Injection molding produces polymer parts by melting material, forcing it into a closed mould, cooling it until sufficiently rigid and ejecting the part.

A typical cycle is

plasticize melt
    ↓
close and clamp mould
    ↓
inject and fill
    ↓
pack / hold pressure
    ↓
cool
    ↓
open and eject

Filling and packing are distinct. During filling, the advancing melt front occupies the cavity. During packing, additional material is pushed in while gates remain open enough to compensate for volumetric contraction during cooling.

The mould must also be clamped strongly enough to resist cavity pressure. If projected cavity area is $A_p$ and average cavity pressure is approximated by $p$, a simple separating-force estimate is

$$F\approx pA_p.$$

For $p=50\ \mathrm{MPa}$ and $A_p=0.010\ \mathrm{m^2}$,

$$F\approx 50\times10^6\times0.010=500\ \mathrm{kN}.$$

Cooling often dominates cycle time because the polymer must lose enough heat before ejection without excessive deformation.

Injection molding is economically attractive for high repetition because one mould can reproduce complex parts rapidly, but that advantage comes with substantial tooling cost and strong requirements on part/mould design.