Injection Molding Machines

Injection molding machines melt plastic and inject it into molds to make everything from bottle caps to car parts. They combine enormous clamping force, hot barrels and nozzles, hydraulic and electric power, and often robots. Operators are most at risk of crush injuries between the mold halves, and burns from hot plastic and parts.

• General Industry
• Manufacturing
• Plastics

How the machine works

• Clamp unit: the two halves of the mold are held together by a clamping system, in a force that can range from tens to thousands of tons. When the mold closes, anything between the halves is crushed.
• Injection unit: plastic pellets are fed from a hopper into a heated barrel, melted by heaters and a rotating screw, and injected through a nozzle into the mold.
• Mold and ejection: the part cools, the mold opens and ejectors push the part out, sometimes onto a conveyor or into a robot.
• Power comes from hydraulic, electric or hybrid drives. Machines can have gates, light curtains, auxiliary equipment such as dryers, chillers and granulators, and robots.
• OSHA applies its machine guarding rules (29 CFR 1910.212) and lockout rule (1910.147). The industry standard is ANSI/SPI B151.1 (for horizontal injection molding machines), with related plastics machinery standards. See Machine Guarding and ANSI Standards.

Main hazards

• Crushing between the mold halves or platens: a hand or body part in the mold area when it closes can be crushed or amputated.
• Burns: hot barrel, heater bands, nozzle, mold surfaces, molten plastic and hot parts or sprues. Purged plastic can spit or spray.
• Hydraulic hazards: high-pressure fluid leaks and injection, hot oil, accumulator energy. See Hydraulic Hazards.
• Electrical hazards in heater circuits and controls.
• Fumes: overheated or degrading plastic can give off irritating or toxic fumes. See Ventilation.
• Noise from hydraulics and auxiliary equipment. See Noise.
• Robots and automation: unexpected movement. See Robots & Automation.
• Heavy molds and loads during changes, lifted by cranes or hoists. See Overhead Cranes & Hoists.
• Slips: spilled pellets and oil on the floor. See Slips, Trips & Falls.
• Stored energy and unexpected motion during maintenance.

Safeguarding the mold area

• Operator gates over the mold area, interlocked so that the mold can’t close, or the ejector or other dangerous motions can’t begin, while the gate is open. Machines typically use redundant interlocks: an electrical gate switch plus a mechanical or hydraulic interlock, and they cross-check each other.
• Rear and side guards and gates so that no one can reach the mold from the back or side.
• Guards that cover the nozzle and barrel to protect against burns and spray, and purge guards.
• Emergency stops at the operator’s station and other points.
• Light curtains or presence-sensing devices where gates are not used, mounted at the right distance.
• Safety blocks or mold safety bars that mechanically hold the platens apart for work in the mold area, where required.
• Guarding on auxiliary equipment: granulators, conveyors, hopper loaders and dryers. See Conveyors.

Daily and routine safety

• Test the safeguards at the start of each shift: open the gate and confirm the machine stops or won’t cycle, check the emergency stops and check that guards are in place. Report any failure and take the machine out of service. See Tagging Out Damaged Tools.
• Operators should be trained on the machine, the hazards, the controls and what to do about problems. See Training Requirements.
• Wear PPE: safety glasses with side shields or a face shield when near the nozzle and during purging, heat-resistant gloves for hot parts, long sleeves, and hearing protection. See Eye & Face Protection and Hand Protection.
• Keep the area clean: sweep pellets and clean oil leaks right away.
• Use proper tools (brass or soft tools, not steel) to remove stuck parts or sprues, with the machine in a safe state. Don’t use your hands.
• Control fumes with exhaust at the nozzle and the proper melt temperature.

Purging and hot plastic

• Purging pushes old plastic out of the barrel. Hot molten plastic can spit, spray or burn. Use guards or a purge shield, wear a face shield and heat-resistant gloves, stand clear and collect purged plastic in a container made for it.
• Never look into the nozzle or hopper throat while heating or injecting, and keep your face away from the hot nozzle.
• Hot plastic on skin: cool with cold water right away and get medical care. Don’t peel off the plastic. See Burn First Aid.
• Moisture in some plastics can cause spitting, so use dried resin as the maker says.

Mold changes and maintenance

• Mold changes, cleaning, adjustments and repairs are the most dangerous tasks. Lock out and tag out the machine, release stored energy (hydraulic accumulators, springs and the clamp), and verify zero energy before anyone enters the mold area or works on the machine. See Lockout/Tagout and Group Lockout.
• Use mechanical blocking (mold safety bars) to keep the platens from closing, as the maker directs, in addition to lockout.
• Use cranes or hoists rated for the mold weight, with lifting points rated for the load, and keep people from under the load. See Eyebolts & Hoist Rings.
• Let hot components, such as heater bands and the barrel, cool or wear heat protection.
• Don’t treat quick jobs, like clearing a stuck part, as minor servicing unless the alternative protection is truly effective. See Minor Servicing & Alternative Energy Control and Clearing Jams.
• Only qualified people should work on hydraulic and electrical systems. See Qualified vs. Unqualified.

Robots, takeout devices and auxiliary equipment

• Part removal robots and conveyors must be guarded or fenced, with interlocks tied into the machine’s safety system. Keep people out of robot reach zones. See Robots & Automation.
• Dryers and material feeders have hot surfaces and moving parts. Granulators (grinders) have knives that can amputate, and must never be hand-fed or entered unless locked out.
• Chillers and cooling water systems create slip hazards and electrical hazards in wet areas.

Injection molding checklist

• All gates and guards in place; interlocks tested each shift and never bypassed
• Emergency stops work; light curtains and mold safety bars are used where required
• Operators trained and wearing eye, hand and hearing protection
• Purging done behind a shield with a face shield and heat-resistant gloves
• Machine locked out, energy released and blocking used for mold changes and repairs
• Robots, granulators and auxiliary equipment guarded and locked out for service

Last updated: October 4, 2026