TROUBLESHOOTING - Liquid Silicone Rubber Injection Molding

Despite all the careful preparation that could have gone into material collection, machine, operation, and manufacturing conditions, issues may arise that significantly impact the consistency of the finished product during the Liquid Silicone Rubber injection molding service. If these problems are not addressed quickly and appropriately, they can cause harm to the unit, mold, and finished product in the injection molding service.

ISSUE FACED BY THE INJECTION MOLDING COMPANIES: LIQUID SILICONE RUBBER PARTS ARE NOT CURING PROPERLY

Definition: LSR parts cling in the cavity or feel tacky when measured with a durometer. Parts have an inadequate stiffness when measured with a durometer.

• Check to see if the mold temperature is stable.
• Examine the heaters and thermocouples.
• Examine the mixing unit
• Ensure that the supply lines and mixer are clean.

• Extend the cure time
• Raise the mold’s temperature.
• Boost the recharge period (better mixing, 80 percent recharge rule)
• Apply backpressure to the screw (better mixing)

• Examine for inhibition.
• LSRs will not cure if they come into contact with the following substances: chlorinated solvents, acetone, methylethylketone (MEK), adhesives in tapes, peroxides and their by-products, sulfur compounds, amines, urethanes, tin compounds, nitrogen, sulfur, potassium, and chloride compounds soluble or partially soluble in silicone, butyl rubbers, chlorinated rubbers, strong acids (such as hydrochloric (such as lithium hydroxide, sodium hydroxide, potassium hydroxide)
• Examine supply hoses for leftover Liquid Silicone Rubber content and meter-mix wiper blades for Sulphur or Tin from nearby organic rubber processing.
• The mixing ratio is not one-to-one.

• The impingement block is curing: Check valves failing to close, resulting in an inappropriate ratio
• Different materials have different viscosities in injection molding service: Screw is not properly mixing, and a static mixer is needed.
• The static mixer is cured or partially cured.
• Air in material
• Meter-mix cavitation 
• At the mold’s bottom, where the clamp is tighter, a C-clamp press may provide air or gas.

Definition: Parts are not curing properly.

• Check for contamination (ex. sulfur-containing compounds)

• Run longer cure time or higher temperature
• Preheat the inserts

ISSUE FACED BY THE INJECTION MOLDING COMPANIES: UNDER-FILLED LIQUID SILICONE RUBBER PART 

Definition: The part is short, has an irregular surface and is underweight.

• Examine the meter-mix
• Examine the screw clearance and the operation of the check-ring.
• The material has a poor viscosity and is sliding through the check ring.
• Clean out the system (pre-cured material in the static mixer, etc.)
• Examine the viscosity of the silicone.
• Examine the silicone shelf life.

• Increase the size of the shot
• Boost the injection rate.
• Boost the injection pressure.
• Reduce the temperature of the mold.
• Boost the injection hold pressure.
• Reduce the temperature of the material
• Increase the vacuum 
• Boost the backpressure.
• Delay the pressure switchover.
• Add a bump
• Mold lube may be added or removed.

• Change to a Liquid Silicone Rubber with reduced viscosity.

• Poor venting (see the Bubbles/Burners/Whitening issue)
• Cold runner sprue that has not been balanced or pre-cured
• Runner and gate must be balanced injection molding service.
• Increase the size of the gates and sprue.
• Enhance ventilation

Definition: A Liquid Silicone Rubber film found at the parting line or in the ejector system.

• Examine the parting lines and moving parts of the mold.
• Get the parting line clean.
• Be sure that the shot size is consistent (weight)
• Examine mold parallelism
• Examine the silicone rheology.
• Examine the silicone shelf life.
• Examine the heaters/thermocouples.

• Reduce the size of the shot
• Clamping force should be increased
• Reduce the injection rate.
• Reduce the injection pressure.
• Reduce the temperature of the mold.
• Reducing the injection hold pressure 
• Clamping force should be reduced.

• Switch to an LSR with a lower viscosity.

• Vents should be smaller.
• Change to a bigger machine.

Definition: White edges are visible, and blisters are visible on the finished part as 

• Clean the mold and make the vents deeper.
• The air in the material: Check the vacuum pump and gaskets, and raise the vacuum time or level.
• Moisture in Liquid Silicone Rubber should be checked.
• Examine the LSR’s shelf life.
• Process the gaskets and make sure they’re in good shape.
• Vacuum should be increased or added.
• Backpressure should be increased.
• Reduce the injection rate.
• Reduce the injection pressure.
• Slow down the screw pace.
• Reduce the temperature of the mold.
• Reduce the clamping force.
• Usage of mold lube should be less
• Inject more often with the plastic injection molding service.

• Change to a Liquid Silicone Rubber with a higher viscosity.

• The air from the meter-mix
• The material is out of proportion: there is too much element B.
• Uneven filling: The runner and gate must be balanced.
• Mineral spirits/solvent in the system has contaminated the system.
• Mold venting should be checked and increased if necessary, injection molding service.
• Runner and gates must be balanced.

Definition: Streaks, orange skin, or visible flow lines in parts

• Vacuum check/addition
• Check the mold lube
• Ventilation should be checked/increased.
• Examine the rheology of silicone.
• Check the shelf life.

• Reduce the temperature of the mold.
• Boost the injection rates.
• Injection pressure should be increased.
• Boost the vacuum.
• Reduce the temperature of the cold runner and the chiller.
• Increase the size of the bump
• Reduce the mold lube
• Clamping force should be reduced.

