Source: medical plastics
Liquid silicon rubber (LSR) is a kind of non-toxic, heat-resistant and high resilience flexible thermosetting material. Its rheological behavior mainly includes low viscosity, rapid curing, shear thinning and high coefficient of thermal expansion.
LSR is divided into glue a and glue B. the ration device is used to control the ratio of them to be 1:1, and then the mixture is fully mixed through the static mixer. After the injection pipe is injected, the injection molding is carried out. The liquid silica gel is injected into the hot runner mold to make silica gel products, which can achieve the advantages of one-time molding, no waste and automation.
LSR is widely used in the medical and health industry, such as making sealed infusion system, ring dialysis filter, hearing aid components, baby nipple, mother's nipple mask, home appliance suction valve, medical device components and so on.
Its products are characterized by good thermal stability, cold resistance, excellent electrical insulation, good elasticity, water and moisture resistance, resistance to acid, alkali and other chemical substances, and will not produce toxic substances when burning.
Forming process
LSR is a two-component liquid material, which is divided into A-component and B-component. The mixer works to fully mix A-component and B-component in an accurate 1:1 ratio. And because part of the products are colored design, so equipped with color pump set and color measurement part. A + B components, additives, colors, etc. are fully mixed before entering the plasticizing system.
The plasticizing screw has the function of homogenization and mixing at the same time. The mixture is injected into the hot mold through the screw. At the mold temperature of 170-200 ℃, the silica gel takes place curing reaction. When using a cold runner system, it is important to note that the runner is cold enough. In order to avoid leakage of glue, 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.
1. Feeding system
The following forms can be selected:
1) The two-way pump can move up and down to feed, which can maintain the pressure well. Because the pumps a and B are connected and controlled by hydraulic and pneumatic synchronously, this kind of feeding system is relatively reliable and accurate.
2) Unidirectional pump is a general type, which can only be used for single feeding. 3) Synchronous single pump with check valve
4) The metering cylinder system is mainly used in combination with single pump.
2. The key parts of liquid silicone rubber injection molding machine 1) due to the low viscosity of LSR, the backflow and leakage of materials should be considered in the processing process, so the sealing of screw is necessary. 2) In order to prevent LSR solidification, a needle valve nozzle is used. 3) Mixing and measuring parts of a and B components.
3. Mold design
In mold design, there are generally the following forms:
1) Hot runner is a waste of materials, simple design, low cost, and mostly used for large products. 2) The cold runner with needle valve can realize automation and short cycle. 3) Cold runner without needle valve
Due to the high LSR expansion coefficient, the expansion will occur when heating, but there is a small shrinkage when cooling, so the components can not maintain accurate side distance in the mold. Therefore, cold runner can be used for processing. LSR should keep low temperature and fluidity. The cold runner adopts closed system. In the injection cycle, the closed system uses "sealing needle" or "needle valve" in each runner to control the accurate measurement of LSR materials. Silicone rubber has remarkable thermal expansion characteristics, the shrinkage is 2% - 4% (curing temperature is 150 ℃), and silicone rubber has the characteristics of compression deformation.
Therefore, the following aspects should be paid attention to in mold design: 1) in order to avoid turbulence and spray, small injection port is used; 2) needle shaped or wing shaped injection port is used; 3) T-shaped guide bolt is used instead of column shaped guide bolt, air gun is used for demoulding, and thermal insulation layer / plate is added; 4) rust inhibitor / oil containing polymerization inhibitor is not used, generally toluene, xylene, etc. can be used.
Analysis of LSR vulcanization
1) The curing 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) high mold temperature will lead to vulcanization, which will cause flow obstruction; 5) laminar flow can avoid bubbles; 6) high speed injection will lead to turbulence; 7) viscosity of materials will change the flow mode; 8) turbulence will lead to white spots.
Common product problems and Solutions
1. Reasons for incomplete solidification of products measures: too low temperature, too intelligent temperature rise time, too short extension time, artificial inspection for solidification of system and pipeline pollution, inspection for mixing problems of cleaning system, inspection for screw, nozzle, mixer, improvement of feeding speed and back pressure, non-1:1 mixing inspection for pump pressure, and improvement of feeding system pressure
2. Causes of the product surface is not mature (the surface is not dry) measures air pollution inspection air path, replacement of oil filter with desiccant vulcanization time can not be well controlled adjustment of vulcanization time mold release agent influence replacement mold release agent surface pollution inspection workers use gloves clean and cloth type cleaning mold
3. Causes and measures for incorrect hardness of products
Temperature and curing time increase the temperature, use thermometer to measure and check the heating rod to check the sensor and its position, extend the time, preheat the embedded parts, keep the pressure too short, extend the time of keeping the pressure, extend the time of shooting the nozzle top mold, mainly extend the time of shooting the nozzle top mold
4. Causes and measures for impurities in products
Black line refueling or die, screw wear cleaning pipeline inspection backflow preventer reduces feeding speed or back pressure
Hard rubber mouth is not mature, extend the time of injection nozzle top mold, cold runner curing, reduce the temperature of cooling system, screw or static mixing pipe, stir curing, regular cleaning
The reasons for the stirring of micro gel increase feeding speed or increase the screw pressure of the back pressure screw curing, screw screw speed too fast, reduce the screw speed, sand line gloves, air guns replace gloves, check the gas path.
5. Causes and measures of product with color spots
Stain
Color adding control to stabilize color adding and increase color adding amount
Static mixer increases screw speed, check color port (too large / blocked)
Flow mark
Color paste dispersion change color paste batch number change glue injection port shape
6. Measures for product deformation or scorching
Too high melting point temperature, too low temperature, too fast injection speed, control injection speed, exhaust gas, expand exhaust port or slot
The shape of the trapped gas injection port uses the gas in the film injection port compound to correctly exhaust the screw backward to reduce the screw backward distance or the product has bubbles to control the injection speed. The vacuumizing and solidification time of the vacuumizing mould is too short to extend the solidification time or to increase the temperature. The top mould of the nozzle adjusts the angle and pressure of the top mould of the nozzle. The temperature is too high to reduce the injection speed. The injection speed is too slow to increase the injection speed. The injection port is too small to increase the injection speed Large injection port
7. Lack of material or shrinkage on product surface
Cause measures
Shortage
Too little feed increase feed pressure check valve check return ring temperature too high reduce mold temperature
Too slow injection speed increases injection pressure, increases injection port and reduces mold locking force
High viscosity increases injection port and flow channel increases exhaust port mould vacuumizes
Flash side
Nozzle closing metering volume control improves die locking force dent nozzle top mold time is too short extend nozzle top mold time
Through the above information, it can be concluded that the main problems of silicone injection are: mixing, metering, screw sealing, mold design.
With proper design and planning, injection molding of LSR material is a production process with good economic benefits and relatively simple operation. Fully understand the principles of injection molding and process design, the manufacturer can avoid problems and carry out efficient production, which will bring high-quality and high-yield industrial effect.