Analysis and Countermeasures of Common Quality Problems in SMC Molding Process
2020-07-23
1057
SMC material molding process is a highly efficient production method in the molding process of fiberglass/composite materials. SMC molding process has many advantages, such as accurate product size, smooth surface, good repeatability of product appearance and size, one-time molding of complex structures, and no need to damage the product during secondary processing. However, there may also be defects in the SMC molding production process, mainly manifested in the following aspects:
Problem 1: Shortage of materials
Shortage refers to the incomplete filling of SMC molded parts, which is mainly concentrated at the edges of SMC products, especially at the root and top of the corners.
Cause analysis
1. Less material discharge;
2. SMC material has poor fluidity;
3. Insufficient equipment pressure;
4. Curing too fast.
Formation mechanism and countermeasures
1. After being heated and plasticized, SMC material has a high melt viscosity, and there is not enough time, pressure, and volume to fill the mold cavity with the melt before the cross-linking and solidification reaction is completed.
2. The storage time of SMC molding materials is too long, and excessive volatilization of styrene causes a significant decrease in the flowability of SMC molding materials.
3. The resin paste has not penetrated the fibers. Resin paste cannot drive fiber flow during molding, resulting in material shortage. The direct solution to the shortage of materials caused by the above reasons is to remove these molding materials during cutting.
4. Insufficient feeding amount leads to material shortage. The solution is to increase the feeding amount appropriately.
5. There is too much air and a large amount of volatile substances wrapped in the molding material. The solution is to increase the exhaust frequency appropriately; Appropriately increase the feeding area and clean the mold at regular intervals; Increase the molding pressure appropriately.
6. The pressure was applied too late, and the molded material had already completed cross-linking and curing before filling the mold cavity.
7. If the mold temperature is too high and the cross-linking curing reaction is advanced, it should be cooled appropriately.
Question 2: Pores
There are regular or irregular small holes on the surface of the product, which are mostly located at the top and middle thin walls of the product.
Formation mechanism and countermeasures
1. SMC molding material is coated with a large amount of air and has a high content of volatile compounds, resulting in poor exhaust; The thickening effect of SMC material is poor and cannot effectively remove gas. The above causes can be effectively controlled by increasing the exhaust frequency and cleaning the mold.
2. The feeding area is too large, and reducing the feeding area appropriately can achieve control. In practical operation, human factors may also cause sand holes. For example, if the pressure is applied too early, it may make it difficult for the gas trapped in the molded material to be discharged, resulting in surface defects such as pores on the product surface.
Question 3: Warping and deformation
The main reasons for this are uneven curing of the molding material and shrinkage of the product after demolding.
Formation mechanism and countermeasures
During the resin curing reaction, the chemical structure changes, causing volume shrinkage and uneven curing, resulting in a tendency for the product to warp towards the side that is cured first. Secondly, the thermal expansion coefficient of the product is greater than that of the steel mold. When the product cools, its unidirectional shrinkage rate is greater than the unidirectional thermal shrinkage rate of the mold. To address this, the following methods are adopted:
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. Properly increase the molding pressure, increase the structural density of the product, and reduce the shrinkage rate of the product;
4. Extend the insulation time appropriately to eliminate internal stress.
5. Adjust the curing shrinkage rate of SMC material.
Question 4: Bubbling
A semi-circular bulge on the surface of a cured product.
Formation mechanism and countermeasures
It may be due to incomplete solidification of the material, excessively high local temperatures, or high volatile content in the material, causing air to accumulate between the sheets and resulting in semi-circular bulges on the surface of the product.
1. Properly increase the molding pressure.
2. Extend the insulation time.
3. Reduce the temperature of the mold.
4. Reduce the feeding area.
5. The surface color of the product is uneven.
Reading source: SMC molded product manufacturer www.jmlicheng.com
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