Question

Injection molding is typically used to make plastic parts. If the pressure at the gate is monitored as a function of time during the injection of a thermoplastic polymer, identify the profile that best describes this event. 

• A. Q
• B. R
• C. P
• D. S

Hint:

In injection molding, the pressure rises rapidly at first as the mold cavity fills, and then stabilizes as the material is packed and held under pressure to ensure a full mold.

Answer 1

The Correct Option is C

In injection molding, the pressure at the gate changes during the injection of a thermoplastic polymer. The process typically involves the following phases: filling, packing, and cooling. The pressure profile depends on the behavior of the material as it enters the mold cavity and is packed into the mold. Let’s analyze the pressure-time profiles:
- Profile P shows an initial rapid rise in pressure, which then levels off. This is characteristic of the injection phase in molding, where the material is forced into the mold cavity, and the pressure quickly increases as the polymer is injected. After filling, the pressure stabilizes during the packing phase, as the material is compacted to fill the mold.
- Profile Q shows a quick rise and then a sudden drop in pressure. This is not typical for the injection molding process, as the pressure typically doesn't drop so sharply once the material is injected into the cavity.
- Profile R shows a slow increase in pressure. This is not a typical behavior in injection molding because the pressure usually rises quickly at the beginning of the process.
- Profile S shows a slow and steady increase in pressure. This is more typical of a steady-state flow or extrusion process, not injection molding, where there is usually a sharp increase followed by a plateau.
Therefore, the correct answer is (C) P, as it best represents the typical pressure profile observed in the injection molding process.