Introduction
Liquid silicone rubber molding troubleshooting is a critical step in manufacturing process, it will affect the final product quality. Although we get great planning in material selection, machine, process and processing parameter, there are still potential issues difficult to avoid. If we cannot control these issues promptly and properly, final damage will occur in the machine, mold and finished product.
In order to isolate potential problems and identify these solutions, we provide a list of the most common problems in LSR molding process. Once we address possible solutions. Firstly, check the process and possible variations in processing conditions. Secondly, check whether a change of materials is necessary. At last, make changes in the mold and machine. All solutions will be categorized into four parts: people, processing, material and equipment.
Imperfect Curing of LSR Parts
The normal performance of this issue is the LSR parts stick in the cavity with tacky feel. Once measure these parts with durometer, they are in a low hardness. Our common solution as following:
People
- Check whether mold temperature achieve required level.
- Inspect the heater systems.
- Check the mixing devices.
- Clean the supply lines and static mixer.
Processing
- Increase the curing time.
- Rise the mold temperature.
- Increase mixing time.
Material
- Check inhibition compounds: LSR materials will not cure properly once contact with inhibitions, such as chlorinated solvents, acetone, peroxides and by-products, sulfur compounds, amines, urethanes, tin compounds, sulfur, nitrogen, potassium, chloride compounds, strong acids (hydrochloric acid, nitric acid, sulfuric acid), strong bases (lithium hydroxide, sodium hydroxide, potassium hydroxide).
- Check residual materials in supply hoses and meter-mix wiper blades.
- Sulpher or tin from nearby processing.
- Check mixing ratio of 1:1.
Equipment
- Incorrect ratio by impingement block.
- Different viscosity in different LSR area, static mixer is required for proper mixing.
- LSR curing or partially curing in static mixer.
- Air in material, meter-mix cavitation, air or gas at the mold bottom with tighter clamp.
LSR Curing Issue in Over-molding
Liquid silicone rubber materials don’t cure properly on the substrates. Normally solutions:
People
- Check contamination from both environment and substrate, except sulfur containing compounds.
Processing
- Run longer cure time.
- Increase mold temperature.
- Preheat the insert substrates.
Under-filled in LSR Molding
This will give rise to short size LSR parts with uneven surface and underweight. We solve this issue by following methods:
People
- Inspect the meter-mix.
- Inspect the screw clearance and checking-ring operation.
- Clean the LSR molding system.
- Check LSR viscosity.
- Check LSR shelf life.
Processing
- Increase LSR shot size.
- Increase LSR injection speed.
- Rise LSR injection pressure.
- Rise mold holding pressure.
- Decrease the mold temperature.
- Decrease the LSR material temperature.
- Raise the vacuum in the mold.
- Raise the back pressure on screw.
- Add mold lube.
Material
- Change to a new grade liquid silicone rubber with lower viscosity.
Equipment
- Improve venting, poor venting result in bubbles, burners, whitening problems.
- Unbalance or pre-curing in cold runner sprues.
- Enlarge injection gate or spure.
- Balance runner and gate.
Flashing in LSR Mold
Once the liquid silicone rubber (LSR) film occurs at the parting line or in the injector system, we should provide the solutions as following:
People
- Check the parting lines and moving parts in mold.
- Clean the parting line area.
- Whether the shot size or weight is consistent.
- Whether the mold is parallel.
- Check the flowability of LSR materials.
- Inspect the LSR shelf life.
- Inspect the heater system.
Processing
- Decrease the shot size in injection process.
- Decrease the injection speed.
- Reduce the injection pressure.
- Reduce the mold temperature.
- Increase the clamping force and decrease the holding pressure in molds.
Material
- Change to a new grade liquid silicone rubber with lower viscosity.
Equipment
- Reduce vents size in mold.
Bubbles, Burners and Whitening
Once air is trapped in LSR molding process, there are white edges or blisters visible on final LSR parts. In this case, we need to solve this as following:
People
- Check air in LSR materials: check vacuum pump, increase vacuum time and vacuum level.
- Check moisture level in liquid silicone rubber.
- Clean the mold and cavities, deepen the vents in molds.
- Check LSR shelf life.
Processing
- Increase vacuum level.
- Increase the back pressure on screw.
- Decrease injection speed.
- Decrease injection pressure in molds.
- Reduce screw speed in injection units.
- Reduce mold temperature.
- Diminish the clamping force.
- Rise LSR injection time.
Material
- Change to a new grade liquid silicone rubber with higher viscosity.
Equipment
- Air form meter-mix in mixing system.
