GZbohangzhineng@163.com
+86-18675873677
Injection Molding is one of the most widely used manufacturing methods for producing plastic parts in volume with high precision. Below is a clear breakdown of each stage of the injection molding process, from raw resin to finished part.
The process starts by selecting a suitable plastic resin (often thermoplastic) in pellet or granule form.
These pellets are often dried or pretreated (to remove moisture) to avoid defects during molding.
The dried resin is fed into a hopper, which directs it into the machine’s heated barrel.
This initial preparation is critical: consistent pellet quality, dryness, and correct dosing influence the stability and quality of the molded part.
Inside the barrel, the screw (or sometimes a plunger) rotates and advances the plastic pellets forward.
Heaters and friction cause the material to melt and gradually become a homogeneous molten plastic.
The screw geometry is divided into zones (feed zone, compression or transition zone, metering zone) to progressively compress and melt the material uniformly.
A check ring or non-return valve prevents molten plastic from backflow during the injection stroke.
Uniform melt temperature, good mixing, and stable consistency are essential to avoid defects like burn, unmelted particles, or flow marks.
Before injection, the mold halves must be closed and locked with enough force (clamping force) to resist the pressure inside.
The clamping unit holds the mold tightly shut during filling and pressure phases, preventing flash or material leakage.
Clamping systems may be toggle, hydraulic, or hybrid types, depending on machine design and tonnage requirements.
The molten plastic is forced into the mold cavity via the nozzle, through the sprue, runners, and gates.
The filling must be fast enough to prevent premature solidification, but also controlled to avoid over pressure or flow turbulence.
Once the mold is filled, the machine maintains a holding pressure to compensate for volumetric shrinkage as the plastic cools and solidifies.
The injection pressure, speed profile, and timing are key parameters to tune based on material and part geometry.
After the cavity is filled, the machine continues to exert pressure (called packing or holding pressure) to force more material into the cavity and maintain density as the melt cools and shrinks.
This phase helps avoid voids, sink marks, and ensures dimensional consistency.
The duration and level of holding pressure must be optimized to balance material usage and quality.
The molten plastic inside the mold gradually cools and transitions to a solid state.
Molds incorporate cooling channels (for water, oil, or other fluid) to remove heat in a controlled way.
The mold temperature, cooling rate, and part wall thickness strongly influence cycle time, warpage, internal stress, and final part quality.
Cooling often dominates the cycle time; therefore, optimizing cooling design is critical for productivity.
Once the part has sufficiently solidified, the mold is opened (clamping unit retracts).
Ejector pins, plates, sleeves, or other mechanisms push the part out of the cavity.
Care must be taken to avoid damage to the part or mold, especially in designs with small drafts, ribs, or undercuts.
Any excess material such as runners, gates, or flash is trimmed or removed, either manually or automatically (in-line).
After ejection, parts may require secondary operations: trimming, machining, surface finishing (painting, plating, texturing), assembly, or inspection.
Many molded parts are ready with minimal finishing if the mold is well designed.
Quality control functions such as dimensional inspection, defect checking, and functional testing validate that parts meet specifications.
When switching materials or at the end of production runs, purging is needed to clean residual polymer from the machine (screw, barrel, nozzles).
The purging procedure ensures there is no contamination or color mixing in subsequent jobs.
While the above describes the standard injection molding sequence, there are specialized variants and advanced techniques:
Insert molding / Overmolding: Embedding metal inserts or existing parts into the mold before injection so plastic flows around them.
Gas-assisted injection: Injecting a gas (e.g. nitrogen) to hollow out thick sections or reduce material use.
Cube (rotating) mold systems: Use rotating middle blocks to perform multi-step operations in one cycle, thereby reducing cycle time.
Fusible core / soluble core molding: Use a core material that can later be dissolved or melted out to form internal cavities.
These techniques expand the design freedom and functionality of molded parts beyond conventional layouts.
Uniform wall thickness and adequate draft angles help with part ejection, cooling consistency, and defect reduction.
Proper gating, runner design, and flow analysis minimize defects like weld lines, sinks, or flow hesitation.
Mold temperature control and cooling channel design are essential to fast, stable cycles.
Scientific molding (recording and controlling process parameters) helps maintain repeatability and reduces variation.
Material selection (flowability, shrinkage, mechanical properties) must match part requirements and the mold’s capabilities.
To reliably deliver high-quality results at volume, the choice of machine and mold partner matters enormously. BOHANG (Guangzhou Bohang Intelligent Technology Co., Ltd.) is a company that offers expertise in injection molding equipment and related solutions. Their machines support core functions such as melting, injection, clamping, and mold handling.
By working with a provider like BOHANG, you can benefit from:
Integrated understanding of both molding machinery and mold tooling
Quality assurance from design, manufacturing to after-sales
Ability to match equipment parameters (tonnage, injection capacity, speed) to your part demands
Reliable support and faster deployment for production
If you're planning a new injection molding project or evaluating machine suppliers, checking out BOHANG’s offerings is a solid place to start.
