MOULDING DEFECTS AND REMEDIES
1
~ HOW MANY KINDS OF DEFECTS ARE THERE?
2
~ HOW MANY KINDS OF DEFECTS ARE THERE?
31 3
DEFECTS 1. Sink Marks 2. Streaks 3. Blistering 4. Weld Lines 5. Gloss difference 4
DEFECTS 6. Jetting 7. Record grooves effect 8. Dull spots near gate area 9. Incompletely filled part ( Short shots) 10. Diesel effect 5
DEFECTS 11. Over sprayed parts (Flashes) 12. Stress whitening/Cracking 13. Ejector marks 14. Deformation during demolding 15. Ejection Grooves 6
DEFECTS
7
DEFECTS
21. Defects on electroplated parts 22. Broken or ragged film hinge 23. Warpage 24. Fish eye
8
DEFECTS
25. Hesitation 26. Over packing 27. Unbalanced flow
9
28. Nozzle drool 29. Nozzle freeze – off 30 Nozzle spitting 31. Screw slip
10
WHAT ARE THE MAIN FACTORS CAUSING DEFECTS?
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SOME MAIN FACTORS CAUSING DEFECTS
The moulding Machine Mould Operating conditions i.e.. Temperature, pressure, injection speed & time Material. Design of product Management
12
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! "#$%&# Sink marks
Sink marks on a cylindrical core with poor temperature control
17
! "#$%&# Sink marks appear as depressions on the surface of a molded part. These depressions are typically very small; however they are often quite visible, because they reflect light in different directions to the part.
18
CAUSE OF SINK MARKS
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
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Moulded part with sink marks (Gating at the thin wall)
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2. STREAKS Streaks are the prominent stretchy marks on the surface of the moldings. CHARRED STREAKS MOISTURE STREAKS AIR STREAKS COLORED STREAKS GLASS FIBER STREAKS
24
2.CHARRED STREAKS
Charred streaks due to excessive residence time in the plasticising cylinder 25
2.MOISTURE STREAKS:
Streaks due to excessive moisture content of the granules 26
2.AIR STREAKS:
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2.COLOUR STREAKS:
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2.COLOUR STREAKS:
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6. Reduce residence time in material hopper 7. Reduce injection rate 8. Round sharp edges of transitions. 9. Reduce depth of engraving 10. Provide for venting
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11. Use smaller colour pigments 12. Check solubility of the used die 13. Use shorter glass fiber
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3.BLISTERS
Hollows created on or in the molded part
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3.BLISTERS
Air traps occur when converging flow fronts surround and trap a bubble of air. The trapped air can cause Incomplete filling and packing, Surface blemish in the final part. Air trapped in pockets may compress, heat up and cause burn marks.
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3.BLISTERS
(hollows created on or in the molded part) voids
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3.BLISTERS / BUBBLES
Bubbles
3.Bubbles
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3.BLISTERS
DELAMINATION
4.Blisters and delamination
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4.WELD LINE
(Witness marks of two or more melt flow fronts at their joining zone)
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5.GLOSS/GLOSS DIFFERENCES
Very glossy impression due to reflection on the polished surface 49
5.GLOSS/GLOSS DIFFERENCES
Only slightly glossy impression due to reflection on a rough surface 50
5.GLOSS/GLOSS DIFFERENCES
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5.GLOSS/GLOSS DIFFERENCES
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5.GLOSS/GLOSS DIFFERENCES
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6.JETTING
The prominent inhomogeneous snake like strands on the surface of molding JETTING
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6.JETTING Jetting occurs when polymer melt is pushed at a high velocity through restrictive areas, such as the nozzle, runner, or gate, into open, thicker areas, without forming contact with the mold wall. Jetting leads to part weakness, surface blemishes, and a multiplicity of internal defects.
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7.RECORD GROOVES EFFECT
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7.RECORD GROOVES EFFECT
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8.DULL SPOTS IN GATE AREA
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9.INCOMPLETELY FILLED PART (SHORT SHOTS) SHORT SHOT
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9.INCOMPLETELY FILLED PART (SHORT SHOTS) INCOMP0LETELY FILLED
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9.INCOMPLETELY FILLED PART (SHORT SHOTS)
A short shot is the incomplete filling of a mold cavity which results in the production of an incomplete part. The flow freezes off before all of the flow paths have filled.
