Trouble Shooting in Plastic Injection molding

7/30/2019 Trouble Shooting in Plastic Injection molding

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TROUBLE SHOOTING IN

INJECTION MOULDINGPROCESS

Presented

By

N DURAI

CIPET-Chennai



Short shotsA ‘short’ is an incomplete moulding. Moulding with narrow

sections or ribs are vulnerable to short shots. Frequently the moulding is

unfilled at the furthest point from the gate

Possible cause:

Insufficient Material injected into the cavity.

Check:

1. Short size2. Injection speed – may need to be increased

3. Injection time – increase

4. Changeover point from ‘Injection’ to ‘ Holding’ pressure

5. Mould temperature - Increase



FLASH

Check:1. Mould clamping pressure – may be too low.

2. Injection pressures may be too high for mould lock.

3. Change over point from injection to ‘hold-on’ pressure is too late.

4. Switches set to bring on mould lock before injection .

5. Mould - for wear damage or an obstruction.

Flash is a thin film of unwanted material forming outside the

designed cavity. It is frequently found among the split line or around the

undersized ejector pins.

Possible Cause:The mould is forced open due to the injection pressure being too

high for the mould locking force or mould tool may be worn.



SINK MARKS

Check:1. Hold – on pressure – may be too low

2. Hold – on pressure – check levels.

3. Material cushion or (pack) size.

4. Mould temperature – may need to be too low.

5. Gate – may need to be increased.

Depressions on the surface often opposite ribs, bosses or change in

section, usually worse when furthest from gate.

Possible Cause:Material shrinkage or flowing back through the gate.



Burn MarksDescription

Burn Marks show up on the finish molded parts as charred or dark plastic

caused by trapped gas and is usually accompanied by a distinctiveburnt smell.

Note: If this problem is allowed to continue without fixing the root cause

it will very quickly cause damage to the molding surface.





Brittleness Description

Brittleness is a condition where the part cracks or breaks at a

much lower stress level than would normally be expected

based on the virgin material properties.

Possible Solutions

1.Check for material contamination.

2.Decrease amount of regrind use.

3.Decrease back pressure.

4.Decrease injection pressure.

5.Decrease screw speed.6.Increase melt temperature.

7.Dry material. Refer to the drying instructions provided by the

material supplier.



DISTORTIONMoulding is misshapen on ejection.

Possible cause:

Moulding is ejected from the mould before properlysolidified.

Check:

1. Cooling time – may be too short.

2. Mould temperature – may be too high.3. Material temperature – may be too high.

4. Mould ejector system – uneven or forced ejection.



Bubbles

Description Bubbles are similar to blisters in that there is air

entrapped in the molded part.

1. Possible Solutions

2. -Decrease injection speed.

-Decrease injection temperature.

-Dry material further.

-Increase injection pressure.-Increase number and/or size of vents.

-Increase shot size.



JETTING:A worm shaped area opposite and away from a gate

with flow or weld lines at the furthest edges.

POSSIBLE CAUSE:

Material enters the cavity too fast. This produces a ‘worm’ that

travels across the cavity and ‘corrugates’ to produce a back filling, the

material then radiates from the gate but does not fuse with the worm, which

is now too cold

CHECK:

1. Injection Speed – this may be too high

2. The gate – may need to alter or change its position



Cracking, CrazingDescription

Cracking 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.

Possible Solutions -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.



Delamination Description Delamination occurs when single surface layers start flaking off

Possible Solutions

-Adjust injection speed.

-Check for material contamination. Incompatible resins or colorants may

have been accidentally mixed causing this condition to be seen.

-Dry material.

-Increase melt temperature.


-Insufficient Blending. Check melt homogeneity and plasticizing performance.





Flow, Halo, Blush Marks

Description

Flow, Halo, Blush Marks are marks seen on the part due to

flow of the molten plastic across the molding surface.

Possible Solutions

-Decrease injection speed.

-Increase cold slug area in size or number.

-Increase injection pressure.

