Air Compressors for Plastic Injection Molding

Injection machine power mainly relies on hydraulic or electric drive; compressed air isn't the main power source. During production, multiple stages need compressed air to assist.

Air Usage Points in Injection Production

Mold Blow-off

Blowing into mold cavity before opening helps product release; 4-6 bar does it. Each cycle blows one to three seconds; single-use volume depends on product size, small ones one to two liters, big ones four to five liters. Blow air must be oil-free and water-free; oil causes oil spots on product surface, especially visible on clear parts.

Ejection Assist

Ejection assist cylinders mount on ejector plates or bars. Lots of factories completely miss this air use when selecting, figure hydraulic ejection is enough. Combined pneumatic-hydraulic ejection is used on big tonnage machines; cylinders handle fast idle stroke, hydraulics handle final ejection force, can shorten cycle by fractions of a second on deep cavity thin-wall products. Pressure 4-6 bar, average per machine very small, 0.02-0.05 m³/min or so.

Robots

Robot supply lines cannot share main pipes with blow-off. When robots and blow-off act at same time, if pickup fails, product drops and smashes the mold, just mold repair over $14,000. Shared piping causes air competition; pressure swings and robot suction loses grip.

Cylinders are sensitive to moisture. Domestic cylinders generally have lower tolerance here than imported, related to seal materials.

Mold Water Channel Purging

When changing molds, cooling water channels need blowing dry; otherwise residual moisture causes water marks on next mold's products and rusts the mold. Needs 6-8 bar, slightly higher than other stages, to completely blow out pooled water. Some factories use nitrogen for purging, claim it prevents mold steel oxidation; more common on precision molds.

Leak Testing

Leak testing pressure range varies big. Regular plastic containers 0.5-2 bar enough; auto intake manifolds 3-5 bar; fuel system parts might need above 10 bar. When selecting leak test equipment mainly look at test cycle time; fast ones under a second, slow ones three to five seconds.

Central Material Feeding

Central feeding transports plastic pellets from silos to each injection machine, either negative pressure suction or positive pressure conveying. Negative pressure uses roots blowers or vacuum pumps, strictly speaking not compressed air territory. Positive pressure conveying uses compressed air, typically 2-4 bar, depends on conveying distance.

Price gaps among central feeding suppliers are pretty big; equipment itself has minor differences, mainly in install and after-sales.

Auxiliary Equipment Pneumatic Valves

Pneumatic valves on dryers, mold temp controllers, chillers have tiny air use; plant-wide total won't exceed 0.5 m³/min, basically negligible.

Air Volume Estimation

Per Machine Calc

Formula is Q = (V × n × 60) / 1000, where Q is average air consumption (m³/min), V is single-use volume (L), n is cycles per minute.

Example, 80T small machine with 20-second cycle and 3L per use calculates to about 0.15 m³/min.

Experience Numbers

Small machines under 100T: roughly 0.1-0.2 m³/min per machine; 100-250T: 0.2-0.4; 250-500T: 0.3-0.6; 500-1000T: roughly 0.5-1.0; above 1000T: 0.8-1.5. All excluding robots; add 0.1-0.3 per robot.

This dataset from Atlas Copco selection guide, widely circulated in industry. Lots of compressor salespeople use it as reference. Runs conservative; selecting by this won't have problems, might have some wasted margin.

Plant-wide Calc

Total demand = Σ(per machine) × simultaneity factor

What simultaneity factor to use is always debated. Textbooks say 0.7-0.85; field engineers see it differently. 24-hour continuous production 0.75 is fine; shops with volatile orders and frequent mold changes, peak can hit 0.9 or above.

Most use points need 5-7 bar; main system pressure recommend 7 bar.

For air quality, product-contacting air needs dry and oil-free. Refrigerated dryers hit about +3°C pressure dew point, enough for most applications. Oil content has ongoing debate; some say oil-free mandatory, others say oil-flooded screw with multi-stage filtration can also meet spec. Cleanroom injection stance is clear: oil-free mandatory; medical and electronics have zero tolerance for oil.

Medical and food grade injection have higher requirements. Oil-free is baseline; some also need sterile compressed air. Config typically oil-free screw plus adsorption dryer plus sterilizing filter, costing about 2.5-3 times regular config. FDA has specific regs; those in this field know them.

Configuration Example

Take a 50-machine injection shop, average 150T, 30 robots, 1 central feeding system.

Air volume: 50 injection machines at 12.5 m³/min, 30 robots at 6, feeding system around 2, other auxiliary 1.5, total 22 m³/min. Times 0.8 simultaneity plus margin, design volume roughly 21 m³/min.

Economy config: 2 units 37kW VFD screw, 3 cubic meter receiver, 25 m³/min refrigerated dryer, three-stage filtration. Two VFD units auto-adjust by load; one fails, other sustains. Downside is no backup. Compressor brands, Atlas Copco stable for imports; Jaguar and Kaishan have good rep domestically, priced at roughly 60%.

Reliability config: 2 units 30kW VFD plus 1 unit 30kW fixed-speed backup, 4 cubic meter receiver, 30 m³/min refrigerated dryer. Auto parts factories basically all config this way; under JIT supply, downtime losses far exceed buying an extra unit. Backup normally off, run no-load once a week to keep condition.

Common Problems

Product surface has water marks that reappear after wiping dry; nine times out of ten moisture in blow air. Summer high temp high humidity, refrigerated dryer capacity drops; pressure dew point rising from +3°C to above +10°C is normal. First check if dryer has problems, then install auto drains at blow points, float type or electronic both work, don't use timer drains. Summer humidity high, refrigerated dryers often can't keep up; if possible use adsorption dryers.

Robot actions slowing needs sorting out. If pressure insufficient, gauge reading low; if flow insufficient, gauge may be normal but cylinder action still slow. Measure pressure at robot supply point vs main pipe; differential over 0.5 bar means piping resistance high, check pipe diameter and elbow count.

Cylinders dying often is mostly air quality issues: water causing rust, oil causing seal swelling. Add FRL unit before cylinders, 5μm filtration enough, change elements regularly.