Electric machines promise energy savings, precision, and cleanliness. Hydraulic machines offer power density and lower capital cost. The choice depends on what you’re making and what you’re paying for electricity. Neither technology dominates across all applications; each has strengths that suit specific production requirements. Understanding the differences enables machine selection matched to actual needs rather than marketing claims.
How Hydraulic Machines Work
Hydraulic injection molding machines convert electric motor energy into hydraulic pressure, then use that pressure to power all machine movements.
Core components: An electric motor drives a hydraulic pump, which pressurizes oil in a reservoir. Valves direct this pressurized oil to cylinders that move the clamp, advance the injection screw, and operate auxiliary functions. Control systems regulate valve positions to achieve desired movements.
Pump technology has evolved significantly. Early hydraulic machines used fixed displacement pumps running constantly, consuming energy whether moving or not. Modern machines use several approaches:
Variable displacement pumps adjust output to match demand, reducing energy consumption during holding phases.
Servo-driven pumps match pump speed to demand, shutting down nearly completely during idle phases. These servo-hydraulic machines represent the current efficiency standard for hydraulic technology.
Accumulator-assisted systems store energy during low-demand phases and release it for high-demand movements like injection, enabling smaller pumps while meeting peak power needs.
Energy consumption patterns: Hydraulic systems consume energy continuously to maintain pressure, even when nothing moves. During the holding phase of the molding cycle, hydraulic systems maintain clamp pressure by keeping oil under pressure. This continuous energy draw is the fundamental inefficiency that electric machines address.
How Electric Machines Work
All-electric injection molding machines use servo motors directly driving ball screws or belt systems for all machine movements.
Direct drive approach: Each axis of motion (clamp, injection, ejection, plasticating) has dedicated servo motors that operate independently. Motors consume energy only when moving against load. When holding position (like maintaining clamp force), motors hold position with minimal energy consumption because ball screws are self-locking.
Regenerative capability: When decelerating heavy movements (like stopping clamp opening), servo motors act as generators, returning energy to the electrical system or storing it in capacitors. This regeneration can recover 15 to 30 percent of acceleration energy depending on cycle characteristics.
Independent axis control: Because each motion has its own drive, movements can overlap freely. Plasticating can begin while ejection completes. Mold opening can occur while cooling continues. This independence often allows cycle time reduction through parallel operations that hydraulic machines cannot achieve without additional pumps.
Energy Consumption Comparison
Energy savings claims vary widely, and context matters enormously. Marketing materials often compare best-case electric scenarios against worst-case hydraulic scenarios, producing impressive but misleading numbers.
All-electric versus conventional hydraulic: In this comparison, electric machines typically consume 50 to 70 percent less energy. The dramatic savings reflect the inefficiency of fixed-displacement pumps running continuously. This comparison remains valid only for machines without modern pump technology. However, many older hydraulic machines still in operation do use conventional pump technology, so these savings are achievable when replacing aging equipment.
All-electric versus servo-hydraulic: Modern servo-hydraulic machines have largely closed the efficiency gap. Real-world comparisons typically show all-electric consuming only 5 to 20 percent less energy than servo-hydraulic equivalents. The remaining advantage comes from the fundamental efficiency of direct drive versus hydraulic transmission. For new machine purchases, this comparison is more relevant than the conventional hydraulic comparison.
Application-dependent variation: Savings vary significantly by cycle characteristics.
Long cooling phases favor electric more (hydraulic maintains pressure throughout; electric just holds position). A part with a 60-second cycle and 45-second cooling time shows large electric advantage.
High-speed cycles favor electric less (more time in motion, less idle holding). A thin-wall packaging part with a 4-second cycle shows minimal electric advantage.
Large clamp forces favor hydraulic (power density advantages emerge at scale). Machines above 1,000 tons often find hydraulic systems more practical.
Calculating payback:
Energy cost difference = (Hydraulic kWh – Electric kWh) × Machine hours × Electricity rate
Simple payback = (Electric premium – Hydraulic cost) ÷ Annual energy savings
With electricity at $0.10/kWh and 6,000 annual operating hours, a machine consuming 15 kWh average on hydraulic versus 12 kWh on electric saves:
(15 – 12) × 6,000 × $0.10 = $1,800/year
If the electric machine costs $30,000 more, simple payback is 16.7 years.
With electricity at $0.20/kWh (common in many regions), the same calculation shows 8.3 years payback.
Energy cost assumptions dramatically affect the economic case for all-electric. Facilities with high electricity rates or sustainability mandates find stronger justification for electric technology than facilities with low energy costs and no environmental reporting requirements.
Precision and Repeatability
Electric machines offer inherent precision advantages from their direct-drive architecture. Whether these advantages matter depends on application requirements.
Positioning accuracy in electric machines typically reaches ±0.01mm or better for injection stroke, cushion position, and mold position. Servo motors with encoders provide closed-loop position control with resolution measured in microns. Hydraulic systems achieve ±0.05mm or somewhat better with servo valves and feedback, but the compressibility of hydraulic oil limits ultimate precision. Under pressure, hydraulic fluid compresses slightly, introducing variability that direct mechanical drive avoids.
