Injection Molding Process Principles
Injection molding is essentially a cyclic process of "Heating & Melting → High-Pressure Injection into Mold → Cooling & Solidification → Mold Opening & Part Ejection." It is the core manufacturing process for mass-producing thermoplastic parts.
1. The Complete Cycle (4 Stages)
Clamping
The moving platen closes and locks the mold halves to withstand the upcoming injection pressure and prevent mold flash (mold separation).
Injection & Holding Pressure (Packing)
The screw rotates to plasticize material, pushing pellets forward to form melt in the front of the barrel.
The screw acts as a piston, injecting the melt into the mold cavity at high pressure and high speed.
After the cavity is filled, it switches to holding pressure to compensate for material shrinkage during cooling.
Cooling
Cooling water circulates through the mold to lower the part temperature until it reaches a rigid enough state for ejection. During this stage, the screw simultaneously performs plasticizing for the next cycle (parallel operation).
Mold Opening & Ejection
The mold opens, and ejector pins/mechanisms push the finished part out, initiating the next cycle.
2. Plasticizing Principle (Inside the Barrel)
|
Barrel Zone |
Function |
|---|---|
|
Feed Zone |
Solid pellets are conveyed forward while absorbing initial heat. |
|
Compression Zone (Transition) |
The screw channel depth decreases; material melts due to shear heat and external barrel heating. |
|
Metering Zone |
The melt is homogenized and metered into the reservoir at the front of the barrel. |
💡 Note: Plastic melting is achieved through a combination of external heater bands and shear friction heat generated by the rotating screw.
3. Filling and Forming Principles (Inside the Mold)
Filling Stage: Melt overcomes resistance from runners/gates to fill the cavity under high pressure. Focus on flow front progression, shear rate, and weld line location.
Holding Stage: As the polymer cools, its volume shrinks. Holding pressure compensates by forcing additional material into the cavity, reducing sink marks and dimensional deviation.
Cooling & Solidification: Lower mold temperatures speed up cooling but increase internal stress/warpage risk. Higher mold temperatures improve surface gloss but extend cycle time.
Ejection: Draft angles and undercut features determine whether side-actions (sliders/lifters) are required for demolding.
4. Key Process Parameters (Core for Setup)
|
Parameter |
Influence |
|---|---|
|
Injection Pressure / Speed |
Fill completeness, short shots, flash, weld line strength. |
|
Holding Pressure & Time |
Compensation effect; affects sink marks, part weight, and dimensions. |
|
Barrel Temperature (Zones) |
Plasticizing quality. Too low = unmelted; Too high = degradation/discoloration. |
|
Mold Temperature |
Surface finish, crystallinity (critical for PA/POM), and warpage. |
|
Cooling Time |
Determines cycle time; must ensure no deformation upon ejection. |
|
Back Pressure & Screw RPM |
Affects mixing homogeneity, splay marks (silver streaks), and degradation risk. |
5. Common Defects & Root Cause Analysis
Short Shot: Insufficient pressure/speed, small gate size, or low melt temperature.
Flash: Inadequate clamping force, excessive injection pressure, or poor mold parting line fit.
Sink Marks: Insufficient holding pressure or time, uneven wall thickness.
Warpage: Uneven cooling, asymmetric molecular orientation, or premature ejection.
Voids/Burn Marks: Poor venting, moisture in raw material, or air trap causing thermal decomposition.