Selection And Inspection of Gelcoat Materials

1.1 Characteristics of High-Quality Mold Gelcoat

High-quality mold gelcoats possess various performance features. In terms of heat resistance, different types of gelcoats, such as epoxy gelcoat (EP), vinyl ester gelcoat (VE), and unsaturated polyester gelcoat (UP), exhibit varying thermal performance. Generally, the heat resistance follows the order: epoxy gelcoat > vinyl ester gelcoat > unsaturated polyester gelcoat. For ambient temperature curing processes, vinyl ester or unsaturated polyester gelcoats can typically meet the requirements. However, for molds requiring elevated temperature curing, such as those used in autoclave processes where temperatures exceed 120°C for prolonged periods, epoxy mold gelcoats are recommended.

In terms of wear resistance, vinyl ester and polyester gelcoats typically have a Barcol hardness of around 40 HBa, while some brands can achieve 45-50 HBa. Adequate hardness is essential for protecting the mold during demolding, but excessively high hardness should be avoided to maintain a balance with toughness and strength, ensuring optimal performance.

Additionally, gloss is a key characteristic of high-quality gelcoats. Gelcoats with Barcol hardness below 30 HBa generally struggle to achieve a high gloss finish. Gelcoats with fewer pinholes exhibit better gloss, and the durability of the gloss is also critical. Some gelcoats may initially exhibit excellent gloss but deteriorate quickly after producing a few products, requiring frequent polishing.

1.2 Gelcoat Quality Inspection Process

The quality inspection of gelcoat must strictly follow the procedure:

1. Production Date: Verify the production date on each container as recommended by the supplier. Ensure the gelcoat is used within its shelf life.

2. Gel Time: Each batch of gelcoat has a specified gel time provided by the supplier. During testing, the sample should be at the standard test temperature of 25°C. Add the specified amount of curing agent, mix evenly, and record the time from addition to gelation. Ideally, the gel time should range from 40 to 60 minutes. Short gel times can lead to air entrapment and pinholes, while excessively long gel times can affect curing and hardness.

3. Viscosity: Test the viscosity using a BROOKFIELD viscometer at 25°C. Select the appropriate spindle, measure at low and high speeds, and convert the readings into viscosity metrics.

By rigorously inspecting production dates, gel times, and viscosity, the gelcoat's quality can be ensured to meet mold-making requirements.

2.Precautions and Methods for Gelcoat Spraying

2.1 Selection and Use of Spraying Equipment

Selecting the right spraying equipment is crucial for gelcoat application. Commonly used tools include handheld multifunctional spray guns and pressure-fed gelcoat spray guns. The curing agent is mixed manually to ensure accuracy, and the spray gun produces a finely atomized fan-shaped spray. While slower than pump-fed systems, the work time depends on the gel time of the material.

Handheld multifunctional spray guns are lightweight, easy to operate, and versatile. They are suitable for spraying product gelcoats, mold gelcoats, exterior gelcoats, resins, paints, and decorative coatings, especially gelcoats containing polyester particles. The structure consists mainly of a gun body, nozzle, and cup.

Operation Steps:

• Connect an air compressor (minimum 8 bar pressure and 0.4 m³/min flow rate) to the gun.

• Select a suitable nozzle based on material properties, check seals, and apply lubricant to the connections.

• Pour pre-promoted gelcoat into the cup, mix thoroughly with the curing agent, and secure the lid.

• Adjust the airflow control valve to optimize atomization before spraying.

• After spraying, clean the gun with solvent (e.g., acetone), and apply lubricant for storage.

Pressure-fed gelcoat spray guns are also suitable for mold and product gelcoats. Proper maintenance and operation are critical:

• Maintain the appropriate distance between the spray gun and the mold to avoid sagging (too close) or poor coverage (too far).

• Move the spray gun at a consistent speed for even thickness.

• Clean the spray gun promptly to prevent clogging, especially the nozzle.

2.2 Thickness Control During Spraying

Gelcoat thickness significantly impacts mold quality. Proper thickness improves strength, durability, and appearance. A recommended mold gelcoat thickness is 1 mm, achieved in two layers of 0.5 mm each, with each layer applied in three passes (0.15-0.17 mm per pass).

Use a wet film gauge to measure the gelcoat thickness. After spraying, place the gauge vertically on the wet gelcoat surface. The wheel indicating the highest point of adhesion determines the thickness.

For product gelcoats, the typical thickness is 400-500 µm. Apply in two layers, allowing each to cure before the next. Spray the first layer with a thickness of 0.15-0.17 mm, followed by a perpendicular second layer of the same thickness, and then a final pass. Ensure the first layer is fully cured before applying the second to prevent cracking or wrinkling.

Tips for Optimal Results:

• Reference the spray area and gelcoat weight to calculate the precise curing agent amount.

• Use M50 curing agent at 1-2% of the gelcoat weight. Adjust based on temperature and area size.

• Add 2-5% styrene to adjust viscosity and compensate for losses during spraying.

The gelcoat layer should be controlled at 0.3-0.5 mm, with 350-550 g/m² application rates. For low temperatures (<15°C) or high humidity (>80%), spraying is not recommended.

After spraying, the gelcoat can cure at room temperature (approximately 1.5 hours) or in a heated room (60°C for 45 minutes). Use a wet film gauge to ensure the thickness meets requirements.