Composites technology

Composites technology

Arising from components nature base and general classification of plastics the fibrous composites (laminates) are created only in the period of intermediate or final product processing.

Taking this fact into account it is necessary to evaluate the production technology as a very important factor which determinates both final properties and the production costs.

Therefore it is necessary to pay the considerable attention to proper choice of technology.

Determining factors for choice of technology

The production technology is given first of all by the product itself . The choice is influenced by several important factors:

  • a) series scale
  • b) dimensions and complexity of product
  • c) surface performance
  • d) properties required incl. weight and strength
  • e) costs limits

Usually there is necessary to make a compromise to fulfil all the factor to some extent or to approach the most important as much as possible.

Series scale is one of basic factors influencing the choice of technology. It is evident that short series is not possible to realize using machine technologies, on the contrary large series are not possible to manufacture economic by hand methods.

Complexity and design: Some technologies allow only certain forms (for instance filament winding), some technologies are able to be applied only for simple shapes (spray-up).

Surface quality: Some technologies allow only one-side perfect surface from mould

Final properties requirements: Majority of mechanical properties are dependent on the reinforcement content and orientation and matrix type. These factors are deeply depending on technology.

 

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Pultrusion

Pultrusion

By this method it is possible to produce very effective continuously GRP profiles of various cross section shape with high reinforcement volume (up to 80%). Reinforcement, mostly glass, but also carbon or other rovings, in some cases combined with fabric or mat tapes giving the transversal strength, is pulled through the resin bath, impregnated and subsequently the excess resin is squeezed out. Impregnated reinforcement is then pulled into the dies. Cross section of die matches the final profile shape. Modification of this process is pressure impregnation of dry reinforcement in the first section of die.

Curing takes place in the die either by means of conductive heat (the dies are heated electrically or by heat media) or by heat caused by means of high frequency field. Cured composite profile is pulled by hydraulic clamps or towing belts with controlled speed and it is cut by the saw to the required length on the end of the line.

Pulforming is the variant of simple pultrusion. It is semicontinuous process – pulled out impregnated reinforcement is placed into two part heated mould where the final shape is reached and composite is cured. Leaf springs with changing cross section along the length are manufactured by this method as an example.

Suitable materials

Reinforcements: Primarily glass rovings, less often carbon rovings, weaved tapes and bands made of various fibres or mats with heavily solved binder, surfacing mats, sometimes printed.

Resins: Low viscosity polyester, vinylester and epoxy resin systems quickly cured by elevated temperature (80-160°C). The matrix usually contains internal release agent, additives for better surface smoothness and better pigmentation, pigments and fillers for instance to reach fire retardation properties.

Pultrusion is suitable for continual production of profiles from very thin and simple ones (1 mm reinforcing strips for hockey sticks) to complex shaped and large size ones (width and height in the range tents of centimeters, wall thickness up to 15 mm, rods and tubes of various diameter.

 

 

 

Other continuous methods

Flat laminate sheets or corrugated (roof or decorative) sheets are also produced using continuous lines. Starting material can be either weaved reinforcements and mats or chopped strands spread onto supporting foil and subsequently impregnated in resin bath. The upper release foil is then applied and material is pulled into shaping and heated curing chamber. Pulling and cutting sections make the end part of the line.

Suitable materials

Reinforcements: Rovings, mats, fabrics, possible their combination, mainly of glass fibres.

Resins: Polyester, rarely epoxy resins

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Spray-up

Spray-up

By this machine technology cut roving and initiated resin are simultaneously applied by special pneumatic gun onto the mould.

The gun is mostly hand-guided, but it is possible to use PC controlled automation.

The moulds are simple, low-cost, practically the same as for hand lay-up, mostly made of composites.

Gelcoat is usually applied (by hand or by spraying) as a first layer. Laminate spray-up is carried out in several layers “wet-to-wet” depending on the part thickness. Each of applied layers is con-

solidated by fin, paddle or bristle rollers of various size and shape removing the entrapped air bubbles at the same time.

Spray-up (SU) ranks also among so called “open” technologies, therefore it is necessary to carry out it in separate air exhausted spraying boxes as the mostly used resins by spray-up are polyesters containing harmful styrene.

Hand lay-up

Hand lay-up

Hand lay-up (HLU) – also called contact moulding – is the oldest, the simplest and still most spread technology. HLU belongs to so called open technologies. The mould (negative-female or positive-male) is first provide with release agent and subsequently very often with gelcoat.

