Practice for Developing a Wet Lay-up - P108

 

Practice for Developing a Wet Lay-up
Practice document
Document Type	Practice
Document Identifier	108
Themes	Materials deposition and consolidation management
Tags	Bagging Consumables Deposition Demoulding Development to production Integrated product development Process step
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Integrated Product Development Practice Practice for Developing a Process Step Integrated Product Development Wet layup

Overview

Wet lay-up is a cost-effective and efficient method for processing composite materials. This technique typically utilizes a single-sided mould that controls both the shape and the cosmetic surface of the part.

Wet lay-up is the process of applying resin to a dry reinforcement material while exposed to the atmosphere. This is typically done with a brush or squeegee.

Significance

Advantages of wet lay-up:

• Low startup costs: The tooling required is relatively inexpensive compared to closed moulding processes. Minimal/no ancillary equipment is required.
• Scalability for large parts: Wet lay-up can accommodate very large parts.
• Versatile and cost-effective: It provides a broad range of capabilities for producing parts efficiently, offering significant cost benefits.
• High-quality finish: A high-gloss finish can be achieved on the tool side of the part, enhancing its appearance.

Practice

Before beginning the composite manufacturing process, proper preparation of the mould is recommended to increase quality and ease of part release. A well-prepared mould surface helps achieve the desired finish, prevents defects, and ensures smooth removal of the part once it has cured. The following steps outline the typical processes for preparing the mould before applying gel coat and resin. These are general, high level procedures. Consult the product supplier for proper application, use, and safety instructions.

Mould Preparation

• Multiple (~four) coats of a mould sealer such as Sealer GP, a high modulus, clear, flexible film that forms a protective shield on the mold or tool surface, are applied to the mould surface. The purpose of the mould sealer is to prevent mechanical bonding. Procedure for applying sealer can be found in the Zyvax product information sheet such as Zyvax Flex-Z TDS.
• Multiple (~four) coats of mould release are applied to the mould surface. The purpose of the mould release is to prevent chemical bonding. While other solvent-based release systems are available, mould release wax is the most commonly used in the industry. An electric buffer used in combination with a microfiber cloth can be used to quickly buff off wax after applied for flat large surface areas.
• Some moulds may require edges to be taped off or have mould release wax brushed onto them to prevent overspray or resin build-up.
• Ensure mould surface has been wiped clean from any contaminants prior to applying gelcoat.

Gel coat Process

Gel coat is a pigmented resin that is applied onto the mould surface to produce a smooth, protective layer for the composite laminate. This layer serves both an aesthetic and functional purpose, providing a durable, glossy finish and protecting the underlying laminate from damage. Gel coat is typically applied by spraying on to the mould. Application with a brush is also possible but may result in less consistent thickness. There are different types and variations of spray equipment on the market, however the common system is a high-pressure external mix pneumatic gun, which means that the resin and catalyst are fed through separate lines exiting the gun tip at separate orifices and merging into one continuous fan pattern which can be adjusted by the operator. A High Volume Low Pressure (HVLP) gun can also be used. HVLP systems are also pneumatic, however, gel coat and catalyst are mixed together and poured into the cup located on top of the gun. The fan pattern can also be adjusted, as well as the material flow. Both are effective in their own way, but for mass produced parts, the external mix gel coat gun is commonly used in industry. The HVLP gun is used for smaller, low production parts.

• Gel coat is applied to the mould surface to create a smooth, durable finish. A mil gauge is typically used to measure the thickness of the gel coat as it is sprayed onto the mould. The ideal thickness for the gel coat is typically between 18-24 mils. It is important to keep in mind that gel coat will shrink as it cures. Applying too little gel coat can lead to a non-uniform surface, which may cause light to show through from the reverse side of the laminate. On the other hand, applying too much gel coat can result in sagging or thick areas, potentially causing the gel coat to release prematurely from the mould surface. Excessive thickness can also create voids in the laminate due to an uneven surface. Proper application ensures a smooth, high-quality finish.

• The recommended initiator (catalyst) percentage for polyester gel coat is typically between 1.25 - 2 %, with a range of 1 - 3 % depending on environmental factors such as mould temperature, gel coat temperature, and ambient conditions. Adjusting the catalyst percentage ensures optimal curing conditions. Refer to the TDS for more information.

