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Liquid silicone rubber mold-HongYe silicone Date:2013-04-23

Liquid silicone rubber mold design features
Thermosetting liquid silicone rubber (LSR) injection pressure mold structure, in general, thermoplastic rubber mold structure, but there are many significant differences. For example, the viscosity of LSR rubber generally low, and thus filling time is very short, even in the low injection pressure. In order to avoid air trapping in the mold with a good exhaust is crucial.
In addition, the LSR rubber in the mold does not like thermoplastic rubber as shrinkage, they are often exposed to heat expansion, cold a slight contraction. Thus, as its products do not always expect to stay on the convex side of the mold, but stranded in a larger surface area of ​​the cavity.
A contraction rate
Although the LSR does not shrink in the mold, demoulding and cooling, but they are often contracted by 2.5% -3%. As to whether the contraction of the number of, to a certain extent depends on the formulation of the rubber. However, from a die angle, the shrinkage may be subject to several factors, including mold temperature, the temperature of the rubber mold release and mold cavity pressure and rubber subsequent compression.
The location of the injection point is also questionable, because the plastic material flow direction shrinkage is usually larger than the shrinkage perpendicular to the flow direction and rubber. Dimensions of its shrinkage of the products also affect the shrinkage of the thicker products are generally thin small. If you need to secondary vulcanization may be able to extra contraction of 0.5% -0.7%.
Parting line
Determine the location of the parting line is one of the first few steps of mold design silicone rubber injection pressure. The exhaust is the main groove located along the parting line to achieve this groove must pass through in Note Caulking material to reach the final of the area. This helps to avoid internal bubbles and reduce the loss of bonding at the strength.
LSR low viscosity, the parting line must be precise, to avoid excess glue. Even so, in the stereotypes of products can often see the parting line. The mold release by the geometric dimensions of the products and sub-surface location of impact. Products designed to be slightly chamfered, help ensure that products consistent with the other half of the cavity of the required affinity.
3 exhaust
With the injection of the LSR, the air stuck in the mold cavity is compressed in the mold is closed and then with the filling process and is discharged through the ventilation groove. Air, if not completely discharged, it will remain in the rubber (so often leads to products is exposed white side). Ventilation groove width lmm-3mm depth of 0.004mm-0.005mm.
Vacuum within the mold to create the best exhaust. This is the parting line design of a washer, and quickly vacuum cavity pumped into vacuum to achieve. Once the vacuum to the rated level, the mold is completely closed, start the injection pressure.
Some injection molding equipment to allow operation in the closing force can change this so that processing can be closed mold at low pressure, until 90% -95% of the cavity, the LSR is full (so that the air more easily excreted), and then switch to a higher closing force, so as to avoid the Silastic expansion occurred Resin.
Four injection points
Molded LSR cold runner system. Can maximize the advantages of this plastic material can be the production efficiency to the maximum limit. Processed products in such a way, you do not remove the plastic injection Road, thus avoiding to increase the labor intensity of the job, and sometimes can avoid wasting a lot of the material. In many cases, no plastic injection channel structure can also shorten the operation time.
Rubber injection nozzle needle valve to the forward flow control, many manufacturers will be provided as standard equipment with the injection of gas control switch mouth, and can be set in the mold parts. Some mold manufacturers specially developed out of an open cold runner system, its size is very small, resulting in extremely limited within the mold space to set up a number of injection points (and thus fill the whole cavity). This technology without the need to make the plastic sprue separation, so that mass production of high quality silicone rubber products possible.
Cold runner system, it is important to form an effective temperature interval between the cavity of hot and cold flow. If the runner is too hot, the plastic material may be before the injection began vulcanization. However, if cooled too fast, it will absorb too much heat from the mold gate area, resulted in complete cure.
For conventional injection runner (such as submerged runner and the conical runner) injection products, suitable for small-diameter plastic injection ports feeding (feeding the mouth diameter is usually 0.2mm-0.5mm) to cast. The low viscosity of LSR rubber as thermoplastic rubber, balanced runner system is very important, so that all the cavity will be rubber evenly filled. The design of runner system simulation software, can greatly simplify the development process of the mold, and filling test to prove its effectiveness.
5 demoulding
By curing the liquid silicone rubber is easy to adhere to the surface of the metal, the flexibility of the products will make demolding difficult. Under normal conditions, the LSR has a high temperature tear strength to make it stripping, even larger products will not be damage. The most common mold release technology including the stripper plate mold release, mold release pin mold release and pneumatic demolding. Other common roller scraping mold export plate mold release and automatic Royal mode.
When using the mold release system, you must keep them in the high-precision range. The top of the gap between marketing and guide pins too much, or parts due to prolonged wear and tear while the gap is larger, it may cause overflow glue. Inverted cone-shaped or mushroom-shaped top marketing effect is very good because it allows large contact pressure, easy to improve the sealing sterile purpose.
6 mold material
Made of mold pallets commonly used non-alloy tool steel (no.1.1730, DIN code C45W) are exposed to high temperature of 170 ℃ -210 ℃ mold pallet of stem, taking into account the impact resistance, and should be used to pre-tempered steel (no. 1.2312, DIN code 40 CrMn-M oS 8 6) manufacturing. Set the cavity of the mold pallet should be used by nitride or to return to B with the heat treatment of steel manufacturing, to ensure that its high temperature performance.
LSR with high filler content, such as oil level the LSR, recommended the higher hardness of the material to create a mold, for example, bright chrome-plated steel or specially developed for this purpose, powdered metal (no.1.2379, DIN code X CrVMo121 155). Design high wear material mold, should be designed to withstand high friction parts into the replacement, so do not replace the entire mold.
Cavity surface finish affect the products is great. The most obvious is that the stereotypes products will be fully consistent with the surface of the cavity. Transparent products with mold should be polished steel manufacturing. Chin / Ni surface treated steel wear resistance is extremely high, and polytetrafluoroethylene (PTFE) / nickel make demolding easier.
7 temperature control
In general, the LSR molded to electric heating is appropriate, strip heaters, cartridge heater or hot plate and heat. The key is to make the entire mold temperature field is uniform distribution, in order to promote the LSR uniform curing. In a large mold, it is cost-effective heating method when push oil temperature control heating.
Insulation board coated mold helps to reduce heat loss. Any part of the hot mold is not appropriate can make between each operation subject to large temperature fluctuations, or cause to run gas. If the surface temperature drop too low, rubber curing rate will slow down, which tend to make the products can not mold release, causing quality problems. Should maintain a certain distance between the heater and the parting line to prevent the template warped deformation, burr formation of excess glue on the finished product.
If the design cold runner mold, hot and cold end to ensure completely separated. Manufactured using a special titanium alloy (eg, 3.7165 TiA16V4]), which is compared with other steel, the thermal conductivity is much lower. For a whole mold heating system, insulation board should be set in between the mold and the mold pallet to the minimum heat loss.
Appropriate design and ideas to ensure that the LSR injection molding, the mold is very important in this. The above mold design principles aimed at rubber filled the cavity, shorten the curing time, and finished product quality, high yield, so that the silicone rubber processors to get a good value for money

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Keywords for the information:Liquid silicone  Liquid Silicone rubber  Liquid silicon 

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