Silicone is widely used in the market because it does not release toxic substances, has a soft and comfortable touch, and has good physical and chemical properties at high temperatures and low temperatures (-60c~+300c). Few other polymers can match it. Strong and powerful elastomer, better than rubber for sealing, excellent electrical insulation, and resistance to chemicals, fuels, oil, water, and good material to cope with adverse environments.
Application
At the application level, industrial and medical fields such as oil seals, keyboard keys, electrical insulation materials, auto parts, daily necessities such as pacifiers, artificial catheters, respirators, frog mirrors, leather shoes, sneaker pads, food containers, etc.
Classification
Silicone can be divided into solid and liquid. The former is processed by hot pressing, while the latter is mainly made of injection molding. Although the liquid is relatively expensive in terms of equipment investment and raw material cost, its production speed is fast, the degree of processing is low, and the waste is less. Based on other factors, the use of liquid silicone injection molding, in the pursuit of precision, speed, and automation of the injection molding production industry, must be a future-oriented trend.
Silicone technology development
From the perspective of injection molding machine manufacturers, the development of LSR injection molding machines is also very promising. The biggest difference between LSR injection molding machines and general plastic injection molding machines is the feeding system. , screw, mold, and control system design, this is another way for domestic injection molding machine manufacturers to expand business opportunities and machine added value. At present, the competition in the ordinary injection molding machine market has become fierce and fierce.
Looking forward to the future market and customer needs, it is a good way to develop a special machine for silicone injection molding. Liquid silicone is divided into A glue and B glue. The ratio of the two is controlled by a quantitative device to be 1:1, and then fully mixed through a static mixer, and injected into the injection tube before injection molding production. The liquid silicone is injected into the hot runner mold to make silicone products, which can achieve the advantages of one-time molding), no waste, and automation.
In the past three to five years, injection molding technology for thermoset liquid silicone rubber (LSR) has developed rapidly. The injection molding design of LSR differs significantly from rigid engineering thermoplastics mainly because the physical properties of the two rubbers, such as low viscosity, rheological properties (fast cure), shear thinning properties, and higher thermal expansion coefficients are quite different. Due to the low viscosity of LSR, it can be filled with faster flow rates during injection molding even at lower injection pressures, but in order to avoid air entrapment, the requirements for mold ventilation are more stringent.
Hot runner molding & Cold runner molding
In general, modern LSRs have faster cycle times (less than 20 seconds in some cases), and in order to take full advantage of this feature, processing machinery, injection molding machines, and part transfer systems must work together. Operates as a highly integrated whole. Cold Runner Forming Modern cold runner systems take full advantage of the shear thinning properties of LSR to truly achieve waste-free, burr-free forming.
Looking at recent years, we found that the dominant position of cold runner molding in the manufacturing industry has risen sharply, which has led to a good momentum of increasing the output of rubber products, reducing scrap, and reducing labor costs. LSR does not shrink in the mold, like thermoplastics. However, due to the high coefficient of expansion, it expands when heated, but contracts only slightly when cooled. As a result, parts often do not hold accurate side margins in the mold, only in cavities with larger surface areas.
Similar to hot runner molding, in cold runner processing, thermoset LSR should be kept at low temperatures and flowability to ensure no material loss. This processing method is most suitable for producing large volume parts of similar size and structure in a clean indoor environment. The ideal model is the round-the-clock operation in equipment with minimal human influence, and progressively larger periods of operation (daily or weekly).
Open system & Closed system
There are two basic types of cold runner equipment in use today, closed systems and open systems, each with advantages and disadvantages. During the injection cycle, the closed system employs “actuator pins” or “needle valves” in each line to control the flow of LSR rubber. On the other hand, the opening system uses “shrinking nozzles” and valves to control the flow of materials according to the injection pressure. Closed systems are typically characterized by lower injection pressures compared to open systems. Adjustable “throttles” in the device allow for fine-tuning of unbalanced runners and different shear thinning properties of the material. The downside is that for certain parts and molds of a given size, additional adjustments to the equipment are required. Open systems utilize high shear rates through nozzles or valves to shut off flow when injection pressure is reduced.
