Compression Molding

Rubber Compression molding

Compression molding is one form of molded silicone rubber parts. Compressing preformed silicone rubber in a mold with a press during the compression process, Silicone rubber is forced into the mold cavity to form the final product during this process, any excess rubber is pushed into the flash and trim gate allowing easy cleanup once the rubber is set.

Compression molding works best for medium hardness compounds in low volume production. Silicone rubber can be difficult to insert into more complex mold designs. And Soft and Hard silicone rubber compounds don’t do well in the compression molding process.

But Silicone rubber compression molding offers advantages over other methods

  • Cost-effective Tooling
  • Maximized cavity count
  • The short set-up time that saves on short production runs
  • Economical process for medium precision
  • the least amount of waste in comparison to other molded rubber processes. 

Compression molding can be a cost-effective solution when:

  • Having silicone rubber industries manufacture the Tooling.
  • the cross-section of the part is very large and requires a long cure time.

Compression molding is used for anything from simple silicone rubber o ring to the most complex silicone parts.

If compression molding isn’t the best fit for your project, Ukong can offer a variety of other molded silicone rubber products through injection or transfer molding.


Injection Molding

Liquid silicone injection molding

Injection molding process involves rapidly injecting an exact amount of Silicone rubber from a tube or cylinder into a closed heated mold, the injection press closes the heated mold and applies pressure to keep the mold closed during the injection and curing cycles, the screw then injects the silicone rubber through the nozzle into the mold. the silicon rubber travels through the mold via a system of runners and sprues and finally enters the mold cavities through a gate, it then fills and packs the cavities to form the desired part rubber.

Injection molding offers economical advantages including:

  • short molding cycle
  • Lower unit cost
  • high dimensional tolerances
  • An absence of flash
  • little scrap or waste.

If you’re interested in what Ukong can do for your company email us at [email protected]


Transfer Molding

Rubber transfer molding

Transfer molding combines the advantages of injection molding with the ease of compression molding. the process of transfer molding involves placing a rubber and a chamber at the top of the mold and then pushing through sprues into a heated mold.

Transfer molding offers several advantages over other methods,

  • shorter production cycles
  • maintains close dimensional tolerances than compression molding
  • provides uniformity to the molded products
  • fast mode setup.



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silicone phone case

Why new silicone product look yellow?

There are three reasons, curing agent, processing technology and raw materials are poor quality

  • Curing agent

The performance of the curing agent can be divided into general and anti-yellow. It is applied in the following two aspects:

The general one is non-anti-yellowing curing agent, it can be used for non-exposed silicone products which are relatively dark colors or low color requirement.

The other one is an anti-yellow curing agent, it can be used for silicone products that need to be exposed to light or certain chemicals or lighter and brighter colors, and a higher requirement of colors.

  • Processing technology

During molding, the clear silicone turns yellow. The main problems are

  1. Mold’s temperature is too high
  2. Vulcanizationtime is too long
  3. Turn yellow after the second Vulcanization
  • Raw materials are of poor quality

Sometimes we add an anti-yellow curing agent to the silicone rubber, but the clear silicone products still look yellow after molding. It may be the silicone rubber material’s anti-yellowing ability is too poor. The only solution is to change the raw material.

Although the excellent weather ability and anti-aging properties of silicone products. However, sometimes silicone products turn yellow after molding. In order to prevent this kind of situation, we should choose the correct curing agent. During processing, we should strictly control the temperature of tools and their curing time, and pay attention to the raw material resistance of yellowing.

silicone phone case

Why clear silicone cases turn yellow over time?

Clear phone cases are normally made from silicone that is popular for its inexpensive and flexible properties. Unfortunately, these silicone turn yellow as they age. The main reason is because of the ultraviolet radiation of the light. If you usually use your smartphone with the case outdoors, the case will turn yellow after long-time exposure to the light. Of course, UV radiation is not the only element that can cause the silicone to age. This natural process is accelerated when they are exposed to excessive amounts of chemicals, light, and heat.

