Forming processes for glass fibre and resin—other methods

Forming processes for glass fibre and resin—other methods

Forming processes for glass fibre and resin--other methods J. M O U N T I F I E L D * In addition to hand lay-up or contact moulding there are many a...

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Forming processes for glass fibre and resin--other methods J. M O U N T I F I E L D *

In addition to hand lay-up or contact moulding there are many alternative methods of glass fibre reinforced plastics (G RP) production designed to meet specific requirements. This article describes in general terms the commonly used commercial processes. Other sophisticated techniques usually covered by patents are not included. No attempt has been made in this article to cover each process in minute detail but rather to acquaint the potential user with the available production methods and principles. In choosing a process many factors have to be considered. Important ones are the type of glass reinforcemenf and resin system to be employed, the size and complexity of shape of the end product and the quantity of mouldings required. Design considerations are equally important and perhaps the final decision on selection of materials and the choice of production depends on these.

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MA TCHED DIE MOULDING Low pressure presses and matched metal die moulds are normally used 'where accurate and large quantity work is required. Such presses are specially designed for moulding reinforced plastics. A standard range is available. This process (Fig 1) provides mouldings with a high degree of finish on both surfaces, high glass content and uniform dimensional properties. Tool costs must be compared with the number of units required and it is suggested that where these are below 1,000 the relatively high cost of tooling is rarely justified. The principal is to arrange glass fibre reinforcement, normally a preform, to which has been added a measured amount of catalysed resin, in the mould. The moulds are then closed and the resin cured under heat

* Fibreglass Ltd, 34 Dover Street, London Wl, England

FIG 1 The matched die moulding process. Glass fibre, normally a preform, and a catalysed resin are arranged in the mould. The moulds are closed and the resin cured under heat and pressure

and pressure. The metal dies can be steam or electrically heated to a temperature of usually around 125°C. Pressure is low and can vary between 50 and 2501b/in 2 . Cure cycle varies between one to four minutes. Shallow mouldings can be successfully produced using tailored woven glass fabric and or random mats. Positioning of the reinforcement by hand however is laborious and as the main advantage of this process is speed this would be pointless. Therefore the glass fibre reinforcement normally incorporated is a 'preform'. A preform is essentially a mat

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4]

of chopped strands bonded together in the shape of the end product. It can be made by joining together a number of tailored layers of random mat, or more usl~ally by blowing or showering chopped roving onto a perforated metal screen so fabricated that the dimensions match those of the male mould over which it must fit. Preforms can be made by several methods. However, the two basic ones are by air preforming and water slurry. Air preforming is the most popular using either the 'directed fibre method' or 'plenum chamber process'. In both cases chopped roving is collected onto a perforated screen and a resinous binder applied which after curing allows the preform to be removed from the screen undamaged and ready for transfer to the press.

Directed fibre method The roving is chopped into the required length, (usually 2in) and the strands blown through a flexible hose controlled by the operator onto the perforated metal screen mounted

Preform scrjen ~ . X exhaust ~}._ Binder spray ~

Rovingkchopper \

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FIG 2 The directed fibre method. Roving is chopped and the strands blown onto a perforated metal screen mounted on a rotating base. The resin binder is sprayed on. The screen supporting the preform is removed to an oven for craig Roving

Roving chopping unit ~;al.,~¢" jPlenum /chamber

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on a rotating base. The resin binder can be sprayed simultaneously or applied on completion of deposition. The strands are retained on the screen by suction. After the binder has been applied, the screen supporting the preform is removed to the oven for curing. The temperature of the oven is approximately 150°C and the curing cycle lasts approximately 2½min. This method is shown in Fig 2.

Plenum-Chamber process In the Directed Fibre method large preforms are easily made. In the Plenum-Chamber process (Fig 3) size of part is entirely governed by the size of chamber and turntable available. The glass roving is fed into a chopper unit at the top of the chamber and the strands controlled by a fibre distributor if one is fitted. They are then sucked uniformly onto the perforated screen. The turntable rotates and after controlled build-up of fibre, a resinous binder is sprayed on and as before, screen and preform moved to the curing ave n.

Water slurry process This method utilises glass fibre chopped strands which are slurried in water containing cellulosic fibres. Water is exhausted through a contoured, perforated s c r e e n - t h e glass and cellulosic fibres being deposited on its surface. The wet preform is transferred to an oven where hot air is sucked through. When dry the preform is sufficiently strong to be handled and moulded. The ratio of glass to cellulose can be varied depending upon strength requirements. The forming cycle is fairly rapid and intricate design shapes are possible.

