Main causes of IM defects
- Material (thermoplastics)
- Injection molding machine
- Molding tool
- 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
- Short shots
- Flash
- Weld lines
- Sink marks
- blistering
- Jetting
- Burn mark
- Warpage
- Streaks
- Gloss differences
- Stress whitening
- Hesitation
- Overpacking
- 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
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