Hand layup prepreg (Autoclave/Out-of-autoclave) processing - A291

 

Hand layup prepreg (Autoclave/Out-of-autoclave) processing
Document Type	Article
Document Identifier	291
Prerequisites	[[A200|]] [[A259|]] Materials deposition and consolidation management (MDCM) [[A201|]] Viscosity (resin)

Introduction

Despite continued advancements in other material deposition methods, hand layup prepreg remains a big part of the composites manufacturing industry. Hand layup involves skilled laminators forming individual prepreg plies layer by layer into various geometries. This process can produce complex and high performance parts, however, it can be costly and operator dependent.

Significance

Hand layup is one of the most widely used prepreg deposition methods. It is capable of producing highly complex features due to advanced human dexterity and can easily adapt to new designs and design changes. Understanding the workers' techniques can help designers better design parts with manufacturability in mind. These insights can not only accelerate new worker training but also shed light on automating the layup process using automated tape laying (ATL) and automated fiber placement (AFP).

Scope

This page provides an overview of the hand layup process. The process is explained from the MSTE perspective to include important variables for hand layup. Challenges associated with hand layup and other key considerations from Thermal and cure/crystallization management (TM), Materials deposition and consolidation management (MDCM) and Residual stress and dimensional control management (RSDM) are also discussed.

Prerequisites

Recommended documents to review before, or in parallel with this document:

• Material deposition - A182
• Materials deposition and consolidation management (MDCM) - A157
• Prepreg - A171

Process Demonstration

The video below provides an introduction to prepreg layup. It provides a brief overview of how to handle the material, how to apply it to the tool, how to perform a debulking step and the effect of common processing issues on the outcome of a part.

Process description

Ply cutting

In the case of hand layup, the room temperature prepreg is cut into the desired size and shape before layup. The cutting is done in a clean room (see Prepreg - A171 for more detail) and can be either manual or automated. Hand cutting prepreg can be done using knives and guides. Templates (typically made from aluminum or steel) can be fabricated to guide the hand cutting. It maybe be unavoidable to have chips or particles being shaved off the template and embedded into the prepreg. The disadvantage of manual cutting includes: labor intensive, time consuming, poor material utilization, and risk of fiber orientation errors. Automated ply cutting is used in most high volume production environments. Among many type of cutters (lasers, steel rule dies, water jets), reciprocating knives and ultrasonic cutters are the most common in the composites industry. The cut prepreg plies are then labelled (often by automated cutting machine), sorted and sent to subsequent layup. This process is commonly referred to as 'kitting'.

Ply deposition

The hand layup is typically performed in a clean room, similar to the environment of the cutting step. The operator then manually deposits each prepreg ply onto the tool or previous ply often with the assistance of a hand held tool (see tool of hand layup below) to firmly pack down the laminate to avoid/reduce air pockets. The sequence and orientation of each prepreg ply and consumable material should comply with the engineering drawing or work order. Backing paper and release film should be removed before the prepreg is applied. Some specifications may require the prepreg ply to be applied with the backing paper side face up or face down. Templates, guides or laser projection units can be used to guide the layup. Templates can be inconvenient when the part is large or has many layers of different sizes and shapes. Guiding holes outside of the effective part boundary can be cut into the prepreg to match the pins on the mold. These "ears" are then machined when the part is cured. Laser projection technologies are widely used these days to projects the outline of the next ply at appropriate location with high accuracy (± 0.015 - 0.04 inches). The projections are generated with CAD software and are capable of taking the complex contours into consideration. During the hand layup, if applicable, sensors such as thermocouples or pressure sensors can be embedded.

Hand layup is prone to defects such as fiber bridging and winkles or wavienss when the part geometry is complex. See Prepreg - A171 for more information on fiber bridging. Squeegees and rollers can be used to remove wrinkles in hand-layup. Hot air guns or irons can soften the prepreg, increase its tack and thus makes it easier for operators to layup with less wrinkles. However, the material temperature should generally not exceed 65.5 °C (150 °F).

