Injection Molding process and Molding Defects

Molding Machine operating
sequence

Injection molding is a cyclic
process. During the injection molding process, the machine undertakes a
sequence of operations in a cyclic fashion. A process cycle is one complete operation
of an injection molding machine.

Process Cycle or basic injection molding
machine operations are shown in the series of diagrams below. 

See below diagram for better understanding.

Molding Machine Process Condition

Importance of process condition:- The quality of the molded part is
greatly influenced by the conditions under which it is processed. Following are condition for molding process condition:-

Before setting process conditions,
you should make sure the molding machine is in proper working order, and that
the mold you plan to use was designed for the particular machine you plan to
use. Follow the step-by-step procedure provided below to control the settings
on your machine.

Following are set points for mold
process condition:-

Top Molding Defects List

Detailed explanation of molding defect is explained below, read one by one:-

 1. Short
shot 

A
short shot is a molded part that is incomplete because insufficient material
was injected into the mold.

Cause of Short Shot molding
Defect

Remedies to reduce short shot
molding defect

 2. Brittleness: 

A brittle
molded part has a tendency to break or crack. Brittleness results from material
degradation leading to shorter molecular chain length ( thus lower molecular
weight). As a result, the physical integrity of the part is substantially less
than the specification.

Cause of Brittleness Defect

Remedies
to reduce brittleness  defect in molding

1. Enlarge the
   sprue, runner, and/or gate.
2. Get a better
   screw design for the material you’re using to achieve a better mixed melt
   temperature.
3. Reduce the back
   pressure, screw rotation speed, or injection speed. since shear heating can
   result in material degradation.
4. Set proper
   drying conditions before molding.
5. Reduce regrind
   material. The brittleness could be caused by too much reground material added
   into the original virgin material.
6. Change to a
   high-strength material since low-strength materials tend to become more brittle
   if processed Improperly.
7. Reduce the barrel temperature and nozzle
   temperature. 

 3. Sink and voids: 

In Sink defect the molten plastic
contract at particular location and leaving a down step on surface.

Cause of Sink & Voids

1. Low injection
   and packing pressure
2. Short hold time
   or cooling time
3. High melt
   temperature or mold temperature
4. Localized
   geometric features 

Remedies to reduce sink &
voids defect in molding

1. Conceal sink
   marks by adding a design feature, such as a series of serrations on the area
   where they occur.
2. Conceal sink
   marks by adding a design feature, such as a series of serrations on the area
   where they occur.
3. Increase the
   size of gates and runners to delay the gate freeze-off time.
4. Add more vents
   or enlarge the vents.
5. Relocate the
   gate to or near a thicker section.
6. Increase the
   cushion at the end of the injection stroke.
7. Increase the
   injection pressure and the holding time.
8. Increase the
   screw forward time and decrease the injection rate.
9. Decrease the
   melt and mold-wall temperatures.
10. Increase the
    cooling time.
11. Check the non-return valve for possible material
    leakage.

 4. Silver Streaks: 

Splay
appear as streaking on the part surface in the direction of flow.

Cause of Silver Steak defect in Molding

1. Moister, air,
   gases or volatile present in the plastic resin or mould surface.
2. Improper
   material handling.

Remedies to reduce Silver streak
defect in Molding

1. Clean the
   surface at the beginning
2. Handle the resin with care plastic, plastic
   resin should be free from other particles.

 5. Sprue sticking: 

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

Cause of sprue sticking defect

1. Sprue too
   small/ diameter too long.
2. Insufficient polish.
3. Sprue bushing
   too hot.
4. Void at base of
   sprue

Remedies to reduce sprue sticking defect

1. Install correct
   size sprue.
2. Polish well in
   the draw direction.
3. Ensure good
   thermal contact between sprue bushing and mold.
4. Use alloy sprue
   bushing  with ample water flow.
5. Increase nozzle
   tip diameter if too small.
6. Reduce
   injection speed specially at the beginning of the shot.

 6. Part
sticking: 

Part
is jetting and not pulling out of the cavity and in rarer circumstances cannot
ejected off the core.