In general, the cavity filling time of an open system is slightly shorter than that of a closed system. Open systems have higher cavity density due to smaller runners and nozzles. The runner requires a natural balance and is strictly matched with the rheological properties of the material itself. Because of the small size of the flow passage in an open system, there is usually no need for an adjustable “orifice”, and a normal valve can be used to control the flow well and obtain the best pressure point. Parting line When designing a liquid silicone rubber injection molding mold, the position of the parting line must be considered first because some channels need to be set inside the parting line and these channels are used to complete the ventilation task. The ventilation holes must be set at the end of the mold where the injection material finally arrives. . Taking these factors into consideration in advance helps to avoid air entrapment and loss of weld line side seam strength.
Liquid Silicone Rubber Molding
Due to the low viscosity of LSR, it is necessary to ensure the accuracy of the parting line and avoid burrs. Nonetheless, the parting lines on the final product are clearly visible. Part geometry and parting line location also affect the release process. In part design, a slight undercut helps to ensure a strong bond between the part being molded and the mold cavity. Shrinkage Although liquid silicone rubber does not shrink during injection molding, due to the high thermal expansion coefficient of silicone rubber, it usually shrinks by 2,,3, after release and cooling.
The exact shrinkage data mainly depends on the material formula, but from a processing point of view, if the designer considers some factors that affect shrinkage in advance, the final shrinkage will change. These factors include the processing temperature, the material stripping temperature, the cavity pressure, etc. Another consideration is the location of the injection port because usually the material shrinks more in the direction of flow than it does in the vertical direction.
In addition, the size of the part is also a factor. Generally speaking, the thicker the part, the smaller the shrinkage. If secondary vulcanization is required in the actual application, an additional 0.5%, and 0.7% shrinkage should be considered. Ventilation When the mold cavity is closed, the air is trapped in it. As the liquid silicone rubber is injected, the air is first squeezed and then gradually driven out of the cavity by the filler. Due to the low viscosity of the LSR, the cavity is quickly filled. During the rapid filling process, if the air cannot be completely driven out of the cavity, it will be entrained in the vulcanized material (usually manifested as a white edge along the perimeter of the part or a small smooth bubble inside).
Typical ventilation ducts are 1,3mm, 0.004, and 0.005mm deep, and have been successfully used in production. The best way to remove air trapped in the cavity is to use a vacuum to drive out the trapped air in the cavity during each injection molding cycle. That is, when designing the parting line to ensure that the mold is closed, the vacuum pump will evacuate all cavities through the clamp under the mold switch. Once the vacuum reaches the desired standard, close the mold and start the injection. Another method of a successful application is to use the adjustment of the clamping force to achieve the purpose of driving out the air. When the clamping force is low, the manufacturer fills the LSR to 90, 95, of the cavity, and then increases the clamping force, and at the same time avoids the backlog of liquid silicone rubber overflowing and causing burrs.
It is very difficult to have a proper machining design for the injection port, where the valve marks are small and strong, and the valve position is not perceptible. However, if the valve is located in a non-critical area or on the inner surface, a lot of trouble can be avoided. For example, as mentioned above, the use of a cold runner system for injection molding of LSR material can save the elimination of injection port marks, thereby avoiding the need for a Labor-intensive production process and waste of a lot of materials. In many cases, a spineless design will also reduce cycle time. If a cold runner system is used, it is important to have an effective isolation temperature between the hot cavity and the cold runner. If the runner is too hot, the material will begin to solidify before injection, and if it cools too quickly, it will absorb too much heat from the mold valve area and prevent solidification from completing. The valve or actuating pin of the closing system is generally designed to be between 0.5 and 0.8mm to ensure the movement space of the pin and the material flowing around it. In the open system, the nozzle and valve are usually smaller (0.2, 0.5mm), so that the flow can be better controlled.
