Hand Layup in Prepreg Factory - A405

 

Hand Layup in Prepreg Factory
Document Type	Article
Document Identifier	405
Tags	Deposition
Prerequisites	Hand layup prepreg (Autoclave/Out-of-autoclave) processing

Introduction

Ply layup is the act of placing the plies and compacting them prior to curing the part. It is a critical step, as an incorrectly placed ply can be difficult to identify and can result in a scrap part. If it slips through, it will affect the part's performance. The methods of layup range from painstaking hand layup to fully automated standard processes (FW, ATL, AFP) and/or custom robotic solutions.

Even so, the most common form of layup still found in industry is still hand layup. As a result, it is imperative to have a good understanding of the process and associated technologies used to assist operators in a factory setting.

Significance

Prepreg hand layup remains a largely labour based process. Due to the dexterity required to apply prepreg plies, along with production volumes that typically do not justify highly automated processes, the use of hand labour remains the mainstay of prepreg part layup. Accuracy of order, orientation of the fibres,  position and compaction are all critical to make a good part.  The accurate ply shape/ outline (be it exactly cut, overcut or even undercut) is also critical to the performance and can be a contributor to the mass of the part. Understanding the limitations and technologies that are available to aid operators in the laminating process is key when designing a process.

Scope

This article covers details related to hand layup in the factory setting, while the specifics of hand laminating are covered here. Details include layup process selection, MSTEP and prepreg, and details, requirements, and goals of a hand layup workstation. The goal of this page is to provide an overview of how the process looks in a factory setting.

Process Selection for Layup

Hand forming and compaction/debulking of each ply into its final shape must be done with care and consistency with the design requirements. The goal is to prepare well-compacted parts built to specification, ready for cure. It is important to perform the layup in the right order, with the correct side up, fibre orientation in the correct direction, and in the right position. 

The majority of prepreg composite parts are assembled/laid up by hand. Tools have been developed to decrease the skill requirements and tediousness of layup, as well as increase the productivity, quality and traceability. The physical layup process, however, is still largely a manual operation. 

The degree of automation to implement is defined early on in the part definition and is typically selected based on return on investment, which in turn is a result of volumes to manufacture and program life. Productivity and efficiency are often measured in kg prepreg/hour of work against the capital and operating costs involved. Layup rates can range greatly, however, general guidlines are provided below:

Approach	kg/ hour deposition (Est. range)
Manual measured hand Layup	0.5 to 3.0
Hand layup with templates	2.0 to 5.0
Laser guided hand layup	3.0 to 10.0
Filament winding	15 to 100+
Auto tape layup ATL	100
Auto fibre placement AFP	75

In each case, the cost of labor versus the capital investment is a key criterion in selecting the process to be used. Once a layup process/method is selected and a process certified, the commitment to maintain the process is established. For this reason, important decisions early on in the bidding process can have large overall impacts on the success of a program. Once started, the opportunities are usually in reducing the labour and increasing the automation level involved in the manufacturing process.

At scale, the introduction of automated technologies and aids for operators can be thought of as belonging to the categories shown in the figure below. The corresponding selection of these is dependent on the desired throughput and quality requirements of the part being made.

Categorizing Prepreg Layup

MSTEP approach in Prepreg Hand Layup

The following figure shows the application of the MSTEP approach to composite processing, specifically to the hand layup of prepreg composite materials.

Hand Layup MSTEP for Prepreg

Hand Laminating Workstation

Most composite layups are done in a cleanroom, as the resin is sticky and dust and foreign objects can easily adhere to the material. UV exposure can may be an issue when working with prepreg and levels are typically restricted. Prepreg enters the clean room with backer plies on both sides and in hermetically sealed kit bags. The part leaves the clean room in a vacuum bag, debulked, ready for cure. The layup room requires cleanroom conditions to avoid contaminating the resin and prepreg. The manufacturing process also creates dust and debris that must be minimized to avoid creating a scrap part. Cleanrooms are accessed by airlock where possible, or by cleanroom “garage” doors, which are tightly sealed and have a fast open and close cycle to minimize the possibility of dirt entering. More about specific requirements for layup clean rooms can be found in the following page:Design Requirements for Hand Layup Workstation.

