I. Overview of Carbon Fiber SMC Molding

1. Carbon Fiber SMC

SMC (Sheet Molding Compound) is a sheet molding compound formed by impregnating short-cut carbon fibers, matrix resin, and additives with a certain ratio using professional molding equipment and a certain process.

2. Carbon Fiber SMC Molding

This refers to the process of cutting carbon fiber SMC sheets according to the requirements of product size, shape, thickness, and weight, then stacking the cut materials into the heated metal mold cavity, and curing them under pre-set process forming conditions;

SMC molding has many advantages, such as simple operation, easy automation, short production cycle, ability to form products with smooth surfaces and complex structures, the ability to embed metal during molding, low waste, etc. In addition, products obtained using carbon fiber SMC molding have excellent performance: excellent mechanical properties, thermal stability, chemical corrosion resistance, low shrinkage rate upon heating, and low water absorption.

II. Carbon Fiber SMC Molding Procedures

1. Material Inspection

The quality of carbon fiber SMC sheets greatly affects the molding process and product quality. Therefore, before pressing, the quality of the material must be understood, such as resin type, fiber content, surface density and unit weight, film peelability, whether the material is expired, whether the fiber impregnation is good, whether there are impurities, whether the film covering is damaged, material hardness, and quality uniformity, etc.

2. Material Cutting

Thaw the material until there is no water mist on the surface of the PE bag before cutting. According to the structure and shape of the product, the material feeding position, and the process, determine the shape and size of the sheet cutting. The size is mostly 70%-100% of the projected area of the product surface. To prevent contamination by external impurities, the upper and lower films are removed only before loading, and the work area is kept clean to avoid introducing foreign objects.

3. Adjust Press Parameters

① Mainly includes pressure, temperature, and molding time, which are determined according to the product thickness, material requirements, and mold conditions;

② The mold installation must be level and ensure that the installation position is in the center of the press table.

4. Clean the Mold

① Thoroughly clean the mold and apply a release agent. Before loading, use a clean gauze to wipe the release agent evenly to avoid affecting the appearance quality of the product. For new molds, they must be degreased before use;

② Close the mold and heat the mold.

5. Confirm Mold Temperature

① Open the mold and measure the upper and lower mold temperatures of the cavity using a contact thermometer (Note: measure multiple points in the front, back, left, middle, and right to ensure that the surface temperature of the mold reaches the pressing temperature required by the material, and the core temperature is about 5 degrees lower than the cavity temperature);

② If the temperature has not reached, close the mold and wait for continued heating;

③ After the temperature is reached, spray the release agent on the upper and lower molds again.

6. Confirm the Amount and Area of Material Added

① Amount of material added: The amount of material added for each product can be calculated using the following formula during the first pressing: Amount of material added = Product volume * density 1.5g/c㎡;
② Material addition area: The size of the material addition 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, and mold structure. Generally, the material addition area is 70%-100%. If it is too small, it will lead to fiber orientation due to excessive flow length, reducing strength, increasing waviness, and even failing to fill the cavity;
③ Material addition position: The material addition position and method directly affect the appearance, strength, and directionality of the product. Under normal circumstances, the material addition position should be in the middle of the cavity. For asymmetrical complex products, the material addition position must ensure that the material flow reaches each end of the mold forming cavity during molding. 7. High-Temperature High-Pressure Molding

After the material block enters the cavity, start the machine and the press descends rapidly. When the upper and lower molds are in contact, slowly apply the required molding pressure. After a period of heat preservation and pressure maintenance, the product molding is completed. During the molding process, the molding process parameters should be reasonably set according to the material characteristics and press conditions.

Molding Temperature: The high and low molding temperature depends on the curing system of the resin paste, product thickness, production efficiency, and the complexity of the product structure. The molding temperature must ensure that the initiation and crosslinking reactions of the curing system proceed smoothly and achieve complete curing. The molding temperature of carbon fiber SMC is generally between 130-140℃. Generally speaking, the molding temperature selected for thicker products should be lower than that for thin-walled products to prevent excessive heat accumulation inside thick products due to excessively high temperatures. Increasing the molding temperature can shorten the corresponding curing time; conversely, when the molding temperature is lowered, the corresponding curing time needs to be extended. The molding temperature should be selected by balancing the highest curing speed and the best molding conditions.

Molding Pressure: The SMC molding pressure varies with the product structure, shape, size, and SMC thickening degree. Simple-shaped products require less pressure, while complex-shaped products require more pressure. The higher the SMC thickening degree, the greater the molding pressure required. Products with high appearance performance and smoothness requirements require higher molding pressure during molding. In short, the determination of the molding pressure should consider multiple factors. Generally speaking, the carbon fiber SMC molding pressure is between 10-25Mpa.

Molding Time: The curing time (also called heat preservation time) of carbon fiber SMC at the molding temperature is related to its properties and curing system, molding temperature, product thickness, and color. The molding time of carbon fiber SMC is usually between 10-15 minutes.

Because SMC is a rapid curing system, the rapid closure of the press is crucial. If the closure of the press is too slow after the material is added, pre-cured patches may appear on the surface of the workpiece, or there may be material shortages or excessive size. While achieving rapid closure, the mold closing speed should be carefully adjusted at the end of the press stroke. When the mold is a certain distance from closure, the closing speed should be slowed down to facilitate air venting.

8. Demolding and Appearance Inspection

① Slowly open the upper mold. After it rises to 10mm, confirm whether the product is stuck in the mold. After confirming that the product remains in the lower mold, move the upper mold to the upper limit position. Open the top mold mechanism and slowly lift to remove the product;

② Inspect the appearance of the product for any missing material, dirt, pores, dry yarn, etc. If the product does not require spraying, simply trim the excess burrs to complete the product production.

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