DOUGH MOULDING (PRE-MIX) Generally described as bulk moulding compounds this process involves the pre-mixing of fibres, usually E glass with resins, fillers, pigment and catalyst before moulding. The putty-like compound can be easily formed into accurate weighed charges for placing into a mould cavity. This compound is formed to final shape between matched metal dies under heat and pressure. Pressures can vary from 100-10001b/in 2 and length of curing cycle can depend upon design requirements such as wall thickness as well as compound curing characteristics. Strengths are lower than with preform moulding and unit dimensions usually limited due to the higher pressures required.

FILAMENT WINDING

FIG 3 The Plenum Chamber procesz The roving is fed into a chopper unit. The fibres are sucked uniformly onto a perforated screen. The turntable rotates and resin is sprayed on. The screen is removed to an oven for curing

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COMPOSITES September 1969

Filament winding is a highly skilled and specialised process utilising multi-ended ravings or single strands would on to a rotating mandrel, (Fig 6). The glass filaments, usually in the form of ravings are guided into predetermined positions to give the maximum strength properties in the required direction. Resin impregnation is achieved by passing the

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FIG 4 Hot press .moulding. The fabrication o f a bonedome type of crash hebnet ushzg a preform (courtesy Fibreglass Ltd} (a)

(b)

The automatic build up o f three preforms simultaneously The buiM up of the chopped glass fibre roving and the application o f binder spray by hand

(c) (d)

Placing the cured pre-forms into the press Addition of measured quantities o f pigmented polyester resin mix (e) Removal o f untrimmed helmet shells from the press

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Gearbox

.Zo/ . . . . . Roving

~ototin!mondrel FIG 5 Eight hundred reinforced plastic seats in the Cinerama Theatre, Tel-A viv, hot moulded

FIG 6 The filament winding process. The roving are guided into predetermined positions. The ravings are initially coated with resin in a bath before rotation

COMPOSITES September 1969

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continuous f'daments through a resin bath containing the conventional thermosetting resin mix, either a polyester or epoxy. It is also possible to apply resin,flfix direct to the mandrel, winding the rovings into this. Preimpregnated rovings can also be used. Filament winding processes fall into two broad categories-helical winding and polar winding. In helical winding the £daments are applied to lay at rovings or single strands wound on to a rotating mandrel (Fig 6). The glass to wind the reinforcement around the ends of the shaped mandrel or onto a sphere. Principal applications include pressure vessels, bottles, high pressure pipes, high strength tubing and chemical tanks. In the early days manufacturers usually built their own plant based on a variation of the familiar lathe - it is now possible to purchase machinery and associated equipment designed specifically for precision f'flament winding.

PUL TR USION The pultrusion method (Fig 8) is a method of producing continuous-length prof'des whereby rovings, or other forms of reinforcement are passed through a resin bath and then through dies to set the prof'de. Final cure is effected by drawing through a continuous oven. Many pultrusion processes are in use based on intermittant and continuouspull methods and a wide variety of pro£des with excellent properties can be made. Principal applications include rod stock, tubing and sections.

CONTINUOUS SHEET PRODUCTION Flat sheeting and shaped panels can be made by hand or press moulding or by continuous process but corrugated or ribbed reinforced plastic sheet is now predominantly machine made. All processes are similar in principle using the familiar sandwich lay-up (Fig 9). The concept is simple and straightforward - glass fibres in the form of chopped rovings or chopped strand mat and resin mix are sandwiched between two continuous layers of release fdm (Cellophane or Melinex), and the resulting package is passed through a series of metering rollers to eliminate air bubbles and consolidate the laminate. The sandwich then passes through ovens where it can be drawn fiat, corrugated or formed into ribbed section panels during curing. After leaving the curing oven the panel is trimmed to the required width and length. Panels with decorative effects can easily be produced, using special films to create textured surfaces. E glass reinforcement is normally used for translucent sheeting and mechanised processes usually demand specially formulated resins and curing systems to obtain the best results. Where the Building Codes impose fire-resistant qualities, fire retardant grades are used. Manufacturers are now adapting equipment to allow the continuous production of panels and sheets based on more complicated prot-des and shapes.