In addition to this, the constant friction during normal use, and the sweat on the hand or the oil on your face cause the oxidation reaction will also turn the cell phone cases yellow.

Rather than just being a stain, the yellowing is part of the material degradation. But yellowing does not affect its basic use such as hardness and service life.


How to remove the yellowness of silicone cases at home?

how to remove yellownessDetergent powderpaint thinner

Take an empty bowl with tissue paper in it.

Put the Detergent powder in the bowl, mix water & detergent powder, add more tissue paper( tissue papers will help to stick to the cover and will remove it better), then add vinegar in the mixture, stir it well. Now put a yellowness silicone cover in it. Leave it for 5 minutes. Then add paint thinner. Leave it for 5 minutes again, stir it well and cover it with tissue, leave it for 20 to 30 minutes. Finally, take the silicone case out, it looks good and transparent again.

silicone overmolding

Overmolding refers to a process used to combine at least two materials to create one object part or design.

Today, two technologies can be used to create overmolded objects: Injection molding and 3D printing

Injection molding is the most common process used for over-molding more recently. Some 3d printing technologies are being used to over mold objects of the two, Overmolded parts typically consist of a substrate and the overmolding material, or sometimes more materials. one material is commonly known as the substrate, it is either partially or fully covered by a second material or materials referred to as the overmolded material.

The substrate material can be just about anything, metal, plastic, glass, and so on. The overmolded material is often silicone rubber or thermoplastic material. The over molding process often results in either a chemically bonded part of the materials are mechanically interlocked.

Overmolding offers several benefits. one, in particular, is that it can eliminate secondary operations when building parts thereby reducing manufacturing costs. It is often used to deliver a specific feel to a product such as softening edges or delivering a more pleasant feel or giving users a firm but soft grip. In brief, the injection overmolding process works like this.

With injection molding technology, substrate parts are molded first, the molded parts are either placed into a different mold or placed back into the injection molding machine, where the second material or materials is injected once cooled this object is removed from the mold, then you have an over molded part.

The 3d printing technology is typically jetting where two or more materials are jetted concurrently onto the part during the layer-by-layer build process. The types of products that are over molded include tool hand grips like drills scissors, Screwdrivers, personal care products, cell phones, and anything needing market differentiation. For more information about the silicone overmolding process, please visit Ukong website.


Silicone products

The silicone has many excellent characteristics and functions. But silicone belongs to the production of synthetic raw materials, the different proportion of ingredients in the materials will have a different functional effect, and what reasons for the aging in silicone rubber products? Following are some reasons for aging in silicone cases.

Contact medium aging

Exposure to moisture in the air, or other conditions, such as the external insulation of electrical power systems, in salt-contaminated areas, is extremely susceptible to salt spray, therefore, it is necessary to study the aging performance under this condition. In addition, some silicone rubber products which come into contact with oil are tested to improve their oil resistance.

The aging of silicone rubber under different kinds of oil is due to the mechanical damage of aging on one hand, and on the other is the degradation of cross-linking system, which results in low molecular weight materials. The presence of fluid can reduce the coefficient of friction, but it can also accelerate the chemical degradation of the silicone rubber, thus accelerating wear.


Thermal oxidative aging

Natural rubber (NR) for capacitor seals, EPM, EPDM, Styrene-butadiene, butyl rubber, silicone rubber (NVQ), and so on, are all of these forms of thermal-oxidative aging during use. NR containing a lot of unsaturated double bonds, under the combined action of heat and oxygen, the VULCANIZATE will degrade, the molecular chain and cross-linking bond will be cracked and broken, resulting in aging.

Thermal oxidative aging is both a radical and an oxidation reaction. Under anaerobic thermal aging conditions, free radical decomposition occurs, and the lower the bond cleavage energy, the faster the decomposition, when molecules break down and form free radicals, then the main chain breaks and forms cross-links, producing all kinds of gases. In ultraviolet aging, molecules absorb ultraviolet light and activate certain functional groups, and decomposition or through the energy transfer in other functional groups, such as decomposition, through the formation of active groups and reaction.

