DEFECT IN PLASTIC INJECTION
MOULDING PROCESS
DEFECT IN PLASTIC INJECTION
MOULDING PROCESS
In plastic injection molding, the molded part
produced are higher potential to have a plastics
defects. These defects may be due to:
 Part design
 Part materials (resin)
 Molds design
 Molds fabrication process
 Molds material
Machine parameters and others
1. BLACK SPOT/DOT
 Generally caused by foreign material in the resin, (look like as a
contamination), or the presence of dead space in the nozzle.
 Sometime it also caused by grease at the side pull or lifter
mechanism.
Countermeasure
1. Clean the screw barrel and nozzle
2. Purging
3. Check the material (plastic resin)
2. BUBLES
 Also known as voids. Generated inside the molded
part, usually due to insufficient mold filling or air trap.
 Example critical products: Lens, light pipe…
Countermeasure
1. Air Vent
2. Sub-Insert
3. Ejector pin
3. BURR/FLASH
 Also known as fin, consisting of thin, flat projection in regions where
polymer melts has flowed into the parting line of the mold.
 Long production run – burr will become flash
Rootcause
1. Parting line no good
2. Low clamping force
3. Injection pressure
too high
Countermeasure
1. Re-fitting
2. Use appropriate
clamping force and
Injection pressure
4. MOULD RELEASE FAULT
 Defect in the molded part due to poor release or improper ejection from
the mold. Usually caused by overfilling of mold with polymer melt and/or
by insufficient draft/taper in the mold.
Defect
1. Pin Mark
2. Drag/Pull Mark
Countermeasure
1. Proper polishing
2. Appropriate cooling time
3. Appropriate draft/taper
Rootcause
1. Pin Mark
 Improper polishing
 Unbalanced ejection
 Cooling time not enough
2. Drag/Pull Mark
 Improper polishing
 Draft/taper not enough
DESIGN FOR UNDERCUTS
(A)
(B)
(A) - Cavity move to top side and slider move to right side
(B) - Angular pin push the part out from core side
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5. SHORT-SHOT FAULT
 Incomplete molded part, with missing portions or members, caused by failure
to obtain complete filling of the flow pathways with the polymer melt.
Rootcause
1. Too thin
2. Injection pressure not enough
3. Air trap
Countermeasure
1. Avoid design too thin
2. Increase injection pressure
3. Sub-Insert
6. SINK MARK
 Sink marks are indentations on product surfaces. It is the most frequent
defect in molding due to volume shrinkage during cooling. This is because
it takes time to cool center portion of thick products so that the skin may
collapse to create sink marks.
Rootcause
1. Part design
2. Injection pressure not enough
3. Holding pressure time not enough
4. Cooling time not enough
7. WELD LINE
 Form at interface of polymer melt flow from two different pathways. Weld line
maybe unacceptable in term of surface appearance or reduced physical
strength.
Rootcause
1. Part design
Countermeasure
1. Air vent
2. Ejector pin
3. Sub-Insert
4. Texturing
5. Increase mould
temperature
8. WARPING
 Warpage is caused by variation in shrinkage throughout the part.
 Three types of shrinkage effects:
 Orientation effects
 Area shrinkage effects
 Differential cooling effects
Rootcause
1. Cooling channel designed
2. Cooling time not enough
3. Part design
4. Imbalanced ejection
Orientation and area
shrinkage effects
Warping caused by
differential cooling
9. SILVER STREAKS
 Gaseous components in the plastic appear at the molding
surface and collapse (also called Moisture Mark)
Rootcause
 Inappropriate drying
temperature and time
 Air trap
10. BURN MARKS
Burn marks are small, dark, or black spots that appear near the
end of the molded part’s flow path or in the blind area where an
air trap forms.
Rootcause
 Entrapped Air
 Material Degradation
11. DELAMINATION
Delamination (sometimes called lamination or layering) is a defect
in which the surface of a molded part can be peeled off layer by
layer.
