Injection Molding process and Molding Defects

 In this article we will learn basic of injection molding process, molding machine and some molding defects. This is next article to previous one- "Plastic Injection molding process Chapter-1". In previous article we have learned about following things:-

1. What is plastic
2. What is polymerization process
3. Morphology of Polymers
4. Classification of Plastic
5. Structure and properties of Thermoplastic
6. Difference between Amorphous Polymer & Crystalline Polymer

Injection Molding Basic study with Molding Defects



Now we will cover short learning about plastic injection molding process. We will cover following points in this post:-

1. Additive, filler and reinforcement effect on polymer property
2. Injection Molding Machine
3. Molding Machine operating sequence
4. Molding machine process condition
5. List of Molding Defects

Additive filler and reinforcement effect on polymers property

Additive, filler and reinforcement are material that are added with polymer compound to change its property, change could be either we increasing or decreasing.

Additives could be coupling agent, Flame retardants, Plasticizers, Colorants (pigments & dyes), Blowing agent. These additives are added to improve interface bonding between polymer matrix, stop spreading of combustion, enhancing flexibility, improve melt flow properties.

Fibers could be reinforcing fibers, like-Baron, carbon, Kevlar, Glass, etc. These reinforce fiber increase tensile strength, increase strength, and reduce shrinkage & warpage.

Fillers could be Conductive fillers (Aluminum powder,  carbon fiber), Extender filler (Silica, Calcium carbonate). These filler improve electric & thermal conductivity, reduce material cost.

Details of additives and filler effect on polymer are explained in below picture


Injection Molding Machine Basic Details

There are 2 main units in Injection molding machine. Clamping unit and Injection Unit. Clamping unit contain Mold & Cavity tool. While injection unit contain Hopper, Barrel & some screws.


Screw used in injection molding are divided into 3 section:-

 1. Feed Zone :- Where the plastic first enter in screw
 2. Transition Zone:- where the plastic is conveyed and compressed in the screw
 3. Metering Zone:- where the plastic is melting completely


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. 

  1. The mold closes and the screw begins moving forward for injection.
  2. The cavity fills as the reciprocating screw moves forward, as a plunger
  3. The cavity is packed as the screw continuously moves forward.
  4. The cavity cools as the gate freezes off and the screw begins to retract to plasticize material for the next shot
  5. The mold opens for part ejection
  6. The mold closes and the next cycle Begins

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:-

  1. As you lower the temperature, higher pressure is needed to deliver the polymer melt into the cavity.
  2. If the temperature is too high, you risk causing material degradation.
  3. If the injection pressure is too low, a short shot could result.
  4. If the pressure is too high, you will flash the mold.

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:-

  1. Set the melt temperature
  2. Set the mold temperature
  3. Set the switch-over position
  4. Set the screw rotation speed
  5. Set the back pressure
  6. Set the injection pressure to the machine maximum
  7. Set the holding pressure at 0 MPa
  8. Set the injection velocity to the machine maximum
  9. Set the holding time
  10. Set ample remaining cooling time
  11. Set the mold open time
  12. Mold a short-shot series by increasing injection volume
  13. Switch to automatic operation
  14. Set the mold opening stroke
  15. Set the ejector stroke, start position, and velocity
  16. Set the injection volume to 99% mold filled
  17. Increase the holding pressure in steps
  18. Minimize the holding time

Top Molding Defects List

  1. Short shot
  2. Brittleness
  3. Sink & Voids
  4. Air Bubble
  5. Silver Steak
  6. Sprue Sticking
  7. Part Sticking
  8. Burn Marks
  9. Dislocation
  10. Black Spot & Speaks
  11. Weld Lines
  12. Jetting
  13. Warpage

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

  1. Insufficiently-sized restrictive-flow areas, such as gates, runners, and thin walls.
  2. Low melt and/or mold-wall temperatures.
  3. A lack of vents to bleed the air trapped inside the cavity.
  4. Insufficient machine injection pressure (resulting from high melt resistance and a restricted flow path), volume, and/or ram speed.
  5. Machine defects such as an empty hopper, blocked feed throat, or a worn non-return (check) valve that causes loss of injection pressure or leakage of injection volume.
  6. Premature solidification of the polymer melt due to Hesitation, poor filling pattern, or prolonged injection time. 

Remedies to reduce short shot molding defect

  1. Strategically increase the thickness of certain wall sections
  2. Fill the thick areas before filling the thin areas. Doing so will avoid hesitation, which causes early solidification of polymer.
  3. Increase the number and/or size of gates to reduce the flow length.
  4. Increase the size of runner systems to reduce resistance.
  5. Place vents at the proper locations, typically near the areas that fill last.
  6. Increase the injection pressure.
  7. Increase the injection speed. Within the machine limits, this will create more viscous heating and reduce the melt viscosity.
  8. Increase the injection volume.
  9. Increase the barrel temperature and/or the mold-wall tem perature.
  10. Check the hopper for sufficient material supply or a clogged feed throat.
  11. Inspect the non-return valve and barrel for excessive wear. 

 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

  1. Improper drying conditions
  2. Improper temperature setting
  3. Improper runner system and gate design
  4. Improper screw design
  5. Weld line weakness 

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 10ÂșC 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:-



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