What are the steps of plastic injection mold injection molding processing
Plastic injection molding is a manufacturing process for producing large volumes of identical parts by forcing molten polymer into a shaped cavity. The method works with a wide range of thermoplastic and thermosetting resins - from commodity grades like PP and ABS to engineering polymers such as PA (nylon) and POM. Because the mould cavity can reproduce fine details repeatedly, the process of injection moulding is especially common in automotive, consumer electronics, medical devices, and household appliance production.
A complete plastic injection moulding process involves more than just the machine cycle. It starts well before the press runs - with part design review, mould fabrication, and material preparation - and continues after the part is ejected, through inspection and secondary operations. The sections below walk through each stage of the injection molding manufacturing process, from mold making to final part handling, so that engineers and procurement teams can follow the full workflow and identify where quality is built in.
How Injection Molds Are Made - From Design to Assembly
Before any resin is melted, the mould itself must be designed, machined, and assembled. The injection mold making process typically follows this sequence:
The mold design stage begins with a 3D CAD model of the target part. Engineers run mold-flow analysis to simulate how molten plastic will fill the cavity, where weld lines may form, and whether the cooling layout is balanced. This analysis is critical for preventing downstream defects and reducing the number of trial-and-error iterations during mould trialling.
Once the design is approved, the mould base - usually made from P20 pre-hardened steel or, for shorter runs, aluminum alloy - is machined on CNC milling centres. Complex geometries that cannot be reached by conventional cutters are finished using EDM (Electrical Discharge Machining). After machining, the cavity surfaces are polished or textured according to the part's cosmetic requirements.
The mould assembly process brings all machined components together: cavity and core inserts are fitted into the mould base, ejector pins and return springs are installed, and cooling water lines are connected and leak-tested. Slides, lifters, and any hydraulic or pneumatic cylinders for side actions are mounted and manually cycled to confirm smooth, interference-free travel. A fully assembled custom plastic injection mould ready for production trials is then mounted onto the press for its first trial shot. Any misalignment identified at this stage - even fractions of a millimetre - can cause flash, short shots, or premature tool wear, which is why experienced mould fitters spend considerable time hand-fitting components before the tool leaves the shop.
Injection Molding Machine Setup Before Production
With the mould installed on the press, the next preparatory step is the injection molding machine startup procedure. The operator first confirms that the mould is securely bolted to both platens and that the ejector stroke matches the mould's requirements. Water hoses are connected to the cooling channels, and the flow rate is set according to the mould specification sheet.
The barrel heaters are then switched on zone by zone, and the machine is left to soak until each zone reaches its target temperature - a process that can take 20 to 40 minutes depending on machine size. Resin pellets are loaded into the hopper during this warm-up period. If the material is hygroscopic (for example, nylon or polycarbonate), it must have been pre-dried in a desilicant dryer to the resin manufacturer's recommended moisture level before entering the barrel; failing to do so almost always produces splay marks on the finished parts.
Once barrel temperatures stabilise, the operator runs several purge shots to flush residual material from the previous production run, then begins dialling in the injection parameters: shot size, injection speed and pressure profile, switchover point, holding pressure, and cooling time. Most facilities record these settings on a process sheet so that the same mould can be set up consistently every time it returns to the press. Understanding how to operate an injection moulding machine at this level is what separates a stable, low-scrap production run from one that constantly needs intervention.
With the mould assembled, the machine warmed up, and the resin dried, the actual moulding cycle can begin. Although different sources break the cycle into varying numbers of stages, the core injection moulding process steps followed on every machine are: (1) clamping the mould shut, (2) injecting molten plastic into the cavity, (3) holding pressure to compensate for shrinkage, (4) cooling the part inside the closed mould, and (5) opening the mould and ejecting the finished part. The seven-step breakdown in this article expands on those five core stages by separating the plasticizing, nozzle-seating, and pressure-release actions into their own steps, giving a more detailed view of what happens during each phase of the injection molding operation.
1.Heating and preplasticizing
Driven by the transmission system, the screw is used to feed the materials from the hopper forward and compacted. Under the mixing action of the external heater, the screw and the shearing and friction of the barrel, the material is gradually melted. A certain amount of molten plastic has been accumulated at the head of the barrel.The backward distance depends on the amount needed for a single injection by the metering device. When the infusion volume reaches the predetermined amount, the screw stops spinning and goes back.
The screw used in this plasticizing phase is often referred to as a reciprocating screw - it rotates to convey and melt the resin, then acts as a plunger during injection. The screw's geometry (flight depth, compression ratio, and L/D ratio) is selected based on the polymer being processed. Amorphous resins like ABS and PC generally work well on general-purpose screws, while semi-crystalline materials such as POM and PBT may benefit from screws with a longer compression zone to ensure uniform melt temperature. Choosing the correct screw configuration for the resin injection molding process has a direct impact on melt homogeneity and, ultimately, part quality.
2.Fits and locks
The clamping mechanism pushes the template and the mold moving part installed on the moving template and the mold moving part on the moving template clamping and locking, so as to ensure that sufficient clamping force can be provided to tighten the mold.
3.injection
When the assembly is completed, the entire injection seat is pushed forward to make the injector nozzle fully fit with the main runner of the mold.
