There is no "cure-all" for eliminating molding problems. Every mold is distinctive and has its own peculiarities, even molds which are exact duplicates. The same holds true for molding presses. Therefore, what works for one mold or press may not work for another.

Studies have determined that about 60% of defects result from machines and equipment, 20% from molds and dies 10% from material, and 10% from operator error.

List of errors problems and troubles in Injection Molding is following:

  1. Short Shot
  2. Bubbles
  3. Flash
  4. Sink Marks
  5. Weld line
  6. High AA (Acetaldehyde)
  7. Dark/brown Marks
  8. Wave like pattern
  9. Haze/Cloudy
  • Flow Lines
  • Parting line visible
  • Gas Burns
  • Parts stuck in mold

Short Size

Weight or volume of product injected during injection molding process is called short size in grams or ounces.

Short size (Single cavity mold) = weight of product (Single cavity) + Weight of runner + weight of Sprue

Short Shot

Term short shot is used when mold cavity is not completely filled.

Possible Causes of Short shot

  1. Causes of Short Shot
  2. Injection Pressure low
  3. Barrel temperature low
  4. Injection speed low
  5. Back pressure low
  6. Low temperature of Mould manifold
  7. Low temperature of Mould cavity

Possible Solutions of mentioned problems

  1. Increase injection pressure and speed (do not both at the same time)
  2. Increase back pressure
  3. Increase barrel temperature
  4. Increase mould manifold temperature
  5. Increase mould cavity temperature
  6. Clean vents


Bubbles on the outer surface is possible caused by following

  1. Moisture
  2. Low boiling point contamination (Additive or filler with low boiling point)
  3. Improper drying
  4. Worn out screw or mixing head
  5. Back pressure low
  6. Screw speed

Possible Solutions of mentioned problems causing bubbles

  1. Properly dry resin before going to injection molding
  2. High speed can cause bubbles so maintain RPM at moderate


Excessive material from parting line is called flash.

Possible Causes of Flash

  1. High injection pressure
  2. High injection speed
  3. Low clamping pressure
  4. Damaged surface

Possible Solutions of the problem

  1. Decrease the injection pressure
  2. Increase the clamping pressure
  3. Decrease holding time
  4. Decrease injection speed
  5. Rematch mold parting line
  6. Check mold surface for any damage or misalignment

Sink Marks

Sink marks can be defined as surface sink/pressed inward. Sink marks mostly appear where thick and thin areas join.

Possible Causes of sink marks

  1. Defective mold design
  2. Uneven wall thickness
  3. Cooling not proper
  4. Low holding pressure
  5. Less holding time
  6. Less injection pressure

Possible Solution of the problem

  1. Recheck mold design
  2. Check coolant channels and coolant temperature
  3. Increase holding pressure
  4. Increase holding time
  5. Increase injection pressure

Weld line

Weld line appears where two melt front meet in the mold during injection.

Possible Causes of weld line

  1. Low melt temperature
  2. Low nozzle temperature
  3. Low mold temperature
  4. Injection time high
  5. Cycle time is low

Possible Solutions of the problem

  1. Increase the melt temperature
  2. Increase nozzle temperature
  3. Increase mold temperature
  4. Decrease injection time
  5. Increase cycle time

High AA (Acetaldehyde) for PET only

Acetaldehyde is an impurity left or produced during processing of PET. AA level is kept less than 10 ppm (2 – 5 ppm) for water as high level of AA causes the change in water taste.

Possible Causes of high level of AA

  1. Contamination in resin
  2. Exposure to high temperature
  3. Thermal degradation
  4. High processing temperature

Possible Solutions of high AA level

  1. Check the material data sheet
  2. Keep from exposure of high temperature
  3. Process at normally low temperature and set heat profile reverse high temperature at feed zoon and lower at nozzle.

Dark/brown marks

Dark or brown marks on the surface of product appear possibly  due to following reasons

  1. Degradation of material
  2. High temperature
  3. High percentage of regrind
  4. Material with less processing temperature
  5. Material stuck in the grooved barrel or flight of screw
  6. Wear and tear in the screw and barrel

Possible Solution of the problems

  1. Decrease melt temperature
  2. Decrease regrind percentage
  3. Check the filler and additives temperature
  4. Check for the foreign material
  5. Check barrel and screw assembly condition

  • Flow Lines
  • Parting line visible
  • Gas Burns
  • Parts stuck in mold

Wave like pattern

Possible Causes of the problem may be one of these

  1. Low melt temperature
  2. Low mold temperature
  3. Molds surface not polished
  4. Low injection pressure
  5. High injection time

Possible Solution of wavy surface

  1. Increase melt temperature
  2. Increase mold temperature
  3. Polish the surface of mold
  4. Decrease the injection time

Flow lines

Lines on the surface of final product in flow direction.

Flow lines possible causes

  1. Poor mold surface finish
  2. Low injection pressure
  3. Low injection speed
  4. Low back pressure
  5. Narrow nozzle orifice
  6. Low mold temperature
  7. Narrow gate size
  8. Material stuck to the path of flow

Possible Solutions of the problem

  1. Polish the mold
  2. Increase injection pressure
  3. Increase injection speed
  4. Increase back pressure
  5. Increase the size of nozzle orifice and gate
  6. Increase mold temperature
  7. Clean the mold runner and sprue

Parting line

Where two halves of mold plates meet is called parting line.

Possible Causes of parting line

  1. High melt temperature
  2. High shot size
  3. High Injection pressure
  4. Molds halve surfaces are not aligned
  5. High Mold Temperature
  6. High Injection speed

Possible Solution of the problems

  1. Lower the melt temperature
  2. Decrease injection speed
  3. Decrease injection pressure
  4. Align and proper set the mold
  5. Decrease injection speed

Gas Burns

Dark or brown marks on the surface are called Gas burns

Possible Causes of Gas burn

  1. Poor venting
  2. High melt temperature
  3. Contamination
  4. High shear rate
  5. Narrow gate or nozzle orifice for shear sensitive materials

Possible Solutions

  1. Increase numbers of vents
  2. Decrease melt temperature
  3. Check for contamination
  4. Reduce the RPM of screw
  5. Check for the gate and nozzle size for shear sensitive materials like PVC

Part sticks in mold

  1. High holding pressure and time
  2. High injection speed
  3. Low mold temperature
  4. Mold not polished
  5. Poor sprue bushing

Possible Solutions

  1. Use mold release
  2. Use air for ejecting of part
  3. Polish the surface of mold, sprue, runners and gate
  4. Decrease holding time and pressure
  5. Lower the injection pressure and speed
  6. Increase mold temperature

We provide Plastic Process Machineries, include Plastic injection molding machinesExtrusion blow molding machinesPET blowing molding bottle making machines,Plastic Product molding MouldsPlastic Machine Auxiliaries, and etc.

Tel: +86-574-28535376 / +86-574-27837982