Injection Moulding Machine Selection Guide: Clamping Force, Screw and Energy

Specifying an injection moulding machine starts with one calculation: projected area of the part in the mould multiplied by process pressure gives the required clamping force. Skip it and the machine either flashes the mould open or, if oversized, wastes energy on every single cycle.

Calculating Clamping Force

The practical method is to multiply the total projected area of part and runner (cm²) by a material-specific unit pressure, generally 300-600 kg/cm². Use the lower end for easy-flow materials such as PP and PE, and the upper end for glass-filled engineering plastics. Add a 10-20 percent safety margin. A part with 150 cm² projected area at 400 kg/cm² needs about 60 tonnes, so with margin a 70-80 tonne machine is right.

Screw and Injection Unit

Screw diameter and L/D ratio determine plasticising capacity and melt homogeneity. General purpose screws use 20:1 to 22:1 L/D; heat-sensitive materials such as PVC use lower ratios and technical plastics higher ones. Shot volume should cover at least 1.3 times part weight, and running below 30 percent or above 80 percent of machine shot capacity causes quality problems.

Machine Classes and Energy Compared

Machine type	Clamping force	Energy use	Cycle repeatability	Typical application
Hydraulic (fixed pump)	50-1,000 t	0.8-1.2 kWh/kg	Moderate	General purpose
Hydraulic (servo pump)	50-1,600 t	0.45-0.7 kWh/kg	Good	Mid to high volume
Hybrid	50-800 t	0.4-0.6 kWh/kg	Very good	Technical parts
All-electric	30-650 t	0.3-0.5 kWh/kg	Excellent	Medical, precision
Extrusion line (pipe)	-	0.25-0.45 kWh/kg	-	Pipe, profile, film

Used Injection Machine Checklist

1. Read total cycles and running hours from the control; above 20 million cycles a mechanical rebuild is close.
2. Have screw and barrel wear measured; a worn screw is the most common cause of inconsistent filling.
3. Compare platen dimensions, tie-bar spacing and mounting hole patterns against your own moulds.
4. Look for scoring and corrosion on tie bars and play in the toggle bushings.
5. Check hydraulic oil temperature, filter condition and pump noise under load, and request an oil analysis.
6. Confirm every heater band works and that temperature control is stable.
7. Run a trial: across 20 consecutive shots, part weight variation should stay below 0.5 percent.
8. Clarify whether robots, conveyors, dryers and chillers are included in the sale.

Engel, Arburg, Haitian, KraussMaffei and Husky are the most commonly encountered manufacturers, and their parts and control support is what sustains used values.

Cost Per Part

• Raw material: 60-75 percent of total part cost in most cases, so reducing runner weight goes straight to margin.
• Energy: moving from hydraulic to all-electric roughly halves kWh per kilogram, which compounds fast in three-shift operation.
• Mould maintenance: plan servicing, cooling channel cleaning and core inspection every 100,000-250,000 cycles.
• Cooling: mould water temperature directly drives cycle time; for chiller capacity see HVAC and ventilation equipment.
• Drying: hygroscopic materials such as PA, PET and PC lose strength if moulded without a dryer.

Regrind and Runner Recovery

Runners and scrap can be ground and reused at a controlled ratio. For technical parts, 10-20 percent regrind is generally accepted; higher ratios degrade mechanical properties. For in-house recovery equipment and external waste plastic lines, see environmental and recycling machinery.

Moulds and Ancillary Equipment

The machine is only part of an injection moulding investment. The mould often matters more: cavity count, cooling channel design, hot runner system and core mechanisms determine both cycle time and part quality. An eight-cavity tool cuts cost per part markedly against a four-cavity one, but demands more clamping force and a larger shot volume.

Ancillaries are equally part of the cell. Material dryers, conveying systems, mould temperature controllers, chillers, robots and take-off conveyors must be planned together. An undersized mould temperature controller is one of the most common causes of unnecessarily long cycles.

When buying used, clarify not only mould compatibility but also whether ancillaries are included. A machine bought without a robot and dryer creates unexpected extra investment in the first months. Check as well whether hot runner controllers come with the machine and whether the number of zones covers your tools.

Energy metering is one of the fastest paybacks in plastics processing. Once a kWh meter is fitted per machine, it becomes visible within weeks which mould and which settings drive unnecessary consumption. Seeing a 20 percent gap between two machines making the same part is common, and the cause is usually mould cooling or a settings error.

New or Used?

In injection moulding, shift count and part precision decide. A new machine leads on energy efficiency and cycle repeatability; a used one lets low-volume or seasonal producers buy a second machine for the same budget.

Criterion	New machine	Used machine
Initial investment	High	Substantially lower
Lead time	8-30 weeks after order	Usually 1-3 weeks
Warranty and service	Manufacturer warranty, commissioning included	Limited or dealer warranty
Technology and efficiency	All-electric drive, closed-loop process control, MES link	One or two generations behind
Spare parts	Secured for many years	Depends on model age
Depreciation	Steep loss in the first years	Much slower value loss
Finance and leasing	Suits leasing and incentives	Limited, needs appraisal

A new all-electric injection machine halves energy per kilogram against older hydraulic machines and markedly improves cycle repeatability. Closed-loop process control lowers scrap. Production data flows straight into the MES, which simplifies the traceability required for medical and automotive parts.

What to ask when buying new matters just as much as the used checklist:

• Lead time: the schedule from order confirmation to commissioning must be in writing.
• Warranty scope: duration, excluded parts and response time should be stated clearly.
• Service network: ask the distance to the nearest authorised service and its response time.
• Parts commitment: stock status and lead time for critical parts belong in the contract.
• Training and commissioning: confirm operator training and acceptance tests are included in the price.

Frequently Asked Questions

What tonnage machine do I need?

Calculate projected area of part and runner in cm², multiply by 300-600 kg/cm² depending on material, and add a 10-20 percent margin. The result is the required clamping force in tonnes.

Does an all-electric machine pay back?

In three-shift production with short cycles, the energy saving typically covers the price premium in three to five years. For single-shift, low-volume work, a servo-hydraulic machine is the more balanced choice.

Will my mould fit any machine?

No. Platen size, tie-bar clearance, minimum mould height, ejector pattern and maximum daylight all have to match. Compare your mould drawings with the machine data sheet before you buy.

How do I detect screw wear?

Weight variation at fixed settings, short shots, black specks and colour carryover are classic signs of a worn screw and barrel. For certainty, measure both diameters; barrel bore enlargement beyond 0.3 mm calls for a rebuild.

What should I check on an extrusion line?

Screw and barrel condition, gearbox noise, calibration tank vacuum performance, haul-off synchronisation and cutter repeatability. The line is a single system, and one tired unit slows everything downstream.

When is a new all-electric machine the right call?

In three-shift production with short cycles and tight tolerances, a new all-electric machine wins on energy and scrap. In a single-shift plant making large, simple parts, a used servo-hydraulic machine is the more balanced investment.

To compare new and used injection, extrusion, blow moulding and granulating machines, browse the plastics and rubber machinery category.