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Common Quality Problems And Countermeasures In SMC Material Molding Process

Jun 30, 2020

The SMC material molding process is one of the most efficient in the FRP/composite molding process. The SMC molding process also has many advantages, such as: accurate product size, smooth surface, good appearance and size repeatability of products, complex structures can also be formed at one time, and secondary processing without damaging products, etc. However, undesirable defects will also occur during the SMC molding process, mainly in the following aspects

(L) Lack of material: Lack of material means that the SMC molded parts are not completely filled, and their production parts are concentrated on the edges of the SMC products, especially the roots and tops of the corners.

Cause Analysis:

(a) Low discharge volume

(b) Poor fluidity of SMC materials

(c) Insufficient equipment pressure

(d) The curing is too fast.

Production mechanism and countermeasures:

① After the SMC material is plasticized by heat, the melt viscosity is large, and there is not enough time, pressure, and volume to fill the mold cavity before the crosslinking and curing reaction is completed.

② The storage time of SMC molding material is too long, and too much styrene volatilizes, which causes the flow performance of SMC molding material to decrease obviously.

③ The resin paste is not saturated with fibers. During molding, the resin paste cannot drive the fiber to flow and cause material shortage. The most direct solution to the lack of materials caused by the above reasons is to remove these molded materials during cutting.

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

⑤The molding material is covered with too much air and a lot of volatiles. The solution is to increase the number of exhausts properly; increase the feeding area burps appropriately to clean the mold for a certain time; increase the molding pressure appropriately.

⑥ Pressing is too late, the molding material has been cross-linked and cured before filling the cavity. ⑦The mold temperature is too high, the crosslinking and curing reaction is in advance, and the temperature should be properly reduced.

(2) Stomata. There are regular or irregular small holes on the surface of the product, and the production parts are mostly at the top and middle thin wall of the product.

Production mechanism and countermeasures:

①The SMC molding material is packed with a large amount of air and has a large volatile content, and the exhaust is not smooth; the thickening effect of the SMC material is not good, and it cannot effectively expel the gas. For the above-mentioned causes, it can be effectively controlled by a combination of increasing the number of exhausts and cleaning the mold.

②The feeding area is too large, and it 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, the gas wrapped by the molding material may not be easily discharged, resulting in surface defects of pores on the surface of the product.

(3) Warpage deformation. The main reason is that the molding material is unevenly cured and off

Product shrinkage after molding.

Production mechanism and countermeasures:

The chemical structure changes during the curing reaction of the resin, causing volume shrinkage. The uniformity of the curing makes the product warp to the side that is cured first. Secondly, the thermal expansion coefficient of the product is larger than that of the steel mold. When the product cools, its unidirectional shrinkage is greater than the unidirectional thermal shrinkage of the mold. To this end, the following methods are used to solve: ① reduce the temperature difference between the upper and lower molds, and make the temperature distribution as uniform as possible;

③Use cooling fixture to limit deformation;

④ Properly increase the molding pressure, increase the structural compactness of the product, and reduce the shrinkage rate of the product;

⑤ Prolong the holding time properly to eliminate internal stress.

⑥Adjust the curing shrinkage of SMC material.

(4) Foaming. The semi-circular bulge on the surface of the cured product.

Production mechanism and countermeasures:

It may be that the material is not completely cured, the local temperature is too high, or the content of volatile content in the material is large, and the air trapped between the sheets makes the semi-circular surface of the product bulge.

① Appropriately increase molding pressure

②Extend the holding time

③ Lower the mold temperature.

④ Reduce the discharge area

(5) Uneven color on the surface of the product

Mechanism and countermeasures

① The mold temperature is not uniform, and the local temperature is too high. The mold temperature should be properly controlled;

②The fluidity of the molding material is poor, resulting in uneven fiber distribution. Generally, the molding pressure can be increased to increase the fluidity of the melt;

③During the color paste preparation process, the pigment and resin are not miscible.