The oxidative decomposition of side-chain organic groups in silicone rubber occurs mainly in the anaerobic high-temperature open environment, which leads to the hardening of silicone rubber, while in an oxygen-free high temperature closed environment, the main chain fracture reaction occurs mainly, the formation of volatile cyclic polysiloxane, resulting in softening of silica gel products. The oxidation of side-chain methyl and the degradation and fracture of the main chain are the main reactions of silicone rubber at high temperatures.

Fatigue Aging

The fatigue aging of silicone rubber case is that the aging phenomenon of silicone rubber products under the action of a certain frequency and periodic stress, which is caused by the change of the molecular structure of silicone rubber material, that is, force and heat effect. (due to the rubber in a number of deformation, hysteresis phenomenon, so that the internal silicone causing heat )


Ozone aging

Ozone content in the atmosphere is very low, in the aging process of silica gel, ozone attack silica gel molecules, silica gel expansion and dispersion resulting in surface cracks. Ozone reacts with double bonds in silica molecules to form Ozonide and peroxide, which in turn regenerates ozonide. The Ozonide is broken down into free radicals by the action of heat and light, leading to a chain growth reaction. In addition, the silicone rubber under stress will produce molecular fracture, but not the formation of Ozonide, the phenomenon of cracking and aging.

By studying the effect of ozone on different kinds of tread rubber, including natural rubber, oil-filled methyl-styrene-butadiene rubber, and the blend of isoprene rubber and cis-1,4-polybutadiene rubber, etc., the service life of silicone rubber decreased and decreased rapidly in the short time of ozone action, which proved that silicone rubber had obvious degradation at the beginning of ozone action. Along with the Ozone Action Time extension, the service life drops slowly. This may be due to the degradation process slowing down during the deepening phase of polymer decomposition.


For customized products, some choose screen printing, pad printing, or transfer printing which including heat transfer printing and water transfer printing. At present, transfer printing is widely used in plastic, hardware, rubber, silicone rubber, PVC, ceramics, solid wood, and other industries, so for soft silicone rubber products. Heat transfer printing and water transfer printing are similar, but what is the difference between heat transfer print and water transfer print?

Silicone bong

Heat transfer printing

The heat transfer printing process is one-time heating of heat transfer printing film, transfer the decorative pattern onto the surface of the product to form a high-quality decorative mask. In the process of heat transfer, heat and pressure are applied to separate the maintenance layer and pattern layer from the polyester substrate, hot-melt adhesive bonds the decorative layer and product surface.

Water transfer printing

Water transfer printing with a special chemical treatment of the film, after printing the required color lines, flat on the surface of the water, the use of water pressure, the color line pattern evenly transfer to the surface of the product, when the coated film is automatically dissolved in water, after cleaning and drying, and then a layer of transparent maintenance coating, this product has shown a completely different visual effect!

The main difference is:

The heat transfer printing process is complex. step 1, needs a professional operator to Create a high-resolution image. Step 2, According to the picture size made dot-type gravure engraving plate. Step 3, The gravure plate is installed on the printing machine and printed on the PET polyester film, which is printed into thermal transfer printing film. Thousands of meters of finished patterns can be printed in one hour. Step 4, The heat transfer printing film installed on the heat transfer printing hot stamping machine for printing, simple operation, the one-time formation of the pattern, without color register. A heat transfer printing hot stamping machine 8 working hours a day can print out thousands of products.

Water transfer printing greatest technical advantage is no needed for special equipment, not limited by the medium, do not need special supplies, no high-temperature heating is required as long as you have a scanner, computer, inkjet printer plus water transfer ink, water transfer paper, you can print any solid object, any surface at will, the image bright color, absolute photo-quality. Small investment, quick to use, easy to operate, applicable to all areas. But the defect is that its processing efficiency is low, high cost, its effect is not good as the heat transfer printing!