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COMPOSITES September 1969

FIG 7 Fume stack produced by the continuous pipe process. The production time was 96min

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FIG 8 The pultrusion process. RovhTg is passed through a resin bath and then dies to set the profile. Final cure occurs in the oven

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Fibre ravings

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AdJustable forming strips Lifting mechanism for top formers

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FIG 9 Continuous sheet production. Ravings and resin mix are sandwiched between layers o f release film. This is passed through rollers to eliminate air and then to a series o f ovens where it is drawn flat, corrugated or ribbed

INJECTION MOULDING Injection moulding is a high production process designed for use with thermoplastic materials (Fig 11). The glass fibre and resin compound is softened in a heating chamber and injected into the mould cavity which is kept cooled below the softening point of the resin. When the resin has solidified the moulding is ejected. This process is useful for high production of small precision parts as complex details are easily moulded.

THE BAG MOULDING TECHNIQUES Vacuum bag moulding

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FIG 10 Corrugated reinforced plastic cladding panels on the water cooling tower at the Scott Bader synthetic resin plant at Wollaston. The panels were made on a continuous sheeting n~ch&e and incorporate a surfacing tissue for hnprovhTg weathering properties

In vacuum bag moulding atmospheric pressure is applied to the resin impregnated lay-up by exhausting air from beneath an impervious flexible sheet sealed to the edge of the mould. The mould surface preparation and lay-up are as for contact moulding. The flexible sheet is then laid over the lay-up, sealed and clamped around the edges of the mould. Vacuum is drawn and the resultant atmospheric pressure forces out entrapped air and excess resin. This process permits rather higher glass content with less risk of air inclusion (Fig 12). la,',,pingring

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FIG 11 Injection moulding. The glass fibre and resin compound is softened in a heating chamber and injected into the mouM cavity. The mould in ejected after the resin has solidified

FIG 12 The vacuum bag moulding method. Atmospheric pressure is applied to the resin impregnated lay-up by exhausting air from beneath an impervious flexible sheet sealed to the edge o f the mould. The flexible sheet is laid over the lay-up and clamped round the mould. Vacuum is drawn and entrapped air and excess resin are forced out

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Pressure bag moulding Similar to the vacuum bag process this method demands a much stronger mould to withstand working pressures and the addition of a pressure plate (Fig 13). After proceeding as before with mould surface preparation and lay-up, a tailored bag usually of neoprene or similar material is placed against the lay-up. Air or steam pressure is then appfied between the bag and plate. Pressure ranges from 20-501b/in 2 .

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Autoclave moulding Autoclave moulding is a modification of the pressure bag technique allowing for the application of heat and pressure simultaneously. In this process, after following the procedure for pressure bag moulding, the complete assembly is placed in a steam autoclave at pressures between 50 and 1001b/in 2. This achieves higher glass content and improved removal of air. In general the bag moulding processes described are used for medium sized mouldings where there is less dependence on the ability of the operator.

FIG 13 Pressure bag moulding. A tailored bag is placed against the lay-up. Air or steam pressure is applied between the bag and plate

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FLEXlBL E PL UNGER MOULDING This process is comparable to the pressure bag method. Fluid pressure is exerted on a lay-up in a heated, precision made, strong female mould (Fig 14). The male portion is a strong flexible plunger of solid neoprene or rubber so shaped that it first makes contact at the bottom of the mould and with additional pressure expands radially until pressure is exerted over the whole mould surface. The tools should be so mounted to allow for pressures up to about 1501b/in 2 . The usual process is to lay reinforcement in the mould onto which the resin is poured. The assembly is then covered with a sheet of non-moisture proof cellulose film. The plunger is forced in and follow-up pressure maintained. This method is particularly useful for small symetrical shapes required at medium production rates.

