Wet Lay-Ups

AMTS-SWP-0014-A-2008

AMTS STANDARD WORKSHOP PRACTICE _________________________________________ WET LAY-UPS

Reference Number: AMTS_SWP_0014_2008 Date: June 2008 Version: A

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Contents 1

Technical Terms ......................................................................................3

2

Scope .......................................................................................................3

3

Primary References.................................................................................3

4

Materials used in Wet Lay-ups ...............................................................4 4.1 Reinforcement ............................................................................................ 4 • Glass Fibre ...............................................................................................4 • Aramide Fibre ...........................................................................................4 • Carbon Fibre.............................................................................................4 4.2 Resin .......................................................................................................... 5 • Polyester Resins ......................................................................................5 • Vinyl esters ...............................................................................................5 • Epoxies .....................................................................................................5

5

Tools required .........................................................................................5 5.1 Moulds........................................................................................................ 5 5.2 Hand tools .................................................................................................. 6

6

Method for producing a hand lay-up .....................................................6 6.1 Preparation of mould .................................................................................. 8 6.1.1 Cleaning of the mould.........................................................................8 6.1.2 Mould release system.........................................................................8 6.2 Environmental preparation.......................................................................... 8 6.3 Preparation of materials.............................................................................. 9 6.4 Surface layer ............................................................................................ 10 6.5 Mixing of epoxy......................................................................................... 10 6.6 Preparation for first layer........................................................................... 11 6.7 Impregnation of layers .............................................................................. 11 6.8 Final layer................................................................................................. 12 6.9 Vacuum bagging....................................................................................... 12 6.10 Cleaning of tools ..................................................................................... 12 6.11 De-moulding ........................................................................................... 13 6.12 Post curing.............................................................................................. 13 6.13 Post processing ...................................................................................... 13

7

Processing of poly-ester resins ...........................................................13

8

Quality control .......................................................................................14 8.1 Determining epoxy quality......................................................................... 14 8.2 Preparation of a test specimen during the lay-up ...................................... 14

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1 Technical Terms 2 Scope This procedure describes the manufacturing procedure of wet lay-ups. The following sections are discussed with reference to other SWPs, where applicable. Materials used in wet lay-ups • Reinforcement (fibres used in the epoxy matrix) • Resins Tools required • Moulds • Hand tools Processing • Processing of epoxy resins Method for producing a wet lay-up • Preparation of the mould • Lay-up method

3 Primary References MIL-HDBK-17-1F: Composite Materials Handbook, Volume 1. Polymer Matrix Composites Guidelines for Characterization of Structural Materials

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4 Materials used in Wet Lay-ups Wet lay up involves the construction of a composite material through integration of resin and reinforcement (fibre) components to form a matrix. The resin provides stiffness (keeps the fibre in position) and structure to the component while the fibre component provides the strength. The following section gives a brief overview of the materials used in Wet Lay-ups. For more detail refer to WP 002 - Raw Materials.

4.1 Reinforcement The choice of a suitable reinforcing fibre depends mainly on the required application. The criteria for selecting the reinforcing fibres are: • • • • •

Density Elasticity Tensile strength Impact strength Cost

The most common types of materials are: • Glass Fibre (“GRPs” glass-fibre-reinforced plastics) • Aramide Fibre (“SRPs” synthetic-fibre-reinforced plastics) • Carbon Fibre (“CRPs” carbon-fibre-reinforced plastics) The different types of reinforcing fibres and their properties can be summarized as follows:

Property

E-glass

Aramid

Carbon

Relative cost of yarn

1

5

10

Density [g/cm³]

2.5

1.4

1.8

Modulus of elasticity [GPa]

70

100

210

Tensile strength [MPa]

2400

3000

4000

Strain at break point

4.50%

2%

1.20%

Impact strength

Better

Best

Fair

Fatigue resistance

Good

Better

Best

Table 4.1-1 Types of fibres and their properties

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4.2 Resin The selection of a resin system depends on the required application. The criteria for selecting the resin are: • • • • • • •

Compatibility with fibres Tensile strength and elongation Shrinkage Cost Chemical resistance Glass transition temperature (Tg) Useable pot life

Types of available resin systems are: • • •

Polyester Resins Vinyl esters Epoxies

5 Tools required 5.1 Moulds Male or female moulds are used to produce the component depending on the finish required. Female moulds are used whenever a smooth external finish is required and male moulds for a smooth internal finish.

