Which statement best describes the primary purpose of the dwell period in a manufacturing cycle?

• A. Cooling the mold surfaces
• B. To allow the plastic to cool and harden
• C. To align the mold halves
• D. To reduce viscosity of the material

Answer: (B)

The main idea is that the dwell period serves as a cooling-and-solidification pause. After molten plastic is injected into the mold, it must sit long enough for heat to escape and the material to harden so the part can be ejected without deforming. This pause lets the plastic reach a solid, dimensionally stable state, which helps prevent defects like warping, sink marks, or trim distortions and improves surface finish.

It’s not about cooling the mold surfaces themselves—that cooling is built into the mold design and cooling channels—nor is it about aligning the mold halves (that happens during closing and clamping). It also isn’t about reducing viscosity; viscosity decreases with heat, while the dwell period is a cooling stage. If the dwell period is too short, the part may still be soft and deform during ejection; if too long, cycle time increases unnecessarily.