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Detailed explanation of SMC FRP molding process and solutions to common problems

May 09, 2020

FRP SMC molding process

The main raw material of SMC is composed of SMC special yarn, unsaturated resin, low shrinkage additives, fillers and various additives. SMC has the advantages of superior electrical performance, corrosion resistance, light weight and easy and flexible engineering design. Its mechanical properties are comparable to some metal materials, so it is widely used in transportation vehicles, construction, electronics / electricity and other industries.

1. Preparation before pressing

preparation stage

preparation stage

(1) SMC quality inspection The quality of SMC sheet material has a great influence on the molding process and product quality. Therefore, the quality of the material must be known before pressing, such as resin paste formulation, resin paste thickening curve, glass fiber content, glass fiber infiltrant type. Single weight, film peelability, hardness and quality uniformity.

(2) Cutting Determine the shape and size of the sheet material cutting according to the product's structural shape, feeding position, and flow, make a model, and then cut according to the model. The shape of the cut is mostly square or round, and the size is mostly according to 40% to 80% of the projected area of the product surface. In order to prevent contamination by external impurities, the upper and lower films are only removed before loading.

(3) Preparation of equipment

①Familiar with the various operating parameters of the press, especially to adjust the working pressure and the running speed of the press and the parallelism of the table.

②The mold must be installed horizontally, and ensure that the installation position is at the center of the press table. Before pressing, the mold must be thoroughly cleaned and coated with a mold release agent. Before feeding, wipe the release agent with clean gauze to avoid affecting the appearance of the product. For new molds, oil must be removed before use.

2. Feeding

Feeding stage

Feeding stage

(1) Determination of feeding amount The feeding amount of each product can be calculated by the following formula during the first pressing

Feeding amount / g = Product volume / cm3X1.8 / g

(2) Determination of the feeding area The size of the feeding area directly affects the density of the product, the flow distance of the material and the surface quality of the product. It is related to the flow and curing characteristics of SMC, product performance requirements, mold structure, etc. Generally, the feeding area is 40% -80%. If it is too small, the glass fiber orientation will be caused by the long process. Decrease the strength, increase the waviness, or even fill the cavity. If it is too large, it is not conducive to exhaust, and it is easy to produce cracks in the product.

(3) Feeding position and method The feeding position and method directly affect the appearance, strength and directionality of the product. Normally, the feeding position of the material should be in the middle of the cavity. For asymmetric and complex products, the feeding position must ensure that the material flow reaches the ends of the molding cavity at the same time during molding. The feeding method must be conducive to exhaust. When stacking multiple layers of sheets, it is best to stack the blocks in a pagoda shape with the top, bottom, and bottom. In addition, the material blocks should not be added separately as much as possible, otherwise there will be air wrap and fusion zone, resulting in a decrease in product strength.

3. Molding

After the material block enters the mold cavity, the press goes down quickly. When the upper and lower molds coincide, slowly apply the required molding pressure, and after a certain curing system, the molding of the product is completed. During the molding process, various molding process parameters and press operating conditions should be selected reasonably.

(1) Molding temperature

The molding temperature depends on the curing system of the resin paste, the thickness of the product, the production efficiency and the complexity of the product structure. The molding temperature must ensure that the curing system initiates and crosslinks smoothly, and achieve complete curing.

Generally speaking, the molding temperature of thick products should be lower than that of thin-walled products, which can prevent excessive heat accumulation in the thick products. If the thickness of the product is 25 ~ 32mm, its molding temperature is 135-145 ℃. And thinner products can be formed at 171 ℃.

The increase of molding temperature can shorten the corresponding curing time; conversely, when the molding temperature decreases, the corresponding curing time needs to be extended. The molding temperature should be selected between the highest curing speed and the best molding conditions. It is generally believed that the SMC molding temperature is between 120-155 ° C.

(2) Molding pressure

SMC molding pressure varies with product structure, shape, size and SMC thickening degree. Products with simple shapes only need a forming pressure of 5-7MPa; products with a complicated shape can have a forming pressure of 7-15MPa. The higher the degree of SMC thickening, the greater the molding pressure required.

The size of the molding pressure is also related to the mold structure. The molding pressure required by the vertical parting structure mold is lower than that of the horizontal parting structure mold. Moulds with smaller clearances require higher pressure than those with larger clearances.

In short, the determination of molding pressure should consider many factors. Generally speaking, SMC molding pressure is between 3-7MPa.

(3) curing time

The curing time (also called holding time) of SMC at molding temperature is related to its properties and curing system, molding temperature, product thickness and color. The curing time is generally calculated at 40s / mm. For thick products above 3mm, for each additional 4mm, the curing time increases by lmin.

4. Press operation

Since SMC is a fast curing system, the rapid closing of the press is very important. If the press is closed too slowly after feeding, it is easy to appear pre-cure patch on the surface of the product, or the lack of material, or the size is too large. While achieving fast closing, the closing speed of the mold should be carefully adjusted at the end of the stroke of the press to slow down the closing process and facilitate exhaust.

Summary: The above is the entire content of the important knowledge point "SMC fiberglass molding process flow" organized by the editor of Ningguang Mould, hoping to help friends engaged in the fiberglass manufacturing industry. If you want to know more about SMC production and other information, please bookmark this site and follow this site for updates.