In precision injection molding, cavity pressure is the most direct and critical physical quantity reflecting state of plastic melt, mold filling behavior, and internal part quality. Pressure sensors, as "eyes" that capture this core information, are essential components for achieving precise timing control (especially V/P switching point), optimizing process parameters, improving part quality, and ensuring production stability. This article will systematically explain their operating principles, selection criteria, and their impact on quality.

 

Choosing appropriate sensor type is the first step to a successful application. It requires comprehensive consideration of measurement requirements, installation conditions, melt characteristics, and cost.

Key selection principles:
For cavity pressure monitoring (V/P switching core): Piezoresistive or high-performance ceramic capacitor types are preferred, as they provide stable and reliable static and quasi-static pressure signals.
For research on high-speed dynamic processes: Piezoelectric is the only choice.
For high-temperature/corrosive melts: Ceramic capacitors offer significant advantages.
Installation Space/Mold Structure: Determines sensor size and mounting method (flush mounting minimizes flow impact).
Signal Processing and Integration: Consider compatibility with injection molding machine control system or external data acquisition system.

 

Real-time cavity pressure data provided by pressure sensors is core link between process parameters and final product quality. They are particularly important in controlling V/P switching point:

Core Function: V/P switching point is critical point for transitioning from injection phase (primarily velocity-controlled) to holding/compression phase (primarily pressure-controlled). Switching too early (underfilling cavity) results in underfill and short shots; switching too late (overfilling the cavity) results in flash, high internal stress, and increased energy consumption.
Sensor Application: In-cavity pressure sensors (preferably near gate or at the end of cavity) directly monitor actual filling status. When cavity fill rate reaches 95%-99% (specific value depends on product structure and material), cavity pressure rises sharply. Using this pressure inflection point or a set pressure threshold as basis for V/P switching is far superior to traditional stroke position or time switching, which are easily affected by melt viscosity fluctuations, mold temperature, and machine conditions.
Quality Improvement: Ensures that each shot is switched at near-complete fill, fundamentally eliminating volume shrinkage defects (sink marks, shrinkage holes) and flash caused by inaccurate switching points, significantly improving product qualification rates.

Core Function: Internal stress (orientation stress, temperature stress, and shrinkage stress) is primary cause of warpage. Uneven filling and improper holding pressure are main causes.
Sensor Application:
V/P Switching Optimization: As described in step 1, precise switching avoids over- or under-pressure and reduces initial orientation stress.
Pack Pressure Profile Optimization: Leveraging real-time sensor feedback, implement multi-stage or decaying holding pressure. During melt cooling and shrinkage, system precisely compensates for drop in cavity pressure to maintain required holding pressure, balancing material replenishment and frozen layer growth. This prevents insufficient holding pressure (inadequate packing, excessive shrinkage, and sink marks) or excessive/excessive holding pressure (forced compression of frozen layer, resulting in high internal stress).
Cavity Pressure Balance Monitoring: Multi-point pressure sensors monitor balance of filling and holding pressure in various areas of multi-cavity molds or complex cavities, identifying and resolving localized stress concentrations caused by runner design or uneven cooling.
Quality Improvement: Effectively reduces internal stress levels in parts, significantly reduces warpage, and improves dimensional stability and consistent mechanical properties.

Core Function: Shrinkage is an inherent material property, but actual amount of shrinkage is significantly affected by holding pressure and holding time.
Sensor Application: Cavity pressure (especially pressure at gate freeze) is used as a Key Process Indicator (KPI). Sensor-based closed-loop control ensures sufficient and consistent holding pressure is applied before gate freeze during each molding cycle. Research has shown that cavity pressure-based holding pressure control can significantly reduce part shrinkage fluctuations to ±0.3% or even lower.
Quality Improvement: Significantly improves dimensional accuracy and batch-to-batch consistency, reduces subsequent assembly issues, reduces reliance on tolerance compensation.

Underfill/Short Shot: If cavity end pressure sensor detects zero or significantly lower pressure at the end of filling, it can immediately trigger an alarm.
Weld Line Strength/Bubble: Monitor pressure and time at which melt front reaches different locations, optimize injection speed profile (multi-stage injection), ensure melt front converges at appropriate speed and pressure, and improve weld line strength. Abnormal pressure curve fluctuations may also indicate air entrapment.
Over-Pack/Flash: Alarm is triggered when the cavity pressure exceeds safety threshold.

