Moulding - Injection Moulding Defects

16.0  Injection Moulding Defects

16.1    Introduction:

Injection moulding defects can occur with three factors. Via moulding machine, mould and the material. A good care to be taken in all these three elements to produce non-defective parts. The following are the normal defects appeared on the parts

16.2      Main factors

16.2.1 Moulding Machine

In this moulding machine, the temperature pressure injection speed and injection time is very important

16.2.2  Mould

A proper design of the mould is a must to avoid defects

16.2.3 Material

            Proper material utilization and process to be checked before loading into the    machine

16.2.4 Management

            Management of all process material and the machine is also an important factor for achieving effective component

16.3    Types of defects:

1. SINK MARK

Sink marks are the localized contractions or the depressions on the surface of the moldings

                                                                

                          

 

Sink marks occur during the cooling process if certain areas of the part are not cooled sufficiently causing them to contract

If these contractions are not compensated and the outside wall is not stable enough due to insufficient cooling, depressions occur

Causal factors of sink marks

1. Insufficient plastic in the mould to compensate for shrinkage due to:

•         Thick sections, bosses, ribs

•         Injection pressure too low

•         Unbalanced gate

•         Injection speed too slow

2. Plastic too hot

3. Article ejected too hot due to insufficient cooling

4. Gate located at thin section

Possible solutions for sink marks

• Relocate gates on or as near as possible to thick sections
• Increase cooling time
• If possible change the mold design to maintain an even wall thickness throughout the part
• Increase injection speed & mold temperature. Do this if the sink marks are away from the gate or in thin walled areas
• Dry the material

2. STREAKS

Streaks are the prominent stretchy marks on the surface of the moldings. There are several streaks available depending on the material and finish

• Charred streaks
• Moisture streaks
• Air streaks
• Colored streaks
• Glass fiber streaks

 

         

Charred streaks due to excessive residence time in the plasticizing cylinder

Moisture streaks due to excessive moisture content of the granules

Colour streaks in the sub-manifold of the cold runner. Colour streaks is also due to an incompatible master batch

Causal factors for streaks

• Temperature too high or residence time too long during pre drying
• Melt temperature too high
• Residence time in plasticizing unit too long
• Insufficient pre-drying of the granules
• Wrong storage of the material
• Temperature at flange too low
• Poor venting

Possible solutions for streaks

• If melt temperature is above processing range reduce cylinder temperature, Screw speed, dynamic pressure
• If the residence time is in critical range reduce the cycle time.
• If  burnt marks appear near gate decrease injection rate avoid sharp corners, edges in gate
• Material to be sufficiently pre dried
• Check material storage and  its packaging
• Reduce residence time in material hopper
• Reduce injection rate
• Round sharp edges of transitions.
• Reduce depth of engraving
• Provide for venting
• Use smaller colour pigments
• Check solubility of the used die
• Use shorter glass fiber

3. BLISTERS/BUBBLES                 

 (hollows created on or in the molded part)

                                     

                          

Air bubbles (solidified while flowing around)               Voids in a thick walled molded part.

Causal factors for blisters

• Decompression is excessively high or fast
• Insufficient dynamic pressure
• Venting problems inside the mould
• Regrind use

Possible solutions for blisters

·         Decrease melt temperature

·         Decrease screw speed

·         Dry the material

·         Increase back pressure

·         Increase mold temperature

·         Ensure regrind is not too coarse

·         Provide additional mold vents

·         Relocate gate

4. WELD LINE

(Witness marks of two or more melt flow fronts at their joining zone)

Weld Lines are created when two or more melt flow fronts  meet possibly causing a cosmetically visible line.  It can also create a weakened area in the finished molded part especially with filled resins 

                                 

Causal Factors of Weld Line

• Low melt temperature
• Injection pressure too low
• Excessive mould lubricant on the mould
• Improper venting in the mould
• Low mould temperature
• Injection speed very slow
• Shift in mould resulting in one wall being too thin
• Poor material flow properties