Moulds are produced from a variety of materials depending on the application: Materials used in the manufacturing of moulds include: • • • • • •

GRP Wood (Superwood / MDF board, chipboard, plywood, solid wood) Hard shell plaster Metal Silicone rubber moulding compounds Prototyping board or polyurethane casting material

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For more detail refer to SWP xxx – Moulds and tooling

5.2 Hand tools Hand tools used in composite hand lay-up are listed for the specific application: 1. Cutting of fibres • Scissors • Roller blade cutters • Blades 2. Mixing of epoxy • Mixing containers • Weighing scale or epoxy dispensers • Mixing sticks or electric mixers • Cups 3. Impregnation • Brushes • Rollers • Squeegees 4. Cleaning • Solvents • Paper towel or cloths • Jugs

6 Method for producing a hand lay-up Hand lay-up is the oldest, simplest, and most commonly used method for composite part construction. This method is primarily applied on minor series, simple geometries and mould construction, as only minimum technical requirements need to be fulfilled. The primary features of hand lay-ups are: • • • •

Low mould costs Low capital costs Ideal for smaller and medium-size series up to about 1000 pieces Wage-intensive owing to high labour content

Fig 6.1 gives a flow diagram of the most common activities during a hand lay-up.

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Mould Preparation

Environmental Preparation

Preparation of Materials

Surface layer (if required)

Mixing of epoxy

Preparing for 1st layer

Impregnation of layers

Mixing of epoxy (If required)

Application of peel ply

Vacuum bagging

Cleaning of tools

Fig 5.1 Flow chart representation of the Wet Lay-up Process

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6.1 Preparation of mould 6.1.1 Cleaning of the mould •

To clean the mould, remove all left over epoxy and other materials after demoulding of the previously moulded part.

•

A simple piece of Perspex is an efficient tool for the removal of dried epoxy from a mould. Care should be taken however not to damage the surface of the mould during the cleaning process.

•

Small imperfections on the mould surface may be repaired temporarily by: o

Filling all dents or holes in the mould with plasticine or a similar filling product.

o

Flattening all bumps (including the plasticine) with fine sanding paper to give a smooth finish to the surface.

6.1.2 Mould release system A Release agent must be applied onto the mould surface to prevent the part from bonding to the mould surface. There are two basic types of release agent available: • Wax-based release agent • Water-based release agent Ensure that the selected release system is compatible with the selected materials. It is best to perform tests before using new materials. When tests are performed, the same materials, application processes and environmental conditions should be simulated in the test in order to provide reliable results. Take care when new moulds are used for the first time. A special procedure is normally required to prepare the mould for the very first usage. Refer to SWP 036 - Release agents for more information on this topic.

6.2 Environmental preparation The criteria for good environmental preparation include the following: Temperature: 1. Effect of temperature on curing of most resin systems • •

Ideal temperature ranges from 25 – 35 ˚C that should be maintained in the laminate. Temperatures below 15 ˚C will impede the curing process.

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At temperatures below 10 ˚C, thin-walled laminates cannot cure completely, even after several days. Curing will only restart once the temperature is raised above 20 ˚C. Resins must be warmed up to room temperature before application. This is necessary to prevent possible condensation occurring on laminates.

2. Effect of temperature on the processing of composites •

Increasing the temperature: o Shortens the processing time (pot life) o Reduces the viscosity (the resin flows more freely)

•

Reducing the temperature: o Increases processing times. o Increases the viscosity. o Impairs the fibers’ wetting properties.

