Injection Molding Cycle Time Calculator

A standard injection molding cycle has four phases: (1) Injection/fill time: polymer melt is injected into the mold cavity, typically 1-5 seconds. (2) Packing/holding time: additional material is packed in under pressure to compensate for shrinkage, typically 5-15 seconds. (3) Cooling time: the part cools to ejection temperature, often the longest phase (10-60+ seconds). (4) Mold open, ejection, and close time: typically 2-8 seconds.

The Rossman cooling time formula: t(c) = (s^2 / (pi^2 x alpha)) x ln((pi/4) x ((T_melt - T_mold) / (T_eject - T_mold))), where s = wall thickness (m), alpha = thermal diffusivity of the polymer (m^2/s), T_melt = melt temperature (degC), T_mold = mold surface temperature (degC), T_eject = ejection temperature (degC).

Thermal diffusivity (alpha) = Thermal conductivity / (Density x Specific heat). For ABS: alpha = approximately 1.0 x 10^-7 m^2/s. For PP: approximately 1.2 x 10^-7 m^2/s. For HDPE: approximately 1.5 x 10^-7 m^2/s. Values are from CAMPUS plastics database and material supplier datasheets.

Cooling time is proportional to the square of wall thickness. Doubling the wall thickness increases cooling time by a factor of 4. This is why thin-wall molding is so effective at reducing cycle times. The Rossman formula directly encodes this squared relationship.

Cycle times vary widely by part size and material. Small thin-walled parts (0.5-1.5 mm) may cycle in 5-15 seconds. Medium parts (2-3 mm wall) typically cycle in 20-45 seconds. Large thick-walled parts (5 mm or more) may require 60-120 seconds. Accurate cycle time prediction is critical for quoting and mold qualification.