• A typical cure time for a gel coat is around 45 minutes. However, this can vary depending on environmental conditions, initiator type and ratio, and thickness of the application.

• To check if the gel coat is ready for lay-up, lightly press a disposable gloved finger onto the cured surface. If a print is visible but does not leave an imprint on the finger, the gel coat is at the correct tackiness for the next step. If the surface is too soft or sticky, it requires more curing time.

Chopped Strand Mat

Chopped Strand Mat (CSM) consists of 25 to 50 mm (1 to 2 inch) long fibres that are randomly distributed in plane. These strands are pressed flat and held together with a binder. When the CSM is wet-out with polyester resin, the styrene in the resin dissolves the binder, making the material more pliable and easier to work with during the lamination process. The most common areal weight of reinforcement used in industry is 1.5 oz/sq ft., which typically results in a material thickness of approximately 1.15 mm (0.045 inches). 1.0 oz/sq ft CSM is also used, and typically results in a material thickness of 0.75 mm (0.030 inches). 1.0 oz/sq ft is typically used as a skin coat to prevent blistering and provide a protective layer for the primary laminate. 1.0 oz/sq ft also works well as the initial ply when complex geometry is present on the mould surface. Because it is thinner than 1.5 oz/sq ft it wets out (saturates) with resin more easily and conforms to the mould surface more readily. While other sizes and thicknesses of CSM are available, they are less commonly used in industry. CSM is not only used for production parts but is also a major component in the construction of tools (moulds). For more information on mould maintenance, see Practice of Mould Maintenance - P109.

Prior to lay-up, ply kitting is essential. The number of plies and the type of material required are determined by the engineered design specifications. For typical production FRP parts, three plies of 1.5 oz/sq ft CSM are used. The surface area for the lay-up includes the flange coverage and a 25 to 50 mm (1-2 inch) runoff beyond the flange. This extra material ensures easier demoulding once the part has cured. For flat surfaces, a simple length x width measurement with a tape measure suffices.

Rolls of material range in widths of 100 to 150 cm (38-60 inch). Material is rolled out onto a designated cutting table. The cutting table should be wide enough to at least accommodate a 120 cm (48 inch) wide roll, and approx. 240 cm (8 ft) in length. Puckboard is typically used as the cutting surface of the table. The plies are cut and folded into a size which can be carried to the work station. For regular production parts, plies are kitted based on a predetermined amount of plies required for each part and placed on shelving in the cutting area. For larger moulds, which require CSM to be butt-jointed, the outer edge of the material, or the straight edge must be frayed in order for the joint to bond properly once resin is applied. The edges are typically torn by hand, removing approximately 25 to 50 mm (1-2 inch) of material. Failure to fray the butt-jointed areas can result in resin richness, causing the part to be brittle and weak.

Lay-up

There are a few different methods of lay-up, each having their own pros and cons. Here are the most common:

1. Spray-up\Chopper Gun

The resin and catalyst are mixed internally in a pneumatic spray gun, similar to a gelcoat gun, but with an additional valve that purges acetone and air through the system after spraying. The resin and catalyst are pumped from a 55-gallon (210 litre) drum and a catalyst reservoir, respectively, via a slave pump. The roving is fed through guides mounted on the spray equipment’s swing arm (boom) and then directed through a guide located at the top of the gun. A small motor controls top mounted blades that when activated, feed the roving into the spinning blades, chopping it into 25 to 50 mm (1-2 inch) long pieces of chopped glass that fall directly into the resin stream, which in turn is distributed onto the mould surface. Unlike Chopped Strand Mat (CSM), there is no binder involved, allowing the glass to be laminated immediately. This method can be used on any mould, but is typically used for larger moulds, as the material is distributed very fast.

The steps are as follows:

• Mould preparation: Ensure that all necessary steps, such as mould cleaning and release application, have been completed prior to beginning the lay-up.

• Skin-Coat application: For moulds with tight radii, a skin-coat of 1 ply of 1.0 oz/sq ft CSM may be applied as the first layer. This thin layer is easier to wet-out and conform, ensuring a smooth finish before proceeding with the rest of the laminate build. The skin-coat is allowed to cool to room temperature after exothermic curing before continuing with additional plies.