Speed consistency matters for sensitive processes. Electric machines maintain programmed injection speeds with minimal variation because the relationship between motor speed and screw speed is direct and predictable. Hydraulic speed depends on oil temperature, which varies with machine warm-up and ambient conditions. Cold oil flows differently than hot oil. Modern hydraulic controls compensate, but electric systems have inherent advantages. For processes where fill rate affects part quality, electric consistency can reduce scrap.
Shot-to-shot repeatability in critical dimensions benefits from electric precision. Medical components, optical parts, and precision technical parts often see Cpk improvements on electric machines. A process running at Cpk 1.2 on hydraulic equipment might achieve Cpk 1.5 or higher on electric, providing more margin against specification limits. The value of this improvement depends on how close existing capability is to requirements and the cost of defects.
Injection speed profiling benefits from electric responsiveness. Complex parts requiring variable injection speed through the fill stroke achieve better profile fidelity on electric machines. The servo motors accelerate and decelerate more quickly and predictably than hydraulic systems, enabling sharper transitions between speed stages.
Which applications require electric-level precision:
Micro-molding with extremely tight tolerances
Medical components with functional dimension requirements
Optical parts requiring surface quality consistency
Electronic connectors with fine-pitch features
Multi-shot applications requiring precise positioning
Many industrial parts don’t require electric-level precision. Over-specifying machines for parts that would process equally well on modern hydraulic machines wastes capital.
Cleanliness and Maintenance
Operating environment differences influence machine selection for certain applications.
No hydraulic fluid in electric machines eliminates oil leak potential, oil mist, and the risk of product contamination from hydraulic system failures. This advantage matters for medical, food-contact, pharmaceutical, and cleanroom applications. For general industrial production, oil management is routine rather than problematic.
Cleanroom compatibility: Electric machines can operate in ISO Class 8 or better cleanrooms without special enclosures. Hydraulic machines require additional containment and ventilation to manage potential emissions.
Maintenance differences:
Hydraulic machines require oil changes (typically annually or per manufacturer schedule), filter replacement, seal inspection, and hydraulic system monitoring. These are familiar maintenance activities with well-established procedures.
Electric machines require servo motor care, ball screw lubrication, belt or coupling inspection, and encoder maintenance. While eliminating hydraulic maintenance, these electrical and mechanical maintenance needs substitute different (though often lighter) requirements.
Maintenance cost comparison varies by application intensity and machine quality. High-quality machines of either type require less maintenance than economy models. Total maintenance costs over machine life typically favor electric machines slightly, though the difference rarely dominates the economic analysis.
Capital Cost and Total Cost of Ownership
Electric machines carry price premiums that must pay back through operational advantages.
Initial cost premium ranges from 10 to 30 percent for equivalent tonnage and capability. Premium magnitude has decreased as electric technology has matured and production volumes have increased. The gap is smaller for small machines (where electric dominates) and larger for big machines (where hydraulic remains standard).
Total cost of ownership calculation should include:
Initial purchase price difference
Energy cost difference over expected life (typically 15 to 20 years)
Maintenance cost difference
Precision advantages that reduce scrap or enable higher-value applications
Resale value differences (electric machines often retain value better)
The economic analysis depends heavily on local electricity costs, operating hours, and how precision differences translate to value for specific applications.
Hybrid Options
Hybrid machines combine technologies, seeking advantages of each. Pure approaches sometimes leave value on the table that hybrid designs capture.
Servo-hydraulic machines use electric servo motors to drive hydraulic pumps, gaining much of the efficiency advantage of electric while maintaining hydraulic force transmission. This approach dominates modern hydraulic machine design and largely closes the energy gap with all-electric. The servo motor runs only when flow is needed, eliminating the continuous energy consumption of conventional pumps. Most new hydraulic machines use this technology.
Electric injection with hydraulic clamping puts electric precision where it matters most (injection) while using hydraulic power density for clamping. This combination suits large machines where full-electric clamping becomes costly and complex. The injection unit benefits from electric precision for shot control, while the clamp system uses hydraulic cylinders that handle large forces efficiently. This hybrid dominates the mid-to-large tonnage range.
Electric clamping with hydraulic injection is less common but exists for applications prioritizing clamp precision and speed while accepting hydraulic injection characteristics. Toggle clamps driven by servo motors provide fast, precise clamp operation while hydraulic injection handles materials or processes where hydraulic characteristics are acceptable.
Two-platen hydraulic machines represent another design variation common in large tonnage ranges. By eliminating tie bars on one side, these machines allow wider mold access for large tools. They remain hydraulic but offer operational advantages for certain large-part applications.
Hybrid machines offer compromise solutions where neither pure approach is ideal. They accept some complexity from managing two power systems in exchange for optimizing each function. The additional complexity requires maintenance personnel familiar with both technologies.
The hydraulic versus electric debate has no universal answer. Application requirements, energy costs, and production environment determine which technology delivers better value. The trend toward electric in smaller machines and servo-hydraulic in larger machines reflects the current technology optimization. Matching technology to actual requirements, rather than following generalized recommendations, produces the best outcomes.
Sources
- Plastics Technology. “Machine Technology Comparison.” https://www.ptonline.com/
- Modern Plastics Encyclopedia. “Injection Molding Machines.”
- Engel. “Machine Technology Overview.”
- Arburg. “Hydraulic vs. Electric Machine Comparison.”
- Nissei. “All-Electric Advantages and Applications.”
- Society of Plastics Engineers. “Machine Selection Guidelines.”