Possible Solutions for Weld Line

• Increase injection pressure
• Increase injection speed
• Increase injection hold
• Increase melt & mold temperature
• Make sure part contains no sharp variation in cross-sections
• Vent cavity in the weld area
• Move gate (Move weld line to invisible area)
• Add over flows

5. GLOSS/GLOSS DIFFERENCES

The gloss of a mould is the reflecting property of its surface when exposed to incident  light. When a ray of light hits the surface its direction will change(Refraction).The smoother the surface of molding the smaller the scattering angle of reflecting ray and rougher the surface the larger the angle. 

Causal Factors for Gloss Differences

• Different cooling conditions
• Shrinkage differences
• Stretching of already cooled areas e.g. Due to warpage
• Molds that are textured or resins that are filled
• Poor polished mold surfaces

Possible Solutions for Gloss Differences

• Clean mold surface
• If the part design allows increase the polish of the molding surface
• Increase melt temperature
• Make sure venting is adequate

6. JETTING

(The prominent inhomogeneous snake like strands on the surface of molding)

Jetting originating at the gate, spreading over entire part.

                           

                     

Causal Factors For Gloss Differences

• When cross section of the molded part is increased rapidly in conjunction with high injection speeds
• Wrong position of the mold where melt fills the cavity from top to bottom

Possible Solutions For Jetting

• Decrease injection speed
• Change the melt temperature, up or down
• Increase the gate diameter
• Move the gate so that when the plastic first enters   the cavity it hits an obstruction such as a rib or wall

7. RECORD GROOVES EFFECT

Very fine grooves on the surface of the moulded part

Causal Factors For Gloss Differences

• Insufficient mould & melt temperatures
• Injection speed

Possible Solutions For Record Grooves Effect

• Increase mould temperature
• Increase melt temperature.
• Avoid small runners
• Increase injection rate
• Increase maximum injection pressure

8. DULL SPOTS IN GATE AREA

Dull 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 widely spread reflections in this area

Possible Solutions for Dull Spots in Gate Area

• Reduce 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.

Causal Factors for Short Shots

• Injected compound volume is too small (shot volume)
• Poor venting
• Injection pressure or rate is insufficient
• Wrong temperature control
• Thin 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

Possible Solutions For Short Shots

• 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 mould is vented correctly and vents are clear.
• Confirm that the non-return valve used is not leaking excessively
• Change part design. Thin areas of the mould 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

10. Diesel effect

Diesel effect due to entrapped air at the end of flow path & poor ventilation near rib.

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.

Causes of 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 Solutions For Diesel Effect

• Alter 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. FLASHES

Flashes 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.

Causal Factors For Flashes

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

Possible Solutions For Flashes

• 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.

12. STRESS WHITENING

Cracking or Crazing is caused by high internal moulded 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. 

Possible Solutions for Stress Whitening

• 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

13. EJECTOR MARKS

Ejector marks are the depressions or the elevations causing abrupt changes in the wall thickness. These may also cause punctures of the surface of part.

Causes For Ejector Marks

• Wrong dimensioning or the design of mould
• High temperature differences within the mould or between mould wall and ejector
• Premature demoulding or high demoulding forces
• Wrong fittings or ejector lengths

Possible Solutions For Ejector Marks

• Check counter bores and contact surfaces of the ejector heads
• Refit ejector
• Change ejector size accordingly
• Increase cooling time
• Change ejector system
• Check draft on core
• Reduce demoulding force if it is high, reduce holding pressure

14. DEFORMATION DURING DEMOULDING

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

Possible Solutions for Deformation during Demoulding

• Reduce holding pressure
• Reduce holding pressure time
• Increase cooling time
• Check ejector dimensions
• Use antifriction layer to reduce demoulding forces
• Adopt direction of polishing to direction of demoulding
• Use mould release agent

15. EJECTION GROOVES

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

Causes For Ejection Grooves

These may appear due to

• Less draft.
• More surface roughness.
• Mold deformation due to excessive cavity pressure.
• Mold is not stiff.

