Liquid Silicone Rubber Injection Molding Troubleshooting

When injection molding Liquid Silicone Rubber certain problems can occur that directly affect the quality of the final product, in spite of the great planning that may have gone into material selection, machine, process, and the processing parameters. If these issues are not controlled promptly and properly, they could end up resulting in damage to the machine, mold, and finished product.

To help isolate and identify potential problems and their solutions, the following is a list of the most common problems encountered during processing. The first step, when addressing problems and possible solutions, is to look at the process and possible variations in a processing conditions. If the process is optimized, next is the material and if warranted a change of material may be necessary; and lastly a change in the mold and machine. The solutions can be categorized as: people, processing, material, and equipment.

 

Problem: Liquid Silicone Rubber parts are not curing properly

Definition: The LSR parts stick in the cavity or feels tacky; when parts are measured with a durometer they have a low hardness.

 

Solution:

People

  • Check if mold temperature is level
  • Check the heaters and thermocouples
  • Check the mixing device
  • Clean the supply lines and mixer

Processing

  • Increase the cure time
  • Increase the temperature of the mold
  • Increase the recharge time (better mixing, 80% recharge rule)
  • Put backpressure on the screw (better mixing)

Material

  • Check for inhibition
    • LSRs will not cure when it is in contact with: 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 acid, nitric acid, sulfuric acid), strong bases (such as lithium hydroxide, sodium hydroxide, potassium hydroxide)
    • Check for the presence of residual Liquid Silicone Rubber material in supply hoses, on meter-mix wiper blades
    • Sulphur or tin from nearby organic rubber processing
  • Mixing ratio is not 1:1.

Equipment

  • Impingement block is curing up: Check valves not closing causing the ratio to be incorrect
  • Viscosities of materials are different: Screw not mixing properly and static mixer required
  • Static mixer is cured or partially cured
  • Air in material
  • Meter-mix cavitation
  • C-clamp press can have air or gas at the bottom of the mold where the clamp is tighter

 

Problem: Parts not curing properly – Insert molding

Definition: Parts are not curing properly.

 

Solution:

People

  • Check for contamination (ex. sulfur containing compounds)

Processing

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

 

Problem: Liquid Silicone Rubber part is under-filled

Definition: Part is short, has an uneven surface, underweight.

 

Solution:

People

  • Check the meter-mix
  • Check the screw clearance and check-ring operation
    • Material is low viscosity and is slipping past the check ring
  • Clean the system (pre-cured material in the static mixer, etc.)
  • Check the silicone viscosity
  • Check the silicone shelf life

Processing

  • Increase the shot size
  • Increase the injection speed
  • Increase the injection pressure
  • Decrease the mold temperature
  • Increase the injection hold pressure
  • Lower the material temperature
  • Increase the vacuum
  • Increase the back pressure
  • Delay the pressure switchover
  • Add a bump
  • Add/remove mold lube

Material

  • Change to a lower viscosity Liquid Silicone Rubber

Equipment

  • Poor venting (see Bubbles/Burners/Whitening problem)
  • Cold runner sprue not balanced or pre-curing
  • Balance runner and gate
  • Enlarge gates and/or sprue
  • Improve venting

 

Problem: Mold flashing

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

 

Solution:

People

  • Check the mold parting lines and moving parts
  • Clean the parting line
  • Check for consistent shot size (weight)
  • Check mold parallelism
  • Check the silicone rheology
  • Check the silicone shelf life
  • Check the heaters/thermocouples

Processing

  • Decrease the shot size
  • Increase the clamping force
  • Decrease the injection speed
  • Decrease the injection pressure
  • Decrease the mold temperature
  • Decrease the injection hold pressure
  • Decrease the clamping force

Material

  • Change to a lower viscosity LSR

Equipment

  • Reduce size of vents
  • Switch to a larger machine

 

Problem: Bubbles, Burners, Whitening

Definition: Air is trapped in the part, white edges, or blisters visible on the finished part.

 

Solution:

People

  • Clean the mold, deepen the vents
  • Air in the material: Check vacuum pump, gaskets, increase vacuum time or vacuum level
  • Check moisture in Liquid Silicone Rubber
  • Check the LSR shelf life
  • Check the gaskets

Processing

  • Increase or add vacuum
  • Increase the back pressure
  • Decrease the injection speed
  • Decrease the injection pressure
  • Decrease the screw speed
  • Decrease the mold temperature
  • Decrease the clamp force
  • First few shots have air bubbles, usually disappear after 3 or 4 shots
  • Use less mold lube
  • Increase the injection time

Material

  • Change to a higher viscosity Liquid Silicone Rubber

Equipment

  • Air from meter-mix
  • Material is off ratio: Too much part B
  • Uneven filling: Need to balance the runner and gate
  • Contamination: Mineral spirits/solvent in the system
  • Check/increase mold venting
  • Balance runner and gates

 

Problem: Flowlines (also called weld lines or knit lines)

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

 

Solution:

People

  • Check/add vacuum
  • Check mold lube
  • Check/increase venting
  • Check silicone rheology
  • Check shelf life

Processing

  • Decrease the mold temperature
  • Increase the injection speeds
  • Increase the injection pressure
  • Increase the vacuum
  • Decrease the cold runner temperature, turn chiller down
  • Add a bump
  • Decrease the mold lube
  • Decrease the clamping force

Material

  • Change to a lower viscosity Liquid Silicone Rubber

Equipment

  • Nozzle shut-off pin not opening far enough and too much shear causing preheating of material during injection
  • Increase gate size
  • Chroming the mold surface

 

Problem: Scorching

Definition: LSR material is pre-curing during injection

 

Solution:

People

  • Check Liquid Silicone Rubber rheology
  • Check Liquid Silicone Rubber’s shelf life

Processing

  • Decrease material temperature
  • Decrease injection speed
  • Decrease injection temperature
  • Decrease mold temperature
  • Decrease back pressure
  • Decrease screw speed

Material

  • Change to a lower viscosity LSR

Equipment

  • Increase mold gates

 

Problem: Pot life

Definition: Material is curing in the injection unit, longer injection and refill times, start-up problems after long break.