• Wet-Out process: Resin and roving are sprayed onto the mould surface. Additional resin is sprayed to touch up any dry spots.

• Additional applications may be necessary to achieve the desired thickness, depending on the part's overall requirements. However, it can be challenging to gauge thickness during application since there is no precise way to measure the amount of material, other than by experience and visual inspection.

• Overspray: This method generates significant overspray, which, if not promptly cleaned, can accumulate on tooling, equipment, tables, walls, and floors.

• Maintenance: A chopper gun has more moving parts, which means more maintenance is required. Regular upkeep is essential to ensure the equipment remains in good working condition.

2. Wet layup

To learn more about the wet layup process, see Wet layup - A296. This method is ideal for small batch production, test panels, and when no spray equipment is available. Resin and catalyst are measured and mixed in chemical resistant plastic pails/containers, and distributed onto the mould manually.

The steps are as follows:

• Mould Preparation: Mould or alternative tooling is prepped and gelcoated.

• Ply kitting: The required materials are kitted and ready for use.

• Preparing resin: A container is placed on a scale, and the scale is zeroed. The predetermined amount of resin is decanted into the container. The catalyst amount is calculated by multiplying the resin weight by the required percentage. For example, 150 g of resin would need 2.25 ml of catalyst if using a 1.5% concentration. A catalyst squeeze bottle with "ml" indicators is typically used for accuracy. The catalyst is added to the resin and mixed thoroughly for 30 seconds to 1 minute.

• Wet-out process: A 50 mm (2-inch) paintbrush is used to evenly distribute the resin onto the mould surface The material is placed onto the mould. Resin is brushed onto the material, ensuring it is fully wetted out. Use metal rollers, typically 10 mm (0.5 inch) diameter x 75 mm (3 inch) wide, and 50 mm (2-inch) chemical-resistant brushes to press the material into the mould. The primary purpose of the rollers is to remove air entrapment and ensure full contact between the material and mould surface. Brushes help with air removal and absorb any excess resin.

• Inspection: After each ply is laminated, perform a visual inspection to ensure there are no missed air voids. The first ply is the most critical, as it is in direct contact with the gel-coated mold surface. Any missed voids can create weak spots in the laminate, potentially resulting in a defective or scrapped part. If the gelcoat is not the final finish, repairs may be made, especially for parts coated with sandable gelcoat that can be body worked and primed.

• Skin-Coat application: For moulds with tight radii, a skin-coat of 1 ply of 1.0 oz/sq ft CSM may be applied as the first layer. This thin layer is easier to wet-out and conform, ensuring a smooth finish before proceeding with the rest of the laminate build. The skin-coat is allowed to cool to room temperature after exothermic curing before continuing with additional plies.

• Repeating the Process: The process continues, repeating the steps until all required plies have been laminated.

Demoulding

Depending on certain conditions, the time in which the part is ready to be demoulded can vary, but typically it ranges from 45 to 60 minutes after the end of lamination. Temperature and catalyst percentage play a big part in the turn around times. To ensure optimal results, it is best to consult the TDS (Technical Data Sheet) of the chemicals being used before beginning part fabrication.

• Measure the laminate’s temperature. Do not attempt to demould if the temperature exceeds 30°C (85°F).

• Tap a plastic wedge between the mould surface and the laminate using a rubber mallet, starting at one corner.

• Place a second wedge at the opposite corner and repeat the process.

• Slide the wedges along the edges of the mould on all four sides to release the flange area.

• Gently tap the wedges further in to help separate the part from the mould. Be cautious not to insert the wedge too far, as this may cause excessive stress on the part, leading to cracks or potential failure.

• Parts with simpler geometry and fewer feature lines are generally easier to demould than those with complex or deep cavities. For stubborn parts, air may be needed to aid in demoulding.

• Demoulding techniques vary depending on the unique characteristics of each part. It is often a process of trial and error, with adjustments made as needed. Careful planning before using wedges is essential to avoid unnecessary damage.

• Once the part has been successfully demoulded, it can be carefully removed and taken to the grinding or trimming area for finishing.