FIG 14 Flexible plunger moulding. Fhdd pressure is exerted on the lay-up in a precision made female mould. The male mouM makes contact at the bottom first and with additional pressure expands radially until pressure is exerted over the whole mould surface

El¢ctrlc motor power to top pletten travel for doyllght

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COLD MATCHED TOOL MOULDING This process, frequently referred to as cold press moulding can be accurately described as the formation of reinforced plastic mouldings in matching tools, operating at low pressures and ambient temperatures. It is not a new process but one that has been revived and developed as the intermediate production method for applications where wet lay-up (contact moulding) laminating is too slow and costly in terms of labour content and where the numbers of mouldings required does not justify the high cost of machined steel matched tools. As with hot press production the process gives mouldings with two good surfaces but uses low cost tools permitted by the low pressure and temperatures. Certain design considerations are necessary to nll successful cold moulding t o o l s - they must be rigid and stable having a

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COMPOSITES September 1969

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control

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Bottom plotten pressure apphed by hydrauhc rams 6m stroke

FIG 15 Cold matched tool moulding rig. The two halves o f the tool are brought together under slight pressure. Release is effected on completion o f the moulding cycle

cavity accurate to the desired shape of the finished moulding with a pinch-off on all boundaries. The mould surface must be suitably chemical resistant and capable of withstanding the temperatures generated by the exerthomic reaction of the resin. The tools are unheated and normally used in the pressure range of 6-251b/in 2. The press or rig (Fig 15) is essentially a means of bringing the two halves of a tool together under slight pressure and effecting release on completion of the moulding cycle. Expensive presses are not essential and as the moulding is cured before pressure is released there is greater flexibility in the choice of reinforcement. Experience has shown that the conventional glass fibre reinforcement materials ie chopped strand mat, woven fabrics and needled mats can be used but continuous filament mat has proved particularly suitable for this process resulting in better conformity to mould contour. Sisal mats and other reinforcements can also be used but

Table 1 Comparison of processes for preparing pre-forms

(Owens-Coming)

Direct labour Material wastage Uniformity Direct labour skill Cost of preform screens Set-up time and cost

Directed fibre

Plenum chamber

Water slurry

most most least most least lowest

moderate moderate moderate moderate moderate medium

least least most moderate most highest

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36o ,bo s6o 6bo 7 ,o 8bo 98o ,ooo Number of mouldlngs off

FIG 16 Number o f mouldings/cost o f moulding using the cold matched tool method" 1 Cold matched tool moulded FGE 5000, 2 - 1 resin/glass ratio 2 Cold matched tool moulded FGE 5000, 3 - 1 resin/glass ratio 3 Cold matched tool moulded E csm, 2 - 1 resin/glass ratio 4 Cold matched tool moulded FGE 5000, 4 - 1 resin/glass ratio FGE is a continuous filament mat supplied by Fibreglass Ltd

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Table 2 This table shows typical strengths of 1/8in thick cold press mouldings using a fast curing polyester resin system incorporating different reinforcement materials. (BIP Chemicals Limited)

Reinforcement

Weight Tensile used strength (per f t 2 ) Ib/in 2

Flexural strength Ib/in 2

Continuous filament

1.5oz 3oz

6100 8000

13200 20600

Chopped strand mat suitable for use in cold press moulding

1.5oz 3oz

5400 7700

10500 19400

Sisal

1.5oz 3oz

2800 3400

6900 8200

Comparison of CSM reinforcement in conventional hand lay-up laminate (post stoved 3 hours at 80°C)

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2000

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60C~0 ' I00~00 of componentsoff

Numl~r

FIG 17 Number o f Components off versus cost o f component in the hand lay-up, cold press and hot press processes when considering combinations the specific characteristics of each material must be taken into account. Special high reactivity polyester resin systems are required for the process- they are generally available and the manufacturers recommendations should be followed. The surface finish obtained is satisfactory for most purposes but it is possible to have a gel coat finish is considered desirable. The physical properties of mouldings produced by this method are generally higher than those produced by hand lay-up and are more consistant (Figures 16,17).

RESIN INJECTION MOULDING 1.5oz 3oz

5100 6100

11500 15800

This process is usually defined as the Marco Process. It requires matching airtight moulds so constructed as to remain rigid when vacuum is applied between them. A resin

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4"/

Table 3 Characteristics of the major processes

Matched-die moulding

Contact moulding Hand lay-up spray-up

Filament winding

Dough moulding

Mat preform pre-preg

Cold press

Maximum part size determined by

Mould size, Transport of part

Machine dimensions

Press rating and size

Press rating and size

Press rating and size

Shape and styling limitations

none

usually surface of revolution

mouldability

mouldability

mouldability

Translucency

yes

yes

no

yes*

yes

Volume of production category

low to medium

medium to high

high

high

medium

Number of finished surfaces and quality

one excellent

one excellent

all excellent

two very good

two good

Typical glass content by weight

20-35%

65-90%

10-35%

25-45%

20-40%

Strength category

medium

very high

low/medium

medium/high

medium

Strength orientation

random except fabric types

follows winding pattern

random throughout moulding

random

random

Resin rich, corrosion resistant surfaces

yes, by gel-coat

yes, by gel-coat

no

yes, by gel-coat

yes, by gel-coat

&/or

&/or

&/or

&/or

surface mat

surface mat

overlay mat**

surface! overlay mat

Practical thickness range (inches)

