How to Unclog Cooling Channels in Plastic Injection Molds

The cooling system is one of the most critical parts of an injection mold. It directly affects cycle time, part dimensional stability, and surface quality. When cooling channels become blocked, heat transfer efficiency drops, cycle times increase, and parts may develop warpage, sink marks, or inconsistent shrinkage. To address these issues effectively, it is important to first identify the cause and location of the blockage, then select the most appropriate cleaning method. This article provides a practical overview of common causes, effective unclogging techniques, and preventive maintenance practices.

Before attempting any cleaning, it is essential to understand why the blockage occurred and where it is located.

1.1 Common Causes

Blockages typically come from three main sources. First, scale deposits form when hard water is used. Calcium and magnesium ions precipitate in the hot channels, gradually reducing the flow area. Second, rust and corrosion can occur in carbon steel molds or in systems with poor maintenance, especially in humid environments. Rust particles can break off and accumulate in bends or manifolds. Third, foreign debris such as sealing tape fragments, metal shavings, or microbial growth can enter the system during installation or prolonged inactivity.

1.2 Diagnosis MethodsSeveral simple checks can help identify blockages. Monitoring the temperature difference between inlet and outlet water is a common method. In a well-functioning system, the difference is usually around 3 to 5°C. If it exceeds 8°C, it often indicates reduced flow due to blockage. Pressure gauges can also reveal problems; an unusual pressure increase in a specific circuit suggests an obstruction. For more complex molds, an industrial endoscope can be used to visually inspect the inside of channels and confirm the type of blockage.

The choice of method depends on the type of blockage and the complexity of the cooling circuit.

2.1 Mechanical MethodsMechanical unclogging relies on physical force to remove deposits or debris. High-pressure water jetting is a widely used method. By applying pressures of 10 to 15 MPa, water can flush out loose scale and particles, especially when directed in the reverse flow direction. For more stubborn blockages, flexible cleaning rods or brushes can be used. These tools are inserted into accessible channels to break up rust or scale, but care must be taken to avoid scratching the inner surfaces. For precision molds, ultrasonic cleaning is an effective option. The mold is submerged in a cleaning tank, and high-frequency vibrations (28 to 40 kHz) create microscopic bubbles that dislodge contaminants without damaging the steel.

2.2 Chemical MethodsChemical cleaning uses acids or alkaline solutions to dissolve scale, rust, or organic buildup. For scale, citric acid or oxalic acid solutions are commonly used. These acids are heated to around 40 to 60°C and circulated through the channels for a few hours to break down mineral deposits. For oil or microbial contamination, alkaline cleaners may be more suitable. After any chemical treatment, the system must be thoroughly flushed with large amounts of clean water to neutralize residual acid or alkali and prevent subsequent corrosion.

2.3 Professional Equipment

For complex molds with intricate cooling circuits or deep-seated blockages, specialized equipment is recommended. Pulsed cleaning systems combine chemical cleaning with hydraulic pulsation, typically at pressures of 20 to 30 MPa. The rapid pressure changes help break up stubborn scale and rust, which is then carried away by the circulating cleaning fluid. This method is highly effective for multi-circuit molds and requires minimal disassembly.

3.1 Verification of ResultsAfter unclogging, it is important to confirm that the cooling system has been fully restored. Measuring the flow rate through each circuit ensures they are balanced, ideally within a 10% variation. Running the mold under production conditions and checking the temperature distribution across the mold surface confirms uniform cooling. Conducting a short trial run helps verify that defects related to poor cooling, such as warpage or sink marks, have been eliminated.

3.2 Preventive MaintenancePreventing blockages is more cost-effective than unclogging them. Using softened or deionized water reduces mineral content and scaling. Regularly testing water hardness and adding corrosion inhibitors is also recommended. Implementing a preventive cleaning schedule—such as every 3 to 6 months for general molds and monthly for high-precision or high-volume molds—helps maintain performance. When a mold is taken out of production for an extended period, all water should be blown out using compressed air to prevent rust. For molds used with corrosive resins or in harsh environments, using corrosion-resistant steels like S136 or applying a protective chrome plating to cooling channels can significantly reduce blockage risks.

Maintaining a clear and efficient cooling system is essential for high-quality injection molding. When blockages occur, a systematic approach of diagnosis followed by targeted mechanical or chemical cleaning is required. However, the most effective strategy is proactive prevention through proper water treatment, regular maintenance, and appropriate material selection. By ensuring cooling channels remain unobstructed, manufacturers can reduce cycle times, improve part quality, and extend the service life of their molds.