Brief introduction of carbon fiber molding process
Compression molding is one of the oldest and most dynamic molding methods in the production of composite materials. It is a method of adding a certain amount of premix or prepreg into the metal mold, and curing it by heating and pressure. The main advantages of the compression molding process are high production efficiency, easy to achieve specialized and automated production and high dimensional accuracy of products, good repeatability, etc., and can form products with complex structures at one time, and can be mass-produced.
Steps of carbon fiber compression molding process:
1. Insert placement
Inserts are generally made of metal, which can enhance the products, and the mechanical properties of products using inserts are greatly improved. There are also inlays added to impart electrical, thermal, or other functional properties to the product. Preheating is preferred before insert placement.
The insert is usually placed by hand, and the placement should be accurate and stable. If the insert is small, it can also be installed with pliers or tweezers. A product can use one insert, or several different inserts can be placed, its position must not be misplaced, not skewed, the insert must be stable, and if necessary, it should be fixed to prevent displacement or removal, otherwise it will not be possible. To the purpose of using the insert, it will cause the scrap of the product and even damage the mold.
The accuracy of the feeding amount will directly affect the size and density of the product, and it should be strictly quantified to uniformly add the material into the mold groove. Quantitative feeding methods are: gravimetric method, counting method and volumetric method. The gravimetric method is accurate but cumbersome, and is mostly used for materials that require precise dimensions and are difficult to feed by volumetric methods, such as crumbly and fibrous materials. The volume method is not as accurate as the gravimetric method, but it is easy to operate and is generally used for powder measurement. The counting method is only used for pre-pressing material feeding.
The mold clamping is divided into two steps. Before the punch does not contact the material, it needs to be low pressure (1.5-3.0MPa) quickly, so that the cycle can be shortened and the plastic change can be avoided. After the punch contacts the material, it should start to close slowly. The mold speed should be changed to high pressure (15-30MPa) and slow speed, so as to avoid damage to the insert and discharge the air in the mold.
In order to remove the air, moisture and volatiles in the mold, after the mold is closed, some need to open the mold for a period of time, this process is called exhaust. The exhaust operation should be fast, and it must be completed before the material is plasticized. Otherwise, the material hardens and loses its plasticity. At this time, even if the mold is opened, the air cannot be exhausted, and even if the temperature and pressure are increased, it is impossible to obtain an ideal product. Exhaust can shorten the curing time, and can improve the mechanical and electrical properties of the product.
The process in which the material changes from a fluid state to a hard infusible insoluble state is called the curing of thermosetting resins. The speed of curing depends on the rate at which the composition of low relative molecular mass in the resin is converted to the product of high relative molecular mass, that is, the curing speed is related to the molecular structure of the resin.
6. Holding time
The process of curing the resin in the mold is always under high temperature and high pressure, and the time required from the start of heating, pressurizing, curing to cooling and depressurizing is called the holding time. The dwell time is essentially the time to maintain the temperature and pressure, which is exactly the same as the curing speed. If the dwell time is too short, that is, premature cooling and pressure reduction will lead to incomplete curing of the resin, reducing the mechanical properties and electrical properties of the product. Heat resistance. At the same time, the product will continue to shrink and warp after demolding.
Demoulding is usually done by the top (out) rod. For products with forming rods or some inserts, special tools should be used to unscrew the forming rods, etc., and then demolding.
8. Clean the mold
Since some residual materials may be left in the mold and fall into the flash during molding, the mold must be cleaned after each molding. If the adhesion on the mold is too strong, it can be cleaned with a copper sheet, or a polishing agent can be used to wipe it, etc. , after cleaning, apply a release agent for the next molding.