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Source: Rubber Technology Network
First, the characteristics of silicone materials
A. The silica gel material is generally gelatinous, somewhat similar to plasticine, colorless, translucent and tasteless.
B. Its main features are: high temperature resistance (up to 300 ° C) and low temperature resistance (lowest -100 ° C), is currently the best cold and high temperature resistant rubber; at the same time excellent electrical insulation, stability to thermal oxidation and ozone High, chemically inert. The disadvantage is that the mechanical strength is low, oil resistance, solvent resistance and acid and alkali resistance are poor, it is difficult to vulcanize, and the price is relatively expensive. Use temperature: -60 ° C ~ +200 ° C. The above is a description of silicone rubber in the manual.
C. Operating temperature: As described above, it is generally set to -40 ° C - 200 ° C, and can reach 230 ° C in a short time.
D. Aging problem:
Oil resistance, acid and alkali resistance are poor, and have little to do with stress.
E. Bonding problem: Before the surface treatment, except the silicone type product is used as the adhesive and the silicone parts are bonded, the other parts must be surface-activated and the vulcanized surface can be cured. Any substance that can be vulcanized or crosslinked with rubber is collectively referred to as a vulcanizing agent, also known as a crosslinking agent. There are many types of vulcanizing agents, and they are still increasing. The vulcanizing agents that have been used include sulfur, selenium, tellurium, sulfur compounds, metal oxides, peroxides, resins, terpenes and amines.
F. Material hardness: The rubber parts can be theoretically selected from 10 to 80 degrees in terms of Shore hardness in the domestic market. It can be easily found at 20 to 70 degrees, and the most commonly used is 40 to 50 degrees.
Second, the production process of silicone products
A. Material shape and color
B. After color mixing, the milky white silica gel is changed into a sheet C of various colors. After mixing and shaping, the raw materials are pressed into strips and cut into strips.
D. Vulcanization molding
E. Trimming, trimming, inspection, packaging.
F. Finished drawing
Third, product size and characteristics
A. Limit size: 15~20MM can be achieved at the thickest point. If it is a sphere, it can be 30mm in diameter. The recommended thickness for general thickness is no more than 3MM. When it is greater than 3MM, it will cost more curing time and increase cost. The thinnest processing can reach 0.2MM, but the design is generally the thinnest 0.3MM, 0.4MM recommended.
B. Relative size: In the thickness difference, the thinnest part and the thickest part are recommended not to exceed 3 times. Such problems are mainly determined by the temperature and pressure requirements of the material vulcanization.
C, shrinkage rate. The shrinkage rate of the silica gel material is related to the hardness of the material. The secondary material provided by the manufacturer is mostly between 1.022 and 1.042. For the material of 40 to 50 degrees, the shrinkage rate of 1.03 is generally taken. Compared to plastics, silicone products do not produce similar apparent surface defects due to shrinkage.
D. Dimensional Accuracy: Because silicone products are mostly multi-holes, the number of holes is quite large compared to plastic products. Therefore, there is no convenience in the size control of plastic products. The general accuracy is plus or minus 0.1, and the high precision product is plus or minus 0.05. When the fit is used for the fit between the hole and the button of the plastic part, the minimum gap is 0.1, and the recommended value is 0.2 on one side.
E. Shape design: For the rubber sleeve parts, the original drawing can be provided to the mold factory according to the outline drawing of the product. The problem of the matching is explained by the mold factory. Under normal circumstances, depending on the size of the product, the fit of the rubber sleeve to the product is generally a negative deviation of 0.2 to 0.5 on one side.
Fourth, the mold structure
A. The upper limit of the mold thickness, tentatively set to 250mm, the size can reach 500X500, the product can reach 400X400 in the mold area. The mold structure is generally divided into upper mold, lower mold, bottom plate, top plate, ejector pin, core, hinge and other auxiliary parts. In the design of silica gel products, attention should be paid to the characteristics of the mold and production process.
B. Demoulding: The release of silicone products can be directly removed. The size requirement for the product with no design defect is mainly the ratio of the circumference of the core and the minimum of the mold at the time of demolding. For products below 55°, it can be 2-3 times, and the part that needs to be demoulded is not There may be sharp corners at the rips and openings. Because the vulcanization requires a temperature of 180 degrees Celsius, the work of demolding is almost entirely manual except for the upper and lower parts of the mold itself. Therefore, it is convenient and quick to take off when designing parts. In order to avoid excessive mold release time, it will affect the vulcanization quality and production efficiency of the parts. Since the release mode of the silica gel is mainly manual, the action stroke of the top plate is also small, so the silicone mold has no oblique top compared with the plastic mold. Generally, the core is not provided.