4. Injection and pressure keeping
Completely after laminating mould clamping and the nozzle, the hydraulic cylinder injection into the high pressure oil, promote relative cylinder screw moved forward, will accumulate in the cylinder head of melt with enough pressure injection mould cavity, to make plastic volume contraction due to lower temperature, to ensure the density of plastic parts, dimension precision and mechanics performance, should be within the mold cavity on the melt to keep certain pressure, to supplement the materials.
After the holding-pressure phase, the melt inside the cavity must cool and solidify before the mould can open. Cooling is typically the longest portion of the entire injection moulding cycle - it can account for 60–70 % of total cycle time depending on wall thickness and resin type. Temperature-regulated water or oil circulates through channels machined into the mould plates, extracting heat from the plastic at a controlled rate.
Uniform cooling is essential. If one region of the cavity cools significantly faster than another, internal stresses develop and the part may warp or show sink marks on thick sections. Mould designers address this by positioning cooling channels at consistent distances from the cavity surface and, in complex geometries, by using conformal cooling channels produced through additive manufacturing. Monitoring coolant flow rates and inlet/outlet temperature differentials during production is a straightforward way to verify that the mould's thermal performance remains stable over long runs.
5 .unloading
When the melt at the gate of the mold is frozen, the pressure can be released.
6.Step back of injection device
Generally, the screw can be rotated and withdrawn after unloading to complete the next feeding and pre-plasticizing process.(under the general condition of the existing injection molding, the action of the ejector seat evacuation of the main gate of the mold has been canceled, and this action is executed when the material with serious salivation, such as PA, is formed).
7.Open mold and push out plastic parts
After cooling and shaping the plastic parts in the mold cavity, the mold locking mechanism opens the mold and pushes out the plastic parts in the mold.
Ejection looks simple in theory, but part sticking during injection molding is one of the most frequent disruptions on the shop floor. A part that refuses to release cleanly from the core can crack, deform, or leave fragments behind that damage the next shot. Common causes include insufficient draft angle on vertical walls, over-packing during the holding phase, inadequate mould surface polish, and excessively high mould temperature on the core side. Applying a proper mould release agent may offer a temporary fix, but the long-term solution is to address the root cause - whether that means re-cutting draft angles, adjusting holding pressure, or improving the ejector pin layout. Operators who understand the injection moulding procedure at this level can often diagnose sticking issues within a few cycles rather than losing hours of production time.
Quality Inspection and Post Injection Molding Assembly
The moulding cycle itself is only part of the story. Once parts are ejected, they enter a post-processing sequence whose complexity depends on the product's requirements.
At a minimum, each shot undergoes a visual check for surface defects - flash along the parting line, burn marks near the end of fill, or silver streaks caused by moisture in the resin. For dimensionally critical components, parts are measured with calipers, gauges, or a coordinate measuring machine (CMM) against the drawing tolerances. Many manufacturers run statistical process control (SPC) on key dimensions so that trends can be caught before out-of-spec parts are shipped.
Secondary operations following the plastic injection molding process steps may include gate trimming (removing the vestige where the runner meets the part), ultrasonic welding, pad printing or hot stamping for branding, and mechanical assembly with fasteners or snap fits. In projects that require plastic injection molding assembly with metal hardware - such as threaded brass inserts or electrical contacts - the inserts can be pressed in after moulding using heat or ultrasonic energy, or they can be placed into the mould cavity before injection in what is known as the insert moulding process for encapsulating metal components directly in the cavity. Insert moulding eliminates a separate assembly step and produces a stronger plastic-to-metal bond because the resin flows directly around the insert during cavity filling.
For those new to the field, it helps to remember that the term "injection molding" covers a family of related processes, not just one. Standard single-material moulding is the most common, but the same fundamental principle of injection moulding - melting resin, injecting it under pressure, and cooling it in a shaped cavity - also applies to insert moulding (where a pre-placed metal or ceramic component is encapsulated), overmoulding (where a second material is moulded onto an existing part), and bi-injection molding with two-shot dual-material cavity tooling (where two resins are injected through separate barrels in a single cycle). Each variant adds steps to the basic workflow but shares the same core plasticizing-and-filling mechanism described above.
Mapping the Complete Injection Moulding Process Diagram
Visualising all of the stages above in a single injection moulding process diagram helps production and quality teams see where each step depends on the one before it. A typical diagram follows a closed loop:
Mould Design → Mould Machining & Assembly → Machine Setup → Resin Drying & Feeding → Plasticizing → Clamping → Injection → Holding Pressure → Cooling → Mould Opening → Ejection → Inspection → Post-Processing / Assembly → (repeat cycle from Resin Feeding for the next shot).
Mapping the flow this way makes it easier to identify bottlenecks. For example, if cycle time is too long, the diagram quickly points attention to the cooling phase - the stage that almost always dominates total cycle time in a plastic molding process. If scrap rate is climbing, the diagram helps trace the issue back to whether it originates in material prep (moisture), in the injection phase (pressure or speed settings), or in ejection (sticking or deformation). Teams that maintain a living process-flow document alongside their standard operating procedures tend to onboard new operators faster and sustain more consistent output over time.