"Plastic goes in, part comes out" is true, but it skips the part that actually matters: what happens inside the machine between those two moments. This guide walks through the real working principle — the two systems that do all the work, why the screw both rotates and slides, why clamping force has to match the part rather than just being "big enough," and the exact sequence of actions the machine runs through, second by second, on every single cycle.
Understand it in 30 seconds
Before the pressures, temperatures, and screw geometry, here is the mental image that makes the whole machine click:
The analogy
Picture an oversized medical syringe. You load it with a dose, push the plunger, and the contents shoot out the needle into a precise location. An injection molding machine works on exactly this principle, at industrial scale, with molten plastic instead of liquid.
Solid plastic pellets are fed into a heated barrel, melted into a thick, honey-like fluid, then forced — under high pressure — through a nozzle into a closed steel mold. The plastic fills every cavity, cools into a solid shape in seconds, and the mold opens to release a finished part.
That single cycle — melt, inject, hold shut, cool, release — is the entire working principle of injection molding. Everything else in this guide is really just an explanation of how each of those five actions is carried out with enough precision to make identical parts, ten thousand times in a row.
Anatomy: the two systems inside every machine
Regardless of size, brand, or configuration, every injection molding machine is built around two systems that do fundamentally different jobs.
Injection unit
Melts the resin and forces it into the mold.
- Hopper — feeds raw pellets by gravity
- Barrel with heater bands — melts the resin
- Reciprocating screw — conveys, shears, and meters the melt
- Non-return (check) valve — stops melt flowing backward during injection
- Nozzle — seats against the mold and delivers the shot
Clamping unit
Holds the mold shut against the pressure of the incoming plastic.
- Fixed platen — holds one mold half stationary
- Moving platen — carries the other mold half and travels
- Tie bars — guide the moving platen and carry the clamping load
- Toggle linkage or hydraulic cylinder — generates the closing force
- Ejector system — pushes the finished part free after opening
The reason both systems need to be powerful is the same: because the cavity inside the mold reaches real, measurable pressure during filling — typically 20–45 MPa — the injection unit needs enough force to push molten plastic in against that resistance, and the clamping unit needs enough force to keep the mold from being pushed back open by it.
How machines are classified
Before looking at the cycle itself, it helps to know the three ways machines differ from one another, because the working principle changes slightly depending on which type you're looking at.
By plasticizing method: plunger vs. screw
The original injection molding machines pushed a solid plug, or plunger, of plastic forward to melt and inject it — a design with a real weakness, since the plug at the center melted more slowly than the plastic touching the hot barrel wall, giving uneven melt quality. The modern alternative, the reciprocating screw, both melts the resin through rotation and then moves forward like a plunger to inject it. Screw-type machines now account for roughly 95% of all injection molding equipment in use, because the screw's mixing action produces far more uniform melt and lets color or recycled material blend in evenly.
By drive type: hydraulic, mechanical, hybrid, electric
Clamping and injection force can be generated by oil-driven hydraulic cylinders (the traditional design, still dominant at larger tonnages), a mechanical toggle linkage that multiplies force through leverage for fast, low-cost clamping on smaller machines, or servo motors in hybrid and all-electric designs that trade some raw force for much higher repeatability and lower energy use.
By operation mode: manual, semi-automatic, automatic
A manual machine requires an operator to trigger each stage of the cycle by hand. A semi-automatic machine runs the full cycle on its own once triggered, but needs an operator to start each new cycle — usually because a part has to be removed by hand. A fully automatic machine repeats the cycle continuously and unattended, the part falling free or being removed by a robot, which is what makes high-volume production economical.
Four physical layouts: horizontal, vertical, angle, turntable
Beyond how force is generated, machines are also built in different physical layouts — and the layout changes how the cycle plays out in practice, especially around part removal.
| Layout | How it works | Best suited to | Main limitation |
|---|---|---|---|
| Horizontal | Clamping and injection share one horizontal centerline; the mold opens sideways | The large majority of production — low center of gravity, finished parts fall free by gravity | Larger floor footprint than a vertical machine |
| Vertical | Clamping and injection share a vertical centerline; the mold opens up/down | Small parts (generally under ~60 g shot weight) where inserts are placed into the mold by hand or robot before each shot | Parts don't fall free, so full automation is harder; unsuited to large/medium machines |
| Angle | Injection direction lies in the same plane as the mold's parting line | Flat parts where a gate mark cannot appear on the visible face | Inserts can tip or fall during cycling; limited to smaller machines |
| Multi-station turntable | Several molds mounted on a rotating table cycle through fixed stations | Parts needing long cooling or heavy insert-loading, where overlapping stations raise throughput | Complex clamping system; clamp force available at each station is limited |
The complete working cycle, step by step
This is the heart of the working principle: the exact sequence of actions the machine performs on every cycle, in order. Each stage exists to solve a specific physical problem — understanding why a stage happens is more useful than just memorizing its name.