 

Solution:

People

  • Check cooling on the barrel, nozzle, and cold runner
  • Disconnect nozzle from the hot mold during breaks

 

Problem: Pre-cured particles in the finished part

Definition: Cured sprue visible in part; cured particles in part.

 

Solution:

People

  • Clean static mixer, screw and injection unit

Processing

  • Decrease the cold runner mold temperature
  • Improve the water flow
  • Decrease the tool temperature at the gate
  • Increase the sprue delay time
  • Cured material in static mixer, manifold, or shutoff valve
  • Increase the cooling of the nozzle so sprue does not cure so fast.
  • Back pressure on screw may be too high, reduce back pressure, reduce shot size

Equipment

  • Material leaking from injection point
    • Decompression of material in cold runner with an open system is insufficient
    • Shut off needles have too much clearance, adjust shut-off needle
  • Poor mating surface of sprue bushing and nozzle
    • Regrind nozzle tip or replace sprue bushing
  • Improve the cooling

 

Problem: Difficulty demolding

Definition: Part not ejecting from the cavity, part not separating from the runner.

Parts stick to the coldest surface; however, this is not always true

 

Solution:

People

Better releasing

  • Make sure meter-mix is on ratio, too much part B causes sticking
  • Spray mold with PTFE mold release, do not use silicone mold release

Processing

Better releasing

  • Increase injection pressures to pack the material
  • Decrease injection hold pressure
  • Shorten cure time
  • Increasing the temperature allows lower molecular weights to come to the surface and help release
  • Decrease the clamp force
  • Longer air blow off or higher air pressure
  • Sprue sticking in the mold – shorten cure time
  • Decrease the mold temperature

Get part to stick

  • Inside of part has a roughened surface; preheat the metal core to complete the cure

Material

Better releasing

  • Check for material contamination

Equipment

Better releasing

  • Optimize the undercuts
  • Adjust the mold surface roughness, a roughened surface releases easier (see Mold Design section). 400 grit glass bead is better than 300 grit
  • Treat the mold surface, use a solution of 5-7% Ivory soap for a release solution
    • Nickel-Teflon coating provides better release
  • Combine ejector pins, brushes, and plates with air eject and/or roller sweep
  • Reduce the surface area; parts tend to stick to largest surface area
  • Parts sticking to robot arm – use Teflon vacuum cups on robot
  • Change draft angle – should be 1-2°

Get parts to stick

  • Polish the surface
  • Increase the surface area
  • Change the draft angle
  • Increase the surface area

 

Problem: Part is over sized

Definition: Error in the dimensions of the injected molded part

 

Solution:

People

  • Check for even mold temperatures

Processing

  • Increase the mold temperature
  • Decrease the injection hold pressure
  • Decrease the shot size

 

Problem: Part is under sized

Definition: Error in the dimensions of the injected molded part

 

Solution:

People

  • Check for even mold temperatures
  • Check the injection parameters

Processing

  • Decrease the mold temperature
  • Increase the injection speed and pressure
  • Increase the injection hold pressure
  • Curing insufficient (increase cure time)

Material

  • Increase the cure time

 

Problem: Irregular cycle times

Definition: Varying cycle times in production

 

Solution:

People

  • Check for cured material in the static mixer
  • Check-ring not seating properly (increase injection speed)
  • Check the hydraulic system of the machine
  • Check the injection parameters

Processing

  • Recharge time irregular
    • Check meter-mix system
    • Increase the cooling on the barrel (material skinover inside barrel)
    • Increase the screw speed
  • Increase the cooling on barrel, nozzle, or cold runner
  • Decrease/optimize the screw back pressure
  • Decrease the mold temperature

 

Problem: Backrind

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

Solution:

Processing

  • Decrease the mold temperature
  • Decrease the injection speed
  • Decrease the injection pressure
  • Increase the back pressure
  • Increase the material temperature
  • Add a bump
  • Decrease the clamping force

Equipment

  • Mold gates are the wrong size

 

Problem: Short material pot life

Definition: Starting difficulties after a long production break, or excessive injection and recharge times at start-up.

Solution:

People

  • Check the cure profile of material

Processing

  • Insufficient cooling on the barrel and/or nozzle
  • System was not purged properly with “B” component
  • Initiate a sprue break
  • Continue cooling of cold runner after turning off heaters

Material

  • Contamination of the material
    • Replace hoses when switching the material

Equipment

  • Move the injection unit away from the mold during short breaks

 

Problem: Poor adhesion of LSR material to insert

Definition: Lack of adhesion between the LSR and the cavity insert

 

Solution:

Processing

  • Increase the mold temperature
  • Increase the injection speed and pressure
  • Increase the cure time

Material

  • Material compatibility

 

Problem: Black streaking in the part

Definition: Black particles in the injected molded Liquid Silicone Rubber part

 

Solution:

People

  • Check-ring spring is too strong, causing too much pressure in the barrel – possibly causing the screw to deflect

Processing

  • Slow injection speed (due to abrasive filler)

Equipment

  • New injection screw and barrel

 

Problem: Post curing

Definition: Defects in the part after the curing

 

Solution:

People

  • Make sure the previous material was purged out prior to running another material

References

  1. dowcorning.com
  2. engineering-dictionary.org
  3. Stritze, B. Custom molding of thermoset elastomers. Carl Hanser Verlag, Munich, 2009.
  4. silbione.com