When a flat ply of prepreg with limited extensibility is formed into a concave or convex surface, the ply needs to deform. Unidirectional prepreg plies can bend or be stretched/compressed in the direction perpendicular to the fiber direction in a very limited amount. Whereas a woven prepreg ply allows the fiber tows to rotate and slide against each other, i.e. shear, so that the ply can extend and contract in the ±45° directions. Given one geometry, the shape of deforming a ply into that geometry is not unique and can depend on the worker's techniques. If the deformation exceeds the prepreg extensibility limit, slits, darts, cuts or folds need to be made to accommodate the material deficits or material excess, which can significantly decrease the structure performance.

Vacuum bagging and compaction

The vacuum bagging in hand layup autoclave prepreg process is similar to that in general prepreg processing. See Prepreg - A171 for more detail. For out-of-autoclave prepreg however, this is an extremely crucial step because vacuum is the sole source of compaction. Out-of-autoclave, OoA (aka. vacuum-bag-only, VBO) prepregs rely on specific features in the prepreg microstructure and processing techniques to achieve low porosity. Intentionally designed partial impregnation (dry areas) create paths for the entrapped air during layup to be removed. These paths, sometimes referred to as "engineered vacuum channels (EVaCs)" allow the gas inside the laminate to migrate and to be removed through the laminate boundaries during de-bulking and early stages of the cure. Therefore, permeable boundaries such as dry fiberglass, cork must be used to allow VBO prepreg to "breath".

Thermal transformation

After vacuum bagging and sufficient debulking, the parts undergo thermal transformation. This is typically achieved through an autoclave. However, increasing out-of-autoclave (OoA) prepregs have gained acceptance because they are more cost efficient, have less overall emission as well as capable of producing less than 1% void content parts with autoclave-quality. With OoA prepregs, the parts are either cured using an oven or heating blankets while being vacuum bagged. Not only can OoA parts be cured with less compaction pressure (1 atmospheric pressure), they can sometime be cured at lower temperature (93°C/200°F or 121°C/250°F) compared to the autoclave counterpart (85 psi with 177°C/350°F). Heated tools can also be used to cure OoA prepreg materials.

Material of hand layup prepreg (Autoclave/Out-of-autoclave) processing

Graphite/epoxy prepreg is the most common material for this process. See Prepreg - A171 for more detail. Depending on the fibre architecture, prepreg weight and matrix materials, the prepreg can have different characteristics. Also if inserts are involved in the layup, the hand layup difficulty can also increase.

As mentioned above in ply deposition, unidirectional or woven prepreg can deform differently to accommodate the part geometry. In general, the heavier the prepreg (measured in grams per square meter, gsm), the harder it is to hand layup because the material is denser and more difficult to conform to intricate features. The right amount of tack from the matrix can help workers laying the material more efficiently. Too little tack can cause the prepreg ply to not adhere to the previous layer or the tool surface, making it difficult to create the desired geometry. Heating may be required to increase the tackiness, which ultimately takes more time. Too much tack can also present challenges if a worker tries to adjust or remove a ply that has already been laid down.

Tool of hand layup prepreg (Autoclave/Out-of-autoclave) processing

Tools for hand layup are typically one sided, open molds on which prepreg is laid on top. Caul plates can be used on the vacuum bag side to provide better surface finish and more uniform part thickness. Depending on the application, tools for hand layup can be made from a wide range of materials such as wood, plastic, metal and composite materials. When selecting tooling materials, aside from making sure the material can withstand the prepreg processing condition (temperature and pressure), part thermal management, residual stress and dimensional control must be considered.

Because the OOA prepreg can be cured at lower temperature and pressure, tools made for autoclave which typically have complex substructures can be simplified . Residual stresses and process induced deformation due to tool-part coefficient of thermal expansion (CTE) mismatch can also be better controlled because the temperature gradient is smaller.

Equipment of hand layup prepreg (Autoclave/Out-of-autoclave) processing

Autoclave

Link to Autoclave

Oven

Link to Oven

Application

• Aerospace
• High performance automotive
• Sport/recreation equipment
• Marine

Advantages

• Human dexterity is capable of laying up intricate and complex geometries which may be impossible for ATL, filament winding or AFP
• Lower capital cost compared to ATL, filament winding or AFP

Disadvantages

• Potential for human error
• Labor intensive
• Limited part size by human reach
• Slow material deposition rate compared to ATL, filament winding or AFP
• Lower accuracy in ply orientation and position compared to ATL, filament winding or AFP
• Less control over compaction pressure and temperature during deposition compared to ATL, filament winding of AFP

Troubleshooting

References

About	Help

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.