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Basic Injection Moulding Trouble Shooting Guide*
Contents
Common sense on trouble shooting
Common moulding faults and possible cause: Short shot, Warpage, Mould flash, Brittleness, Excessive shrinkage, Sinks/Voids, Mould Sticking, Drooling, Silver streaks/Splash marks, Flow marks/Jetting, Odor, Low gloss, Weak weldlines, Un-melted granules, Rough surfaces/Orange Peels/Wrinkles, Discolouration/Colour fading, Brown stains/Black streaks, Screw slippage, Burn marks.Shop floor safety guidelines
Common Sense On Trouble Shooting
- Determine whether problem occurs consistently or not. (In most case, you cannot control consistency.)
- Identify possible causes (refer to table below).
- Take corrective actions
- Install control measures, monitor whether new process is capable to prevent problem reoccurrence or not.
- If non go back to step 1. If yes, finalize settings, document changes, and notify all concerning parties for the modifications.
Short shot
Possible causes:
Process- Insufficient injection pressure/ injection rate insufficient material feed
Excessive cooling of the melt
Mould- Unbalanced multi-cavity mould
Insufficient air venting blocks resins flow Runners, gates, or vents too small
Material flow length too long.
Material- Material viscosity too high
Foreign material clogging nozzle and/ or gates
Mould temperature too low
Machine- Feed hopper blocked
Barrel has no resins left
Undersized cylinder heating capacity
Material leaks/
back flow
Warpage
Possible
causes:
Ejected part too hot/ in-balance cooling
Improper balance core and cavity temperature
Poor knock out mechanism balance
Over packing in gate area because of high injection pressure
Holding time too long
Moulded in strains / internal stress built up
Small undercut in mould Varied part’s wall thickness
Improper balanced multiple gates
Material flow too long, insufficient gates Low rigidity on part desig
Mould Flash
Possible causes
Injection pressure/injection rate too high
Barrel temperature too high
Insufficient clamping pressure
Mould sliding cores clearance too large Imprecise mould alignment
Foreign material caused clearance on mould mating surface
Mould temperature too high Air venting design too large
Material over packed the mould cavity
Mould parting defects after age
Hopper temperature too high
Brittleness
Possible causes
Material thermally degraded-
Screw speed too high
Residence time too long
Backpressure too high
Material not predicted
Regrind content too high
Materian contamination
Use of improper colorant
Inadequate radii at corner, notch, or thread
Voids or notch on part after moulding
Inappropriate hopper temperature
Mould temperature too low
Moisture contend too low after moulding (for PA)
Excessive Shrinkage
Possible
causes:
Insufficient cooling
Injection rate too low
Packing pressure (injection, holding, back) too low
Mould temperature too high
Inappropriate hopper temperature
Runner / Gate size too small
Varied part’s wall thickness
Insufficient additives (nucleator)
Sink Marks / Voids
Possible
causes:
Insufficient injection pressure
Insufficient holding time
Insufficient backpressure
Insufficient material feed
Hopper and melting temperature too high
Insufficient sprue/runner/gate size
Improper gate location / material flow length too high
Material leaks / back flow
Non-uniform part’s wall thickness
Mould temperature too high for sinks and too low for voids
Insufficient air venting for voids
Mould
sticking
Possible causes:
Over packing, injection pressure too high
Holding pressure too high
Insufficient cooling
Regrind content too high
Insufficient tapers/draft angle on mould/sprue
Insufficient mould release/resins lubricant used
Undercuts at mold/moving slides failure
Material experiences ”positive shrinkage”
Core & - Cavity Polished surface on core
Insufficient knockout action
Surface irregularities in the mould
Sprue- Nozzle and sprue bushing miss-aligned
Sprue bushing not polished
Drooling
Possible causes:
Nozzle temperature too high
Melt temperature too high
Material too moist
Shut off valve malfunction
Silvery
Streaks Splash Marks
Possible causes:
Material too moist
Overheated material
Residence time too long
Air trapped in melt
Check if excess mould release agent on mould cavity
Reduce regrind
Flow marks Jetting
Possible causes:
Material melt front too cold
Irregular injection rates
Inconsistent nozzle heating
Big variance in part’s wall thickness
Mould too cold
Gate size/design inappropriate
Odor
Possible causes:
Melt temperature too high
Material contamination
Residence time too long
Material had been overheated or even burnt at hold up spots within the machine
Low gloss
Possible causes:
Material too cold
Rough cavity surface/poor cavity polishing
Injection pressure too low
Sprue/Runner/Gate size too small
Excess mould release agent/lubricant on mould surface
Weak Weld lines
Possible causes:
Inadequate venting at weld(s)
Mould temperature too low
Melt temperature too low
Material too viscous (setting too quickly)
Injection speed too low
To small sprue, runner or gate size
Gate location(s) too far from weld
Excess mould release agent on mould surface
Un-melted Granules
Possible causes:
Material too cold
Insufficient plasticizing capacity, slow plasticizing
Melt contains granules with higher melting pressure