• Switch to a Liquid Silicone Rubber with a lower viscosity.

• Since the nozzle shutoff pin does not open far enough and there is too much shear, the material is preheated during the injection.
• Increase the size of the gate
• Chroming the mold’s surface

Definition: During injection molding service, the LSR material is pre-curing.

• Check the rheology of Liquid Silicone Rubber.
• Check the shelf life of Liquid Silicone Rubber.

• Reduce the temperature of the material
• Reduce the injection rate.
• Reduce the temperature of the injection
• Reduce the temperature of the mold.
• Reduce the back pressure
• Reduce the screw speed.

• Switch to an LSR with a lower viscosity.

• Increase the number of mold gates

Definition: Material is curing in the injection unit, resulting in longer injection and refill times and startup issues after a long break.

• Check the barrel, nozzle, and cold runner for cooling.
• During breaks, disconnect the nozzle from the hot mold.

Definition: In part, cured sprue and cured particles are visible.

• The static mixer, screw, and injection unit should all be clean.

• Reduce the temperature of the cold runner mold.
• Enhance the flow of water
• Reduce the temperature of the tool at the gate.
• The sprue delay period should be increased.
• Static mixer, manifold, or shutoff valve with cured material
• Increase the nozzle’s cooling time, so the sprue does not cure as quickly.
• Reduce back pressure and shot size if screw back pressure is too high.

• Material leaking from the injection point 
• Material decompression in a cold runner using an open device is inadequate.
• Shut off the needles. If you have too much clearance, you should change the shutoff needle.
• Sprue bushing and nozzle have a poor mating surface.
• Replace sprue bushing or regrind nozzle tip
• Enhance the cooling system

Definition: A component that does not eject from the cavity or separate from the runner. Parts tend to stick to the coldest surface, but this isn’t always the case.

• Improved releasing
• Make sure the meter-mix is set to the right ratio; too much part can trigger sticking.
• Do not use silicone mold release; instead, spray the mold with PTFE mold release.

• Improved releasing
• To pack the material, increase the injection pressures.
• Reduce the injection pressure hold.
• Cut the cure time 
• As the temperature rises, lower molecular weights rise to the surface and aid in the release.
• Reduce the clamping force.
• Higher air pressure or a longer air blow off
• Shorten the cure time by sprue sticking in the mold.
• Reduce the temperature of the mold.
• Get part to stick
• The inside of the part has been roughened; preheat the metal core to finish the cure.

• Examine the material for contamination.

• Undercuts should be optimized.
• Adjust the roughness of the mold surface; a roughened surface is easier to release (see Mold Design section). A 400 grit glass bead is preferable to a 300 grit glass bead.
• For a release solution, treat the mold surface with a solution of 5-7 percent Ivory soap.
• The nickel-Teflon coating improves release.
• Combine air eject and roller sweep with ejector pins, brushes, and plates.
• Reduce the surface area; sections appear to stick to the surface area with the most surface area.
• Using Teflon vacuum cups on the robot arm to prevent parts from sticking to it.
• Change the draft angle – it should be 1-2 degrees.
• Getting the pieces to stick 
• the surface should be polished
• Increase the area of the surface
• Adjust the draft angle.
• Increase the area of the surface

Definition: An error in the injected molded part’s dimensions.

• Examine the mold for consistent temperatures.

• Increase the temperature of the mold.
• Reducing the injection hold pressure is a good idea.
• Reduce the size of the shot

Definition: An error in the injected molded part’s dimensions.

• Examine the mold for consistent temperatures.
• Examine the injection parameters

• Reduce the temperature of the mold.
• Increase the injection pressure and pace.
• Injection hold pressure is increased.
• Insufficient care (increase cure time)

• Extend the cure time

Definition: In processing, different cycle times are used.

• In the static mixer, check for cured material.
• The check-ring is not correctly seated (increase injection speed)
• Examine the machine’s hydraulic system.
• Examine the injection parameters

• Time to recharge is irregular.
• Make sure the meter-mix system is in working order.
• Increase the barrel’s cooling time (material skin over an inside barrel)
• Increase the screw rotational speed.
• Increase the cooling amount applied to the barrel, nozzle, or cold runner.
• Reduce or optimize screw backpressure.
• Reduce the temperature of the mold.

Definition: Liquid Silicone Rubber film at vents; parting line distortion-usually a ragged indention

• Reduce the temperature of the mold.
• Reduce the injection rate.
• Reduce the injection pressure.
• Backpressure should be increased.
• Raise the temperature of the material
• Add a bump
• Clamping force should be reduced.

• The mold gates are the wrong size.

Definition: Difficulties restarting a machine after a long period of inactivity or prolonged injection and recharge times at startup.

• Examine the material’s cure profile.

• The barrel and nozzle were not properly cooled, and the “B” part was not properly purged.
• Start a sprue break.
• After turning off the heaters, continue to cool the cold runner.

• The material has been contaminated.
• When changing the material, replace the hoses.

• During short breaks, move the injection device away from the mold.

Definition: Black particles in a Liquid Silicone Rubber component that has been injected molded.

• The check-ring spring is too strong, creating excessive pressure in the barrel and likely deflecting the screw.

• Injection speed is slow (due to abrasive filler)

New barrel and injection screw

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