Why is your plastic dryer for injection molding not reaching the dew point?

plastic dryer for injection molding

2026/01/26 By le zhan

The temperature has been precisely set, the injection volume optimized, and the cycle time configured, but the parts still exhibit problems such as splashing, bubbles, or inconsistent mechanical properties. Often, the common cause is moisture. If the plastic dryer for injection molding fails to reach the specified dew point, it cannot completely remove moisture from hygroscopic resins. This results in cycle stalls and poor surface finish. As a Topstar engineer, I will explain the real reasons why the dryer fails to reach the dew point, how to verify the actual situation, and how to restore reliable drying performance.

Determining whether the plastic dryer for injection molding is reaching the dew point?

Before concluding that the target dew point is not being achieved, the problem needs to be verified. Engineers often rely too much on control panel readings or data from a single sensor, neglecting to verify the actual dew point of the drying air at the hopper and resin surface.

Therefore, you need to verify the following:

Dew point measurement method: There are many methods for measuring dew point, such as the chilled-mirror method, capacitive sensor method, and non-dispersive infrared (NDIR) humidity sensor method, all of which map to the dew point. The chilled mirror method is the gold standard, and engineers should use it for verification. Inexpensive capacitive sensors may drift and give inaccurate readings in dirty, high-temperature environments.

Probe location is essential: The dew point temperature should be measured at the hopper inlet and the dryer return air/circulation line. Readings at the regeneration outlet differ from the dry air temperature at the resin surface. If the sensor is located inside the regenerator or near the heater, it may overestimate performance.

Transient errors: Rapid changes in the feeding or purging cycle can cause transient fluctuations in dew point readings. Wait a few minutes for it to stabilize, then record the dew point values over time to identify fluctuations.

Regular sensor calibration: Sensors with a dew-point deviation of 5–10°C can skew measurements. Schedule calibration according to the manufacturer’s guidelines and calibrate after each maintenance.

Actual testing: With the plastic dryer for injection molding in a stable operating state, place a chilled mirror dew-point meter at the hopper opening. If the chilled mirror dew point meter shows a high dew point while the hopper dryer shows a low one, trust the chilled mirror dew point meter reading and suspect problems with the sensor, controller, or the installation location.

1. Most Common Root Causes: Desiccant and Regeneration Problems

The performance of the desiccant is central to the operation of plastic dryers for injection molding. If the desiccant fails, becomes contaminated, or regenerates improperly, the dryer will not achieve the target dew point.

Main Failure Modes:

Desiccant Degradation or Failure: Over time, the desiccant becomes saturated or undergoes chemical degradation. Once saturated, a breakthrough occurs, and the dew point rises sharply. The typical lifespan of the desiccant depends on the operating temperature and the degree of contamination; silica gel, molecular sieves, and specialized beads each have different lifespan curves.

Insufficient Regeneration Temperature: The desiccant must be heated to the appropriate temperature and held for a sufficient time to remove adsorbed moisture. Check that the regeneration heater reaches the recommended temperature and that the regeneration time is adequate.

Poor Purging or Insufficient Purge Flow: Some dryers use heated purging to remove moisture. If the purge flow is too low, regeneration will be ineffective.

Desiccant Contamination by Oil or Particulate Matter: Compressor oil or dust can coat the desiccant particles, reducing their drying capacity. Pre-filters and oil separators are essential when using compressed air.

Rotor or Wheel Failure: In continuous-rotor desiccant dryers, misalignment or motor failure can reduce dwell time and cause moisture carryover.

How to Diagnose and Repair?

First, perform a visual and weight inspection. For bead-type desiccants, take a small sample (if safe) and check for color or weight changes. Molecular sieves usually show little change in appearance, but their capacity will decrease. Also, perform a replacement test. If you suspect desiccant failure, replace the desiccant cartridge or bed with a known good one and compare the dew point. If the dew point returns to normal, the desiccant needs to be replaced, or the desiccant regeneration schedule needs to be adjusted. Then inspect the regeneration cycle, verifying the heater output, the regenerator air path, and the purge valve operation. Finally, install pre-filters and oil separators; if compressed air is used in the circuit, add appropriate coalescing filters and oil/water separators upstream of the hopper dryer.

2. Airflow, Leakage, and Distribution Issues in a Plastic Dryer for Injection Molding

Even with good desiccant and properly functioning heaters, poor airflow or leaks can reduce the dew point. The plastic dryer must have a closed-loop, well-ventilated airflow path between the blower, the drying hopper, and the return air outlet.

Common problems include:

Air leaks in the hopper or piping: Leaks draw in surrounding humid air, increasing the load and hindering the achievement of low dew points. Common leak points include aging gasket seals, sight glass fittings, hose clamps, quick connectors, and hopper windows. Even small leaks can transport significant amounts of moisture under pressure differential.

Filter blockage or dead zones leading to insufficient airflow: Dirty intake filters, clogged diaphragms, or blocked piping reduce airflow, increasing dwell time on the wet side. Conversely, excessively high airflow can minimize contact time with the desiccant system if the regeneration time is not adjusted, resulting in insufficient drying.