3. Advantages of a slightly raised processing temperature: A higher temperature results in a lower viscosity that is of particular advantage in hand lay-up operations: • • •

Improved fiber wetting properties – better impregnation is possible . Lower fiber to resin ratios can be obtained – lighter final product. Improved bonding between fiber layers.

The optimal process conditions can be obtained by heating up the resin and moulds.

Humidity: •

Relative humidity should not exceed 70 %.

Dust free environment •

The environment must be kept free of dust and dust emitting sources to prevent contamination of the air.

6.3 Preparation of materials The fibres for the entire lay-up schedule should be dimensioned beforehand and laid out in such a way as to be easily accessible. Refer to SWP 015 – Cutting of Materials for more information.

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6.4 Surface layer The surface layer (also referred to as the “overlay” or “gel coat”) is the finishing layer of the composite component. The surface layer can be applied in different ways, depending on the viscosity of the resin being used. The most common application methods are: • • •

Brushing into the mould Spraying into the mould Spreading out with a squeegee

The surface layer serves the following purposes: • • • • •

Gives the component its aesthetic exterior finish (smooth, coloured and glossy). Protects the laminate against moisture. Protects the laminate against ultra-violet light. Provides a product with enhanced chemical- and weather resistance. Closes the pinholes in the GRP surface – preparation time for further processing (e.g. applying a paint layer) is significantly reduced. Without a surface layer the GRP surface exhibits small pinholes that have to be filled up before the surface can be painted or sprayed.

The type of material selected as surface layer depends on the required specification. Existing gel coats include the following: • • •

Epoxy gel coat (EP) Polyester gel coat (PU) Vinyl-ester gel coat.

Note when using a polyester gel coat: When more than one layer is required for a thicker surface skin, consecutive layers must be applied “wet-in-wet” to prevent adverse reaction between layers. After an adequate curing time (two to three hours depending on the curing time of the surface layer), the laminating step can be initiated without the risk that the next coating of resin will react with the surface layer. (A chemical reaction may destroy the surface layer beyond repair). Refer to SWP 036 – Surface Coatings for further information.

6.5 Mixing of epoxy The mixing of epoxy should be started when all other preparation has been completed as the epoxy has a limited useable pot life. Consult the product datasheet for processing information. Refer to SWP 007 – Mixing of Resins for more information.

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6.6 Preparation for first layer Before the first layer of fibre is laid on the surface layer, it is primed using epoxy to promote bonding between the surface layer and the fibres. Apply only as much resin as is needed to impregnate the first layer half-way. Where parts form a sharp angle, a gorge is created. It should be filled with a mixture of resin / cotton flocks / Q-cell to avioid fabric peeling away and leaving a cavity in the specific corner.

6.7 Impregnation of layers The reinforcing fibres are layered one on top of the other according to the lay-up schedule. The layers should be laid wet in wet; this means that consecutive layers are laid on top of the others before the epoxy has gelled to promote bonding between the layers. Impregnation method: • • •

• • • •

Position the first layer, making sure that the direction of filaments are correct (The designer calculated the part’s strength assuming that the orientation of the fibres during manufacturing of the part will be correct.) Dab the canvas (using a brush) against the mould to impregnate it using the resin underneath. When there is no more resin underneath the layer, new resin should be applied. Do not keep the canvas too dry – impregnating the next layer will take longer than necessary; do not make it too wet – excessive resin is not removed easily! Apply the second layer, impregnating it by using the resin from the previous layer. When there is no more resin underneath the layer, apply new resin according to the aforementioned guidelines. Apply the rest of the layers as described above. When applying the last layer, only add resin if there is no resin remaining from previous layers that can be used.

