the filament winding machines are numerically controlled with higher degrees of freedom for placing the fibres at required position for meeting the complex design configurations of the products. Fibre orientation is the decisive factor in the strength of the composites. MATERIAL S.A of Brussels, Belgium has developed a user friendly pattern generation software: CADWIND for obtaining custom fibre orientation and high quality of filament wound components. CADWIND calculates from the given strength requirements, the fibre lay-up on the mandrel and generates automatically the part program for any winding machine. The laminate structure is reproduced on the winding machine exactly as calculated by CADWIND.
 
The CADWIND design software tool creates 3D mandrel models and also interface for input of mandrel models from CAD systems .It also calculates the required laminate for axis symmetric and non-axis symmetric mandrel geometries and stores the laminate structure as an interface for finite element method programs.

Optimization of winding angle variation is possible with this software tool. Computer numerical controlled multi-axis filament winding machines using CADWIND software can wind any irregular shapes with no axis of symmetry.

Source: Extract from an article by, Mr. Muttana Suresh Babu, Mr. Gudavalli Srikanth & Mr. Soumitra Biswas

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