Trimming

FRP parts are typically produced in a mould that features a trim or scribe line etched into its surface. This trim line transfers onto the gelcoat, providing a guide for where excess material should be removed.

• A pneumatic grinder or die grinder equipped with a diamond cutting blade are typically used to make the initial rough cut on the FRP part. Carefully trim the excess material, staying close to the trim line without cutting past it.

• For finer trimming, use a pneumatic mini grinder or sander with a 36-80 grit sanding disc. The lower the grit number, the coarser the disc. Coarser grits remove material more quickly but may chip the gelcoat edge, while finer grits are ideal for finishing and smoothing up to the trim line.

• Depending on the complexity of the trim line, other tools or attachments may be required.

• Thickness gauges can help measure the overall thickness of the part. One drawback of hand lamination is the potential inconsistency in part thickness.

• Once trimming is complete, carefully dust off the part to remove any debris.

Troubleshooting

Problem: Part sticking to mould - unable to demould

Cause: The mould prep was performed incorrectly; steps were missed; poor tool/mould design. Application of the sealer is the most important step as it is designed to bond directly to the mould surface, filling in the porosity found within the surface, thus preventing mechanical bonding. It increase the performance of the mould release and longevity of the mould.

Solution:

If the part is sticking, but eventually releases:

• Strip the mould with acetone to remove any previously applied release agents.
• Inspect both the part and the mould for damage, repairing as necessary.
• Apply Sealer GP (~ four coats) following the Zyvax Flex-Z TDS application guide instructions.
• Apply four coats of high-temperature mould release wax.
• Mould is now ready for production.

If the part is permanently stuck in the mould:

• This is the worst-case scenario. Since the part has become mechanically locked to the mould surface, the part and mould are typically scrapped, causing financial loss and taking a mould out of production. The best-case scenario is to fabricate a new mould from the original plug. This process takes time and labour, leading to financial losses and production delays. Always follow the proper procedures and instructions at all times.

Problem: Delamination

Cause: Delamination can result from a variety of factors, including contaminants on or within the material being laminated, equipment malfunctions, and environmental conditions. Common causes include moisture, dust, debris, or the presence of unwanted adhesives, all of which can interfere with the bonding process.

Solution:

• Store materials in a clean, dry, and dust-free environment. Ensure rolls are placed on racks and kitted materials are stored on shelves or tables away from contaminants.
• Inspect materials thoroughly before lay-up. Check for dampness and contaminants by feel. If the material is damp, discard it, as moisture prevents proper bonding with the resin and can result in incomplete curing and part failure. If needed, blow off or remove any dust from the materials.
• Ensure the mould surface is free from water, dust, and debris before applying the gelcoat.
• Check for water in the air lines. Some shop systems include automatic purging to release trapped moisture. If this feature is not available, manually open the air lines daily to remove any accumulated moisture.
• Test new products and approve them before releasing them for full-scale production. Fabricate test panels to gather data and ensure the material performs as expected.

Problem: Gelcoat or resin slow or not curing

Cause: The most common cause of slow or incomplete curing is insufficient initiator (catalyst) flow or an incorrect mixture. This could be due to a malfunction in the spray equipment, particularly with the slave pump, or because the ratio is too low. Air locks in the slave pump can also cause inconsistentflow into the spray stream. Additionally, low ambient, mould, or resin/gelcoat temperatures may hinder the curing process.

Solution:

• Inspect the spray equipment for blockages. Remove the pin holding the catalyst pump arm to the slave arm and manually pump the system until any air locks are cleared and the catalyst flows consistently.
• Temperature plays a critical role in successful part fabrication. Always refer to the chemical Technical Data Sheets (TDS) before use. Perform gel time cup tests on both new and expired chemicals to confirm their effectiveness.
• Avoid spraying resin on moulds that have been stored outside and are below room temperature, or using resin from drums that have been exposed to cold conditions. Cold resin can separate from its components, leading to pooling in mould cavities.
• Check the manufacturer’s recommended shelf life for resins and gelcoats. Most have a shelf life of 3-6 months.
• When mixing resin manually, ensure to mix for at least 30 seconds to 1 minute to ensure the catalyst is thoroughly incorporated.
• Maintain catalyst percentages between 1% and 3%. Always refer to the catalyst TDS and adjust based on ambient and material temperatures, as these significantly impact curing performance.

