I. Properties of Silica Gel Materials:

 A silica gel is generally gel-like, somewhat similar to plasticine, colorless, translucent and tasteless.  B its main characteristics are high temperature resistance (up to 300℃) and low temperature resistance (down to-100℃). it is the best cold-resistant and high-temperature resistant rubber at present.  At the same time, it has excellent electrical insulation, high stability to thermal oxidation and ozone, and high chemical inertness.  Disadvantages are low mechanical strength, poor oil resistance, solvent resistance, acid and alkali resistance, difficult vulcanization and higher price.  Operating temperature:-60℃ ~+200℃.  The above is the instruction of silicone rubber in the manual.

 C. operating temperature: as mentioned above, it is generally set at-40℃-200℃, which can reach 230℃ in a short time.  D. ageing issues:

 Oil resistance, acid and alkali resistance are poor, which has little to do with stress.

 E. bonding problem: before surface treatment, in addition to using silica gel products as adhesives to bond with silica gel parts, all bonding with other parts must undergo surface activation, and the adhesive surface can only be bonded after vulcanization treatment.  Any substance that can undergo vulcanization reaction or crosslink with rubber is collectively referred to as vulcanizing agent and crosslinking agent.  There are many kinds of vulcanizing agents, and they are also increasing. The vulcanizing agents that have been used include sulfur, selenium, tellurium, sulfur-containing compounds, metal oxides, peroxides, resin, quinones, amines, etc.

 F material hardness: according to Shore hardness, rubber parts can theoretically be selected between 10 and 80 degrees in the domestic market.  Can be easily found in the general 20 to 70 degrees, the most commonly used is 40 to 50 degrees.

Second, silica gel products production process:

 The production process of general silica gel products:

 A raw material shape and color

 B after color matching and mixing, the flaky materials with various colors are changed by milky white silica gel

 C after mixing and shaping, the raw materials are pressed into strips and cut into strips.

 D vulcanization molding

 E trimming, trimming, inspection and packaging.

 F. finished product drawings

 Three, product size and characteristics:

 A. limit size: the thickest part can be 15~20MM, and if it is a sphere, the diameter can be 30 mm.  Generally, the recommended thickness is not more than 3MM. when it is more than 3 mm, it will take more curing time and increase the cost.  The thinnest part can theoretically reach 0.2MM, but the thinnest part is generally 0.3MM and 0.4MM is recommended in design.

 B. relative size: in terms of thickness difference, it is recommended that the thinnest part and the thickest part should not exceed 3 times.  This kind of problem mainly depends on the requirements of temperature and pressure when the material is vulcanized.

 C. shrinkage.  The shrinkage of silica gel materials is related to the hardness of the materials. Most of the secondary materials provided by manufacturers are between 1.022 and 1.042. For materials with 40 to 50 degrees, the shrinkage is generally 1.03.  Compared with plastic, silica gel products will not produce similar obvious surface defects due to shrinkage.

 D. dimensional accuracy: because silica gel products are mostly one mold with many cavities, the number of cavities is very large compared with plastic products.  Therefore, size control is not as convenient as plastic products.  General precision is plus or minus 0.1, and high precision products are plus or minus 0.05.  When the fit is used for the fit between the hole of the plastic part and the key, the minimum clearance is 0.1 on one side, and the recommended value is 0.2 on one side.

 E. Outline Design: For rubber sleeve parts, it is generally enough to provide the original drawing to the mold factory according to the outline drawing of the product. The mold factory will decide on its own to explain the cooperation problems.  Under normal circumstances, depending on the size of the product, the matching between the rubber sleeve and the product is generally a negative deviation of 0.2~0.5 smaller than that of the single side.

 Four, mold structure:

 A. the upper limit of the mold thickness is 250mm temporarily, the size can reach 500X500, and the product can reach 400X400 in the mold area.  pattern

 The structure is generally divided into an upper die, a lower die, a bottom plate, a top plate, a top rod, a die core, hinges and other auxiliary parts.  When designing silica gel products, attention should be paid to the characteristics of its mold and production process:

 B demolding: the demolding of silica gel products can be directly and strongly removed.  For products without design defects, the size requirement for strong stripping mainly lies in the perimeter ratio of the maximum part of the die core to the minimum closing part during stripping, which can be 2-3 times for products below 55 degrees, and there can be no cracks and sharp corners at the openings at the parts requiring forced stripping.  Because vulcanization requires a temperature of 180 degrees Celsius, the demolding work is almost entirely manual except for the vertical separation of the mold itself.  Therefore, the convenience and rapidity of detachment should be considered in the design of parts.  So as not to affect the vulcanization quality and production efficiency of parts due to too long demoulding time.  Because the demoulding method of silica gel is mainly manual and the action stroke of its top plate is also very small, the silica gel mold has no inclined top compared with the plastic mold.  In general, core pulling is not provided.   

