Injection Molded partsDefects & RemediesPart II

8.DULL SPOTS IN GATE AREADull spots in the gate area are primarily caused by Small gates High injection rate.

High injection rates and small gate cross sections and bye passes create strong orientations of the molecules during injections. Since there is not enough time for the relaxation directly behind the gate the peripheral layers are frozen while they are still strongly oriented. Such peripheral layers can be stressed with a very minimal degree and crack under the slightest impact of high shear stress. The hot melt inside flows towards the wall and forms extremely fine notches.Dullness is due to widly spread reflections in this areaPossible SolutionsReduce injection rate.Increase gate diameter.Round of transition from gate to cavity.Move the position of gate to unimportant area.

9.Incompletely filled part (Short shots)Failure to fill the mould or cavities of the mould completely is termed as a short shot.

Injected compound volume is too small (shot volume)Poor venting. Injection pressure or rate is insufficient.Wrong temperature controlThin cross sections. Where in which the melt freezes pre maturely due to unfavorable flow conditions.improper balance of plastic flow in multiple cavity moulds.Insufficient gate opening

Causal Factors

Increase injection pressure,speed. Increase melt and mold temperature. Increase nozzle temperature. Ensure that the manifold and nozzles have reached the set temperature.

Increase shot size and confirm cushion. Make sure mold is vented correctly and vents are clear. Confirm that the non-return valve used is not leaking excessively. 7.Change part design. Thin areas of the mold may not fill completely, especially if there is a thick to thin transition, or there is a long rib that cannot be vented very well. If the part design allows it, change in these areas can improve the situation.

Possible Solutions

Burn Marks or Dieseling show up on the finish molded parts as charred or dark plastic caused by trapped gas and is usually accompanied by a distinctive burnt smell.10.DIESEL EFFECT

This is purely a venting problem, the trapped gas is compressed and gets heated to a high degree causing the burn on the plastic. It can occur at blind holes,fillets,narrow ribs,near places where several flow fronts meet, end of flow paths etc.Possible SolutionsAlter gate position and/or increase gate size. Decrease injection pressure. Decrease injection speed. Decrease melt and/or mold temperature. Improve mold cavity venting. Vents may become smaller over time due to wear and they will need to be brought back to their original depth.

11.FLASHESFlashes are often seen near sealing faces, out of vent grooves, or down ejector pins. It appears as thin or sometimes thick sections of plastic where it would not be on a normal part.

Permissible gap widths exceeded,due to insufficient mold tightness, exceeding production tolerances,damged sealing faces.Clamping force too high or too low.Internal mould pressure too high.Insufficient viscosity of melt.

Causal Factors

Increase clamp pressure. Check mold venting. Vents may have been ground too deep for the material being used. Check sealing surfaces to ensure that they seal off properly by "blueing" them in under clamp tonnage. Check ejector pin bore diameter to pin diameter tolerances. The tolerances may be too large allowing plastic to flash down the opening. The tolerances may be too large for the material being used and can occur due to wear over time.

Possible Solutions

12.STRESS WHITENINGCracking or Crazing is caused by high internal molded in stress or by an external force imposed upon the part. They can also be caused by an incompatible external chemical being applied to the finished parts The cracks often don't appear until days or weeks after the parts have been molded.

Decrease injection pressure. Dry material. Increase cylinder temperature. Increase mold temperature. Increase nozzle temperature. Modify injection speed. If the material is partially crystalline then it may help to reduce the mold and/or melt temperature. If the material is amorphous then it may help to increase the mold and/or melt temperature. Possible Solutions

13.EJECTOR MARKSEjector marks are the depressions or the elevations causing abrupt changes in the wall thickness these may also cause punctures of the surface of part.

Wrong dimensioning or the design of mold.High temperature differences within the mould or between mold wall and ejector.Premature demoulding or high demoulding forces.Wrong fittings or ejector lengths.Possible SolutionsCheck counter bores and contact surfaces of the ejector heads.Refit ejectorChange ejector size accordingly.Increase cooling timeChange ejector system.Check draft on core.Reduce demoulding force if it is high, reduce holding pressure.

14.DEFORMATION DURING DEMOULDINGA residual pressure builds up inside the moulded part which is suddenly released during demoulding.It may be impossible to demould the part.

Reduce holding pressureReduce holding pressure time.Increase cooling time.Check ejector dimensions.Use antifriction layer to reduce demoulding forces.Adopt direction of polishing to direction of demolding.Use mould release agent.Possible Solutions

15.EJECTION GROOVESEjection grooves are produced during demoulding and constitute damage to the surface of the molded part.They mainly occur on the structured surfaces.