Clamping
The clamping unit closes the two mold halves and locks them under force, before any plastic is injected. Closing speed typically follows a slow–fast–slow profile — slow to protect the mold and any inserts, fast across the open middle travel, slow again just before full closure to avoid impact damage.
Clamping force: 50–55,000+ kN, sized to the part (see Section 7)Pre-molding (plasticizing)
With the mold already closed, the screw rotates and pushes pellets forward through the heated barrel, melting them through a mix of friction and heater-band temperature. Melted material accumulates ahead of the screw's non-return valve, which pushes the screw itself backward. A travel switch stops this retreat at a preset point, which is what sets the exact shot size for this cycle.
Screw speed: 30–150 rpm · Barrel temperature: 180–320 °C (resin dependent) · Back pressure: 0.5–1.5 MPaReverse (suck-back)
Once plasticizing stops, the screw makes a small additional retreat without rotating. This relieves pressure built up at the nozzle, preventing melt from oozing out of the open nozzle before injection — a defect foundries call "drooling" or "salivation." Machines running a fixed-feeding routine may skip or shorten this step to save cycle time.
Purpose: prevent nozzle drool · Distance: a few millimetres, position-switch controlledNozzle advance
The entire injection unit slides forward on its carriage until the nozzle seats firmly against the mold's sprue bushing, sealing the path between barrel and cavity so no pressure is lost at the joint.
Injection (filling)
The screw stops rotating and drives straight forward like a plunger, pushing the metered melt through the runner system and gate into the cavity. Injection speed is usually programmed in multiple stages — fast through open areas, slowing near full cavity fill to avoid jetting or flash.
Injection pressure: 30–180 MPa · Cavity pressure: 20–45 MPa · Fill time: 0.5–5 secHold pressure
The instant the cavity is full, pressure switches from high injection pressure down to a lower, sustained holding pressure. This keeps feeding melt into the cavity as the plastic cools and shrinks, compensating for that shrinkage before the gate freezes shut — skipping this stage is one of the most common causes of sink marks.
Holding pressure: 30–70% of injection pressure · Holding time: 1–10 secNozzle retreat and cooling
The injection unit pulls back from the mold, and the part solidifies inside the closed cavity while internal cooling channels carry heat away. On modern machines, the screw begins plasticizing the next shot during this same window, overlapping stages 2–3 with cooling to shorten the overall cycle.
Mold temperature: 20–120 °C · Cooling: typically 50–60% of total cycle timeMold opens, part ejects, cycle repeats
Once the part is rigid enough to hold its shape, the clamping unit opens the mold and the ejector system pushes the part free — it either falls by gravity (horizontal machines) or is lifted out by hand or robot. On a fully automatic machine, the mold then closes again immediately and the entire sequence restarts at Step 1.
Total cycle time: typically 5–60+ sec, depending on part wall thicknessInside the injection unit: how the screw actually melts and meters plastic
Step 2 in the cycle above says "the screw melts the pellets" as if that were one simple action. It isn't — the screw is doing three different jobs as material travels along its length, and a single valve decides whether melt is allowed to move backward or only forward.
Three zones along one screw
A reciprocating screw is divided, by its geometry, into three functional zones: a feed zone near the hopper, where pellets are simply conveyed forward; a compression zone in the middle, where the channel depth gradually shrinks, compacting the pellets and squeezing out air while friction begins melting them; and a metering zone near the tip, where the now-molten plastic is homogenized to a uniform temperature before it reaches the nozzle. The ratio of the screw's length to its diameter (L/D ratio) and the ratio of channel depth at the feed end versus the metering end (compression ratio) are both chosen to suit the resin being processed.
Key parameters
L/D ratio: typically 18:1 to 24:1 · Compression ratio: typically 2:1 to 4:1 · longer L/D and higher compression ratios generally suit resins that are harder to melt uniformly.