Useful hints: • Sometimes it is also difficult to determine whether a layer has been impregnated completely. Glass and aramide fabrics change their colour, glass changes from its original opaque white to a clear or greenish colour, depending on the colour of the resin. This change in colour can be more easily determined when applied on moulds with dark surfaces. Carbon fibres do not change their colour, so their state of impregnation is extremely difficult to determine. The only sign might be the vanishing gaps between single fibres. • When trying to push up the fibre volume or fibre-to-resin fraction, always keep in mind that a lay-up which has not been impregnated sufficiently will break. A high fibre volume fraction is good to have but not at the expense of strength due to layers which cannot connect to each other because one of them has not been impregnated completely.

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By using a foam roller, the process can be made much more efficient, but only makes sense for parts with a large surface area, e.g. skins for aircraft wings. Squeegees are used to spread the epoxy evenly over the entire area and will remove air bubbles that may form between the layers.

6.8 Final layer After the final layer of fibre has been applied a layer of peel ply is applied on the surface. This layer does not bond with the epoxy system being used and will ensure a good finish of the component as well as protection from the effects of air during curing. The peel ply should have a trace lining to make it easy to detect when the layup procedure is completed. Note: The peel ply must be removed before the part enters service or before bonding in an assembly.

6.9 Vacuum bagging After the lay-up is completed, a vacuum bag may be used to enhance the properties of the final product. The technique of vacuum bagging has the following benefits: • • •

It applies even pressure over the whole lay-up. Removes air by creating a negative pressure in the lay-up. Improves the epoxy/fibre ratio by removing excess epoxy through the perforated first layer.

Refer to SWP016 on Vacuum Bagging.

6.10 Cleaning of tools Tools are cleaned after lay-up using an appropriate solvent. Normally acetone works well for cleaning epoxy tainted tools. Cleaning must be performed before the resin starts curing. Cleaning of brushes: • • • • •

Wipe off excess epoxy with paper towel. Use a small quantity of acetone (20ml) and work it through the brush thoroughly with a dabbing motion. Dry the brush by it against paper towel or with a swinging action. Repeat the above procedure twice, each time with fresh acetone. Store the clean brush in a closed acetone container. (This container should only contain enough acetone to cover the brush hairs.) Replace the acetone in this container weekly to remove the build-up of epoxy in the container.

Cleaning of tools: • •

First, take cloth (paper) and remove excessive resin. Decant the required amount of acetone into a cup or similar holder.

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AMTS-SWP-0014-A-2008

Close storage container properly. Clean tool thoroughly. If required, apply fresh acetone to tool. Dump contaminated acetone into a dedicated waste container.

Warning: Acetone is extremely flammable with a low flash point. Containers for acetone, especially when open may NOT under any circumstances be placed or stored near open flames, where welding or grinding is in progress, smoking areas or electrical switches.

6.11 De-moulding After the part has cured sufficiently, it can be de-moulded. Care must be taken to ensure the curing process has completed, as de-moulding forces may cause damage to parts when the resin is still in a soft, semi-cured phase. Select the most suitable technique for de-moulding. Refer to SWP010 on the Demoulding, Trimming and Machining of Composites.

6.12 Post curing Depending on the epoxy system used and the properties required, post curing may be necessary to obtain maximum strength and raise the Tg (Transition glass temperature) of the final product. Refer to SWP009 on the Curing of Composites.

6.13 Post processing Post processing is done after the curing process is completed. This involves the cutting and trimming of the component to the required dimensions prior to final finishing. Refer to SWP10 on the De-moulding, Trimming and Machining of Composites.

7 Processing of poly-ester resins When using poly-esters as the resin system for hand lay-ups, the same processing techniques and tools can be used as with epoxies. Poly-ester resin systems normally have a very short pot life compared to epoxy resins which results in shorter processing times. The next guidelines may be helpful in the processing of polyesters to help extend the pot life: o o o o o

Mix smaller amounts of resin at a time. Spread out resin quickly to avoid exothermic run-away reactions. Control room temperatures as per material datasheet. Add the minimum allowable catalyst as per material datasheet. Apply resin with squeegees to hasten the out-spreading process.