"Process Fingerprint": Cavity pressure-time curve contains comprehensive information about material, machine, and mold conditions, and is the only true "fingerprint" of process. Save pressure curve of qualified products as a benchmark.
Mold/machine/material batch changes: By reproducing baseline pressure curve under new conditions (rather than simply copying process parameters), a qualified process can be quickly established, significantly reducing mold trials (by up to 30%-50%) and shortening production setup time.
Process monitoring and traceability: Real-time monitoring of pressure curve to ensure it is within qualified range enables online quality assessment and problem tracing.

 

The only reliable basis for optimizing V/P switching: Cavity pressure is the only parameter that directly and in real time reflects actual melt filling status and material properties within mold cavity. Position- or time-based switching is indirect and susceptible to interference. Pressure sensors are cornerstone of scientific, precise, and repeatable V/P switching.
Core input for closed-loop quality control: Cavity pressure curve is strongly correlated with key part quality indicators (dimensions, weight, internal stress, and cosmetic defects). Using pressure data to adaptively adjust process parameters (switch point, holding pressure/time, injection speed) is a key step towards achieving true closed-loop quality control and intelligent injection molding.
Process Reproducibility and Knowledge Accumulation: Pressure curves, as "process fingerprints," greatly improve efficiency and reliability of process transfer and replication, and are valuable intellectual assets for companies.
Accelerating Domestic Substitution: Chinese companies, represented by Amperon, have achieved significant breakthroughs in ceramic capacitive sensor technology, demonstrating strong competitiveness in high-temperature resistance, integration, cost control, service responsiveness. They are gradually replacing imported brands (such as Sensata's general-purpose products), particularly in price-performance ratios and for specific harsh operating conditions. Future competition will focus on higher accuracy, improved dynamic performance, more intelligent integration (such as wireless and temperature-integrated), more comprehensive overall solutions.

V/P Switching Control: Preferable installation is near gate (to reflect main filling pressure) or at the end of cavity (to confirm complete filling). A combination of the two is optimal.
Packaging Pressure Control/Shrinkage Monitoring: Area near gate is critical, as packing pressure is transmitted through gate. Monitor pressure at gate freeze point.
Multi-cavity balancing/complex cavities: Place multiple sensors in critical areas (such as weld lines, thin-walled areas, and areas away from gates).
Ensure flush mounting: Sensor's pressure-sensing surface must be perfectly flush with mold cavity surface. Any protrusions or depressions will interfere with melt flow, distorting data or even damaging sensor. Using dedicated temperature and pressure measurement inserts is recommended.

Shielding and Grounding: Use shielded cables and ensure a good single-point ground to prevent electromagnetic interference (EMI) noise. This is especially critical for piezoelectric sensors.
Amplifier Matching: Select a low-noise, high-input impedance charge amplifier for piezoelectric sensors. Ensure that power supply and signal range of piezoresistive/ceramic capacitive sensors match those of acquisition equipment.
Regular Calibration: Establish a regular calibration schedule (e.g., semi-annually or annually, or based on usage intensity) to ensure measurement accuracy. Utilizing TEDS functionality simplifies calibration information management.

Focus on key characteristic points: filling start point, pressure inflection point (V/P switching point), peak pressure, holding pressure decay curve, and gate freezing point pressure.
Curve Comparison Analysis: Comparing current curve with "golden curve," focusing on differences in shape, key point values, and time position, is the most effective tool for diagnosing process fluctuations and root causes of defects.
Correlation with Quality Indicators: Develop a quantitative relationship model between cavity pressure characteristic parameters (such as peak pressure and pressure at the end of holding) and key part dimensions and weight.

Cleaning: Inspect and clean sensor's pressure-sensing surface after each mold opening to prevent carbonization of plastic residue from affecting measurement or damaging surface.
Overload/Shock Prevention: Avoid mechanical shock or severe overload on sensor due to residue or misoperation during mold closing.
Temperature Management: Pay attention to sensor's nominal operating temperature range and avoid prolonged over-temperature operation. Proper mold cooling helps extend sensor life.