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Silicone Rubber Mold Industry and Prospects

At present, the molding silicone rubber is two-component liquid silicone rubber, divided into condensation and addition. Condensation mold silicone rubber main components include: Silicone (raw rubber) of end groups and part of side groups are the pendant hydroxyl groups, fillers, crosslinking agents and vulcanization accelerator. Addition mold silicone rubber main components include: Silicone (raw rubber) of end groups and partial side groups are vinyl groups, hydrogen silicone oil (Crosslinking agent), platinum catalyst (catalyst), and white carbon black (filler).

The specific comparison of the condensation and the addition mold silicone rubber:

Table 1. Systematic characteristics based on vulcanization mechanism

Condensation type
Addition type
Vulcanization speed adjustment method
Vulcanizing agent type, dosage
heating
By-product during vulcanization
Ethanol, etc.
no
Factors hindering vulcanize
no
yes
Electrical insulation
In the curing process will be a temporary decline

stable

Corrosion of the prototype
yes
no
Heat resistance
good
excellent
Deep vulcanization
Most can be vulcanized
All can be vulcanized
Cold, water, weather resistance
The two are roughly the same

Strength
low
high

Table 2. Systematic characteristics based on ingredients

Condensation type
Addition type
main ingredient
Containing polysiloxane
Containing silanol group-containing polysiloxane
Crosslinking agent
Hydrogen-containing polysiloxane
Alkoxysilane or Alkoxy polysiloxane
Catalyst
Platinum compound
Organotin compounds
Depth hardening agent
unnecessary
Water, alcohols, polyols, etc.
Reaction inhibitor
Tetramethyltetravinylcyclotetrasiloxane, Acetylene alcohol, etc.
Low molecular weight siloxane containing compounds
Reinforcing agent
Reinforced white carbon black, Vinyl silicone resin
Reinforced white carbon black
additive
Pigments, release agents, heat-resistant agent


The Limitation of Prototype Design and Manufacturing

As the silicone rubber mold technology is to use the prototype as a model to manufacture molds, and therefore the prototype design and manufacture is an important part. The prototype shape, size and surface roughness etc. will directly affect the quality of silicone rubber mold manufacturing. It can be said that is impossible to obtain an ideal silicone rubber mold without designing and manufacturing an ideal prototype. The constraints of manufacturing factors generally don’t be considered in traditional prototype design. In order to meet the silicone rubber mold accuracy requirements, using empirical data to compensate for the size of the prototype, and not based on scientific analysis, resulting in a lower one-time success rate of mold design. The raw materials are very limited for traditional prototype manufacturing: If hand-processing the prototype, the raw materials commonly used wood, clay and other easy-processing molding of non-metallic materials; if machining the prototype, the raw materials commonly used in low-melting point alloy easy to process. Moreover, the prototype manufacturing process is directly related to its geometry, the more complex curved surface, the greater difficulty of processing. Therefore, there are shortcomings such as long production cycle and high cost for mechanical prototypes; there are shortcomings such as long production cycle, low precision, and poor interchangeability for the Manual processing prototype. It is obvious that this prototype design and manufacturing mode cannot meet the current fierce competition in the manufacturing industry.

The Limitations of the Vulcanization Process of Silicone Rubber mold

Vulcanization is one of the main processes of silicone rubber mold manufacturing and it is also the final processing step. In this process, silicone rubber undergoes a series of complex chemical changes, from liquid gel to high elastic body, in order to obtain more perfect physical properties, Mechanical properties and chemical properties. Therefore, the vulcanization has great significance for silicone rubber mold manufacture and application. Mold silicone rubber vulcanization process conditions including temperature and time, these factors on the curing quality of rubber materials have a very important impact, and can be called two elements of curing.

From the vulcanization mechanism shown in Table 1, it is known that the condensation type silicone rubber necessarily produces low molecular weight by-products such as ethanol or water or hydrogen during the vulcanization process, so its vulcanization process is usually divided into two stages: the first-stage vulcanization and secondary vulcanization. As described above, the vulcanization containing the prototype at room temperature is the first-stage vulcanization, also known as primary vulcanization. In this vulcanization process, in order to prevent the vaporization of small molecules by-product expansion in the contact surface mold with the prototype forming bubbles, holes and so on, the vulcanization must be carried out at room temperature; The vulcanization time must be long enough to make the shape of the silicone rubber mold does not undergo plastic deformation In the process of removing the prototype from the mold. Low-molecular by-product has a bad influence on the silicone rubber mold. After first-stage vulcanization, two-stage vulcanization at high temperature is needed to sufficiently remove the small molecular by-products remaining in the mold, thereby remarkably improving the compression set of the mold, stabilizing the physical and mechanical properties thereof and improving the heat resistance.