Causes of Part sticking Defect

1. Check mold for
   undercuts.
2. Less booster
   time.
3. Less cylinder
   and nozzle temperature.
4. Less injection
   pressure.
5. Less mold
   cavity temperature.
6. Increased clamp
   pressure and mold close time.

Remedies to reduce part sticking
defect

1. If possible add
   undercuts in core to allow the part to pull out of the cavity.
2. Increase the
   cylinder and nozzle temperature.
3. Increase
   injection pressure.
4. Increase mold
   cavity temperature.
5. Decrease clamp
   pressure and mold close time

 7. Burn marks: 

Burn marks are
small, dark or black spots that appear near the end of the flow path of a
molded part or in the blind area where the air trap forms.

Causes of Burn marks defect

1. Entrapped Air
2. Material
   Degradation

Remedies to reduce Burn mark
defect in Molding

1. Place an adequate
   venting system throughout the mold to help vent out the entrapped air.
2. Enlarge the
   sprue, runner, and/or gate.
3. Reduce the
   injection pressure.
4. Reduce the
   injection speed.
5. Reduce the
   screw rotation speed.
6. Decrease the
   barrel temperature.
7. Check the band heaters on the barrel and nozzle,
   and calibrate the thermocouple.

 8. Flash: 

Flash is a defect
where excessive material is found at locations where the mold separates,
notably the parting surface, movable core, vents, or venting ejector pins.

Cause of Flash defect in molding

1. Low clamp force
2. Gap within the
   mold Molding conditions i.e. high melt temperature high injection pressure will
   cause flash.
3. Improper
   venting 

Remedies to reduce Flash defect
in molding

1. Set up the mold
   to seal properly. A mismatch or undesirable gap between the cavity and core
   sides of the mold will result in flash.
2. Make sure the
   mold plates are strong enough to avoid deformation during molding.
3. Check for
   adequate venting
4. Clean the mold
   surface.
5. Mill out the
   surface to keep the sealing pressure of land area around the cavities high
   enough.
6. Adjust the
   machine clamp force.
7. Decrease the
   barrel temperature and nozzle temperature.
8. Reduce the
   injection and packing pressure to reduce the clamp force requirement.
9. Reduce the feed
   setting (stroke length) to reduce metering (over-fill).
10. Increase the
    injection time or slow down the injection speed

 9. Brown streaks: 

Brown
specks or streaks are dark spots or dark streaks found on the surface or
throughout a molded part.

Cause of brown streak defect in
molding:- Material degradation & Material contamination 

Remedies to reduce brown
streaking defect in Molding

1. Clean the
   ejectors and slides.
2. Improve the
   venting system. If the black specks are found at the end of flow paths or blind
   spots, they are likely caused by a poor venting system.
3. Clean or polish
   any nicked surface on the runner system to keep dirt from lodging in these
   areas.
4. Clean the mold
   before molding.
5. Size a proper
   injection machine for a specific mold.
6. Check for
   scratched or dented barrel/screw surfaces that trap material.
7. Check for local
   overheating by a run-away heater band or a malfunctioning temperature
   controller.
8. Make sure no
   contaminated materials, such as dirty regrind, are blended into the original
   material.
9. Put the cover
   on the hopper and all bins of material. Airborne dirt can contaminate the
   original material, leading to black spots.
10. Lower the
    barrel and nozzle temperature.
11. Purge and clean
    the injection unit.
12. Avoid recycling rejected parts with black specks
    and black streaks

 10. Discoloration: 

Discoloration is a color defect characterized by
a molded part’s color having changed from the original material color.

Cause of dislocation defect in
Molding

1. The material
   staying in the barrel too long.
2. The barrel
   temperature being too high, causing the color to change.
3. Contamination caused by reground material,
   different color material, or foreign material

Remedies for Dislocation defect
in Molding

1. Enlarge the
   sprue, runner, and/or gate.
2. Check for
   adequate venting.
3. Use a different
   size injection molding machine
4. Clean the
   hopper completely.
5. Purge the
   injection unit completely if there is any material changing.
6. Reduce the
   barrel temperature and nozzle temperature.
7. Maintain proper housekeeping for origin
   materials and regrind materials storage to avoid contaminated materials

 11. Black
Spots/specks: 

Black
specks and black streaks are dark spots or dark streaks found on the surface or
throughout a molded part. Brown specks or streaks refer to the same type of
defect, except the burning or discoloration is not as severe.

