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INJECTION MOULDING - TROUBLE SHOOTING

The common faults that happen during injection moulding are the following. Click on them to view the probable causes and suggested remedy.

 

 
Fault Probable Causes Suggested Remedy
Silver streaking
  • Mixture of coarse and fine granules
  • Non uniform plastic temperature
  • Exceeding plasticizing capacity
  • Air tapped between the granules at the hopper end
  • Intermitent flow in the cavity · Injection speed too fast
  • Mould temperature too low
  • Injection pressure too high
  • Moisture and volatiles in the material
  • Material too hot
  • Contamination of foreign material
  • Gates and runners too small
  • Excessive condensate / lubricants on the mould material
  • Adjust injection rate
  • Use uniform granulation
  • Reduce temperature of nozzle
  • Preheat material
  • Reduce rear cylinder temperature and use uniform granulation
  • Balance gate position , relocate gates,
  • Vent Mould, reduce injection pressure, reduce cylinder temperature
  • Increase mould temperature
  • Reduce injection pressure
  • Preheat the material adequately, use a hopper dryer
  • Decrease the mould cycle time, decrease the cylinder temperature
  • Check the moulding powder for contamination ,check the feed system, purge the cylinder ,Check the regrind content and quality.
  • Increase the size of the runner and the gates.
  • Wipe the mould , use lesser quantity of lubricants, change the lubricant and try
  • Increase or decrease the injection rate.
Black streaks
  • Frictional heat caused by movement of cold granules past one another
  • Air trapping in the mould
  • Plunger off-center causing friction burning of material
  • Contamination
  • Use external indication, increase rear cylinder temperature , vent mould properly, relocate gates.
  • Reduce injection pressure.
  • Relocate plunger
  • Purge the cylinder
Short shots
  • Cold material
  • Cold mould
  • Insufficient pressure
  • Insufficient feed
  • Poor venting
  • Insufficient plunger forward time
  • Insufficient injection speed
  • Improper balance of plastic flow in multiple cavity moulds.
  • Insufficient feeding
  • Melt flow restricted
  • Back pressure in cavity
  • Unbalanced runner design
  • Cold slug clogging gates, runners and sprue.
  • Un uniform mould temperature· Un uniform melt cushion
  • Machine capacity not enough
  • Increase plastic temperature
  • Increase mould temperature
  • Increase pressure and temperature
  • Increase feed
  • Increase number and size of vents
  • Increase plunger forward time.
  • Increase injection speed
  • Correct unbalanced coixlition
  • Check the material in the hopper, check the feed throat for resin agglomerates, machine capacity not enough
  • Increase the gate, runner, and sprue size, use the nozzle with a higher diameter hole, try with a higher MFI material, check the nozzle for blocks.
  • Provide adequate venting, reduce the clamping force.
  • Balance the runner design , cavity location
  • Increase the size of cold slug well, provide cold well for sub runners.
  • Rearrange the cooling line of mould to provide a uniform cooling
  • Maintain a uniform melt cushion, check the back flow valve and, repair if needed
  • Use a bigger machine.
Flash
  • Material too hot
  • Excessive feed
  • Erratic feed
  • Poor parting line or matting surface
  • Erratic cycle time
  • Injection pressure is too high
  • Insufficient clamping force
  • Foreign matter or flash acting as high spot on mould land.
  • Mould platens are warped , Clearance in vents, knock-outs, etc. are too great
  • Insufficient venting causing material from cavity to flow out
  • Injection pressure unevenly distributed in the mould
  • Reduce material temperature, increase cycle time, decrease mould temperature
  • Reduce feed
  • Check the feeding system.
  • Reface the parting time , matting surface
  • Maintain uniform cycle time.
  • Decrease the injection pressure, decrease the hold on pressure, decrease the injection hold time,
  • Use wherever necessary bigger machine if available, reduces number of cavities; adjust the mechanism to increase the clamping force.
  • Inspect mould land and clean carefully.
  • Check the clearance and adjust
  • Inspect vents and clean in case of blockage, enlarge vents, if needed.
  • Cavity and runner layout should be balanced
Sprue sticking
  • Excessive pressure
  • Nozzle not aligned with sprue bush
  • Sprue puller too small
  • Material too cold at nozzle and sprue unable to breakaway.
  • Nozzle orifice greater than sprue bush orifice.
  • Material too hot, Sprue breaking at component end.