The removal of low-molecular-weight byproducts in the second-stage vulcanization process follows the diffusion mechanism. It can be seen from the molecular kinetic theory that increasing the vulcanization temperature can accelerate the diffusion process. A sufficient amount of fresh air in the oven can also accelerate the diffusion process, in the traditional silicon rubber mold technology, for a specific shape and size silicone rubber mold, the two-stage vulcanization temperature and time, the specific value of fresh air drums the inlet speed is determined empirically. It is clear that this does not meet the requirements of the lean manufacturing of the modern manufacturing industry. With the addition of the molding silicone rubber in the vulcanization process without the formation of small molecular by-products, so the vulcanization process can be completed in one time, or divided into two stages to gradually complete. Typical silicone rubber mold manufacturing technology commonly used two-stage vulcanization method. Obviously, the two-step vulcanization method based on empirical data is conducive to improve the uniformity of entire silicone rubber mold vulcanization degree, and thus to ensure the physical and mechanical properties of the mold, to improve its compression set and dimensional stability, to extend its service life. However, two-stage vulcanization process to increase a production process compared with the primary vulcanization process. It increases energy consumption and equipment use rate, thereby increasing the production cost and extending the production cycle. If using primary vulcanization molding method, and requires each compound of silicone rubber mold to achieve the vulcanized state without the occurrence of sulfur deficiency, the traditional empirical model is difficult competent.

Research Prospects

In order to greatly reduce the development cycle of models and prototypes, manufacturing has begun to use rapid prototyping technology to create complex shapes model and the prototype parts. In the past a short period of ten years, there have been a variety of rapid prototyping technologies. These rapid prototyping technologies can directly generate physical entities according to CAD data model, their common feature is by adding materials rather than removing materials to generate a prototype parts, so that the prototype production costs reduced to 1/3~1/4 for traditional methods, the production cycle reduced to 1/4~1/5 for traditional methods.

Therefore, rapid prototyping as a vulcanization master mold is one of the development trends of silicone rubber mold technology. The main idea of concurrent design technology is to consider the constraints of manufacturing factors in the early design stage of the product, and timely provided to the designer as the basis for design and modification of the program, in meeting the premise of functional requirements, to make product can be quickly and economically manufactured, to improve a success rate of product design and manufacture, to reduce product costs and shorten the product cycle. Therefore, it is one of the development trends of silicone rubber mold technology to develop computer-aided design of silicone rubber molds based on rapid prototyping by using concurrent design technology. CAE technology of rubber vulcanization molding helps to guide the formulation design of rubber compound, improve product quality, and optimize the vulcanization process conditions. Therefore, to carry out the numerical simulation of thermal vulcanization process of the mold silicone rubber material, and then to control the chemical structure of the silicone rubber material and to predict quantitatively physical and mechanical properties of silicone rubber mold by forward simulation method, or the reverse simulation method is used to optimize the vulcanization process Conditions, design formula of polymer materials, and so on, will also be one of the trends of silicone rubber mold technology.

As the mold silicone rubber has many excellent properties, its application is very extensive. In the arts and crafts industry, mold silicone rubber can be used for the production of polymer resin arts and crafts, such as all kinds of imitation gold and silver, imitation jade agate, imitation red wood carving paint, imitation ivory etc. round carving and embossing wall, process clock, lamps, imitation stone furniture, murals and cultural relics and other products are copied using silicone rubber mold production. In other arts and crafts industries such as gypsum, clay, magnetite products, low-temperature metal antique artifacts, Baptist paint, and so on. Those who need to use the mold production products can use the mold silicone rubber production.

High-frequency embossed leather is another field of application of soft silicone rubber mold. This method is very popular at home and abroad. It is directly with silicone rubber in the design of the prototype processing into a working mold, and then places a variety of plastic powder, leather or sheet in the mold, in the high-frequency embossing machine to impose a certain pressure, and the use of high frequency electric field to plastic heating forming. This mold manufacturing process is simple, updated quickly, low cost, life up to 10,000 times and can be a variety of exquisite imitation patterns, such as snake skin, crocodile skin, and pangolin and so on. Currently on the market in the sale of many imitations with reptile patterns of women's bag is the use of high-frequency embossed method of production. Mold silicone rubber non-toxic, can be used in the food industry in the production of some molds. In environmental protection can produce anti-noise earplugs, because this earplug is directly formed in the ear cavity, its shape and ear exactly the same, so comfortable to wear, silencer effect. Silicone rubber is also used portrait production in foreign countries. Face stripping directly on the human, the system was very realistic portraits.
Silicone rubber mold

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