Gelcoat is specially formulated, mostly pigmented non-reinforced surface layer with 0.3-1 mm thickness. It is applied either by hand with brush or hair roller or by spraying. Gelcoat provides the aesthetic point of view and creates the protection against environmental impacts (water, weather, chemicals).

When the gelcoat is partly cured (non-liquid, but still sticky) the laying of reinforcing material can start. Individual reinforcing layers are properly impregnated by initiated resin by brush or the matrix is applied by hair roller, excess resin and air bubbles are removed by fin or paddle rollers.

The laminate is cured by normal temperature mostly without applying of pressure, only in the case when it is necessary to build up the sandwich structure using lightweight core materials (foams, honeycombs etc.)  vacuum pressure under the foil is used.

Curing some types of resin by the elevated temperature is recommendable to enhance their thermal resistance keeping the step by step temperature increase schedule.

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Centrifugal casting

Centrifugal casting

Hollow rotational shape parts, namely subterranean pipes, are produced using equipment which is a combination of spray-up and centrifugal casting.

Spray equipment is moving along the central line in rotating mould – tube of given inner diameter and it distributes the mixture of chopped fibres, initiated resin and various fillers according to computer program. Laid material is gradually densified by means of centrifugal force developed by fast rotation of the mould.

As a last inner layer (so called liner) there is applied high flexible resin or chemical resistant resin for majority of pipes for sewage waters and corrosion applications.

Cured tubes is easy to take out of the mould thanks to shrinkage of composite.

Suitable materials

Reinforcement: Mostly chopped glass roving

Resins: Common polyester resins orthophtalic type are used for basic layers, chemical resistant vinylester resins for corrosion applications and high flexible (up to elongation 50%) special resin as a liner for abrasive resistant tubes used for sewage water.

Fillers: Mixture of sand and calcium carbonate (up to 35% by volume)

Centrifugal casting (Hobas process) is up to some extent the competitive technology for pipe production by filament winding, but it is optimal for thick-wall subterranean tubes.

Standard products are 6 m sections in diameters within 250 – 1500 mm.

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Casting

Casting

Casting technologies are used for production of non-reinforced composites. The principle of the process is casting of resin usually in mixture with fillers, optionally with pigments, additives and curing components into the moulds under the conditions depending on the final product properties and application field.

Cultured marble

Cultured (synthetic) marble is produced by mixing of low viscosity resins with fillers the choice of which is very wide starting from calcium carbonate (chalk) over various inert fillers (crushed stone, mica, crushed mirrors etc.), pigments, additives and initiating compounds and subsequent casting into the moulds which are provided by gelcoat. Gelcoat is mostly transparent based on high quality ISO/NPG resins which are resistant to hot water, detergents and temperature changes.

A filler content of 75-80% is usual in cultured marble. To reach that relatively high filler content (desirable both from the cost point of view and aesthetic one) and ensure the full impregnation of all filler particles without air pockets and in the same time to preserve good flow properties it is recommendable to use the mixture of fillers of various particle size as follows:

  • 0,0 – 0,1 mm 16%
  • 0,1 – 0,25 mm 9%
  • 0,25 – 1,0 mm 25%
  • 1,0 – 4,0 mm 50%

Mixing can be made in dependence on the production volume in small mixers up to large autmated mixing and casting machines.

To reach the deep onyx effect it is necessary to use fine ground aluminium trihydrate and transparent pigments.

Moulds made from reinforced plastics, epoxy compounds, steel, aluminium, polymer concrete or silicone elastomer provided with suitable release agent can be used with regard to part dimension and series size. The material of mould must withstand the exotherm and post cure temperature.

Polymer concrete

The technology is basically similar to that used by cultured marble manufacturing. Because the products are mostly used for industrial applications there are not used neither gelcoat nor pigments.

On the other hand there is wider choice of fillers, namely cheap fillers of a coarser particle size and a higher filler load are normally used – sand, crushed stone, calcium carbonate, limestone, talc and mica. Moulds are mostly heated to speed the curing cycle and vibration is used to remove entrapped air bubbles.