These may appear due toLess draft.More surface roughness.Mold deformation due to excessive cavity pressure.Mold is not stiff.Increase cooling timeIncrease wall thickness.Increase drafts.Use lower surface roughness.Increase mould stiffness.Possible Solutions

16.FLAKING OF SURFACE LAYERDelamination occurs when single surface layers start flaking off the molded part due to insufficient bondage between adjacent layers.

Check for material contamination. Incompatible resins or colorants may have been accidentally mixed causing this condition to be seen. Dry material. Increase melt temperature. Increase mold temperature. Insufficient Blending. Check melt homogeneity and plasticizing performance Possible Solutions

17.COLD SLUGCold slugs are formed when melt solidifies in the gate or in the nozzle before the cavity is filled.

The causes may beTip of the nozzle too coldNozzle too long or unsuitable heater bandPossible SolutionsCheck nozzle designCheck nozzle temperature regulation systemIncrease nozzle temperatureMove injection unit backwards

18.FILAMENTATIONDue to insufficient cooling of the gate or sprue area, the part does not break cleanly from the gate area. The shape may range from a short pointed cone to a filament of several centimeters. The premature retracting can crack the layer which is still in fluid state in to a form of filament.

Provide sufficient cooling time during the cycle. Excessive heat in the gate area. Check thermocouple in the nozzle or decrease the temperature of the hot runner manifold and nozzle. Increase cooling at the gate area. Ensure that you have controllable turbulent flow in the gate area. Reduce melt temperature.Increase holding pressure timePossible Solutions

19.DARK SPOTSDark spots are similar to burn marks or brown streaks but generally not as dark or severe. It may cause the part to be a darker shade than the virgin pellets and is often found nearest the gate area, however it can also appear as dark streaks throughout the part.

Tool Wear, thermal damage, dirtCausesPossible SolutionsCheck hopper and feed zone for contamination. Decrease back pressure. Decrease melt temperature. Decrease nozzle temperature. Move mold to smaller shot-size press. Provide additional vents in mold. Purge heating cylinder. Shorten overall cycle.

20.PLATE OUTPlate out is a surface defect on the mold surface e.g on inserts which eventually causes production of defective parts It usually occurs in materials such as POM,PP,PETABS,PC,PSU,PE . Materials with additives such as flame retardants, lubricants and colorants are also subject to plate out

Wrong or excessive use of lubricantsIn compatibility of colorant ,lubricant and base material.Drying times too long.High material stress, such as excessive melt temperatures, shear stress, shear rates.CausesUse suitable lubricantCheck metering of lubricant.Use antifriction layers for moving mould elements.Check compatibility of base material with additives/colorants/lubricant.Check mixing ratio.Reduce temperatureEnsure sufficient venting.Eliminate moisture

Possible Solutions

Defects on electroplated parts (pimples,blisters,insufficiant layer adherence)Almost all defects on injection molded parts are also visible on electroplated parts.These defects look more prominent than on part without plating.21.DEFECTS ON ELECTROPLATED PARTS

Reduce injection rateIncrease mould wall temperature when using amorphous materials.Increase melt temperature.Avoid wall thickness variations.Avoid deformation during transportationsAvoid internal stresses.Do not use releasing agent.

Possible Solutions

Film hinges are moving and permanent joints. Their function is based on the elastic properties of the material.The failure of film hinges is mainly caused by overly stressed plastic. The film hinge may break partially or completely. Over stressing can also result in whitening.22.BROKEN/ RAGGED FILM HINGE

If the film not filledIncrease melt temperature.Increase injection rate.Increase mold wall temperature.Increase wall thickness of film hinge.Use easier flow materialMove position of gating away from hinge. If too much force required to operate hinge.Reduce mold wall thicknessUse material of low modulus of elasticity.Check hinge design If breaks immediately or after few uses.Remove weld line from film hinge area.Reduce mold wall temperature.Use material of higher viscosityEnsure parallel flow of flow front Possible Solutions

Warpage is the deviation of the mould part from its required shape. Warping, Part Distortion is shown up as parts being bowed, warped, bent or twisted beyond the normal specification outlined on the drawing23.WARPAGE

Article ejected too hot.Variation in section thickness or contours of the screw.Excessive area discharged or packed into the area around the gate.Non-uniform mould temperature due to improper positioning if the cooling channels in the mould.Excessive feed.Injection pressure too high.Insufficient cooling time.Poorly designed or operated cooling system.Unbalanced gates on articles with more than one gate.Holding time is more.Causal Factors

Possible Solutions

Adjust melt Temperature (increase to relieve molded-in stress, decrease to avoid over packing). stress, decrease to avoid over packing). stress, decrease to avoid over packing). Check gates for proper location and adequate size. Check mold knockout mechanism for proper design and operation. Equalize/balance mold temperature of both halves. Increase injection-hold. Increase mold cooling time. Relocate gates on or as near as possible to thick sections. Try increasing or decreasing injection pressure.

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