Silicone mold is one of the simplest methods in rapid tooling. Generally, the 3D printing prototype is re molded with silica gel, but the service life is very short, only about 10-30 pieces. It has good elasticity and replication performance. The draft angle can not be considered when using silica gel compound mold, which will not affect the dimensional accuracy. It has good segmentation. It can directly conduct overall pouring without dividing the upper and lower molds, and then cut along the predetermined parting line and take out the mother mold.

RTV silicone rubber can be divided into addition molding and condensation type. The raw material and formula adopt special mold silica gel, which is sold in the form of double packaging. Component A is rubber and component B is catalyst. When preparing, the room temperature, strength and hardness of the mold shall be considered to determine the weight preparation proportion of AB component. The ratio is mainly related to room temperature. For example, when the room temperature is 20-25 degrees, a: B = 100:1.5.

I Operation method of mold silica gel

1. Mix the mold silica gel and curing agent evenly. The appearance of the mold silicone is a flowing liquid. Component A is silicone and component B is curing agent. For example, take 100g of silicone and add 2G of curing agent (Note: the silicone and curing agent must be stirred evenly. If they are not stirred evenly, the curing and drying of the mold will be uneven, which will affect the service life and turnover times of the silicone mold, and even cause the mold to be scrapped.

2. Vacuum exhaust bubble treatment. After the silica gel and curing agent are stirred evenly, the vacuum extraction and bubble exhaust link is carried out. The vacuum pumping time should not be too long, generally not more than 10 minutes. Otherwise, the curing of silica gel will produce cross-linking reaction, which cannot be painted or poured, resulting in material waste.

3. Brushing operation process. Pour the silica gel discharged with bubbles onto the master mold by brushing or pouring (apply isolation agent or release agent on the master mold before brushing or pouring). The mold must be brushed evenly. After 30 minutes, stick a layer of sand cloth or glass fiber cloth to increase the strength and tension of the mold; After the second brushing, paste a layer of gauze or glass fiber cloth, and then apply silica gel. The treated mold has long service life, many times of mold turnover, long service life, cost saving and efficiency improvement.

4. Fabrication of external formwork. The general method is: surround the mold with rubber board or wood board. One is to fill the mold frame with gypsum. The other is to brush the external mold with resin, brush a layer of resin, brush a layer of fiberglass cloth, then brush and paste, and repeat two or three layers to complete the external mold.

5. The proportion of silica gel (silicone glue, silicon) and curing agent reflects the situation. It is suggested that the addition amount of curing agent should not exceed 5%. The larger the addition proportion of curing agent, the faster the curing time, the more brittle the mold and the shorter the service life. According to the product and process requirements, an appropriate amount of silicone oil can be added to improve the mold making effect. Note that the silicone oil should not be excessive (5% - 10%), otherwise it will affect the tensile force and hardness of the silicone mold.

II Precautions for making silica gel mold

1. Component B is a catalyst, which is easy to be hydrolyzed by moisture, so the cover should be tightly covered after use.

2. Once components a and B are mixed, the chemical reaction starts immediately, and the viscosity gradually increases, which can not be stopped. In order to avoid waste, they should be prepared with use according to the dosage, and should be used immediately after preparation without delay.
3. The mix proportion of group A and B is related to the speed of chemical reaction and the performance of die. The more component B, the faster the reaction, the higher the strength and hardness of the product, but the toughness decreases. Therefore, accurate weighing is required.

4. Before preparation, mix component a up and down evenly, and then weigh.

5. The pouring method is suitable for relief, and the brushing method is suitable for the production of three-dimensional molds. For casting relief molds, a frame with a length and width of 4-6cm larger than the length and width of the original carving shall be made first. The border should be flat and the inner surface should be smooth. When pouring, place the frame on the clean glass plate, and then place the prototype in the frame, leaving a gap of 2-3cm on each side (mold silica gel 25-35 yuan / kg).

Solution to bubbles in the manufacturing process of silica gel mold:

(1) Improve the design of flash groove and exhaust system;
(2) Increase the pressure of the press;
(3) Reduce the amount of release agent and spray it evenly;
(4) Material control moisture;
(5) Try adding some defoamer;
(6) Use cold runner;
(7) The exhaust slot can be added on the die by vacuum pumping with a vacuum machine