Solid surface

There is very hard and resistant version of cultured marble imitating the natural stone or creating the special effects. Starting material is a mixture of special resins, especially acrylic and polyester characterized by high thermal and chemical resistance which are able to be cured to high hardness in combination with fillers. High quality and high purity aluminium trihydrate and/or chips of cured pigmented resin are used as fillers in some cases mixed with pigments.

Solid surface technology differs from cultured marble process in two main points:

  1. a) gelcoat is not used as a surface layer
  2. b) mixing of resin, fillers, pigments and curing agents is made under the vacuum

As a result there is gained homogenous non-porous and very hard material which is grinded and polished after curing. It is possible to cut it or machine it, individual parts can be bond together without glue line and thanks to gelcoat absence it is possible to repair eventual surface defects by sanding and polishing.

Suitable materials

  • Cultured marble:high quality ISO/NPG gelcoats, common polyester resins ortho- or isophtalic type, fillers and pigments
  • Onyx: as by cultured marble, alumina trihydrate as a filler
  • Polymer concrete: as by cultured marble, without gelcoats, non-expensive fillers
  • Solid surface: acrylic or high quality ISO/NPG polyester resins (possibly modified with acrylics), Inert fillers, namely aluminium trihydrate and polyester chips

Casting technologies are suitable for cultured marble production – sanitary wares (bath-tubes, wash basins, WC and bathroom components, window-sills), polymer concrete- industrial application, sewage water tubes, drain systems, pavement and sidewalk parts, industrial floors and solid surface –   bathroom equipment, kitchen and office counters, table desks and decorative applications.

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Press moulding

Press moulding

Cold press moulding

This technology uses low pressure 0.3 – 10 kg/cm2 by normal temperature. The moulds are not heated so that they can be made of low cost materials (GRP, metal sheets, laminated chipboard).

Moulds consist of two parts, the product is smooth on both sides. The pressure comes by the easiest way from hand clampers or using hydraulic cylinders in metal frame or in platten press.

Suitable materials

Reinforcements: Mostly fabrics based on glass or other fibres of various weight, weaving style and fibre orientation.

Resins: Polyester or epoxy resins curing by normal temperature.

The technology is suitable as low investment cost production with series 100-5000 pcs.

Vacuum bag technology

If it is necessary to increase the reinforcement volume and to drain of resin excess to improve mechanical properties or to build in rigid sandwich materials – foam or honeycomb, there is recommendable to use vacuum pressure.

Using the soft sandwich materials which need complete impregnation with resin it is also possible to use vacuum, but it is necessary to choose considerably lower vacuum level.

Impregnated material is applied to the mould by the same way as by HLU. Peel ply (tear-off) synthetic fabric is laid as a last on the structural layers what makes easier the subsequent operations. It is possible to tear it off easily from the cured composite any time and to continue with following steps (laminating, gluing, painting) on the fresh surface without degreasing or grinding.

Next layer is perforated release film followed with bleeder (breather) fabric or mat what absorbs excess resin and enables bleeding of air bubbles by vacuum force. In the end the elastic foil or flexible bag is applied which is glued by sealing putty or tape around the mould circumference to ensure vacuum function.

Evacuation and pressure are of the low level (cca 0.3-0.9 bar), the moulds can be relatively simple and low cost as it is by hand lay-up. Curing is mostly carried out by normal temperature , if prepreg are used there is necessary to place whole mould assembly into heated tunnel or chamber to reach the recommended temperature.

Suitable materials:

Reinforcement: Fabrics and tapes based on glass, carbon or synthetic fibres, their combination or so called hybrid reinforcement (mixture of different fibre types) of various weight.

Resins: Polyester or epoxy resins

For the more demanding applications there are used so called prepregs, i.e. reinforcement preimpregnated by suitable resins, which needs the curing at higher temperatures. Fabric prepregs are characterized by high reinforcement volume, but it is necessary take into account their limited shelf life – several days till few weeks depending on matrix type and storage temperature

(by -18°C until 12 months).

Vacuum bag technology is used for small to medium size series, the technology does not need high cost except cost for vacuum pump. Both-side smooth products incl. sandwich structures with high reinforcement volume and good mechanical properties can be obtained.

Autoclave moulding

Moulding in autoclave is most sophisticated and most expensive technology for series production of large-sized structures. Prepregs cured by elevated temperatures are used as a starting material.