  • Sprue held up in sprue bush due to inadequate taper
  • Hot material ,
  • Excessive material
  • Insufficient draft · Improper fit between sprue bushing and nozzle.
  • Under cuts or rough surface.
  • Long plunger dwell
  • Reduce pressure
  • Realign the nozzle with a sprue bush.
  • Increase the undercut, Increase the size of sprue puller.
  • Increase the nozzle temperature until the sprue breaks from the nozzle.
  • Change the nozzle with a lower Dia orifice or increase the dia of sprue bush orifice
  • Decrease the cylinder temperature, increase the molding cycle time, and increase the cooling time in the mould.
  • Give adequate taper in the sprue bush, use mould release agents, remove burrs on the sprue.
  • Decrease material temperature.
  • Decrease size of the sprue extended nozzle and short sprue bushing
  • Increase draft angle.
  • Hole size in nozzle should be smaller than in the sprue bushing.
  • Eliminate undercuts and polish the surface.
  • Reduce plunger forward time
Sink marks
  • Not enough plastic in the mould to allow for shrinkage due to thick sections, bosses, ribs etc.
  • Not enough feed
  • Injection pressure too low
  • Plunger forward time to short
  • Unbalanced gates
  • Injection speed too slow
  • Plastic too hot· Variation in mould open time
  • Excessive cushion in front of the plunger
  • Gate location, it's size and design· Insufficient packing.
  • Part sections uneven
  • Product too hot when ejected.
  • Restriction in flow
  • Local hot spot in the moulds.
  • Redesign and increase injection pressure
  • Increase feed
  • Increase injection pressure.
  • Increase plunger forward time.
  • Restrict flow through gates near sprue.
  • Raise mould and cylinder temperature
  • Reduce cylinder and mould temperatures, decrease cycle time.
  • Use of timers for mould open time
  • Reduce feed
  • Locate the gate at the thick section of the part, increase the gate size and reduce gate land
  • Increase the injection pressure.Increase the hold pressure time, check back the flow valve for leakage.
  • Redesign the part with an even wall thickness
  • Decrease the cylinder temperature, increase the cooling time in mould, reduce the mould temperature.
  • Increase the gate, runner and sprue size, provide a proper venting in the mould, balance flow and filling in cavity.
  • Redesign the cooling pattern in the mould
Part sticking to the mould.
  • Injection pressure or cylinder temperature too high
  • Excessive feed
  • Plunger forward time too long
  • Undercuts in mould
  • Variation of mould open time
  • Improper finish
  • Rough surface of the mould
  • Inadequate release taper in mould cavity.
  • Ejection system not designed properly
  • Undercuts in the mould cavity.
  • Misalignment of core and cavity
  • Mould made of soft porous steel.
  • Overpacking of the plastic material.
  • Uneven pressure in cavities.
  • Parts sticking to injection side.
  • Mould deposit causing sticking.
  • Reduce pressure or temperature accordingly
  • Reduce feed
  • Reduce plunger forward time.
  • Eliminate undercuts, polish and provide sufficient draft
  • Maintain constant mould open time.
  • Polish rough cavity surface to correct finish, adjust mould temperature to obtain proper shrinkage
  • Polish the mould well
  • Provide a release taper
  • Check the ejector system , provide extra ejector pins
  • Eliminate undercuts with a proper design, polish and provide sufficient draft.
  • Check the alignment of the core and cavity.
  • Replace the mould with a hardened metal.
  • Reduce the injection pressure, reduce the feed material, reduce the screw forward time.
  • Maintain a constant feed cushion, balance the gates and runner for uniform filling.
  • Use a mold release spray, adjust the mould temperature to retain the components on the ejection side. use an undercut runner to keep the parts on the ejection side
  • Check and clean the mould deposits.
Flow lines and folds
  • Cold material
  • Cold mould
  • Gates to small
  • Non uniform section thickness.
  • Increase plastic temperature
  • Increase mould temperature
  • Enlarge gates.
  • Eliminate heavy bosses and ribs.
Poor surface finish
  • Cold material
  • Cold mould
  • Slow injection
  • Poor flow within mould cavity
  • Poor mould cavity finish
  • Contamination of material
  • Material wet
  • Mould surface too cold
  • Poor mould surface
  • Inadequate injection pressure.
  • Insufficient hold pressure time.
  • Increase plastic temperature
  • Increase mould temperature
  • Increase injection speed.
  • Redesign parts.
  • Polish mould cavity surface to correct finish
