How To Improve The Surface Quality of RTM Product Gel Coat and Problem Solving

 

01 RTM/LRTM Process Requirements for Gel Coat Performance

The RTM/LRTM process is a FRP molding process developed on the basis of hand lay-up process and widely used. It was promoted as early as 30 years ago, but it was limited by the level of mold making and material matching, and it was not fully applied until recent years.

The surface quality of RTM/LRTM products is one of the most intuitive and direct factors to measure the entire FRP product. We will talk about the following issues on how to improve the surface quality of the product:

1. Surface quality of the mold and mold processing
2. Selection of gel coat model
3. Requirements for fiber layer
4. Requirements for curing agent
5. Gel coat construction
6. Common problems

Mold Production Requirements

 

Selection of Mold Gel Coat

For the closed mold molding process, the repeated heat resistance of the mold and the requirements of the cavity pressure put forward higher performance requirements for the mold gel coat than the ordinary hand lay-up process. We recommend that users use vinyl-type mold gel coats with higher heat deformation temperature and higher elongation at break.

 

▪It is recommended to use a special curing agent for vinyl, which can greatly reduce the appearance of bubbles and ensure better mechanical properties of the product.

 

▪In most cases, we use a small spray gun to spray the mold gel coat. If the atomization effect is not good, it is not recommended to dilute the mold gel coat by yourself, because both acetone and styrene will affect the final quality of the product. It is recommended to heat it or contact the manufacturer to adjust the viscosity.

 

▪The Proportion of Curing Agent is Not Less Than 1%.

 

▪It is recommended to complete the construction of the mold gel coat in 3 times, and the thickness of each layer is recommended to be 0.4mm-0.3mm-0.2mm. The proportion of curing agent is recommended to be gradually increased by 1.5-2.0. The previous layer of mold gel coat should be dry before the next layer of construction, which greatly helps the degassing of the mold gel coat and prevents sag.

 

 

Layering cConstruction of Mold

▪Selection of mold resin: low shrinkage, heat resistance.

▪Selection of reinforcing fiber: surface mat, chopped mat, mat cloth combination, strong core mat.

▪The reinforcing layer should be constructed in steps, and you should not rush for success.

▪Full curing of the mold is crucial.

 

▶Surface Treatment of Mold

▪Water grinding: professional water mold pad and water grinding sandpaper, correct water grinding method.

▪Drying

▪Professional polishing treatment, pay attention to prevent polishing from leaving swirl marks caused by improper polishing and affecting the surface quality of the product.

▪Professional sealing treatment, which is very helpful for the demolding effect of the mold in the future.

▪Professional mold release agent construction, a process that cannot be sloppy.

 

2 Selection of Gel Coat for RTM/LRTM Process

In order to truly obtain better product quality and brighter FRP product surface, for FRP products such as boats, vehicles, radar covers, and engine covers that have obvious performance requirements and are used in all-weather environments for a long time, we recommend customers to choose isophthalic neopentyl glycol type gel coat. The excellent performance gel coat can provide stable protection for your products for a long time.

Main performance parameters of Noco series gel coat NC and NS

 

 

The main characteristics of the Noco series gel coats NC and NS:

 

▪Good degassing reduces the probability of pinholes on the surface.

▪Good leveling reduces the degree of uneven curing of the gel coat and reduces shrinkage lines on the surface of the gel coat.

▪Appropriate film curing time can prevent the occurrence of color separation and wrinkling problems of the gel coat.

▪Appropriate post-curing time ensures that the gel coat can be cured to a considerable extent during the reaction process after resin infusion, thereby ensuring the demoulding time and obtaining better hardness in a short time after demoulding, and effectively reducing the appearance of fiber lines on the surface of RTM/LRTM products.

▪It should be noted that for isophthalic neopentyl glycol gel coat, it gives long-term performance to the FRP surface. In many cases, orthophthalic gel coats often perform better in operating elasticity and initial hardness. This is due to its relatively simple chemical chain, but it cannot provide long-term excellent performance.

 

 

3. Selection of Curing Agent for Gel Coat

▪Requires low water content

▪Requires low hydrogen peroxide content

▪Curing agent ratio 1.5-2.5%

 

 

4 Gel Coat Construction Requirements

▪ Requirements for spraying equipment: Correct selection of gel coat spraying equipment

 

 

Importance of oil-water separation facilities

▪Construction temperature requirements: water bath heating assistance, mold preheating

 

 

▪Gel coat thickness: Gel coat wet film thickness is an important factor in ensuring product surface quality. We recommend that the gel coat wet film thickness should not be less than 0.4mm.

 

Gel coat finger drying standard: Due to the influence of temperature and humidity, the gel coat finger drying time is often difficult to grasp

 

 

5 Common Problems and Solutions

▪ Demolding:

Solution: A better combination of gel coat leveling and thixotropy, coupled with an appropriate curing system, can effectively reduce the occurrence of demolding problems. For RTM/LRTM products, due to the need for fiber laying, there is often a longer construction time interval. The milder curing properties of isophenyl neopentyl glycol gel coats can also help prevent such problems.

 

 

Pinholes:

The generation of pinholes: The degassing process of gel coat consists of two stages, front degassing and back degassing. The generation of pinholes is often due to poor back degassing effect. Water and hydrogen peroxide in the curing agent are the culprits of pinholes.

Solution: Improve the degassing effect of the gel coat itself, use better quality curing agent, and use the correct construction method.

 

 

Wrinkling:

Poor curing is the root cause of wrinkling of gel coat. The most common problems are too thin gel coat, low curing agent ratio, and styrene accumulation in the groove.

Solution: Ensure the thickness of the gel coat and the ratio of curing agent, and properly blow the deep groove.

 

▪Texture:

Solution: Improve the initial hardness of the gel coat after demolding.

 

▪Repair Color Difference:

Due to different curing conditions, repairs are prone to color difference.

 

Solution: It is recommended to add repair fluid during repair

Try to stabilize the ratio of curing agent as much as possible

Bake the repaired area with a hair dryer

It is recommended to polish at least 6 hours later