AI and Cavity Pressure: Ending Molding Defects

Injection molding factories are eliminating plastic part defects like sink marks and warpage by combining in-mold cavity pressure sensors with melt-viscosity artificial intelligence (AI). This article provides an overview of how real-time pressure curves track plastic behavior inside the mold, how AI detects viscosity changes, and how automated systems adjust holding pressure shot-by-shot to guarantee consistent part quality.

The Root Cause of Sink Marks and Warpage

Sink marks and warpage occur primarily during the packing and holding phases of the injection molding cycle. If the holding pressure is too low, the plastic shrinks excessively as it cools, pulling the surface inward to create sink marks. If the holding pressure is too high or uneven, it locks internal stresses into the part, causing it to warp and twist once ejected.

Traditionally, molding machines use a fixed holding pressure based on the assumption that the plastic resin remains completely uniform. However, melt viscosity—the thickness and flow rate of the melted plastic—constantly fluctuates due to ambient factory temperatures, humidity, and batch-to-batch variations in raw materials, especially when using recycled plastics.

Reading the Cavity Pressure Curve

To see what is actually happening inside the mold, factories install piezoelectric sensors directly into the mold cavity. Every time a part is made, these sensors generate a cavity pressure curve. This graph visualizes the exact pressure the plastic exerts against the mold walls from the moment injection begins until the part cools.

To make defect-free parts, the cavity pressure curve of every single shot must match an ideal "master curve." If the curve peaks too early or drops off too quickly, the part will have defects.

Detecting Melt-Viscosity Shifts with AI

Before the holding phase even begins, the machine's AI monitors the initial injection phase. As the screw pushes molten plastic into the mold, the AI analyzes the relationship between injection speed, screw pressure, and the rising slope of the cavity pressure curve.

This data reveals the exact melt viscosity of that specific shot in milliseconds:

  • High Viscosity (Thicker Melt): The plastic flows slowly and resists entering the cavity. The cavity pressure rises later than normal.
  • Low Viscosity (Thinner Melt): The plastic flows too easily into the cavity. The cavity pressure spikes much earlier than normal.

Shot-by-Shot Holding Pressure Adjustment

Once the AI calculates the viscosity of the current shot, it does not wait for the next cycle to make corrections. Instead, it adjusts the current cycle's holding pressure in real time to force the current pressure curve to align with the master curve.

  1. V/P Switchover Adjustment: The AI alters the exact millisecond the machine switches from the injection phase (Velocity) to the holding phase (Pressure), ensuring the cavity is never over-packed or under-filled.
  2. Dynamic Pressure Profiling: If the AI detects a thicker, high-viscosity melt, it automatically increases the holding pressure to push enough material into the mold, preventing sink marks.
  3. Stress Prevention: If the AI detects a thinner, low-viscosity melt, it immediately lowers the holding pressure. This prevents the machine from cramming too much plastic into the mold, which eliminates internal stress and prevents warpage.

By equalizing the pressure inside the mold cavity on a shot-by-shot basis, the AI ensures that the molecular density of every plastic part is identical. Regardless of how much the raw material fluctuates, the finished parts cool at a uniform rate, permanently eliminating sink marks and warpage.