Hand Layup

Manual prepreg part manufacturing by hand layup is the most laborious way to make parts. Plies are positioned by hand using a ruler and marking the location where the files should be placed. The part must be placed with the designated side up to ensure fibres, the key structural criteria, are positioned correctly.  

Hand Layup Prepreg, operator using stencile

Hand layup using nothing but basic tools such as a measuring tape, rulers, and straightedges, is largely artisanal manufacturing.  In parts with higher requirements for performance, quality, etc., a high degree of skill and understanding is required to achieve the desired results.   

Pros:

• Low capital cost 
• Low barrier to entry approach
• No expensive templates or templating tools to make and maintain 
• Suitable for low-volume production 
• Allow for introduction of digital tools   

Cons: 

• Low/slow production 
• Highly skilled operators required 
• Repeatability can be a challenge 
• Traceability can be low, and undocumented tribal knowledge can affect repeatability. 

Template Assisted Hand Layup

Making paper, sepia or other material templates with marking lines is a method to make repeatable parts without the higher cost of tools. Manual templates are typically Mylar or other material with slots cut in them to mark the position of layup. Templates are typically cut on a 2D cutting table to obtain the necessary accuracy. Features on the template and associated tool are required to align the template with the tool. Once done, the ply position is marked on the existing surface, and once the template is removed, the next ply is applied.

Template assisted layup

Pros: 

• Higher degree of productivity and traceability
• Low cost and relatively high accuracy
• Some degree of skill required, much lower than hand hand-measured layup alone

Cons: 

• Template manufacturing can be costly, with long lead times, and difficult to maintain
• Template storage, management, including revision upgrades, and maintenance is costly
• 25-year maintenance of files and templates can be cumbersome
• Any design changes involve manufacturing of new templates

Laser Assisted Hand Layup

To increase productivity, accuracy and traceability of composite part manufacture, laser projectors were developed to replace the most error prone and slow step in hand layup.  With a laser projector, the outline of the ply is projected from the laser on to the layup surface, so that the operator does not need to measure and mark the ply. The operator merely positions the ply within the laser light being drawn, then moves on to the next ply. This eliminates a considerable amount of non-value added operations, increasing throughput and labour efficiency. An early analysis by Lockheed Corporation identified the savings graphically. 

Laser Assisted Hand Layup

With a laser templating system, the tool has a minimum of four tooling target locations against which the part is positioned. High-accuracy targets are placed in the precision-machined tooling locations. The targets are covered in silver retro-reflective targets.

When the laser light hits the retroreflective target, it receives the signal reflected back to the laser, resulting in an auto-collimated location of the tooling. Once the four or more tooling balls are located, the laser can triangulate the outline of the ply from the CAD file.

Once the operator has placed the ply, a simple controller (handheld, foot pedal, or other) moves the projection to the next ply to allow the operator to position the subsequent plies. The typical laser projection system covers an area 4.57 m (15') x 4.57 m (15') when the laser projector is 4.57 m (15') from the surface. The laser works best perpendicular to the surface.

Various laser systems continue to innovate and offer additional features, such as:

1. Multi-tasking is a benefit where one projector system can project on multiple parts in parallel. This allows the investment in a laser projection system to be shared multiple-fold over many part, offering multiples times in productivity. An adaptation of the analysis above becomes;
2. Text projection on the part, to give additional information to the operator, 
3. Retro-reflective control, using the return signal of retroreflective material as a controller, 
4. Using the computer driving the laser to give additional information and control over the layup operation. 

Virtek Laser Assisted Layup System

Pros:

• Higher degree of productivity, traceability, and quality 
• Lower degree of skill required  
• Can be done in a paperless fashion 
• No templates to store 

Cons: 

• Capital cost 
• Computer infrastructure 
• Basic training required (minimal IT knowledge required) 

Learn more about laser guided systems here: Laser Projectors, Laser Assisted Layup technology.

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.

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

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.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

In the context of knowledge in practice, knowledge refers to the systematic use of science based knowledge in composites manufacturing practice.

There is a distinction between experience based knowledge and science based knowledge:

• Experience based knowledge (‘know-how’) is an understanding of potential outcomes and their relationships that is founded on pragmatism and experience accumulated over time in individual programs, companies and in the industry more broadly.
• Science based knowledge (‘know-why’) is an understanding of potential outcomes and their relationships, based on the important processing physics, that is mature enough to be codified using the appropriate governing laws and constitutive equations.