Main processes of SMC compression molding process
1. Preparation before compression molding
(1) Quality inspection of SMC
The quality of SMC sheet has a great influence on the compression molding process and product quality. Therefore, it is necessary to understand the quality of the material before pressing, such as resin paste formula, resin paste thickening curve, glass fiber content, glass fiber infiltrant type, unit weight, film peelability, hardness and quality uniformity, etc.
According to the structure shape of the product, the feeding position, and the compression molding process to cut the shape and size of the sheet and the sample material . The shape is mostly square or round, and the size is usually 40%-80% of the projected area of the product surface. In order to prevent contamination by external impurities, the upper and lower films will removed right before loading.
(3) Preparation of equipment
a. Familiar with the various operating parameters of the molding machine, especially to adjust the working pressure, the operating speed and the parallelism of the table.
b. The mold must be Install level, and ensure that the installation position is in the center of the molding machine table. Before pressing, the mold must be thoroughly cleaned and the release agent must be applied. Wipe the release agent evenly with clean gauze before feeding, so as not to affect the appearance of the product. For new molds, the oil must be removed before use.
2. loading the raw material
(1) Determination of the feeding amount
The feeding amount of each product can be calculated by the following formula during the first pressing: feeding amount = product volume×1.8
(2) Determination of feeding area
The size of the feeding area affects the density ,the flow distance and the surface quality of the product directly. It is related to the flow and curing characteristics of SMC, the performance requirements of the product, and mold structure. The general feeding area is 40%-80%. Too small will cause glass fiber orientation, reduce strength, increase waviness, and even fail to fill the cavity due to the long process; too large is not conducive to exhaust and easy to produce internal cracks in the product.
(3) Feeding position and method
The feeding position and method affect the appearance, strength and directionality of the product directly. 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 each end of the mold cavity at the same time during molding. The feeding method must be conducive to exhaust. When stacking multi-layer sheets, it is best to stack the material blocks in the shape of a pagoda according to the upper part and the lower part. In addition, try not to add the material blocks separately, otherwise it will produce air entrapment and welding areas, resulting in a decrease in the strength of the product.
Before feeding, in order to increase the fluidity of the sheet, can preheat the sheet to 100°C or 120°C. This is particularly advantageous for forming deep-drawn products.
When the material in the cavity, the molding machine moves down quickly. When the upper and lower molds are matched, the required molding pressure is slowly applied, and after a certain curing system, the molding of the product is finished. During the molding process, various molding process parameters and press operating conditions must be reasonably selected.
(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, the crosslinking reaction proceeds smoothly, and achieves complete curing.
Generally speaking, the molding temperature of thick products should be lower than that of thin-walled products, so as to prevent excessive heat accumulation in thick products. If the thickness of the product is 25-32mm, its molding temperature is 135-145°C. Thinner products can be molded at 171°C. The increase of the 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 in a trade-off between the highest curing speed and the best molding conditions. It is generally believed that SMC molding temperature is between 120-155°C.
(2) Forming pressure
SMC molding pressure decided by the product structure, shape, size and SMC thickness. Products with simple shapes only need a molding pressure of 25-30Mpa; for products with complex shapes, the molding pressure can reach 140-210Mpa. 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. A mold with a smaller fit clearance requires higher pressure than a mold with a larger clearance. Products that require high appearance performance and smoothness require higher molding pressure during molding. 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 of SMC at the molding temperature (also called holding time) is related to its properties, curing system, molding temperature, product thickness and color.
The curing time is generally calculated as 40s/mm. For products thicker than 3mm, some people think that for every 4mm increase, the curing time increases by 1min.
4. Compression molding operation
Since SMC is a fast curing system, fast closing of the press is very important. If the press closes too slowly after feeding, pre-cured patching is likely to appear on the surface of the product, or there is a shortage of material, or the size is too large. While achieving rapid closing, the mold closing speed should be carefully adjusted at the end of the press stroke to slow down the closing process and facilitate exhaust.