Cause of Black spot/Speaks
defects in Molding:- Material degradation & Contamination

Remedies to reduce Black
spots/specks defect in molding

1. Clean the
   ejectors and slides.
2. Improve the
   venting system. If the black specks are found at the end of flow paths or blind
   spots, they are likely caused by a poor venting system.
3. Clean or polish
   any nicked surface on the runner system to keep dirt from lodging in these
   areas.
4. Clean the mold
   before molding.
5. Size a proper
   injection machine for a specific mold.
6. Check for
   scratched or dented barrel/screw surfaces that trap material.
7. Check for local
   overheating by a run-away heater band or a malfunctioning temperature
   controller.
8. Make sure no
   contaminated materials, such as dirty regrind, are blended into the original
   material.
9. Put the cover
   on the hopper and all bins of material. Airborne dirt can contaminate the
   original material, leading to black spots.
10. Lower the
    barrel and nozzle temperature.
11. Purge and clean
    the injection unit.
12. Avoid recycling rejected parts with black specks
    and black streaks.

 12. Weld line: 

A weld line
(also called a weld mark or a knit line) is formed when separate melt fronts
traveling in opposite directions meet. A meld line occurs if two emerging melt
fronts flow parallel to each other and create a bond between them.

Cause of weld line defect in
molding

1. High injection
   pressure and speed.
2. High melt and
   mold-wall temperature.
3. Formation of
   the weld lines closer to the gate.
4. A temperature difference of less that 10ºC
   between the two emerging melt fronts.

Remedies
to reduce weld line defect

1. Increase the
   wall thickness.
2. Adjust the gate
   position and dimension or decrease the part thickness ratio.
3. Increase the
   size of gate and runners.
4. Place a vent in
   the area of the weld/meld line.
5. Change the gate
   design to eliminate weld/meld lines or to form them closer to the gate at a
   high temperature and under high packing pressure.
6. Increase the
   melt temperature, injection speed, or injection pressure.

 13. 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, snake-like jetting stream causes contact points to form between the
folds of melt in the jet, creating small scale “welds”.

Cause of jetting defect in
Molding

1. High injection
   speed and pressure
2. Small gate and runner

Remedies to reduce Jetting defect
in Molding

1. Direct the melt
   against a metal surface.
2. Use an overlap
   gate or a submarine gate.
3. Enlarge the
   size of the gate and runner or reduce the gate-land length.
4. Slow down the
   melt with a gradually divergent flow area.
5. Adjust the
   ram-speed profile.
6. Adjust the barrel temperature to increase or
   decrease the melt temperature incrementally

 14. Warpage: 

Warpage is a distortion where the surfaces of
the molded part do not follow the intended shape of the design.

Cause of Warpage defect in
Molding

1. Part ejected
   while too hot
2. Shrinkage
   differential due to non-uniform shrinkage
3. Melt
   temperature too low
4. Shrinkage
   differential due to non-uniform wall thickness
5. Insufficient
   pack and hold times or pressures
6. Ejection system
   poorly designed
7. Differential
   mold cooling 

Remedies
to reduce Warpage defect in Molding

1. Balance runners
   and gates
2. Increase runner
   and gate size.
3. Redesign
   ejection system for balanced ejection force
4. Balance mold
   temperature
5. Redesign part
   with uniform wall thickness
6. Decrease melt
   temperature
7. Decrease mold
   temperature
8. Increase
   cooling time
9. Increase
   injection rate
10. Increase pack
    pressure
11. Increase/decrease
    injection time
12. Increase melt
    temperature to provide better pack out of part
13. Increase
    cooling time
14. Add pack and
    hold time or pressure 

All molding defect picture is in below presentation:-