Structure of layers is practically the same as by vacuum bag technology. The mould covered with flexible foil or bag is placed into the heated autoclave. First step after moulding temperature is reached is evacuation by vacuum cca. 0.8 bar, then follows the application of pressure approx.

6 bars. Reinforcement volume can be obtained up to 60%.

Pregreg layers can be laid-up either by hand or plies are laid by special computer controlled equipments in the case of big series (airplane or space products)

Suitable materials

Reinforcements:Prepregs based on glass, carbon or aramid fibres ( for air and space applications boron or siliciumcarbide fibres are also used) mostly with epoxy matrix cured at 120-200°C.

Autoclave moulding is used for most demanding large-size parts for air and space applications, racing cars and cladding panels of transport means manufactured in medium to big series.

Hot press moulding

Processing is based on curing in match metal dies by elevated pressure and temperature.

The moulds should have highly polished or better hard chromium plated surfaces. Moulds are mostly electrically heated and they are fixed in hydraulic presses able to reach 10-300 kg/cm2 press force.

Base materials are: prepregs – SMC (sheet moulding compounds) or DMC (dough moulding compounds) or premixes – BMC (bulk moulding compounds).

Prepregs of SMC type are mixtures of chopped glass fibres, resins (mostly polyester or vinylester) partly cured into so called stage B, pigments, fillers and various additives for better flow, surface smoothness and influencing some properties, for instance fire retardation or shrinkage.

SMC pieces placed into mould are able to flow under the pressure and elevated temperature and fill up totally all mould cavity and subsequently to cure completely.

Suitable materials

Prepregs(SMC) in sheet form covered from both sides with polyethylene foil delivered in rolls.

Polyester and vinylester resins of various types are used as matrix. Terephtalic resins are used for special types of DMC or BCM.

Hot press moulding is one of most productive manufacturing method for large-scale series (2000 – 10000 pcs) of small to medium size parts. Short production cycle ranking in minutes, high quality and dimensions reproducibility and the possibility of automation of the process are advantages of this technology. High investment costs for hydraulic presses and mould manufacturing are disadvantages of this processing method.

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Filament winding

Filament winding

By filament winding technology the reinforcement, usually glass but also carbon or aramid rovings impregnated with resin are wound onto mandrel having the form of the final product.

Using this technology it is possible to manufacture hollow composite products – tubes, tanks and vessels of various size even of changing form.

The motion of laying arm along the lengthwise axis by contemporaneous mandrel rotation and the position of laying eye enable the precise laying of fibres and allow to produce even relatively complex parts. Filament winding process is fully controlled by computer.

Reinforcement obtains by special equipment located on the roving creel the certain prestress what makes easier the exact placing on the mandrel. Winding angle is controlled by the ratio of mandrel rotation and movement speed of deposit carriage within 90° (circumferential winding) over cross winding with various angles up to 0°, for instance axis reinforcement of drive shafts and tubes. Thanks to dismantle and removable mandrels it is possible to gain by so called integral winding the complete vessels inclusive bottoms and spherical caps only with small polar outlets.

Suitable materials

Reinforcements: Mostly rovings – glass roving with various tex, for more sophisticated applications also carbon tows. Nowadays is also used special roving Twintex (Vetrotex), which consists of glass and thermoplastic fibres. Beyond the direct glass roving it is possible to use special light-weight roving Spheretex based on glass, carbon or aramid fibres with incorporated expanded thermoplastic microbubbles.

Resins: Polyester resins of various types, vinylester and epoxy resins. Postcuring by elevated temperature is necessary when use the filament wound tanks for storage of foodstuffs or drinking water to reach the maximum curing degree and to obtain the rest styrene level as low as possible.

Filament winding is one of most progressive method for manufacturing of hollow composite parts.

Investment cost is given by the value of winding equipment and by the cost of mandrels.

Tubes with diameter from 150 to 3000 mm and tanks of 1 to 100 m3 volume are currently produced.

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Injection and infusion technologies

Injection and infusion technologies

Injection and infusion technologies

These technologies belong to so called close mould ones, which reduce evaporation of harmful styrene from polyester resins during processing and curing. With respect to strict regulations concerning quality of working conditions and reduction of styrene emissions to the environment close moulding methods are gaining the major importance and expansion contrary to hand lay-up and spray-up.

All modifications of these technologies come out of liquid resin systems which impregnate dry reinforcement placed in the mould using injection or injection in conjunction with vacuum.

