The uniqueness of liquid silica gel (LSR) mould

Silicone rubber molding, especially liquid silicone gel (lsr) injection mold opening is very challenging. Many people think that the technical difficulty of rubber molding is low. The opposite is true. In many cases, lsr moulds require more precision and expertise than thermoplastic molds.

一、Prevent flying edge

Both thermoplastic moulds and LSR moulds have very complex geometric shapes. The main difference between them lies in the requirements of the coordination and relationship between the cavity inserts. Many thermoplastic materials can be fitted with a clearance of 0.002 inches without flanging problems. But the same gap will cause many LSR materials to fly edges, even if the fit is close to 0.0001 inches, or less. This kind of matching requires different degrees of engineering, machining and technological precision. A simple example of differences in fit and association between LSR and thermoplastic injection moulds is the use of thimbles. The ejector pin is often used in plastic moulds. The ejector pin can pass through the parting line on some parts so that half of the ejector rod is on the part and half on the parting line closing device. In the thermoplastic forming of this design, flying edges are usually not produced. Thimbles can also be used for LSR moulds, but they must have a tapered closing design and cannot invade the parting line closing area. Trace debris or rubber left on the seat can cause very serious production problems. Another feature of LSR machining is the requirement of vacuum. Even if the thermoplastic material can be vented during the molding process, the mold usually has sufficient gaps (vents) in the mating and related areas to release these gases as well as the air contained in the cavity area. In addition, LSR molding requires a chemical curing process. Liquid silicon is usually divided into two components. One component has a catalyst and the other has a cross-linking agent. When the two components are mixed, a chemical reaction occurs and some gases are produced. More importantly, the residual air in the mold cavity should be removed before injection to ensure that the mold can correctly discharge the gas produced during the curing process. Because LSR mold parts work closely together to prevent flying edges, appropriate vents must be added to the mold design to allow air to escape from the cavity during filling. If rubber parts need to be controlled within a strict range of specifications, the exhaust should be controlled within a depth tolerance of 1/1000000 inches. Another difference between thermoplastic and LSR mold design is temperature control. Because thermoplastic forming needs to cool the material in the mold, temperature control is usually achieved by circulating oil or water. The temperature of thermoplastic moulds is usually below 80 degrees Celsius. For LSR, moulds that must be heated to between 160C and 220C to solidify the material, the most common way is to use an electric heater to heat the mold.

二、Differences between mold and product design

Compared with thermoplastic materials, there are many differences between mold and product design of LSR materials. The rules to be followed by many thermoplastic materials do not apply to LSR door-to-door. 

This is particularly important in the application of LSR secondary forming. The general secondary molding is to mold the LSR onto the thermoplastic substrate, or one grade of LSR to another LSR. Such products usually require many small adjustments to allow for a more user-friendly mold design and molding process. In the design process, it is necessary to ensure that there are proper shutoff valves, proper ventilation, proper gate location and, most importantly, proper matching of substrates and secondary forming materials. Thermoplastic matrix resin needs to withstand half of the mold temperature of LSR. If the thermoplastic begins to deform at a temperature lower than the curing temperature of LSR, the project will fail. Another problem that needs to be discussed earlier is the bonding method of the two materials. Do we use self-key LSR, mechanical key or both? These projects will affect the design of the mold. Once the product has passed all approvals, it is difficult to change, so these factors need to be discussed as soon as possible.

三、Cold runner and hot runner

Both thermoplastics and LSR can be designed without runners, but there are significant differences in design. Thermoplastics require hot runners to maintain the temperature and viscosity of the material between ejections. LSR, by contrast, requires a cooling channel to prevent the material from solidifying before reaching the heating cavity. Once the two reactive components are mixed, LSR can begin to solidify at room temperature. From the point of view of maintaining the thermal separation between the material conveying system and the mold body, the engineering concepts of the two are similar. When using LSR, the barrel and cooling platform (runner system) on the molding machine use a water mixture to keep cool. The typical temperature range for these components is 15 to 38 degrees Celsius. The most critical area is the point of contact between the cold plate nozzle and the mold, where needle valve gates are usually used. If the area is not properly designed, forming defects can easily occur, such as the appearance of a "wet" gate due to the cold of the area, or an extended gate due to the heat of the area. As the mold size becomes larger and the number of mold holes becomes higher and higher, the temperature sensitivity and exhaust characteristics of LSR make the challenge more difficult. This is why most LSR mold makers don't like more than 64 cavities. The economic incentive to use runnless LSR moulds is even greater than that of thermoplastics for several reasons.

First of all, because the solidified LSR waste can not be reused.

Second, because LSR is often a high-priced material.

Third, because the curing cycle of LSR is usually longer than the cooling cycle of thermoplastics. When forming a small LSR part, the runner may be thicker than the part, so the toolmaker has to wait for the runner to solidify after the part itself is ready for demoulding. In addition, hot runner LSR systems are more difficult to automate because they require the removal of molded parts and runner waste. These wastes must also be collected and eventually removed from the molding area.

四、Quotation of LSR mould

Today's market requires mold manufacturers to provide mold quotations within two or three days, usually based on very limited information. Incomplete information provided at the time of quotation may lead to profitability problems in the future. We have had a lot of examples where injection moulders asked for a certain number of cavities, and then we found out that they didn't have enough presses to run the mold. Some key information will also be ignored, such as customers forgetting to tell us if they need to be fully automated. The part drawings provided are sometimes incomplete or not the latest version of the design. Sometimes the drawings are not completely clear. All of these variables require extra time.

Injection moulders who have used thermoplastics may think that there are hundreds of injection moulders around the world that can produce high-quality LSR moulds. In fact, there are only a few companies that meet the above criteria.

For the reasons discussed above, we strongly recommend working with familiar rubber moulds and working with mature LSR mold manufacturers. This is also applicable to the injection molding machine and its matching metering mixing device. Although many people would say that molds are the most important part of the molding process, a successful LSR project requires four key elements: 

Good material

Good metering mixing device. 

Good molding machine. 

A good mold

If the process needs to be automated, it also needs to include the appropriate automation company. According to our experience, mold manufacturers should have the ability and personnel to manage a complete turnkey project. The mold factory should also have space to allow the entire project to be assembled and tested in its facility. Even if you are only manufacturing moulds, mold manufacturers must prepare mold testing equipment before shipment. Even for the best mold design concepts, there may be some machining problems and some changes need to be made to the mold. In order to do this effectively, all tools should be tested before shipment. The time to market of most products is critical, so anything that can be done at the mold factory can reduce time consumption, which means saving customers money. Therefore, it is very important to evaluate the response time of the mold factory when making design changes. Ask potential mold manufacturers if they have the personnel and equipment to make these quick responses. Also consider whether mold manufacturers have accurate measuring equipment to provide quality LSR molds with tolerances of 1/10 inches and 1/1000000 inches. Whether the mold manufacturer follows the prescribed process to ensure consistency from one cavity to another.