  • Check the feed material for contamination, purge the injection cylinder, clean the feeding system, check the quality of regrind added
  • Preheat the material, use hopper dryer
  • Increase the mould surface temperature
  • Polish the mould
  • Increase the injection pressure
  • Increase the hold pressure time
Warpage
  • Excessive orientation in the material which manifests itself in differential shrinkage in the direction of flow and vertical to it. Orientation increases as material becomes too viscous, the wall becomes thinner, runner becomes narrower and longer
  • Temperature distribution in cylinder is uneven
  • As a result of over - loading of moulding due to excessive or too long follow up pressure
  • Components ejected too hot
  • Port design with uneven thickness
  • Mould coating uneven
  • Distortion due to internal strain
  • Ejection knockout pins working unevenly.
  • Gate location not prope
  • Distortion of heavy section only.
  • Avoid orientation
  • Watch cylinder design
  • If possible use follow -up pressure lower than injection pressure, shorten follow-up pressure.
  • Reduce the cylinder temperature, increase the mould cooling time, place in the jig after moulding, and increase the mould temperature.
  • Design the part with an even wall thickness and strengthen it with ribs.
  • Maintain a uniform temperature in the mould, use a cold mould.
  • Increase / decrease the injection pressure, increase the mould cooling time, increase the overall moulding cycle.
  • Check the knock out pin levels and set evenly, increase the knock out area.
  • Gate location should be in such a way that the material flows from the thick to thin section, gate runner system design for balanced filling.
  • Increase the mould cooling time, maintain uniform melt temperature, slowly cool the part in warm water, place in a shrink jig after ejection.
Fracture cracks
  • Internal stress due to metal inserts with coefficient of thermal expansion different from the plastic material.
  • Internal stress due to improper mould design
  • Embrittement due to thermal degradation
  • Parts ejected too fast
  • Ejector system defective.
  • Internal stress due to cold metal insert.
  • Use higher molecular weight materials, increase wall thickness, and anneal the molded parts.
  • Check the mould design and change if necessary.
  • Reduce the cylinder temperature, use faster molding cycle.
  • Give more cooling time in the mould.
  • Check the ejection system for a smooth operation.
  • Warm the insert before use, thicken the section in the area of crack, use toughened materials.
Brittleness of component
  • Material is too hot
  • Material too cold
  • Mould surface too cold
  • Reground material in excess quantity
  • Bad quality regrind
  • Rate of injection too slow
  • Original part design too thin
  • Product not conditioned
  • Reduce the cylinder temperature, shorten the over all cycle time, machine is too big, change to a smaller machine.
  • Increase the cylinder temperature, increase the nozzle temperature, decrease the screw speed,
  • Increase the mould temperature, control the flow of coolant to the mold
  • Avoid using regrind, use less percentage of regrind
  • Check regrind for contamination, moisture degraded polymer, check the regrind melt index.
  • Increase the injection rate, maintain the minimum feed cushion.
  • Redesign the part to strengthen the weak areas
  • Condition the part as per the raw material manufacturer's recommendation.
Bubbles and voids
  • Internal shrinkage after the outer surface sets.
  • Material too hot
  • Insufficient venting in the mould resulting in entrapment of air
  • Condensation on cavity surface
  • Moisture in material
  • Injection rate to fast
  • Restriction to flow of material
  • Increase the injection pressure, rate, time, Increase the size of the gates, runners, and sprue, increase the screw forward time.
  • Decrease the cylinder temperature, reduce the moulding cycle time.
  • Provide vents at the point of final fill, decrease the injection speed, decrease the mould clamping force, increase the gate size, change location.
  • Reduce the flow of coolant through the mould, wipe the cavity surface with a solvent
  • Preheat the material thoroughly, use hopper dryer
  • Decrease the injection rate.
  • Eliminate restrictions, increase the gate and runner size.
Discoloration
  • Contamination of material
  • Wet material
  • Degradation of material, colorant, additives
  • Degradation of material due to gas entrapment
  • Too much stress on component
  • Charred material in cylinder or nozzle coming out with material.
