Korean Injection Molding — Why the Drive System Is the Machine’s Critical Path
The planetary gearbox injection molding machine selection problem is uniquely Korean in its scale and consequence. Korea is one of the world’s largest producers of plastic injection-molded components — automotive interior parts, electronic enclosures, medical device housings, and packaging materials for the consumer goods sector. Korean injection molding machine (IMM) manufacturers and the Korean factories that operate Japanese and European IMMs both face the same fundamental machine economics: cycle time is revenue, and cycle time is governed by the servo drive system.
The transition from hydraulic to all-electric injection molding machines — which Korea has adopted faster than most markets, driven by the energy cost consciousness of Korean manufacturing — places the servo gearbox at the centre of machine performance. In a hydraulic IMM, a single hydraulic pump serves all functions sequentially. In an all-electric IMM, each axis has a dedicated servo motor and gearbox, and all axes can operate simultaneously — the clamp can be closing while the screw is plasticising material for the next shot. This parallel operation multiplies throughput but also multiplies the number of gearbox selection decisions on every machine BOM.
Korean all-electric IMM manufacturers — including those supplying to the domestic automotive sector (Hyundai, Kia, Samsung SDI battery cell holders) and the electronics sector (LG Electronics housing components) — typically specify 5–8 servo axes per machine. With machine volumes of 200–2,000 units per year per major Korean OEM, gearbox specification decisions have significant BOM and quality impact.
Five Servo Axes — Why Each Requires a Different Gearbox Specification
The fundamental error in planetary gearbox injection molding specification is treating the five servo axes as equivalent servo drives that need the same gearbox. They are not. Each axis has a unique combination of peak torque, continuous torque, speed, backlash sensitivity, radial load, and thermal duty cycle that points to a different series and frame.
| Axis | Output Speed | Peak / Cont. Torque | Backlash Need | Primary Constraint | La Corée toujours puissante |
|---|---|---|---|---|---|
| ① Injection axis | 10–200 mm/s linear | 3–8× / 1.0× | P1 (shot weight) | Peak torque + axial load | EP-AF P1 (high axial) |
| ② Screw rotation | 5–200 rpm | 1.5× / 0.8× | P2 (speed only) | Continuous torque × hours | EP-AB P2 or EP-BPG |
| ③ Clamp (toggle) | 50–300 mm/s linear | 2× / 0.6× | P1–P2 (position) | Cycle count × impact | EP-AB P1 |
| ④ Ejector | 20–150 mm/s linear | 2.5× / 0.5× | P2 (position only) | Compact, moderate torque | EP-AB P2 (smaller frame) |
| ⑤ Rotary table | 1–30 rpm index | 1.5× / 0.7× | P0 (part precision) | Index accuracy | EP-AFH or EP-AB P0 |
The table makes the over-specification cost immediately visible. If a Korean IMM designer specifies EP-AFH (≤1 arcmin standard, highest precision) on all five axes because it is a simple, safe default, they are paying the premium on axes ②③④ where P1 or P2 is fully adequate. Correct axis-by-axis specification delivers equivalent machine performance at significantly lower gearbox BOM cost.
Injection Axis — Peak Torque Ratio and the Axial Load from Melt Back-Pressure
The injection axis drives a ball screw that converts servo rotary motion to the linear force that pushes the injection screw (plunger) forward, injecting molten plastic into the mould cavity at high pressure (typically 800–2,500 bar melt pressure). This is the highest peak torque axis in the machine — and it is the axis most commonly under-specified by Korean IMM designers who size on continuous torque rather than peak.
The peak torque profile of the injection axis differs from all other machine servo axes: during the filling phase, the servo motor delivers continuous torque to maintain injection velocity against rising melt pressure. At the transition from filling to packing (the “cushion” point), the servo must instantaneously provide 3–5× the filling torque to compress the melt against the closed mould face. This peak is brief (50–200 ms) but occurs on every shot — at 8 seconds cycle time and 6,000 hours per year, it occurs approximately 2.7 million times per year.
