Ribs should be 50-60% of the nominal wall thickness. Thicker ribs cause sink marks on the opposite surface due to differential shrinkage during cooling.

Cooling typically accounts for 60-80% of the total cycle time. The other phases (fill, pack, hold) combined usually represent only 20-40% of the cycle.

The pack phase adds additional material (2-5%) into the cavity to compensate for volumetric shrinkage as the plastic cools. This prevents sink marks and voids.

Hot runner systems keep plastic molten in the manifold, eliminating runner waste entirely. This can save 10-30% material compared to cold runners that must be reground or discarded.

Tunnel gates (also called submarine gates) enter the cavity below the parting line at an angle. During ejection, the gate shears off automatically, eliminating the need for manual trimming.

Burn marks occur when trapped air compresses rapidly during fill, reaching temperatures high enough to ignite the plastic (diesel effect). This is prevented by adding vents or reducing injection speed.

Draft angle (slight taper on vertical walls) is needed because the part shrinks onto the core during cooling. Without draft, the part would grip the core, requiring excessive ejection force that could damage the part.

Nylon (PA) is hygroscopic, meaning it absorbs moisture from the air. If molded wet, the moisture causes hydrolysis (molecular chain breakdown), resulting in surface defects and reduced mechanical properties.

Cooling time increases with the square of wall thickness. Doubling wall thickness quadruples cooling time (4×). This is why uniform, minimal wall thickness is critical for cycle time optimization.

The minimum draft angle for smooth surfaces is 0.5-1°. Textured surfaces require additional draft (1° per 0.025mm texture depth). Deep ribs and shutoffs may need 3-5° draft.