For the above two processes, if you still do not know how to choose, Ukong suggests you choose the heat transfer printing when you do a large number of silica gel products transfer printing. While water transfer printing process for a small number of silicone products. The heat transfer printing process is complex but printing has bright colors, ever-changing, small color differences, good reproducibility. Water transfer printing operation is the simple but slightly lower printing quality.


silicone vibrator

Generally, the surface of silicone products is typically quite sticky. Sometimes this can present problems in an end user’s application, especially when buying glossy silicone products, such as silicone tubes, custom silicone molds, silicone sheets, etc. However, there are several ways to reduce this problem.


Powder with Talcum

The simplest way is through the addition of talcum powder, wiping this uniformly across the product’s surface. Silicone swim caps with talcum are very effective, it is easier to wear and take off the silicone cap after powder, and talc can be washed off.


Spray hand-feeling electrostatic oil on the surface

It is a common method to spray hand-feeling electrostatic oil on the surface. In general, the silicone products we have used for a long time will be sprayed with hand-feeling electrostatic oil, such as silicone phone cases, silicone protective covers, silicone vibrator and silicone facial cleansers, etc., this process will not affect the use efficacy, and will not cause harm to the body skin, so this process in the silicone products industry is more widely used.

This process is mainly to spray a layer of hand-feeling oil on the surface of the product to enhance the surface feel of the product and the isolation layer. Because it has a certain adhesion, there will be no demoulding after baking at high temperature, so it is adopted by many consumers, however, the main problem with this kind of process is that it is a little difficult to go through some strict environmental monitoring and certification. Because the ink materials do not meet the environmental requirements.


Produce with anti-static silicone rubber material

anti static silicone rubber

In addition to spray handle oil, silicone rubber manufacturers can also use materials to solve the problem of dust, that is, the selection of anti-static silicone rubber processing, because dust-proof and anti-static silicone rubber is a two-component silicone material, it can be cured at room temperature or at elevated temperature. It is not limited by the shape and thickness of the silicone products which can endure high temperature for a long time. It can be deeply cured, to achieve the effect of non-stick dust.

Anti-static silicone rubber has excellent resistance function, usually after high-temperature vulcanization of the static silicone rubber product surface resistance of 106-109Ω, but also through customer demand modulation to higher performance. Compare with the gas-phase silicone, they are no different. so it is suitable for molding and extrusion and liquid injection molding process and so on, usually, this kind of silicone can be used for silicone suction cup, silicone medical hose, and food-grade silicone products, etc.


Made by Matte Surface mold

silicone mold

custom silicone mold

Silicone products made by the matte surface mold are not active dust absorption. Matte surface silicone is relatively rough to touch, its advantage is effectively isolated from the dust absorption.


How to do if the silicone products accidentally get a lot of dust?

  1. The slight dust can be wiped with a dust-free cloth, and the dust on the surface of the product can be removed.
  2. The more serious dirt and dust can be wiped with white electric oil, or directly soaked in white electric oil liquid and wiped with a clean dust-free cloth.
fluorosilicone vs silicone

For sealing and protection products, Fluorosilicone is suitable for use in aerospace, automobile, electromechanical, electronic, mechanical, and architectural fields, it can be used in kerosene at-55 °C ~ 220 °C and in steam and open flame at 150 °C, and in the strength of more than 100 kg under the pressure of heat resistance, oil resistance, ozone, and other harsh environment continued to use.

Fluorosilicone is called trifluoro propyl methyl polysiloxane, also known as special silicone rubber, its main material is carbon tetrafluoride and ethylene which combine trimethyl trifluoro propylcyclo trisiloxane, and an amount of tetramethyl tetravinyl ring, tetra-siloxane, and a small amount of blocking agent was cured and mixed under the catalyst. Compared with ordinary silicone rubber, the impact properties are improved effectively.