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10.DIESEL EFFECT / BURNED MARKS
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10.DIESEL EFFECT / BURNED MARKS
BURN MARKS
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10.DIESEL EFFECT / BURNED MARKS
BURNS
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10.DIESEL EFFECT / BURNED MARKS
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11.FLASHES
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11.FLASHES
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11.FLASHES Flashing occurs when a thin layer of material is forced out of the mold cavity at the parting line or ejector pins location. This excess material remains attached to the molded article, and normally has to be manually removed.
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11.FLASHES FLASHES
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12.STRESS WHITENING
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13.EJECTOR MARKS
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14.DEFORMATION DURING DEMOULDING
DEFORMATION
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15.EJECTION GROOVES
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16.FLAKING OF SURFACE LAYER
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17.COLD SLUG
COLD SLUG NEAR THE GATE
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18.FILAMENTATION
Long filamentation of a cold runner gating system near the machine nozzle
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Long filament near a hot runner gate
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19.DARK SPOTS
Severe black specks
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19.DARK SPOTS
Poor screw flights design ,dead spots lead to black specks
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20.PLATE OUT
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PLATE OUT NEAR THE GATE
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21.DEFECTS ON ELECTROPLATED PARTS
PIMPLES AND BLISTERS ON A MOULDED PART
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21.DEFECTS ON ELECTROPLATED PARTS
BLISTERING DUE TO DRAWN –IN AIR (COMPONENT WITH AND WITHOUT ELECTROLITIC LAYER)
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22.BROKEN/ RAGGED FILM HINGE
STRESS WHITENING ON A FILM HINGE
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22.BROKEN/ RAGGED FILM HINGE STREE WHITENING
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23.WARPAGE
MOULDED PART WITH WARPAGE ON THE EDGE
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23.WARPAGE
WARPAGE WARPAGE IN THE FLANGE AREA
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23.WARPAGE
Variation in mould temperature will cause component to warp
133
23.WARPAGE
Warped component 134
23. WARPAGE Distortion of the part caused by non-uniform shrinkage of the plastic.
135
23.WARPAGE
Typical warpage in most radiator end tanks.
136
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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. 137
CAUSAL FACTORS FOR WARPAGE
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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.
141
24. FISH EYES
Remedies for Fish eyes 1.
Reduce regrind material Contact material suppliers to get the recommended levels of regrind to use.
2.
Optimize melt temperature
3.
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. 142
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.
143
25. HESITATION
144
25. HESITATION
Remedies for Hesitation 1. 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. 2. Move the polymer injection location to a place that will cause greater pressure to be applied where the hesitation occurred.
145
25. HESITATION
Remedies for Hesitation 3. It is useful to have thin ribs/bosses as the last point to fill 4. Increase the wall thickness where the hesitation occurred, to reduce the resistance to flow. 5. Use a less viscous material 6. Inject more quickly 7. Increase the melt temp 146
26. OVERPACKING
Over packing is when extra material is compressed in one flow path while other flow paths are still filling.
147
26. OVERPACKING
148
26. OVERPACKING
Remedies for Over packing 1. Thicken or thin parts of the model to act as flow leaders or deflectors. 2. Move the injection location to a position that will define similar length flow paths. 3. Divide the cavity into imaginary sections, and use one injection location for each section. 4. Remove unnecessary gates. 149
27. UNBALANCED FLOW
Unbalanced flow is plastic completely filling some flow paths in the mold before other flow paths have filled.
150
27. UN BALANCED FLOW
151
27. UN BALANCED FLOW Remedies for Unbalanced flow 1.
By altering the thickness of regions within the part, flow can be fastened or delayed in certain directions to help balance flows.
152
28. NOZZLE DROOL
153
28. NOZZLE DROOL
154
29. NOZZLE FREEZE - OFF
155
29. NOZZLE FREEZE - OFF
156
30 . NOZZLE SPITTING
157
31. SCREW SLIP
158
