Hydraulic, Electric, and Hybrid Plastic Injection Molding: Which Process is Right for You?

Design engineers have a variety of options when it comes to choosing the plastic injection molding process that is best suited for their specific application. Each of the three main methods (hydraulic, electric and hybrid) offers unique advantages and disadvantages. To make the right choice for your project, it is important to fully understand how these methods differ and what they can offer you.

Hydraulic Injection Molding

Hydraulic Plastic Injection Molding Machines (IMM’s) Depending on your specific needs, electricity costs and personal preferences, a hydraulic press may still be the best choice for the job.

These types of machines use hydraulic cylinders to clamp the two halves of the mold together under high pressure. The plastic substrate pellets are then melted and the liquid is injected into the mold cavity. Once the plastic cools and hardens, the two halves of the mold are separated, the part is removed, and the process is repeated.

To prevent the mold from being pushed open by the injection pressure, causing excess material to “flash” around the part, the clamping device must be able to provide enough clamping force to keep the mold closed during the injection process. For parts with thin wall sections and deep draw depths, a clamping force of approximately 3-4 tons per square inch of cavity is required. For thick-walled sections and shallow draw depths, about 2 tons per square inch is required.

Today’s hydraulic IMMs are capable of controlling clamping forces up to and exceeding 8,000 tons and can create parts weighing more than 50 pounds. Hydroforming is a popular choice in the automotive industry, which needs to produce large, heavy parts such as bumpers.

Advantages
Hydraulic injection molding is the preferred choice for driving core pulls, ejectors and valve gates, and for thick-walled parts that need to be held for long periods of time. Some of its advantages over all-electric machines include:

Greater clamping force for large parts
Better injection rates
High wear and tear resistance
Larger shot sizes
Better ejection capabilities
Can be equipped with accumulators to compensate for slower clamp movement
Lower initial purchase price
Low cost and high availability of replacement parts, resulting in lower maintenance costs
Easier to control advanced projects

Disadvantages
Because they are so powerful, hydraulic presses consume a lot of energy even when idle. While a typical electric motor might consume about 2.55 kWh during an injection molding process, a hydraulic press might consume 5.12 kWh. They require higher molding temperatures and more cooling time, and they are not suitable for cleanroom use due to the risk of fluid leaks. Hydraulic IMMs are noisier and less accurate than all-electric presses. However, as clamping forces are increased and control systems are improved, the precision performance of hydraulic presses will improve.

Electric Injection Molding
These machines are powered by digitally controlled high-speed servomotors rather than hydraulics, resulting in faster, repeatable, more precise and energy-efficient operation. Motor operation is highly predictable, so once the ideal injection process is achieved, it can be replicated very consistently, resulting in higher-quality parts. Once a job is programmed into a motor, its digital control allows it to run virtually unattended, which reduces labor costs and improves profits.

Advantages
Electric injection molding is well suited for cleanroom applications because there is no risk of oil contamination. Because of its high precision, the process is also well suited for small to medium-sized parts and medical products such as petri dishes and syringes. Other benefits include:
Tight accuracy and repeatability, reducing scrap ratesNever-leaking, clean processesReduced downtime typically associated with hydraulics30% to 70% energy savingsQuieter operation; reduces motor noise to less than 70 dBAHigher rapid injection speeds up to 800 mm/sec and faster clamping motionShorter startup times and up to 20% faster cycle timesLess material waste, lowering unit costsNo consumables, such as oils, needing replacement or cleaningLower power requirements significantly reduce operating costs

Disadvantages
While electric motors are faster, cleaner, and more energy efficient, they cannot match the clamping forces produced by hydraulic presses and have a higher initial cost. They are ball screw-driven toggle clamps, both of which are wearing parts that can be expensive to replace or difficult to find on the used market. And, despite offering great accuracy, the high positioning accuracy of electric motors can make them less forgiving than hydraulic presses.

Hybrid Plastic Injection Molding
Combining the best of both worlds, hybrid injection molding machines have been on the market for decades now, combining the superior clamping force of hydraulic presses with the precision, repeatability, energy savings, and low noise of electric motors. This results in better performance for both thin-walled and thick-walled parts. These machines have become increasingly popular over the past few years due to their efficiency and ease of use.

Nearly 66% of the energy in a hydraulic press is used to recover the screw. So while hybrid presses cost more upfront, their electric screw rotation can result in significant savings that can then be passed on to the customer. Our hybrid machines have all the power offered by a fully hydraulic machine, with nearly the same energy efficiency as a fully electric machine.

Advantages
Hybrid plastic injection molding is an energy-efficient and long-lasting process without as much demand, all of which results in a faster return on investment.
Some of their specific advantages include:
Servo pumps allow for continuous adjustment for product design versatility
Tends to use two clamping systems on the toggle
Median upfront cost for all three options (hydraulic, electric, hybrid) but can offer the greatest long-term savings
Closed-loop processes with faster response times
Lower temperatures require less cooling and extend oil and machine life

Disadvantages
Because all hybrids are different, it can be difficult to match the correct press to the product and to provide a reliable replacement machine for certain applications. If any maintenance is required on the hybrid machine, the supervisor must have knowledge of both hydraulic and electric presses.