  • Check the moulding material, purge the heating cylinder, check the feed system and hopper
  • Preheat the material, use a hopper dryer
  • Decrease the overall moulding cycle, use small machine, check the residence time of the material, reduce the screw speed, use colorants and additives which will withstand processing temperature.
  • Provide proper ventings, relocate the gates, and decrease the mould clamping force.
  • Check the ejection system
  • Purge the cylinder completely, remove and clean the nozzle.
Nozzle droofing and foaming
  • Nozzle temperature very high
  • Material too hot
  • Nozzle orifice too large.
  • Moisture in the material.
  • Decrease the nozzle temperature,
  • Decrease the cylinder temperature, decrease the cylinder moulding time
  • Use a nozzle with a small orifice, use the reverse taper nozzle, use a nozzle with shut off valve.
  • Preheat the material, use hopper dryer
Scorching
  • Material over heated
  • Thermocouple burnt out
  • Temperature controller malfunctioning
  • Regrind not of good quality.
  • Improper shutdown procedures.
  • Moisture in material
  • Gas entrapment
  • Rate of injection too fast
  • Constant degrading
  • Reduce the cylinder temperature, decrease the moulding cycle time, and remove dead spots in the cylinder, screw, and nozzle assembly.
  • Check all thermocouples and change, if necessary.
  • Check for the sticking of relays, check for the controllers connected to the wrong heaters, check if the relays are functioning properly.
  • Segregate and check regrind critically for contamination, excess moisture or degraded polymer.
  • Purge the machine thoroughly, clean the nozzle and start moulding afresh
  • Preheat the material , use a hopper dryer
  • Vent the cavities properly, relocate the gate position,
  • Decrease the injection speed, decrease the injection pressure, increase the size of gates.
  • Use a colorant which can withstand the processing temperature,
Weld lines.
  • Material too cold
  • Mould too cold
  • Injection pressure too low
  • Gas entrapment
  • Part too thin at weld
  • Capacity of the machine lower
  • Contamination of material
  • Poor flow of melt
  • Increase the cylinder temperature, increase the overall moulding cycle
  • Increase the mould temperature, throttle the flow of coolant in the mould
  • Increase the injection pressure, increase the screw forward time, increase the rate of injection, increase the size of gates, runners and sprue, use multiple gates and runners.
  • Provide vent in cavities, relocate the gate, decrease the mould clamping force.
  • Thicken the part at the weld line
  • Change to a bigger machine.
  • Check the material, check the feeding system.
  • Use a higher MFI material, use an external lubricant
Dimensional variations
  • Inconsistent moulding cycle
  • Feeding system malfunctioning particularly in the plunger type machines.
  • Inconsistent melt cushion while injection.
  • Malfunctioning of non return valve
  • Uneven back pressure adjustment
  • Malfunctioning of temperature control system.
  • Insufficient plasticizing capacity
  • Incorrect runner, gate dimensions
  • Uneven mould filling
  • Distortion during ejection
  • Incorrect mould dimensions
  • Un uniform mould temperature
  • Packing of cavity not enough.
  • Maintain uniform moulding cycle
  • Check the feed system and adjust for uniform feed.
  • Maintain a uniform melt cushion, check the non-return valve for wear and tear.
  • Check and replace the valve if necessary
  • Arrange for a uniform backpressure.
  • Check the heater bands, check the thermocouples.
  • Use bigger machine
  • Check and provide a proper runner gate system.
  • Design the mould for uniform filling of cavities.
  • Check the knock-out pin system and correct it
  • Machine the mould cavity and core for proper dimensions with proper allowances for shrinkage.
  • Maintain a uniform mould temperature
  • Use a higher injection pressure. Increase the screw forward time.
Jetting
  • Melt injected directly into the cavity
  • Melt temperature is not enough
  • Adjustment of the injection rate
  • Gate size to small
  • Direct the material flow to hit the wall and flood the cavity.
  • Increase the cylinder temperature, increase the moulding cycle time.
  • Increase or decrease the injection rate.
  • Increase the gate size and flare the gate
Gate smear
  • Mould temperature not enough
  • Injection rate too fast, gate too restricted.
  • Increase the mould temperature
  • Decrease the injection rate, increase the gate size, flare the gate, change the gate location.

 

 

 

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