0.030 to 1.000

0.010 to 2.000

0.060 to 1.000

0.030 to 0.250

0.060 to 0.500

Common moulding tolerance on thickness (inches)***

-+0.020

-+0.010

-+0.002

-+0.008"**

+0.020

Local thickness increase

as desired

as desired

as desired

usually 2 ~< : 1

usually 2 ~< : 1

Metal inserts and/or edge stiffeners

possible

possible

possible

possible

possible

Bu ilt-in cores

possible

possible

possible

possible

possible

Minimum radii for ease of moulding (inches)

0.500

0.125

0.030

0.125

0.250

Undercuts

yes

no

yes

no

no

Minimum recommended draw angle (degrees)

2

3

1

1

2

Holes moulded in to avoid material waste

yes large

yes (to suit wind pattern)

yes

yes

no

Trim in mould

yes (rough trim)

yes

yes (except fine flash)

yes (except fine flash)

no

Moulded in signs and labels

easy

easy

difficult

difficult

possible

Combination with thermoplastic liners for corrosion resistance *Except pre-preg.

yes

yes

no

no

no

48

**Except pre-preg.

COMPOSITESSeptember 1969

***Dependent upon laminate thickness.

trough is constructed around the base of the male mould into which the female mould dips (Fig 18). The reinforcement, which can include chopped strand mat and woven roving fabric is laid up dry on the male mould, resin mix is poured into the trough and vacuum is applied at the top of the female mould. Resin is drawn up slowly and displaces the air ahead of it. Particular care is required to ensure that resin can flow freely and areas of tightly packed reinforcement must be avoided. Modification of the process allows resin to be pressurised alone or a combination of pressure and vacuum may be employed. The process is best used for the production of large symetrical mouldings such as boats. The glass content is usually low resulting in mouldings of medium strength.

CENTRIFUGAL CASTING The centrifugal casting or moulding process produces cylindrical seamless tubes of consistent quality, uniform void free wall thickness and with excellent interior and exterior surfaces. The tubes can be made to precise tolerances. The process essentially requires a hollow mandral or mould, a means of rotating and heating the mould, and the necessary equipment for pumping or spraying resin into the positioned glass fibre reinforcement (Fig 20). Although basically Sight ,glass ~

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FIG 18 Resin injection mouMing (Marco process). A resin trough is constructed around the base o f the male mould into which the female mouM dips. The reinforcement is laid on the male mould, resin is poured into the trough and vacuum applied to the top o f the female mould. Resin is drawn up slowly and displaces air ahead of i t

Glass fibre reinforcement

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Heat chamber ,

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Heat chamber Glass fibre reinforcement

FIG 20 The centrifugql casting process. Glass reinforcement and binder are positioned inside the mandrel. The heated mould is rotated and resin uniformly distributed. Centrifugal action forces the glass and resin against the walls and rotation continues until the .finished tube is cured and ready for removal simple, high volume production can be fully automated and requires expensive equipment. The interior of the hollow mandrel or casting tube should be polished to a high degree of t'mish. The glass reinforcement, normally chopped strand mat incorporating a high solubility mat binder is positioned inside the mandrel taking care to avoid lapping. The heated mould is then rotated and selected resin uniformly distributed throughout the reinforcement; centrifugal action forces the glass and resin against the walls of the rotating mould and spinning continues until the f'mished tube is fully cured and ready for removal. Glass contents can be varied from about 25-40% depending upon the resin system selected and end product requirements. Principle applications are storage tanks, pipes, ducting and tubing. Tubes up to 20ft long and 36in diameter with adequate wall thickness are feasible with this method.

REFERENCES 1

FRP Design Data, Fibreglass Ltd

FIG 19 The hull o f an all purpose dinghy, the Jiffy, especially designed for and produced by the cold matched die process. The photograph shows the boat being released from the mould prior to extraction and trimming

COMPOSITES September 1969

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