Possible Solutions For Ejection Grooves

• Increase cooling time
• Increase wall thickness
• Increase drafts
• Use lower surface roughness
• Increase mould stiffness

16. FLAKING OF SURFACE LAYER

Delamination occurs when single surface layers start flaking off the moulded part due to insufficient bondage between adjacent layers.

              

Possible Solutions For Flaking Of Surface Layer

• 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

17. COLD SLUG

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

Causes for Cold Slug

The causes may be

• Tip of the nozzle too cold
• Nozzle too long or unsuitable heater band 

Possible Solutions

• Check nozzle design
• Check nozzle temperature regulation system
• Increase nozzle temperature
• Move injection unit backwards

18. FILAMENTATION

Due 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.        

Possible Solutions For Filamentation

• 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 time

19. DARK SPOTS

Dark 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. 

Causes for Dark Spots

Tool Wear, thermal damage, and dirt are the causes for the dark spots.

Possible Solutions

• Check 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 OUT

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

Causes for Plate Out

• Wrong or excessive use of lubricants
• In compatibility of colorant ,lubricant and base material.
• Drying times too long.
• High material stress, such as excessive melt  temperatures, shear stress, shear rates.

Possible Solutions For Plate Out

• Use suitable lubricant
• Check metering of lubricant.
• Use antifriction layers for moving mould elements.
• Check compatibility of base material with additives/colorants/lubricant.
• Check mixing ratio.
• Reduce temperature
• Ensure sufficient venting.
• Eliminate moisture

21. DEFECTS ON ELECTROPLATED PARTS

Defects on electroplated parts (pimples, blisters, insufficient 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.

Possible Solutions Defects on Electroplated Parts

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

22. BROKEN/ RAGGED FILM HINGE

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.

        

Possible Solutions For Broken/ Ragged Film Hinge

If the film not filled

• Increase melt temperature.
• Increase injection rate.
• Increase mold wall temperature.
• Increase wall thickness of film hinge.
• Use easier flow material
• Move position of gating away from hinge.

If too much force required to operate hinge.

• Reduce mold wall thickness
• Use 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 viscosity
• Ensure parallel flow of flow front

23. WARPAGE

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 drawing

  

Causal Factors for 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.

Possible Solutions for Warpage

• 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.

24. FISH EYES

Fish eyes are a surface defect that results from un melted material being pushed with the melt stream into the cavity and appearing on the surface of a molded part.

Remedies for Fish eyes

• Reduce regrind material
  Contact material suppliers to get the recommended levels of regrind to use.
• Optimize melt temperature
• Modify screw design
  Contact material suppliers to get the right screw design information to avoid improper melt mix or overheating that leads to material degradation.

25. HESITATION

Hesitation is when flow slows down or stops along a particular flow path.

If plastic filling a cavity has the option of filling either a thin section or a thick section, the plastic will tend to fill the thick section first as this route offers less resistance to flow.

Hesitation can occur in ribs and in thin section of parts that have significant changes in wall thickness.

Remedies for Hesitation

• Move the polymer injection location away from the area of hesitation so that the bulk of the cavity fills before the melt reaches the thin area.
• Move the polymer injection location to a place that will cause greater pressure to be applied where the hesitation occurred.
• It is useful to have thin ribs/bosses as the last point to fill
• Increase the wall thickness where the hesitation occurred, to reduce the resistance to flow.
• Use a less viscous material
• Inject more quickly
• Increase the melt temp

26. OVERPACKING

Over packing is when extra material is compressed in one flow path while other flow paths are still filling.

Remedies for Over packing

• Thicken or thin parts of the model to act as flow leaders or deflectors.
• Move the injection location to a position that will define similar length flow paths.
• Divide the cavity into imaginary sections, and use one injection location for each section.
• Remove unnecessary gates.

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