INJECTION AXIS — BACK-PRESSURE AXIAL FORCE
Melt back-pressure acts on screw face areaF_axial = P_back × A_screw
P_back = back-pressure setting (MPa, typically 5–30 MPa)
A_screw = screw cross-section area (mm²)
Example: Ø50 mm screw, P_back = 15 MPa:
A_screw = π × 25² = 1,963 mm²
F_axial = 15 × 1,963 = 29,450 N (≈3 tonnes)
This axial force acts on the injection axis gearbox
output shaft during the entire plasticising phase
(typically 2–4 sec per cycle).
At 2.7M cycles/yr → 5.4M–10.8M sec/yr
of sustained axial loading on output bearing.
The axial force from melt back-pressure is the specification item most frequently omitted in injection axis gearbox selection. Korean IMM designers who select the injection axis gearbox on output torque alone — which is correct for the rotary drive — miss the axial bearing load entirely. The Korea Ever-Power EP-AF high-rigidity series is the standard recommendation for injection axis drives precisely because its enlarged output shaft and upgraded bearing arrangement provides substantially higher axial load capacity at the same frame size and torque rating as EP-AB.
A 500T Korean IMM producing PP automotive bumper sub-components had repeated injection axis gearbox bearing failures at 14–18 months. The original specification (EP-AB140 P1) met the torque requirement but ignored the Ø60mm screw back-pressure axial force of approximately 42,000 N. Switching to EP-AF140 (same frame, 2.3× higher axial capacity) resolved bearing failures completely — 28 months continuous operation at the time of this writing with no bearing issues.
✓ Peak torque at packing (3–5× cont.)
✓ Axial force from back-pressure
✓ Cycles/year × peak torque duration
✓ Specify EP-AF (not EP-AB) for axial capacity
✓ Backlash P1 adequate (shot weight, not CNC precision)
Screw Rotation Axis — The Highest Thermal Duty Cycle in the Machine
The screw rotation axis drives the injection screw in rotation to plasticise (melt) the polymer resin during the recovery phase of each cycle. Unlike the injection axis — which operates at high torque for a brief burst — the screw rotation axis operates at moderate torque for the entire recovery period, which may represent 40–70% of the total cycle time in efficient moulding.
This continuous moderate-torque operation makes the screw rotation axis the highest thermal duty cycle drive on the machine. At 60% of cycle time in continuous three-shift Korean production (6,300 hours per year), the screw drive gearbox accumulates approximately 3,780 operating hours per year — comparable to a high-cycle conveyor drive rather than an intermittent servo axis. The temperature correction from Module 3 of Art15 applies directly: at elevated Korean summer ambient in a plastics factory, the screw drive gearbox housing temperature can reach 75–85°C, reducing grease life below the 20,000-hour catalogue rating.
Backlash grade on the screw rotation axis is genuinely irrelevant — the axis controls screw rotational speed, not position. The screw back-drives slightly at each shot (axially, not rotationally) but the rotation axis gearbox sees only the torque from material shear and the screw’s flight helix resistance. P2 (≤5 arcmin) is the correct specification; the additional cost of P0 or P1 on this axis provides zero functional benefit.
The EP-BPG energy-saving series (≥97% efficiency, IEC worm-replacement flange) is a strong candidate for the screw rotation axis when the machine uses an induction motor for screw drive — common on Korean medium-size IMMs where only the injection and clamp axes are servo-controlled. The BPG’s IEC-standard flange fits the motor without adapter, the sealed grease construction handles the continuous thermal duty, and P2 backlash is standard. For fully electric machines where the screw drive uses a servo motor, EP-AB P2 at the appropriate frame provides the same thermal capability with the servo motor adapter interface.
Clamp Axis — Billion-Cycle Life Requirement and Toggle Impact Load
The clamp axis closes and opens the mould on every cycle. For a Korean IMM running at 8-second cycle time in three-shift continuous operation, the clamp axis completes approximately 2.7 million open-close cycles per year. Over a Korean IMM’s expected 15-year service life, this accumulates to approximately 40 million clamp cycles — each one a full-stroke motion from mould-open to mould-closed and back.