C. Fixing the core:
Unlike plastic molds, the core of a silicone mold is typically removed along with the product part when the part is opened. Therefore, the fixing and accurate positioning of the core are more difficult to handle than the plastic mold, and the steel of the core is required to be higher. Generally, the cores are made together and all the cores are fixed at one time. Or pass the core through the aids so that it can be quickly positioned. Because there is a lot of pressure in the mold clamping process, it is necessary to prevent the core from moving.
D. Core-pulling: Generally, it can't move automatically, but it can be lifted off. The core is only used for small size structure. The setting of the core is similar to that of a plastic mold, but it is manually removed when the mold is opened. In addition, the core drawing has to occupy a large space in the mold, which is not only complicated in processing, but also has low utilization rate of the mold space. So do not use the core as much as possible.
E. Shape: Silicone products have great toughness when the surface is intact, but once there is a crack, the crack will expand rapidly under the action of external force. That is to say, silicone products are very sensitive to cracks. In consideration of these problems, it should be noted in the design of the silicone parts that all positions cannot have sharp corners to avoid stress concentration and cracks. The R angle at the opening should not be less than 0.5F. Self-demolition: the main function of the self-demolition: to accommodate excess rubber; to facilitate the exhaust of the mold; easy to trim the flash. The self-demolition edge generally consists of a thin edge at the parting surface and a correspondingly strong tear edge. The thickness of the thin side portion is generally 0.1 to 0.2, the thickness of the torn edge portion is generally 0.8, and the width is generally 1-2 MM.
G. Inserts: Various types of inserts can be placed in the silicone products, but several problems should be noted: First, the surface of the embedded parts should be pre-treated, mainly surface vulcanization or surface activation. Otherwise the insert and the rubber will be difficult to connect securely. The second is the fixed positioning problem of the insert, which can be fixed in one direction in the vertical direction, but the other directions must be fully fixed. This prevents the insert from moving during the full mold process. The third is the thickness of the periphery of the insert. For parts that are fully encapsulated (all surfaces are covered so that the insert cannot be positioned), the insert perimeter must be at least 0.5 mm thick. For inserts with positioning, the thickness of the surrounding adhesive layer should be above 0.4MM.
H. Other auxiliary processes:
Injection molding (encapsulation) process:
Similar to injection molding, the parts to be glued are first fixed, and the first layer of mold is closed to control the raw materials without contaminating the parts not designed for injection. The second layer mold is further combined to form a rubber compound.
: Firstly pre-form the rubber compound
Into the mold. This process can be used in the case of full encapsulation.
Dipping: The rubber (rubber) material is a fluid, and the workpiece is immersed in the raw material multiple times. The inner layer material is vulcanized and then immersed once until the design thickness is reached.
Epoxy: The raw material has good fluidity, similar to paint, and vulcanized at room temperature. The mold is simpler, generally manual operation, and is applied once when the first layer is fast curing.
L. Multi-color products: A graded vulcanization 2 sets of molds, B manual color matching, one-time vulcanization, requiring parts of the color separation part to have a certain height, so as to avoid color mixing conductive adhesive bonding, secondary vulcanization
Silicone and silica gel (rubber) can be processed without direct vulcanization. 5. Hardness measurement and tools
The most commonly used instrument for measuring rubber hardness is called the Shore (also known as Shore) hardness tester. A metal ram is pressed into the surface of the material with a spring and measured how deep it can penetrate. The instrument measures a penetration depth of zero to 0.100 inches. A reading on the scale of zero means that the indenter penetrates the extreme depth, while a reading of 100 means that the penetration depth is zero. Shore hardness testers with various hardness ranges and degrees of automation.
One of the most common scales used is the Shore A scale. The Shore A hardness tester has a relatively blunt indenter and a medium spring. When the reading is above 90, the Shore A hardness tester becomes less accurate. For such harder materials, a Shore D hardness tester is used. It has a sharp indenter and a spring that is very elastic and can penetrate deeper depths.
When measuring harder plastics, use a sharper and more elastic hardness tester, such as a Rockwell hardness tester. At the other extreme, the Shore 00 hardness tester was used to measure soft gel and foam rubber.
Most materials can withstand the initial pressure, but over time, they will yield due to creep and relaxation. The hardness meter readings can be read immediately or after a certain delay time, usually 5 to 10 seconds. Instant readings always show higher (or harder) readings than delayed readings. Delay readings are more representative of the hardness of the material and its elasticity. A weaker, less elastic material that is more susceptible to creep than stronger, more elastic materials.
In order to ensure the validity of the data, accurate test steps are required. In order to obtain accurate readings, you must have a test piece that is flat and thick enough to prevent the head from being affected by the support surface. The required thickness is usually 0.200 inches, but for hard materials with less deformation, when the thickness is thin, it can be accurately tested.