Injection molding defects

 

Main causes of IM defects

 

  1. Material (thermoplastics)
  2. Injection molding machine
  3. Molding tool
  4. Product design

 

Material

The material factor that causes defects are

  • Granule type and its size
  • Plastic grade - Easy flow & Hard flow
  • Moisture absorption - Pre-heating time

 

Injection molding machine

Molding machine factor that causes defects are

  • Selection of machine with sufficient capacity - Shot capacity, Plasticizing capacity & Clamping capacity
  • Machine controls - Temperature, pressure & time

 

Tool design

Tool design factor that causes defects are

  • Rigid
  • Hard
  • Strong
  • High polish
  • Proper Gating
  • Proper venting
  • Uniform cooling
  • Balanced ejection

 

Product design

Product design factor that causes defects are: 

  • Texture
  • Lettering
  • Ribs design
  • Loading Metal Inserts
  • Holes / cutouts & their position
  • Uniform wall thickness of the part
  • Undercuts – External / Internal (Local / All-round)

 

Types of IM defects

 

  1. Short shots
  2. Flash
  3. Weld lines
  4. Sink marks
  5. blistering
  6. Jetting
  7. Burn mark
  8. Warpage
  9. Streaks
  10. Gloss differences
  11. Stress whitening
  12. Hesitation
  13. Overpacking
  14. Unbalanced flow

 

Short shots

Failure to fill the mold or cavities of the mold completely is termed as a shot. A short shot is the incomplete filling of a mold cavity which results in the production of an incomplete part. The flow freezes before all the flow paths have filled.

Factors that cause short shots are given below.

  • Poor venting
  • Insufficient gate opening
  • Wrong temperature control
  • Insufficient injection pressure
  • Injected material volume is too small (shot volume)
  • Improper balance of plastic flow in multiple cavity molds
  • Thin cross-section wherein the melt freezes pre-maturely due to unfavourable flow conditions

 

Short shot

 

Flashes

Flashes are often seen near sealing faces, out of vent grooves, or down ejector pins. It appears as thin or sometimes thick sections of plastic where it would not be on a normal part. Flashing occurs when a thin layer of material is forced out of mold cavity at the parting line or ejector pin's location. This excess material remains attached to the molded article and normally has to be manually removed.

Factors causing flashes are given below –

  • Insufficient viscosity of the melt
  • Internal mold pressure too high
  • Clamping force too high or too low
  • Mold exceeding production tolerances damaged sealing faces
  • Permissible gap width exceeded due to insufficient mold tightness

 

 

Weld lines

Weld lines are created when two or more melt flows fronts meet possibly causing a cosmetically visible line. It can also create a weakened area in the finished molded part especially with filled resins.

Factors causing weld line are –

  • Slow injection speed
  • Low melt temperature
  • Low injection pressure
  • Low mold temperature
  • Improper venting in mold
  • Poor material flow properties
  • Excessive mold lubricant used
  • The wrong type of gate or gate location

Weld lines

 

Sink marks

Sink marks are localized contractions or depressions on the surface of the molding. Sink marks appear as depression 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.