-Increase melt temperature.


-Increase nozzle temperature.

-Increase size of sprue/runner/gate.



Voids Description

Voids are hollows created in the part. They are normally found in thick

sectioned parts caused by material being pulled away from the hot center section

towards cold mold walls leaving a void in the center .

Possible Solutions -Clean vents.


-Decrease melt temperature.

-Dry material.

-Increase injection pressure.

-Increase injection-hold.


-Increase shot length.

-Increase size of gate.

-Increase size of sprue and/or runners and/or gates.



Warping, Part Distortion

Description

Warping, Part Distortion is shows up as parts being bowed, warped, bent

or twisted beyond the normal specification outlined on the drawing.

Possible Solutions-Adjust melt Temperature (increase to relieve molded-in stress, decrease to avoid

overpacking). 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.



WINDOWS (FISH EYES)

Possible Cause

Unplasticised (Unmelted) particles.

Check:

1. Barrel temperature – may have to increase.

2. Cycle time – may have to increase

3. Screw speed – may be too high

4. Back pressure – may have to increase.

Transparent sections in random places sometimes the shape and

size of granules in the direction of flow. They also occur as natural

particles when a dry coloured moulding is cut.



Sprue StickingDescription

Sprue Sticking generally occurs in a cold runner mold when the sprue is

staying in the mold.

Possible Solutions

-Check mold for undercuts and/or insufficient draft.

-Decrease booster time.

-Decrease injection pressure.


-Decrease injection-hold.-Decrease mold close time.

-Decrease nozzle temperature.

-Increase core temperature.

-Open the gates.

-Ensure that the correct design of nozzle tip for the material is being used.



POOR SURFACE FINISHThis ranges from dull patches on a shiny finish to scratches, pit

marks and a rough finish.

Possible cause:

Implementation on mould surface or inability of melt to replicate

the mould contours.

Check:

1. Tool – polish, damage or rust marks.

2. Injection speed – may be too low

3. Mould temperature – may need to be increased

4. Injection pressure – cavity may be under – pressurised.

5. Melt temperature – may need to increase.



CONTAMINATION

Check:1. Material in hopper for contamination

2. Hopper magnet for ferrous metals.3. Raw materials form manufacturers.

4. Melt temperature – may be high

5. Screw tip, screw and nozzle for evidence of ‘Dead Spots’

due to local overheating.

Foreign material has contaminated the melt, either in the form of

bubbles, crackers, marks, marks or lumps.

Possible cause

If raw material bins or hoppers have been left uncovered it is

possible for foreign particles to enter the melt. This can also cause damage

to the mould and the screw.



Splash marks

Check:1. Material – that drying requirements have been met the moisture

content of material in the hopper.2. Melt temperature may be too high.

3. Decompression setting – may be drawing hot air in from the nozzle.

Sometimes called ‘Scuffing’ or ‘Mica Marks’. These areas or

marks on the surface of the mouldings, like silver streaks, in the

direction of flow.

Possible Cause:

They are usually caused by collapsed bubbles in the melt which have

resulted from unescaped moisture, but can also be caused by overheating the melt.

Most frequently found in hygroscopic materials such as Nylon, ABS, Acrylics, etc.



Flow marksMarks at or near the surface in the form of streaks in the direction

of filling.

Possible cause:

Vague boundary lines between varying viscosities of material.

Check:

1. Injection speed – may need to be increased.

2. Melt temperature – may need to be increased.

3. Mould temperature – may need to be increased.

4. Gate – type and position may need to be changed.

WELD LINE



WELD LINE

Check:

1. Injection speed too low.

2. Mould temperature – may be too low.

3. Mould vents – may be blocked.

4. Hold – on pressure may be too low.

5. Shot volume – may be too small.

6. Barrel temperature – may be too low.

A surface mark, often a line where cold fronts of material meet

eg: After flowing round a core pin, or along thicker sections.

Possible Cause:Lack of fusion between cold fronts of material



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Trouble Shooting in Plastic Injection molding

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