Cooling Time

Contraction (Shrinkage) is the dimensional reduction a molded part undergoes as it cools from melt to solid and reaches room temperature. It is an inherent property of each resin and must be compensated at mold design time by scaling the cavities. ## Types of shrinkage - Volumetric: occurs during cooling inside the mold, partly offset by hold pressure. - Linear mold shrinkage: measured 24 h after demolding, the catalog value in %. - Post-shrinkage: continues for up to a week or more, especially in semi-crystalline resins. ## Typical values by resin - PP: 1.2 – 2.5 % - HDPE: 1.5 – 3.0 % - PA (Nylon): 1.0 – 2.5 % - POM: 1.8 – 2.5 % - ABS: 0.4 – 0.7 % (amorphous, very low) - PC: 0.5 – 0.7 % - PS: 0.3 – 0.6 % ## Factors that affect shrinkage Wall thickness, mold temperature (higher T → more crystallinity → more shrinkage in semi-crystalline), hold pressure, hold time, flow orientation, and reinforcement (glass fiber cuts directional shrinkage by 50 – 70 %). ## Related issues Warpage from uneven directional shrinkage, sink marks in thick areas with poor packing, and internal voids.

Cycle Time is the total time an injection molding machine takes to produce a complete part, measured from one mold close to the next. It is the most critical economic indicator of the process: every second saved multiplies by the number of cavities and the annual volume. ## Phases of the cycle 1. Mold close and clamp 2. Injection (dynamic filling of the cavity) 3. Hold (packing pressure) 4. Cooling and plasticizing in parallel 5. Mold opening 6. Part ejection and robot motion ## Typical values per part - Small parts (<10 g): 5 – 15 s - Medium caps and containers: 8 – 25 s - Large housings (>200 g): 25 – 60 s - Technical parts with inserts: 30 – 90 s Cooling usually accounts for 50 – 70 % of total cycle. ## Factors that affect cycle time Wall thickness (quadratic relation with cooling), resin type (crystalline > amorphous), cooling-channel design, injection profile, robot/EOAT efficiency, and dead time from difficult ejection. ## Reducing cycle time Optimize conformal cooling, tune injection velocity profile, balance cavities, switch to valve gates on hot runners, parallel plasticizing with opening, and remove unnecessary robot moves.

Molding Cycle is the complete sequence of phases that produces one injection-molded part, from mold close to the next mold open. Each phase contributes its time and together they determine press productivity and part cost. ## Phases of the cycle 1. Clamp close and full tonnage applied 2. Injection: the screw forces molten material into the cavity along a velocity profile 3. Hold (packing): constant pressure to compensate for shrinkage during initial cooling 4. Cooling + plasticizing: the screw rotates to prepare the next shot while the part cools 5. Clamp open 6. Ejection and robot / EOAT motion ## How to calculate cycle time Cycle time is the sum of every phase: Cycle time = clamp close + injection + hold + cooling + mold open + ejection Plasticizing (screw recovery) runs in parallel with cooling, so it only counts when it is longer than the cooling phase. Cooling dominates and scales with the square of the thickest wall: doubling wall thickness roughly quadruples cooling time. This wall-thickness rule is the single biggest lever on overall cycle time. ## Cycle time example A representative thin-wall part on a 150-ton press: | Phase | Time | |-------|------| | Clamp close | 1.0 s | | Injection | 1.5 s | | Hold | 4.0 s | | Cooling | 8.0 s | | Mold open + eject | 2.0 s | | Total cycle | 16.5 s | ## How does material affect the cycle? Semi-crystalline resins (PP, PA, POM, HDPE) release more latent heat as they crystallize and usually need longer cooling than amorphous resins (ABS, PC, PS) at the same wall thickness. Melt temperature, mold temperature and the resin's ejection (heat-deflection) temperature all set the minimum cooling time. ## Optimization Conformal cooling channels that follow part geometry, a multi-stage injection profile, plasticizing in parallel with opening, valve gates on hot runners for clean closure, and elimination of dead time on the robot side. ## How long is an injection molding cycle? Thin-wall packaging parts cycle in 3–15 s, while thick technical or engineering parts can take 30–60 s or more. Cooling sets the floor: the thicker the wall, the longer the minimum achievable cycle. ## Which phase takes the longest? Cooling, almost always. It typically accounts for 50–70 % of the total cycle, which is why conformal cooling and wall-thickness reduction give the biggest gains. ## What is the difference between molding cycle and cycle time? "Molding cycle" describes the sequence of phases; "cycle time" is the numerical total in seconds reported on the cell's OEE. The cycle is the what, the cycle time is the how long.

Overall cycle time is the real, average time it takes to produce one shot in actual production — including everything that happens between consecutive parts, not just the machine's ideal molding cycle. Where cycle time usually means the clean, repeating machine cycle, overall cycle time is the figure you get by dividing real run time by parts made, so it captures the losses the nameplate cycle ignores. ## What it includes beyond the ideal cycle - The machine molding cycle: fill, pack/hold, cooling time, recovery, mold open, part ejection, mold close. - Operator/automation time: extra seconds in a semi automatic cycle for the operator vs a fully automatic cycle; insert loading, gate cutting, inspection. - Micro-stops and variation: door interlocks, alarms, slow part drop, robot handshakes — small delays that don't show in the "ideal" cycle. - Allocated downtime: depending on definition, brief stoppages and the amortized share of changeovers or scheduled stops. ## Ideal vs overall - Ideal/machine cycle: the best repeating time the press achieves running clean and automatic. - Overall cycle time: actual output rate, always ≥ the ideal — the gap is the improvement opportunity. This distinction matters for costing and capacity: quoting on the ideal cycle but running at a longer overall cycle is how a job loses money. Reducing it means attacking the losses (automate the semi automatic cycle, cut micro-stops, speed handling) as much as the machine cycle itself. ## Why it matters Overall cycle time is the honest basis for capacity planning, machine-hour cost and OEE performance: it ties the theoretical cycle to what the cell really delivers per hour and per shift. ## Related terms - See also: cycle time, molding cycle, cooling time, automatic cycle, scheduled stop ## What is overall cycle time in injection molding? The real average time per shot in production — the machine molding cycle plus operator/automation time, micro-stops, handling and allocated downtime — found by dividing actual run time by parts produced. ## What is the difference between cycle time and overall cycle time? Cycle time usually means the clean, repeating machine cycle; overall cycle time is the real average including operator time, micro-stops, handling and small losses, so it is always equal to or longer than the ideal cycle. ## Why is overall cycle time important for costing? Because quotes and capacity must be based on the real output rate, not the ideal machine cycle; if you price on the ideal cycle but actually run a longer overall cycle, the job runs over budget.