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8 Quality control The best measure of quality control is the repeatability of a process. If the same conditions can be created uniformly throughout a process, the quality of the product will mostly remain consistent. Improved consistency is achieved in an environment where moisture and temperature is accurately controlled.

8.1 Determining epoxy quality The preparation of test specimens is required to determine the quality of the mixing process of the resin used during the lay-up. A small epoxy sample is cast for each epoxy mix. Each sample should have a reference number to trace it back to the part manufactured with this batch. After sufficient curing, the sample is inspected using the following criteria: •

The surface of the test sample is scratched using a sharp object to determine the shoe-hardness of the epoxy. Inconsistent scratch depth and surface hardness indicates an incomplete reaction of the epoxy components. This is caused as by inconsistent or incorrect mixing procedures.

•

A scratch test must be performed by a person with experience on the physical properties of the different epoxies, as each epoxy will exhibit unique properties and it is up to the person performing the test to deduce whether the desired physical properties have been achieved.

The test sample will also indicate if the curing process is still incomplete.

8.2 Preparation of a test specimen during the lay-up A test specimen for critical parts should be prepared during their actual lay-up process. The reference number on each of the specimens should be traceable to the following: • • • • •

Material batch numbers. Epoxy mixing ratio. Date, time and environmental conditions. Lay-up information. Operator details.

These specimens can then be subjected to a destructive test to establish the quality of the lay-up. The specimens must be assembled with the same epoxy batch mixture and subjected to curing pressure, temperature and the time identical to the actual part. Refer to SWP001 on Test Specimens.

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Appendix 1: Problems and remedies found in hand lay-ups Problem

Possible Cause

Remedy

Resin stays soft and sticky.

Insufficient amount of catalysts added Incorrect mixing Resin not mixed properly.

The component needs additional heat curing. Reject part Post-curing at increased temperature. Reject part Note the optimum operating temperature. Hot cure components, Resin, mould and ambient temperature should be at the specified temperature. Cut off air on outside surface, using a peel ply or a wax containing resin. Clean surface with solvent (acetone). Clean surface with solvent (acetone). Possible reject Grind and degrease the surfaces to be bonded. Hot cure the component. Use correct choice material approved for the application. Coat the surfaces, or use UVstabilised overlay filler. Fill and grind.

Resin stays soft and sticky in places. Pot life inconsistent with specifications. Resin does not cure fully within the specified curing time.

Processing temperature too high or too low. Processing and curing time too low.

Sticky or greasy surface.

Contact with air during curing process.

Joins split when loaded. Laminates exhibit stress whitening when loaded. Transparent or white components darken. Small holes in surface.

Fabric plies are badly joined and can be pulled apart. be pulled apart.

Poor joining between the laminate and the UP pregel, or curing incomplete. Streaks in the resin

Mouldings, filling compounds or moulding compounds “burn up”.

*Notes:

P E -

Incorrect mixing ratio or high air humidity. Inadequate pre-treatment of surface. Brittle resin system. Unsuitable glass fabric. Ultraviolet light/sunlight causes yellowing. Component was laminated without overlay directly in mould. The old ply had already cured. Fabric layers to coarse – making poor contact Incorrect sizing on fabrics Pre-gel and epoxy resin were laid up wet in wet.

Most probable cause is wet fillers or too high air humidity. The hardeners are too reactive and cause heat to build up in the mixture.

Lay-up laminates wet-in-wet or grind the already cured ply. Integrate a finer material between every ply of course material. Use compatible materials The pre-gel must loose its stickiness. A Better solution is to cure the gel at 20 ˚C. Dry fillers for several hours at 50 – 100 ˚C in an oven before processing. Immediately cool the quantity, if possible transferring it to a dish. Use slow hardeners or lay up several plies.

Polyester system Epoxy system

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Notes* P E P E P,E P,E

P

E P,E P,E

P,E P,E

P,E P,E P,E E

P,E

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