- Unbalance ratio of compound A&B, too much compound B.
- Balance the runner and gate for even filling.
- Contamination from mineral spirits or solvent in system.
- Increase mold venting.
Flow Lines or Knit Lines
The common appearance of this issue is streaks, orange skin or flow lines on LSR parts surface. We need to fix this problem by following solutions.
People
- Increase vacuum level.
- Check mold lube.
- Increase mold venting.
- Check LSR rheology.
- Inspect LSR shelf life.
Processing
- Increase mold vacuum.
- Increase injection speed.
- Rise injection pressure.
- Decrease mold temperature.
- Decrease cold runner temperature.
- Reduce mold lube.
- Diminish the clamping force in mold.
Material
- Change to a new grade liquid silicone rubber with lower viscosity.
Equipment
- Increase gate size.
- Open up nozzle shut-off.
- Chroming the mold surface.
Scorching
The liquid silicone rubber materials will pre-cure in injection process, our common solutions are:
People
- Check the rheology of LSR materials.
- Check shelf life of LSR.
Processing
- Decrease LSR materials temperature.
- Decrease the injection speed and temperature.
- Reduce the mold temperature.
- Reduce the back pressure on screw.
- Diminish the screw speed in injection unit.
Material
- Change to a new grade liquid silicone rubber with lower viscosity.
Equipment
- Increase mold gate size.
Pot Life
In longer injection and refill times, LSR materials are curing in injection units. This will give rise to start-up issues. Therefore, we require common solution:
People
- Check cooling situation on barrel, nozzle and cold runner.
- Disconnect nozzle from mold during break time.
Pre-cured Particles in LSR Parts
It is visible with cured sprue and particles in LSR parts. We can fix these problems by following methods:
People
- Clean static mixer, screw and injection unit in system.
Processing
- Decrease the mold temperature.
- Decrease temperature at mold gates.
- Increase the cooling at nozzle area.
- Increase the sprue delay time.
- Reduce back pressure on screw.
- Reduce shot size.
Equipment
- Insufficient decompression of LSR materials in cold runner system.
- Adjust shut-off needle with suitable clearance.
- Regrind nozzle tip or replace sprue bushing.
- Improve the cooling system.
Demolding Issues
There are different demolding issues, such as ejection error, incorrect separation from the runner, LSR parts sticking on surface. We can fix these issues by solutions:
People
- Correct ratio on meter-mix (too much compound B get sticking).
- Apply PTFE mold release.
Processing
- Increase injection pressure.
- Decrease holding pressure in injection mold.
- Decrease curing time.
- Increase the material’s temperature.
- Rise air blow or air pressure.
- Decrease mold temperature.
- Preheat the metal core.
Material
- Check material contamination.
Equipment
- Optimize the undercuts.
- Adjust mold surface roughness for easier release.
- Treat mold surface for better release.
- Nickel-Teflon coating.
- Reduce parts surface area.
- Change draft angle to 1°-2°
Oversize in Dimensions
LSR parts have error in the dimensions with oversize after injection molding. This issue requires the following solutions:
People
- Check whether the temperature of mold is even.
Processing
- Increase the temperature of mold.
- Decrease the shot size.
- Decrease holding pressure in mold.
Under-size in Dimensions
LSR parts dimensions are undersize, we need to fix this issue by solutions:
People
- Check the parameters of injection.
- Check whether the temperature of mold is even.
Processing
- Increase injection pressure and speed.
- Rise holding pressure of mold.
- Increase the cure time for sufficient curing.
- Decrease the mold temperature.
Irregular Cycle Time
LSR injection molding has regular cycle time in production. Sometime, irregular cycle time will cause various defect in production. We should fix by following solutions:
People
- Check LSR materials in static mixer.
- Check the injection parameters.
- Increase injection speed.
- Inspect the hydraulic system
Processing
- Check meter-mix system.
- Increase cooling of barrel, nozzle, cold runner.
- Increase screw speed.
- Decrease back pressure of screw.
- Decrease temperature of mold.
Poor Adhesion between LSR and Insert
The LSR materials have poor adhesion with insert, result in defects on final LSR over-molding parts. We can solve this problems by following ways:
Processing
- Increase the temperature of mold.
- Increase injection speed and pressure.
- Rise the curing time.
Material
- Select materials with better compatibility.
Conclusion
In conclusion, Miwo has the most professional engineering team in China. We can assist our clients develop innovative solution to their silicone challenges. Liquid silicone rubber is an excellent choice for various applications. For specific information about LSR molding, contact us directly for technical information.