For low-viscosity LSRs, the feed diameter is slightly smaller if the material is injected through a conventional injection port, such as a submersible valve or a conical valve. (The diameter of the injection port is usually between 0.2 and 0.5mm.) Demoulding Unless it is a special formulation, generally cured LSR tends to stick to the metal surface, which brings some difficulties to demolding. Even so, the current hot tear strength of LSR rubber can still meet the demoulding requirements, and there is basically no loss after demolding. The most commonly used demolding technology equipment, includes fractionation column trays, ejector pins, and air ejectors. Other methods with more applications include drum sweepers, exclusion towers, and robotic operations. When using an ejector system, the ejector system must be kept within close tolerances. Excessive clearance between the ejector pin and sleeve, or excessive wear on the components, can cause component burrs to appear. The contact pressure of the reverse cone or mushroom ejector is relatively large, which can improve the airtightness of the system, so it is very effective. Mould material Generally, retainer plates are made of non-alloy machined steel (no. 1.1730, DIN code C45W).
Since the formwork is exposed to high temperatures, it should be made of pre-tempered steel (no. 1.2312, DIN code 40 CrMnMoS 8 6) to increase the punching resistance. The template with the cavity is preferably made of elastic hot steel with good temperature resistance. For highly filled LSR materials like oil-resistant grades, tougher materials are recommended, for example, chrome-plated steel and powdered metals have developed considerably for this application (steel no. 1.2379, DIN code X 155 CrVMo 12 I ). When making molds for abrasive materials, care should be taken to use special inserts or replaceable tooling so that worn components can be replaced individually without having to replace the entire mold.
The quality of the mold cavity surface has an important impact on the quality of the part. Simply speaking, the cast part will accurately replicate the original appearance of the mold cavity. Polished steel is very important for transparent parts. Surface-treated titanium/nickel steel is highly resistant to wear, while PTFE/nickel is easier to release. liquid silicone rubber materials have abrasive properties to some extent, so aluminum materials are best not chosen. When economic conditions permit, the best metal materials are selected to obtain better compatibility and facilitate the processing of crude products into final products.
In the temperature control LSR forming process, the typical heating method is electric heating, usually using an electric heating wire heater, heating tube, or heating plate. During the one-shot curing process of liquid silicone rubber molding, uniform distribution of temperature in the mold is very important. In large molds, the most economical heating method is the “oil temperature control method”. Wrapping the mold with an insulating sheet also helps reduce heat loss. If the surface temperature drops too fast, the curing speed of the material will be reduced, the release of the part will be inhibited continuously, and the quality of the part will be affected. Keeping a certain distance between the heater and the parting line can greatly avoid the bending and deformation of the template, but it will make the cast parts appear burrs.
If the mold is designed for a cold runner system, proper isolation at the hot and cold interface is essential. Titanium alloys like 3.7165 (Ti-Al 6V4) have poor thermal conductivity compared to other steel materials, so they are good materials for thermal isolation. For integral mold heating systems, an insulating layer should be placed between the mold and the template to minimize heat loss.
Simulation Design In the LSR runner system, liquid silicone rubber will fill all mold cavities uniformly. In such a system, the balance of LSR runner planning is very important. The use of computerized logistics dynamics simulation software to design shunt valves and vents can help mold improvement and avoid the high consumption of trial and error methods. The experimental results can be demonstrated with filling studies, however, proper simulation requires the engineer to know the mechanical reactivity of the injection molded LSR formulation. Experiments using finite element analysis for component design can ignore high-stress areas.
summary
With proper design and planning, injection molding of LSR material is an economical and relatively simple production process. By fully understanding the principles of injection molding and process design, manufacturers can avoid problems and carry out efficient production. It is believed that LSR’s excellent cavity filling performance and rapid vulcanization characteristics will surely bring a high-quality, high-output industrial effect.