1. DEFECT IN PLASTIC INJECTION MOULDING PROCESS

2. 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

3. 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
Clean the screw barrel and nozzle
Purging
Check the material (plastic resin)

4. 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
Air Vent
Sub-Insert
Ejector pin

5. 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
Parting line no good
Low clamping force
Injection pressure
too high
Countermeasure
Re-fitting
Use appropriate
clamping force and
Injection pressure

6. 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
Pin Mark
Drag/Pull Mark
Rootcause
Pin Mark
Improper polishing
Unbalanced ejection
Cooling time not enough
Drag/Pull Mark
Draft/taper not enough
Countermeasure
Proper polishing
Appropriate cooling time
Appropriate draft/taper

7. DESIGN FOR UNDERCUTS (A) (B)
- Cavity move to top side and slider move to right side
- Angular pin push the part out from core side

8. 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
Too thin
Injection pressure not enough
Air trap
Countermeasure
Avoid design too thin
Increase injection pressure
Sub-Insert

9. 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
Part design
Injection pressure not enough
Holding pressure time not enough
4. Cooling time not enough

11. 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
Part design
Countermeasure
Air vent
Ejector pin
Sub-Insert
Texturing
Increase mould
temperature

12. 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
Cooling channel designed
Cooling time not enough
Part design
Imbalanced ejection
Orientation and area shrinkage effects
Warping caused by differential cooling

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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.

19. 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

20. 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

21. To solve, use an overlap gate or a submarine gate as shown below.

22. 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.

24. Common Defect in Injection Molding
Causes
Flashing
Injection Pressure too high
Clamping force to low
Mold fitting not good
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
Ejector Pin Mark
Cooling time too short
Ejection force too high

25. END

26. QUIZ 1 Next week, Week 7 (27 March 2017) 5 simple questions ½ hour
5% of carry marks

27. ASSIGNMENT Individual assignment. Pages: min 3pages : max 5pages
Find an actual existing plastic made product (injection moulding) with defect(s).
Identify, explain and elaborate the root cause(s).
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