Problem: Resin/laminate curing during layup

Cause: Improper planning and timing, excessive catalyst percentage, incorrect catalyst selection, high ambient or booth temperatures, or expired resin can cause resin to begin curing prematurely during layup.

Solution:

• Proper preparation is essential before fabricating any FRP part. Consider the following questions:
  • How many plies are required?
  • How many employees are needed to perform the layup efficiently?
  • Is there enough material readily available for the entire layup process?
  • Are the ambient and booth temperatures within acceptable ranges? These considerations must be addressed well in advance to avoid rushed or incomplete layups.

• Always refer to the resin and catalyst TDS before starting the fabrication process to ensure the correct products and mixing ratios are being used.
• If using resin that has surpassed its shelf life, conduct a gel time cup test to confirm its viability before proceeding with the layup.
• Ensure the catalyst percentage is adjusted according to ambient conditions, booth temperature, resin temperature, and mould temperature. These factors significantly affect curing rates and working time. For example, LHPD2222 PE resin mixed with 1.5% MEKP-9 catalyst at room temperature typically has a gel time of 25-30 minutes. Ensure there are enough qualified workers available to complete the layup within this window and keep the laminate void-free.

Problem: Air Voids, defects and cracked parts

Cause: Air voids can become trapped between the mould surface and the first ply, as well as between subsequent plies during layup. The first ply is the most critical for removing air voids, as failure to do so can create weak spots beneath the gelcoat, leading to cracking and surface defects. Defective parts can result from various factors, including human error, expired or defective resin/gelcoat batches, or equipment malfunction.

Solution:

• For molds with tight radii, a skin-coat of 1.0 oz./sq ft CSM can help improve the layup process. This material is thinner and more flexible than 1.5 oz/sq ft CSM, making it easier to conform to tight areas and eliminate voids. Metal rollers with a 6 mm (0.25-inch) diameter roller head are ideal for removing trapped air in these difficult-to-reach spots.
• Conduct visual inspections after each ply is laminated and before applying the next one. This ensures that any entrapped air is identified and removed in a timely manner, preventing defects in the final part.
• Cracking often occurs during the demoulding process, especially when excessive force is applied. Inexperienced operators may push wedges too far, causing undue stress on the laminate. To avoid this, parts should be trimmed during layup, prior to curing, to facilitate easier demoulding. Proper training and experience are key to achieving crack-free parts. Every mold has unique requirements, and demoulding techniques should be tailored accordingly.
• Spray equipment will eventually malfunction. Management should implement a proactive maintenance program, while operators must remain vigilant in monitoring and maintaining equipment during use. Regular upkeep ensures optimal performance and reduces the risk of defects caused by equipment failure.

About	Help

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.

A measuring instrument used to determine the thickness of materials in mils (thousandths of an inch). It is commonly used in industries like manufacturing and quality control to measure the thickness of coatings, films, plastic sheets, and other materials requiring precise measurements.

A unit of measurement typically referring to one-thousandth of an inch (0.001 inches). It is commonly used to refer to the thickness of materials and coatings (e.g., gel coat thickness).

An acronym that stands for Technical Data Sheet. These are documents that provide information about specific details of a product. In the composites industry, this typically refers to material and processing properties. They are typically provided by the product manufacturer.

Chopped strand mat - A reinforcement material made of short, randomly oriented glass fibers bonded with a resin.

Areal weight (or fibre areal weight, AW ) refers to the mass/weight of fibre per unit area. (Typically in g/m² gsm ) or ounces/yard² (often just called ounces). Areal weight depends on tow size and fibre architecture (weaving, density, etc.).

An acronym for Fiber-reinforced polymer, which is another term used for a composite material composed of fibre and polymer. This is sometimes expanded to GFRP or CFRP for glass fibre reinforced polymer or carbon fibre reinforced polymer, respectively.

The process of preparing and organizing individual layers (plies) of composite materials, such as glass fibre or carbon fiber, into a specific sequence and shape before they are applied to a mould. Ply kitting is important for accurate material placement and reaching the optimal structural integrity of the finished laminate.

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.