Most Korean IMMs use a toggle mechanism for the clamp axis — a linkage that amplifies the servo motor force to achieve the required clamp force (typically 100–5,000 kN for Korean production machines). The toggle produces a characteristic velocity profile: slow at mould-open and near close (for mould protection), fast through mid-stroke, and a sudden deceleration at full-clamp lock. This deceleration creates a brief impact load on the gearbox output — a torque spike at the end of each clamp stroke that can reach 2–2.5× the continuous rated torque.
The gearbox design life calculation for the clamp axis must account for this peak torque cycle count. Using the L10 bearing life formula from Art16 with the actual equivalent dynamic load (a weighted combination of peak and continuous torque contributions over the cycle) rather than just the continuous torque produces a more accurate service life prediction — and typically shows that the EP-AB P1 specification is adequate for standard Korean IMM clamp applications, while heavy-clamp high-speed machines may warrant EP-AF P1 for the additional bearing load capacity.
CLAMP AXIS — EQUIVALENT DYNAMIC LOAD OVER CYCLE
Fast traverse (3 sec): T_cont = 120 N·m
Deceleration (0.3 sec): T_peak = 280 N·m (2.3× cont.)
Clamp dwell (3 sec): T_hold = 30 N·m
Open stroke (1.7 sec): T_cont = 100 N·mEquivalent dynamic torque (L10 weighted):
T_eq = [(T₁³×t₁ + T₂³×t₂ + …) / t_total]^(1/3)
T_eq = [(120³×3 + 280³×0.3 + 30³×3 + 100³×1.7) / 8]^(1/3)
T_eq = [(5.18M + 65.9M + 0.081M + 1.70M) / 8]^(1/3)
T_eq = [9.11M]^(1/3) = 208 N·m
vs peak-selected T = 280 N·m (34% over-spec if using peak)
vs cont-selected T = 120 N·m (42% under-spec if using cont. only)
This calculation is the correct basis for clamp axis gearbox selection. Using only the peak torque (280 N·m) oversizes the gearbox by 34%; using only the continuous torque (120 N·m) undersizes it by 42%. The equivalent dynamic load method, which is standard in the Korea Ever-Power EP application engineering process, correctly identifies 208 N·m as the effective selection torque.
Rotary Table and Insert Station — Where Precision Actually Matters
Korean IMMs producing multi-component parts — overmoulded connectors, insert-moulded metal components, two-colour cosmetic parts — use a rotary table or index plate that rotates the mould between injection stations. The rotary table is the one axis in the injection molding machine where backlash genuinely matters for part quality.
The indexing accuracy requirement comes from the part geometry: for a two-colour injection part, the second-colour gate must land within ±0.3–0.5 mm of the first-colour feature edge. At a typical rotary table radius of 200–400 mm, this translates to a required index accuracy of:
θ_max = Δx/r = 0.3/300 = 0.001 rad = 3.4 arcmin
Gearbox budget (40% of total): 1.4 arcmin→ P0 (≤1′) adequate with margin
→ EP-AFH (≤1′ standard) eliminates grade selection step
→ P1 (≤3′) marginal — worst case may exceed budget
The EP-AFH ultra-precision series is the standard specification for Korean IMM rotary table drives. Its ≤1 arcmin standard (no grade code, no unit-to-unit variation within a grade band) provides the accuracy margin that P1 cannot reliably deliver on every production unit. The non-standard ratios available in EP-AFH (i=3 to i=100 in single stage) accommodate the carousel geometry without requiring a non-standard ratio order that would extend lead time.
For Korean IMMs producing insert-moulded automotive connectors, including compact staging mechanisms using EP-ADS compact series for tight-space index drives where the metal insert must align with a ±0.1 mm tolerance in the mould cavity, P0 is mandatory regardless of the rotary table radius — the insert position error adds directly to the final part dimensional tolerance and cannot be corrected downstream.
