1. Properties of Liquid Silicone
Liquid Silicone Rubber is a non-toxic, heat-resistant, high-recovery flexible thermosetting material. Its rheological behavior is mainly characterized by low viscosity, rapid curing, shear thinning and high thermal expansion coefficient.
LSR is a dual-liquid rapid vulcanization material with platinum as a catalyst, which can be produced by injection molding, mass, rapid vulcanization, and repetitive mechanical production.
Its products show good thermal stability, cold resistance, excellent electrical insulation properties, and no toxic substances are produced when burning. Therefore, it has become an irreplaceable material in the production design of health products, automobiles, baby products, medical products, diving products, kitchen utensils and seals.
2. molding process
LSR is a two-component liquid material, which is divided into A component and B component. The work of the mixer fully mixes the A component and the B component in a precise 1:1 ratio. And because some of the products are colored designs, they are equipped with a color pump set and a color metering part. A+B components, additives, colors, etc. are fully mixed and then enter the plasticizing system. This plasticizing screw has the functions of homogenization and mixing at the same time. The mixture is injected into the hot mold through the screw, and the silica gel undergoes a curing reaction at a mold temperature of 170-200 °C. When using a cold runner system, it is important to note that the runner must be sufficiently cold. In order to avoid glue leakage, the needle valve is installed on the surface of the mold part. When the glue injection is completed, the needle valve immediately closes the nozzle.
2.1 Feeding system
The following forms can be selected:
1) Bidirectional pump
The feeding can be moved up and down, and the pressure can be well maintained. Since the A and B pumps are connected and controlled by hydraulics and pneumatics synchronously, this form of feeding system is more reliable and accurate.
2) One-way pump
It is a general type and can only be fed in a single item
3) Synchronized single-phase pump with check valve
4) Measuring cylinder system
Mainly used in conjunction with single-phase pumps
2.2 Key components of liquid silicone rubber injection molding machine
1) Due to the low viscosity of LSR, the backflow and glue leakage of the material should be considered during the processing, so the sealing of the screw is necessary.
2) In order to prevent LSR from curing, a needle valve nozzle should be used.
3) Mixing and metering parts of A and B components
2.3 Design of the mold
In mold design, there are generally the following forms:
1) Hot runner
It is a waste of materials, has a simple design and low cost, and is mostly used for large products.
2) Cold runner with needle valve
It can be automated and the cycle is short.
3) Cold runner without needle valve
Due to the high coefficient of expansion of LSR, which expands when heated and shrinks slightly when cooled, the part cannot maintain precise side margins in the mold. Therefore, cold runner processing can be used. LSR should maintain low temperature and fluidity. The cold runner adopts a closed system. In the injection cycle, the closed system uses a “sealing needle” or “needle valve” in each flow channel. ” to control the accurate metering of LSR materials.
Because silicone rubber has significant thermal expansion characteristics, the shrinkage rate is 2 to 4% (vulcanization temperature is 150 ° C), and silicone rubber has the characteristics of compression deformation.
Analysis of LSR flow/vulcanization:
1) The vulcanization chemical reaction of liquid silicone rubber requires a certain reaction time.
2) Ideal flow, with a diameter of 2mm, 170cm, and a flow distance of more than 100cm.
3) Minimum thickness 1/1000mm
4) Excessive mold temperature will lead to vulcanization, resulting in flow obstruction.
5) Laminar flow can avoid air bubbles
6) High-speed injection can cause turbulent flow
7) The viscosity of the material will change the flow pattern
8) Turbulence causes white spots.
Therefore, the design of the mold should pay attention to the following aspects:
1) To avoid turbulence and jetting, use a small injection port.
2) Use a needle or wing-shaped injection port
3) Use T-shaped guide bolts instead of cylindrical guide bolts, use an injection air gun when demoulding, and add heat insulation layers/plates.
4) Do not use rust inhibitor/oil containing polymerization inhibitor, generally toluene, xylene, etc. can be used
In addition, cured LSR tends to stick to metal surfaces, and the most commonly used demolding techniques include ejector pin ejection and air ejection.
Through the above information, it can be seen that the main problems solved by silicone injection molding lie in its mixing and metering parts, as well as the sealing of the screw and the design of the mold.
