Home > News > Content

Matters Needing Attention In Mold Gel Coat Operation

Nov 17, 2020

Before using the mold gel coat on the mold, prepare a spray gel coat test sample. The conditions for spraying gel coat on the template should be the same as the conditions for spraying gel coat on the actual mold. After inspection, the injection model should be retained as other data as the historical record of this mold.


The first coating of the mold gel coat is the most important, because this is the surface that determines the surface quality of the final part. If there are pores in the membrane after the membrane has gelled, the gel coat should be removed and the problem should be corrected. Once the film is clean and free of pores, pits or pinholes, a second coating can be prepared. Some mold manufacturers use different colors when preparing the first and second coatings. The color contrast can show how much gel coat is polished off during post-processing, maintenance and repair operations. The color of the mold gel coat is preferably dark (black, green or orange) to facilitate the detection of defects.

In pursuit of high-quality spraying, mold release wax should be applied to a clean glass plate first. The board usually has a size of 12 inches * 12 inches. Then, using the same equipment, apply the catalyzed gel coat to the production mold with the same technique. Use a wet film measuring instrument to ensure that the wet film thickness of the gel coat is 18 ± 2 mils (450 ± 500 microns). After curing, the gel coat film should be pasted or covered with release tape. The surface of the gel coat should be inspected visually for defects. Polishing the surface and wiping with contrast paint can show surface defects and/or dense internal holes.

If the sprayed surface looks good, you can use the same batch of materials, equipment and production mold technology. If the sample does not look good, the spraying should be adjusted until a high-quality sample with no holes and defects on the surface is obtained. This sample test is suitable for large or very important spraying, such as yacht spraying.

The preparation of the mold gel coat requires spraying two smooth layers, each with a thickness of 18±2 mils (450 microns). Each layer of coating should be sprayed three times and each spray is perpendicular to the previous spray. The first layer of coating needs to be gelled and properly cured to avoid cracking when spraying the second layer of coating. Generally, this time gap is 90 minutes at 77ºF (25ºC) with 1.8% catalyst. The second layer of coating can be in a different color and can be used as a reminder that the first layer of coating has been polished through during mold use.

Laminating should start after the second layer of mold gel coat has cured for 60 to 90 minutes or up to 3 hours, depending on room temperature, air flow, humidity, catalyst type and concentration. If the gel coat cures too completely, it may shrink and release from the original model. The gel coat should be covered with a surface layer at least on the same day as spraying to avoid premature release of the gel coat. A too long delay can also cause dust or dirt to accumulate and prevent proper lamination and bonding.

Spraying mold gel coats below 25ºC can cause under-curing, which can cause distortion and/or loss of gloss on the surface of FRP molds.