Rootcause
• Incompatible materials
blended together
• Too much mold release
agent being used during the
molding process
• Low melt temperature in the
cavity
• Excessive moisture
• Sharp corners at the gate
and runner
12. DIMENSIONAL VARIATION
•Dimensional variation is a defect characterized by the molded
part dimension varying from batch to batch or from shot to shot
while the machine settings remain the same.
Rootcause
Dimensional variation can be
caused by:
• Unstable machine control
• A narrow molding window
• Improper molding conditions
• A broken check ring (within
the injection unit)
• Unstable material properties
13. DISCOLORATION
Discoloration is a color defect characterized by a molded part's
color having changed from the original material color.
Rootcause
This defect can be caused by either material degradation or
contamination from the following problems:
• The material stays in the barrel too long
• The barrel temperature is too high, causing the color to
change
• Contamination was caused by reground material, different
color material, or foreign material
14. FISH EYES
•Fish eyes are a surface defect that results from unmelted
materials being pushed with the melt stream into the cavity and
appearing on the surface of a molded part.
Rootcause
• Low barrel temperature: If the barrel temperature is too low to melt the
materials completely, the unmelted pellets will merge with the melt stream,
marring the surface of the part.
• Too much regrind: The shape and size of regrind is irregular compared with
original material, which can trap more air and cause the material to blend
unevenly.
• Material contamination: If a high-melt-temperature material is blended into
the original material, the blended material may stay in pellet form and
cause fish eyes during the molding process.
• Low screw rotation speed and back pressure: If the screw rotation speed
and the back pressure settings are too low, there might not be enough
frictional heating to melt the material completely in the barrel before
injection.
15. FLOW MARKS
A flow mark or halo is a surface defect in which circular ripples or
wavelets appear near the gate.
Rootcause
Flow marks are caused by cold material near
the gate or lack of compensated material during
the packing stage. The problem usually can be
attributed to:
•• Low melt temperature
•• Low mold temperature
•• Low injection speed
•• Low injection pressure
•• Small runner stem and gate
16. 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.
The buckled, snakelike jetting stream causes contact points to
form between the folds of melt in the jet, creating small-scale
“welds”
Problems cause by jetting
Jetting leads to part weakness, surface blemishes, and a host of
internal defects. Contrast this with a normal filling pattern, in which melt
advances in a progressive pattern from the gate to the extremities of the
cavity
To solve, use an overlap gate or a submarine gate as shown below.
17. HESITATION
Hesitation (or a hesitation mark) is a surface defect that results from the
stagnation of polymer melt flow over a thin-sectioned area, or an area of
abrupt thickness variation. Hesitation can be eliminated by changing the
part thickness or moving the gate location.
• When polymer melt is injected into a cavity of variable thickness, it
tends to fill the thick and less resistant areas.
• As a result, polymer melt may stagnate at thin sections until the rest of
the part is filled and the stagnated polymer melt starts moving again
(figure above).
• However, if the duration of hesitation is significant, polymer will solidify
prematurely at the stagnated point.
• When the solidified melt front is pushed to the part surface, a surface
defect such as a hesitation mark occurs.
Common Defect in Injection Molding
Defect
Causes
Flashing
•
•
•
Warping
• Non-uniform cooling rate
Bubble
• To much moisture in material
Shot Mold
• Air trap
• Injection Pressure not enough
Sink Mark
• Injection Pressure too low
• Non-uniform cooling rate
Ejector Pin
Mark
• Cooling time too short
• Ejection force too high
Injection Pressure too high
Clamping force to low
Mold fitting not good
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END
QUIZ 1
• Next week, Week 7 (27 March 2017)
• 5 simple questions
• ½ hour
• 5% of carry marks
ASSIGNMENT
• Individual assignment.
1. Find an actual existing plastic made product
(injection moulding) with defect(s).
2. Identify, explain and elaborate the root cause(s).
3. Suggest how to countermeasure the problem(s).
• Pages: min 3pages : max 5pages
• Front page: Download at portal.
• 5% of carry marks
• Submission due date : 17 April, before
4pm