Insert moulding: ≤0.1mm → P0 mandatory
Single-colour index: ≤1.0mm → P1 OKEP-AFH: ≤1′ standard → all cases ✓
(no grade selection needed)
Injection Molding Cycle Life — Why the Peak Torque Multiple Is the Critical Selection Criterion
Injection molding machines accumulate cycle counts that no other Korean industrial machine approaches. A Korean food packaging IMM running at 8-second cycles in three-shift continuous operation completes approximately 2.7 million cycles per year. Over a 15-year machine life, this is 40 million cycles. For the gearboxes on the injection and clamp axes — which both experience a peak torque event on every cycle — the cumulative peak torque event count is the dominant fatigue life driver.
Korea Ever-Power EP series gearboxes are rated with both a nominal rated torque (for continuous operation) and a peak torque rating (typically 2–3× the rated torque, for brief events not exceeding a defined duration and count per hour). For injection molding applications, the relevant question is whether the peak torque events — each lasting 50–300 ms at 2–3× rated torque — accumulate fatigue damage in the gear teeth at a rate that limits service life below the catalogue value.
| Paramètre | Catalogue Assumption | Korean IMM Reality | Verdict |
|---|---|---|---|
| Peak events per hour | ≤1,000/hr | 450/hr (8s cycle) | ✓ Within catalogue |
| Peak torque multiple | ≤3× rated | 2.3–4× rated | ⚠ Confirm per machine |
| Peak duration per event | ≤200 ms | 50–300 ms | ✓ Within catalogue |
| Annual peak count | ~1M/yr | 2.7M/yr | Confirm cycle life basis |
The peak torque multiple is the most critical parameter to confirm. If packing-phase injection force results in a torque multiple above 3×, Korea Ever-Power application engineering recalculates service life using the actual peak/continuous ratio for your specific machine specification.
Optimised IMM Gearbox BOM — Cost Comparison: Default vs Axis-Specific Specification
The following BOM comparison illustrates the cost impact of correct axis-by-axis specification vs the common Korean OEM default of specifying identical gearboxes across all servo axes. This example uses a 200T Korean all-electric IMM with five servo axes.
| Axis | Default spec (EP-AFH × 5) | Optimised spec | Cost saving / axis |
|---|---|---|---|
| ① Injection | EP-AFH 140 ≤1′ (axial NG) | EP-AF140 P1 (high axial ✓) | +₩120,000 (correct spec) |
| ② Screw rotation | EP-AFH 090 ≤1′ (over-spec) | EP-BPG P2 (thermal duty ✓) | −₩480,000 saved |
| ③ Clamp | EP-AFH 115 ≤1′ (over-spec) | EP-AB115 P1 (equivalent dynamic ✓) | −₩360,000 saved |
| ④ Ejector | EP-AFH 060 ≤1′ (over-spec) | EP-AB060 P2 (compact ✓) | −₩280,000 saved |
| ⑤ Rotary table | EP-AFH 090 ≤1′ ✓ (correct) | EP-AFH 090 ≤1′ (same, correct) | No change |
| Net BOM saving per machine (optimised vs all-AFH default) | −₩1,000,000 | ||
A Korean IMM manufacturer producing 300 machines per year at ₩1,000,000 BOM saving per machine through correct axis-by-axis gearbox specification realises ₩300,000,000 per year in component cost reduction — while simultaneously improving injection axis reliability by switching from EP-AFH (not rated for back-pressure axial load) to EP-AF (rated for it). Correct specification simultaneously reduces cost and improves reliability. This is the engineering case that Korea Ever-Power application engineers present to Korean IMM OEM procurement teams.
Frequently Asked Questions — Planetary Gearbox for Injection Molding Machines
Specify Your IMM Gearbox BOM with Korea Ever-Power
Korea Ever-Power performs axis-by-axis torque calculation — including back-pressure axial load, equivalent dynamic clamp torque, and screw rotation thermal duty cycle — and provides an optimised five-axis gearbox BOM for Korean injection molding machines. Same working day, in Korean.
Éditeur : Cxm