 C. fixing of mould core:

 Different from plastic mold, the core of silicone mold is usually taken down together with the product parts when opening the mold to take out the parts.  Therefore, it is more difficult to handle the fixing and accurate positioning of the mold core than the plastic mold and requires higher rigidity of the mold core.  Generally, the mold cores are made together to fix all the mold cores at one time.  Or the mould core can be quickly positioned through an auxiliary tool.  Because there is a lot of pressure in the mold closing process, it is necessary to prevent the mold core from moving.   

 D core pulling: generally, it can't run automatically, but it can be forced off. core pulling is only used for small-sized structures.  The setting of core pulling is similar to that of plastic mold, but it is taken out manually when the mold is opened.  In addition, core pulling needs to occupy a larger space in the mold, which is not only complicated to process in the silica gel mold with more than one mold, but also has low utilization rate of mold space.  Therefore, core pulling is not required as much as possible.   

 E shape: silica gel products have great toughness when the surface is complete, but once cracks exist, the cracks will expand rapidly under the action of external force.

 Zhang.  In other words, silica gel products are very sensitive to cracks.  Considering these problems, attention should be paid to the design of silica gel parts: all positions should not have sharp corners, so as to avoid cracks caused by stress concentration.  The angle r at the opening cannot be less than 0.5

 F. self-stripping: the main function of self-stripping: to accommodate excess glue;  It is convenient to exhaust the mold.  It is convenient to trim the flash.  Self-dismantling edges are generally made up of

 A circle of thin edges at the parting surface of the part and the corresponding relatively strong tear edges.  The thickness of the thin edge portion is generally 0.1~0.2, the thickness of the torn edge portion is generally 0.8, and the width is generally 1 ~ 2 mm.   

 G inserts: various inserts can be placed in silica gel products, but several problems should be paid attention to: first, the surface of the embedded parts should be pre-treated.

 Mainly surface vulcanization or surface activation.  Otherwise, it will be difficult to firmly connect the insert with the glue.  The second is the fixing and positioning of the insert, which can be fixed in one direction in the vertical direction, but must be fixed in all other directions.  So as to prevent the insert from moving in the whole mold process.  The third is the glue thickness around the insert.  For parts that are fully encapsulated (all surfaces are encapsulated, so the insert cannot be positioned), the periphery of the insert should be at least 0.5MM thick.  For inserts with positioning, the thickness of the peripheral glue layer should be above 0.4 mm.   

 H. other auxiliary processes:

 Injection (encapsulation) process:

 Similar to injection molding, the parts to be injected with glue are fixed first, and the first layer of mold is closed to control that the raw materials will not pollute the parts not designed for injection.  Then the second layer of mold is closed, and the rubber material is pressed into shape.   

 I. glue rolling process: firstly, glue material is manually preformed into a mold.  This process can be used in the case of whole bread crumb rubber.

 J gum dipping: the raw material of the glue (rubber) body is fluid, and the workpiece is immersed into the raw material for many times.  Soaking the inner layer raw material again after vulcanization,

 Until the design thickness is reached.   

 K glue dripping: the raw materials have good fluidity, similar to paint, and are vulcanized at normal temperature.  The mould is relatively simple, generally operated manually, in the first

 Apply next time when one layer is fast curing.

 L multicolor products: 2 sets of molds for a divided vulcanization, b manual color matching, one vulcanization, requiring a certain height of the color separation part of the part.

 In order to avoid the combination of mixed color conductive adhesive, the secondary vulcanized silica gel and silica gel (rubber) can be directly vulcanized and bonded without treatment.

 Five, hardness measurement and tools:

 The most commonly used instrument for measuring rubber hardness is called Shore (also known as Shore) durometer.  Use a spring to press a metal pressure head into the surface of the material and measure how deep it can penetrate.  The penetration depth measured by this instrument is zero to 0.100 inches.  A reading of zero on the scale means that the pressure head has penetrated the limit depth, while a reading of 100 means that the penetration depth is zero.  There are various Shore durometers with different hardness ranges and automatic degrees.

 One of the most commonly used scales is the Shore A scale.  Shore A durometer has a blunt pressure head and a spring with medium elasticity.  When the reading is above 90, the Shore A durometer becomes inaccurate.  For such hard materials, shore d durometer is used.  It has a sharp pressure head and a spring with strong elasticity, which can penetrate deeper.

 When measuring harder plastics, hardness meters with sharper indenters and stronger elasticity, such as Rockwell hardness meters, are used.  At the opposite extreme, a Shore 00 durometer is used to measure soft gel and foam rubber.

 Most materials can withstand the initial pressure, but will yield over time due to creep and relaxation.  The durometer reading can be read immediately or after a certain delay time, usually 5 to 10 seconds.  Immediate readings always show higher (or harder) readings than delayed readings.  Delayed readings are more representative not only for the hardness but also for the elasticity of the material.  A weaker and less elastic material is more prone to creep than those with stronger and more elastic properties.

 In order to ensure the validity of the data, accurate testing steps are required.  In order to obtain accurate readings, you must have a specimen with a flat surface and thick enough to prevent the pressure head from being affected by the supporting surface.  Usually the required thickness is 0.200 inches, but for hard materials with small deformation, when the thickness is relatively thin, it can also be accurately tested.