SMC compression molding process

Introduction of SMC compression molding process

SMC material molding process is one of the highest production efficiency in FRP/composite material molding process. The SMC molding process has many advantages, such as: accurate product size, smooth surface, good product appearance and size repeatability, complex structures can be formed at one time, and secondary processing does not need to damage the product. However, in the process of SMC molding production, bad defects will also appear, mainly in the following aspects:

?Basic conditions for compression molding

1. The molding material is added when the mold is opened

2. During the molding process, the molding material needs to be cured quickly at a higher temperature

3. Product molding needs to maintain a high molding pressure, and the molding pressure is generally applied by a hydraulic press

4. The size and shape of the product are mainly guaranteed by the mold cavity in the closed state

? Compression molding equipment: presses, should have

1. Enough countertops

2. Sufficient itinerary

3. Sufficient pressure

4. Reasonable speed

Common quality problems and solutions

SMC material molding process is one of the highest production efficiency in FRP/composite material molding process. The SMC molding process has many advantages, such as: accurate product size, smooth surface, good product appearance and size repeatability, complex structures can be formed at one time, and secondary processing does not need to damage the product. However, in the process of SMC molding production, bad defects will also appear, mainly in the following aspects:

(1) Lack of material

??Lack of material means that the SMC molded parts are not completely filled, and its production parts are mostly concentrated on the edge of the SMC product, especially the root and top of the corner.

Mechanism and countermeasures:

??① After the SMC material is plasticized by heat, the melt viscosity is high, and there is not enough time, pressure and volume to fill the cavity with the melt before the cross-linking and curing reaction is completed.

??②The storage time of SMC molding materials is too long, and the styrene volatilizes too much, resulting in a significant decrease in the flow properties of SMC molding materials.

??③ The resin paste has not soaked the fiber. When molding, the resin paste cannot drive the fiber flow and cause material shortage. The most direct solution to the shortage of materials caused by the above reasons is to remove these molding materials when cutting materials.

??④Insufficient feeding amount causes material shortage. The solution is to appropriately increase the amount of feed.

??⑤ Too much air and a lot of volatile matter are wrapped in the molding material. The solution is to appropriately increase the number of exhausts; appropriately increase the feeding area and clean the mold for a certain period of time; appropriately increase the molding pressure.

??⑥The pressure is too late, and the molding material has completed cross-linking and curing before filling the cavity.

??⑦If the mold temperature is too high, the cross-linking and curing reaction will be advanced, so the temperature should be lowered appropriately.

(2) stomata

???There are regular or irregular small holes on the surface of the product, most of which are located at the top of the product and the thin wall in the middle.

???Mechanism and countermeasures:

???① There is a lot of air and volatile content in the SMC molding material, and the exhaust is not smooth; the thickening effect of the SMC material is not good, and the gas cannot be effectively driven out. For the above causes, it can be effectively controlled by increasing the number of exhausts and cleaning the mold.

???② The feeding area is too large, which can be controlled by appropriately reducing the feeding area. In the actual operation process, human factors may also cause trachoma. For example, if the pressure is too early, it may make it difficult for the gas wrapped in the molding material to be discharged, resulting in surface defects such as pores on the surface of the product.

(3) warping deformation

???The main reason is the uneven curing of the molding material and the shrinkage of the product after demoulding.

???Mechanism and countermeasures:

???During the curing reaction of the resin, the chemical structure changes, causing volume shrinkage, and the uniformity of curing makes the product tend to warp to the side that is first cured. Secondly, the coefficient of thermal expansion of the product is larger than that of the steel mold. When the product is cooled, its one-way shrinkage rate is greater than the one-way heat shrinkage rate of the mold. To this end, the following methods are adopted to solve it:

???1. Reduce the temperature difference between the upper and lower molds to make the temperature distribution as uniform as possible;

???2. Use cooling fixtures to limit deformation;

???3. Appropriately increase the molding pressure, increase the structural compactness of the product, and reduce the shrinkage of the product;

???4. Properly extend the holding time to eliminate internal stress.

???5. Adjust the curing shrinkage of SMC materials.

(4) Blistering

???A semicircular bulge on the surface of a cured article.

???Mechanism and countermeasures:

???It may be that the material is not fully cured, the local temperature is too high or the volatile content in the material is large, and the air is trapped between the sheets, which makes the semicircle on the surface of the product bulge.

???① Properly increase the molding pressure

???② Extend the holding time

???③Reduce mold temperature

???④Reduce the discharge area

(5) The surface color of the product is uneven

???Mechanism and countermeasures:

???① The mold temperature is uneven and locally too high, and the mold temperature should be properly controlled;

???②The fluidity of the molding material is poor, resulting in uneven distribution of fibers, which can generally increase the molding pressure to increase the fluidity of the melt;

???③Pigments and resins are not well miscible during the color paste blending process.

???In fact, molding a good product is determined by four elements: material, mold, press and operator. Only good materials can mold good products. Although it is not the only condition, it must be the prerequisite. Therefore, this paper makes some tentative thoughts on the material selection and process control of SMC. Although the reason is relatively simple, some problems in the actual production process are often ignored.

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