Compared with ordinary silicone products, the outstanding feature of Fluorosilicone is performance and service life. The normal service life of ordinary silica gel products, such as reaching the bearing capacity of Fluorosilicone rubber, will accelerate the aging capacity, the tensile strength will drop, the deformation will be larger, and so on. Silicone elastomers are also known as silicone rubbers which are composed of silicon, carbon, hydrogen, and oxygen. They are non-reactive, stable, and resistant to extreme environments and temperatures from −55–300°C (−67–572°F). Fluorosilicone also has these characteristics.

Fluorosilicone rubber distinguishes itself from silicone because it also contains additional beneficial properties common to fluorocarbons. The addition of these compounds enhances FVMQ’s capacity to resist solvents, fuel, and oil. Fluorosilicone has a slightly lower maximum service temperature hot air resistance than ordinary silicone.

The important features of Fluorosilicone products are as follows:

  1. Resistance to oil, solvents, and chemicals

Compared with methyl vinyl silicone rubber, Fluorosilicone has excellent oil resistance, solvent resistance, and chemical resistance, and good oil resistance and solvent resistance even compared with fluoro rubber. It can be said that Fluorosilicone rubber is the only elastomer that is resistant to non-polar medium at-68 °C ~ 232 °C. The resistance of Fluorosilicone to gasoline containing methanol is also good. Even in the gasoline/methanol mixed material, the hardness, tensile strength, and volume of the vulcanizate have little change, the physical properties have hardly changed after 500h long time immersion testing.

  1. Heat resistance

The high-temperature decomposition of Fluorosilicone is the same as that of ordinary silicone, i. e. side-chain oxidation, main chain fracture, side-chain thermal decomposition, and various compound reactions. As the decomposition products can also cause the main chain fracture, so the heat resistance, usually worse than ordinary silicone rubber, at 200 °C temperature has begun oxidation aging. However, by adding a small number of heat stabilizers such as iron, titanium, and rare earth oxides, it can be improved remarkably and has enough heat resistance even at 250 °C. The influence of temperature on Fluorosilicone is greater than that of silicone but less than that of fluoro rubber. The service life of Fluorosilicone rubber was also studied at 150 °C × 2000H, 175 °C × 5000H and 200 °C × 4000H, the results were second only to that of methyl vinyl silicone rubber.

  1. Cold tolerance

Fluorosilicone and ordinary silicone products are good low-temperature performance. Because Fluorosilicone rubber is a linear polymer with soft SI-O as the main chain, its low-temperature property is better than that Fluororubber with C-C as the main chain. Among them, Fluorosilicone (LS-2370U) has better low-temperature characteristics, the brittleness temperature reaches -89 °C, and the general Fluororubber is -30 °C.

  1. Electrical properties, radiation resistance

The electrical properties of Fluorosilicone are similar to those of ordinary silicone rubber, but its special value is that it changes little under harsh conditions such as high temperature, low temperature, humidity, oil, solvent, chemicals, ozone, and so on. The radiation resistance of Fluorosilicone is not outstanding, but the radiation aging resistance is better than methyl vinyl silicone rubber.

  1. Physical and mechanical properties

Fluorosilicone rubber and ordinary silicone rubber, vulcanizate mechanical strength (especially tear strength) is relatively low. Therefore, improving the strength of Fluorosilicone rubber is also an important research topic.

  1. Other features

Fluorosilicone rubber weather aging resistance is very good, even after exposure for 5 years, still maintain good performance. Ozone is one of the most common gases produced during the aging of elastomers, but no cracks or cracks are found in Fluorosilicone rubber by dynamic or static tests. In addition, Fluorosilicone rubber mold resistance, physiological inertia, an anticoagulant is also very good.

Ukong uses compression molding, silicone injection molding process to manufacturer all kinds of silicone products including Fluorosilicone rings, ordinary silicone products.