Improper return air path: If the return air bypasses the desiccant, the circulation loop will be ineffective.

Hopper design and distribution: Uneven airflow within the hopper can expose some material to higher-humidity air. Uneven distribution of hot air or a lack of diffusion devices can lead to material channeling.

Testing methods and countermeasures:

Smoke test method: Use a small smoke generator or stage smoke machine to detect leaks at seams and gaskets. This method is quick and effective.

Differential pressure check: Measure the pressure across the filter to determine if it is clogged.

Flow measurement: Use an anemometer or calibrated flow meter to confirm the blower output and adjust as needed.

Check distribution plates and diffusion devices: For hopper dryers, ensure that the upward-blowing diffusion device or distributor is intact and not clogged. Consider adding flow balancing baffles.

Address airflow issues first; many dew point mysteries stem from poor sealing or distribution, not just the chemical properties of the desiccant.

Airflow, Leakage, and Distribution Issues in a Plastic Dryer for Injection Molding

3. Heater, blower, and mechanical failures leading to performance degradation

Mechanical failures of heaters, blowers, or valves are common causes of dew point problems. Without proper heat regeneration or circulation, the system cannot remove moisture from the desiccant or deliver dry air to the hopper.

Therefore, you need to check the heating elements and power supply. Resistive heaters age, and their elements can fail or increase in resistance. Check the power supply voltage, contactors, solid-state relays (SSRs), and the on/off status of the heater. Some malfunctioning heaters may still heat the regenerator, but fail to reach the required desorption temperature. Simultaneously, check the condition of the blower motor and impeller. Worn blower bearings and dust accumulation on the impeller will reduce airflow. Insufficient airflow minimizes the contact area and increases the dew point. Additionally, check the settings of the variable frequency drive or motor controller; sometimes the controller will limit the speed due to fault codes.

Furthermore, pneumatic or electric valves used to direct regeneration air may be stuck or leaking. If the purge valve fails, the regeneration cycle will not effectively remove moisture. A faulty check valve allows mixing of wet and dry fluids during the cycle.

Diagnosis and Treatment Methods:

Electrical Inspection: Use a clamp meter to confirm that the heater current and fan current match the values shown on the nameplate. Verify the gating signal of the solid-state relay and the operation of the contactor.

Visual and Mechanical Inspection: Check the heater for soot, burn marks, and brittle insulation; check that the impeller moves freely and the bearing clearance is correct.

Valve Function Test: Manually or by command cycle the valves and observe if they operate promptly. Replace faulty solenoid valves or actuators.

4. Problems Caused by Resin Characteristics, Capacity Mismatch, and Process Issues

Sometimes, the plastic dryer for injection molding itself is not faulty, but the upstream process choices prevent it from handling excessive material. Therefore, it is necessary to understand the moisture characteristics of the resin and how the feeding strategy and throughput affect the hopper dryer’s processing capacity.

Considerations when processing materials:

Resin Type and Hygroscopicity: PET, PA, PBT, and PVC are all hygroscopic and readily absorb moisture. Materials stored in a single environment or under poor storage conditions will have higher moisture content upon arrival and require longer drying times.

Wet Recycling vs. Dry Recycling: Regrind and recycled materials contain higher moisture content and fine powders. If the amount of regrind added exceeds 10-20% without considering the additional drying load, it may exceed the capacity of the hopper dryer.

High Throughput or Overloading the Hopper: Increasing the material quantity, cycle speed, or the number of receivers will increase the drying load. Each dryer has its designed processing capacity; exceeding this capacity will prevent reaching the dew point target. Material Storage Methods: Open packaging bags, containers, or humid loading environments will increase moisture content. Hygroscopic resins should be stored in sealed containers with desiccants or vacuum-sealed packaging.

Troubleshooting and Solutions:

You can use a moisture analyzer to test the moisture content of the granules before drying. If the resin arrives with excessive moisture, increase the pre-drying time or use a dryer feeder. Alternatively, adjust throughput or increase capacity. If the load frequently exceeds the dryer’s capacity, add parallel dryers, increase the size of the dryer, or pre-dry the material in batches in a centralized dryer.

Improve material handling by using sealed conveying devices, vacuum loaders, and closed-loop receivers to minimize moisture absorption between pre-drying and the hopper. For critical resins, use a dryer hopper with closed-loop dew-point control. Also, minimize the proportion of recycled material, or pre-treat the recycled material separately to reduce moisture content.

Achieving Target Dew Point

Achieving the target dew point is a systemic issue – sensors, desiccant condition, airflow sealing, mechanical integrity, control systems, and process requirements must all be coordinated. You can use cold-mirror verification, follow the troubleshooting steps above, and implement a maintenance roadmap to ensure your plastic dryer for injection molding reliably maintains the target dew point. If needed, Topstar can provide site-specific checklists, on-site verification, or dryer capacity analysis tailored to your resin blend, throughput, and production layout.

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Why is your plastic dryer for injection molding not reaching the dew point?

Determining whether the plastic dryer for injection molding is reaching the dew point?