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