Why the non-return valve matters
At the tip of the screw sits a check valve, usually a sliding ring. While the screw is rotating during plasticizing, this valve stays open, letting melt flow past the screw tip and accumulate in the space ahead of it. The moment the screw stops rotating and drives forward for injection, the rising pressure forces the ring backward, sealing the valve shut — so the screw now behaves like a solid plunger, pushing the entire accumulated melt forward with nowhere for it to leak back into the screw channel. Without this valve, injection pressure would simply push melt backward along the screw instead of forward into the mold.
Inside the clamping unit: why force has to match the part
Section 5 noted that clamping force ranges from 50 kN on a small machine to over 55,000 kN on the largest. That huge range isn't about machine "size" in a loose sense — it follows directly from a calculation every process engineer runs before a job ever starts.
The calculation
Clamping Force = Projected Area × Cavity Pressure × Safety Factor
Projected area is the area of the part (plus runners) as seen looking straight into the open mold — not its surface area or volume. A flat, wide part needs far more clamping force than a tall, narrow part of the same weight, because it presents a much larger area for the cavity pressure to push against.
Why force scales with projected area, not part weight
What happens if force is too low
If clamping force can't fully resist the cavity pressure pushing outward during filling, the mold faces separate by a tiny amount at the parting line. Plastic finds that gap and leaks out, producing flash, short shots, and dimensional inconsistency — and over time, accelerated wear on the mold's mating surfaces. This is exactly why "bigger is always safer" isn't really how machine sizing works: the right answer is the force that matches the part's projected area and the resin's required cavity pressure, not simply the largest machine available.
How the cycle is controlled
None of the steps above happen by themselves — every stage is triggered, monitored, and limited by a control system, which is what actually makes the machine's behavior repeatable from one shot to the next.
The position of the screw is tracked by travel (limit) switches: one stops the screw's retreat during plasticizing at exactly the right point to set shot size, another confirms the injection unit has fully advanced before injection is allowed to start, and another confirms the carriage has retreated before the mold is allowed to open. Barrel heater bands are divided into multiple zones along the barrel's length, each with its own temperature sensor and controller, so the resin reaches a uniform melt temperature rather than overheating near the nozzle and underheating near the hopper.
An operator interacts with this system through three linked layers: a control keyboard or touchscreen, where the process recipe — injection speed and pressure profile, hold pressure, hold time, screw speed, back pressure, cooling time — is programmed and stored; an electrical control cabinet, which houses the programmable logic controller and monitors temperature, current, and voltage at each barrel zone in real time, flagging any zone that drifts outside its set range; and the hydraulic (or servo-electric) power system, which actually executes each commanded motion at the pressure and speed the recipe specifies. Once a recipe is set and proven, the machine reproduces the exact same sequence of pressures, speeds, and switch points on every cycle without operator input — which is the entire reason injection molding can hold tight tolerances across runs of tens of thousands of identical parts.
A short history: from plunger to screw
The working principle described above — especially the screw, the non-return valve, and the clamping calculation — is largely the product of one engineering leap, not a gradual accumulation of small tweaks.
"Swapping a plunger for a rotating screw, on its own, is the single change that explains how almost every machine on a factory floor today actually works."
Frequently asked questions
What's the difference between the clamping unit and the injection unit?
The injection unit melts plastic and forces it into the mold; the clamping unit holds the two mold halves shut against the pressure that the injection unit generates. They have to be sized together — injection pressure determines cavity pressure, and cavity pressure determines how much clamping force is needed to keep the mold from opening.
Why does the screw both rotate and slide forward?
Rotation is how the screw melts and meters plastic, conveying and shearing pellets as it turns. Sliding forward, without rotating, is how it injects — once the non-return valve seals shut, the screw acts as a solid plunger, pushing the accumulated melt into the mold. One mechanism performs both jobs in sequence within a single cycle.
What does the non-return valve actually do?
It's a check valve at the screw tip that stays open during plasticizing, letting melted plastic flow past the screw and accumulate ahead of it, then closes the instant the screw drives forward for injection. Without it, injection pressure would push melt backward along the screw's channel instead of forward into the cavity.
Is clamping force the same thing as injection pressure?
No. Injection pressure (30–180 MPa) is what the screw applies to push melt through the nozzle and runners. Clamping force (measured in kN or tons) is what the clamping unit applies to keep the mold shut against the resulting cavity pressure (20–45 MPa). Clamping force is calculated from cavity pressure and the part's projected area, not from injection pressure directly.
Why does the screw retract slightly before injection even starts?
This is the "reverse" or suck-back step: a small retreat that relieves built-up pressure at the nozzle so melt doesn't ooze out before the injection unit advances and seats against the mold. Skipping it on machines without this feature can cause "drooling" at the nozzle between shots.