High pressure injection

(RTM – resin transfer moulding)

Moulds are robust construction made of metal or polymer concrete to bear high internal press.

Dry reinforcement is laid into the mould, usually provided with gelcoat. Preformed reinforcement can be used in the case of big series production. Reinforcement based on various fibres should have the structure which enables the easy flow of resin system to achieve the complete impregnation within short time.

The mould is then closed with similar rigid upper part and it is fixed by quick-closing clamps. The injection gun is placed into inlet, the rein is fed from special equipment – high pressure pump with coupled mixing of resin and initiator. The resin is injected until reaches outlets usually placed close to periphery of the mould. This indicates that the reinforcement is fully impregnated in overall part volume.

Matrix system is chosen to cure quickly within 10-20 minutes using exotherm heat.

Suitable materials

Reinforcement: Glass mats from endless fibres (for instance Unifilo) or stitch mats or special complex glass mat + internal tenuous mat made of synthetic or glass fibres (type Rovicore or Combiflow),  sometimes in combination with surfacing light weight mats.

Resins: Mostly polyester resins with built-in accelerator, often extended with low-cost fillers (calcium carbonate, alumina trihydrate) to reduce the cost or to tailor some properties as fire resistance or shrinkage (special types Class A for automotive parts ready-to-use without additional surface treatment.

Thanks to short production cycles and high reproducible quality pressure RTM is suitable for large series – hundreds till thousands of mouldings. It is necessary to take into account of relatively high investment cost for injection equipment and high cost of  moulds. The cost for mould counter- piece can be reduced by using so called distant wax foils of various thickness corresponding with the product thickness. In this case it is not necessary to make the second model (plug).

Vacuum-injection technology

(VARTM – vacuum assisted resin transfer moulding, RTM light)

This technology is a modification of classic RTM when vacuum helps to impregnation process.

Two basic settlement are used:

  1. a) injection into the mould centre – vacuum is applied around the circumference of mould
  2. b) resin is pushed into circumferential channel, vacuum is applied in the middle of mould

Usage of less robust mould construction because of lower press (0,4 – 1 bar) is an advantage of this technology. It enables the production of bigger pieces.

Suitable materials

The same as in the case of high pressure RTM technology + gasket profiles and strips to ensure vacuum tightness.

Vacuum impregnation

(vacuum infusion, VIP – vacuum infusion process)

Technology is similar to RTM light, but there is no need to use injection equipment.

Three modification are used:

1) Vacuum impregnation using flexible upper mould part. Lower mould part is stiff like by RTM light with exhaustion channel, upper part mostly made of GRP has flexible to a certain extent what controls the pressure. Closing force and impregnation is caused by 0.6 – 0.8 bar vacuum.

Resin is fed from container or it is spread onto dry reinforcement before the mould is closed.

This method is suitable for both side smooth parts. Into the product could be incorporated core materials – foams or honeycombs – to create the sandwich structure.

2) Vacuum impregnation under the flexible foil. This technology is very similar to vacuum bag technology. The lower mould part is the same type as by hand lay-up. Instead of second mould part the flexible foil is used tightened it with sealing tapes to the mould edges.

Initiated resin is aspirated from container, in case of large structures the matrix is distributed to distant places by perforated tubes. Vacuum is applied on the mould circumference using channels created by gasket profiles.

3) SCRIMP method is very similar to the method 2), but the resin distribution is provided by means of special net placed over the all surface of dry reinforcement charge. As the last layer is again used the flexible foil.

Suitable materials

Reinforcement: Reinforcing materials of all kinds from fabrics based on glass, carbon or aramid fibres or combination of them with various weight, weaving style and different orientation to special stitched or complex mats. It is possible to insert foam or honeycomb materials to make the sandwich structures or to use semi-sandwich materials of Coremat type. Also peel plies, tapes and surface mats are used.

Resins: Low viscosity polyester and epoxy resins. Matrix systems with long term open time are used for large mouldings with respect to the long impregnation time.

Vacuum infusion methods are particularly suitable for large structures as hulls and decks of boats, big mouldings used as body of locomotives and wagons, wind mill blades etc. where other methods would be considerably time absorbing (hand lay-up) or unbearable expensive (RTM or hot press moulding). On the other hand the working cycles are longer, in some cases even up to several hours. Infusion processes are suitable for small and middle series.

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