Insufficient backpressure
Barrel not well insulated
Screw type is inappropriate for the material plasticizing
Rough Surfaced Orange Peels Wrinkles
Possible
causes:
Rough cavity surface
Material flow too slow-
Insufficient melt temperature
Insufficient mould temperature
Insufficient injection pressure
Insufficient injection speed
Screw forward time too slow
Gate size too small
Discoloration Color Fading
Possible causes:
Material suffers thermal degradation
Melt temperature too high
Colorant melt temperature too low
Brown Stains Black Streaks
Possible
causes:
Material oxidized or contaminated
Screw/barrel not cleaned
Excess oil/grease on mould surface; probably from knockout pins
Screw Slippage
Possible causes:
Screw/Barrel worn out
Feed hopper blocked
Too much lubricant in material
Material too wet
Burn Marks
Possible causes:
Injection speed too high
Temperature settings too high
Not enough mould venting or venting blocked
Material too heat sensitive
Gate size/design inappropriate
Core shift causes excessive material heat up from friction when passing narrow section
Safety Inside the Injection Moulding Workshop
Injection moulding machines are powered my electricity. They are fast, power and dangerously hot. Despite the fact that they are build to fun safely, users still have to treat them with respect because the high clamping tonnage and high temperatures needed to process the plastics are just as effective as crushing or destroying the operator hands, arms, legs and fingers.
Be cautious and pay attention when operating injection moulding machines, or else accidents could happen.
Do not reach over or under protective guards when the machine is running.
Do not climb or crawl into the machine when the machine is operating.
Wear face shields, long gloves and other protective clothing when purging the barrel and when working around the injection nozzle or the mould where hot plastic may be expelled.
If a piece of plastic is caught inside the mould when the latter closes, hit the machine emergency stop button. Do not slide open the safety gate and grab for the plastic. Wait until the clamping action has stopped. Damage moulds can be replaces, but not crushed hands and fingers.
Before starting your work shift, walk around the injection moulding machine that you had been asked to operate. Make sure that all safety guards are in place, observation windows are clear and not broken or missing, and all safety interlocks are in working order and properly adjusted.
Do not fix or adjust the injection-moulding machine when its motors are running.
Stand clear when a maintenance mechanic works on your machine. Do not attempt to help unless requested specifically.
Ejector pins, ejector plates and boxes, cores and core activator are moving parts. Pinch points are created as these devices travel. Watch your hands and fingers.
Safety devices
Every injection-moulding machine has built-in electrical, hydraulic and mechanical safety devices. Find out where they are located and check to make sure that they are operational, as operator on an earlier shift may have disabled some of them.
Clothing and Safety Glasses
Your facilities will no doubt have certain rules and regulation on what you should wear on the job to safeguard your safety. Be particularly cautious about the types of shoes you wear as split hot plastic can easily burn through thin shoes and sharp plastic parts, runners or spruces can pierce soft soles. Wear safety glasses and ear protectors even if they deem to be awkward and inconvenient. Protect yourself with these gears for your own safety.
Motor stop button
If you have to reach into the mould space for some reason, stop the motor that drives the whole machine. The motor or emergency stop button is purposely located close at hand as part of the requirements under government safety regulations.
Do not reach into the material hopper or feed throat while the machine is running. The rotating screw can grind fingers off efficiently. Again, shut off the motor first.
Hot Plastic
Melted plastic is extremely hot. Do not under-estimate the temperatures and pressures that are at work in your moulding machine.
Purging
Purging is the expelling of hot plastic material from the injection barrel into the open air. Wear long gloves, long sleeves, a facemask and safety glasses. Purging of hot runner moulds is particularly hazardous.
Electrical Heaters and Wiring
The injection barrel is heated by relatively high voltage electricity (normally 220 VAC). Be very careful of the heated portion, the heater band at the nozzle is often exposed, so look out for it.
Be alert to other electrical hazards on your molding machine and around you in the shop floor. Make it a habit to watch out for electrical boxes with covers removed, open control panel doors, broken or damaged wiring conduit, frayed or spliced electrical cords. Report them accordingly
Caution: Scrap Grinders
Grinders can throw pieces out from the feeding chute back in your face, so be careful if they operate near you. If you have to clean the grinder, turn it off and disconnect the power completely. Do not just shut off the switch. Better still, pull out the power plug and hang it up where you can keep an eye on it. Avoid someone coming along and turned the machine back on while you have your hands and fingers inside the grinder.