Injection Mold Heating, Insulation, and Cooling Analysis

Heating, insulation, and cooling of the mold—as well as the clamping structure—are integral parts of composite mold design. The structural design of the temperature control system directly affects the product’s surface appearance, internal quality uniformity, and overall forming efficiency.

Compression molding is one of the oldest and most widely used plastic molding methods and remains a key process for studying material properties. It offers advantages such as simple molding equipment, low investment cost, and relatively simple mold structures. Even in today’s highly automated manufacturing environment, compression molding is still widely applied. However, systematic summaries of key mold design skills for compression molding are relatively limited. This section focuses on the design principles of mold heating, insulation, and cooling.

Heating Pipe Design Requirements

Steel heating tubes are the most commonly used heating method in plastic molding dies and are almost universally adopted. Heating pipes can be designed with single-end wiring, double-end wiring, or other configurations. Common materials include seamed pipes, seamless pipes, and stainless steel pipes.

Steel heating tubes feature low heat loss, high thermal efficiency, simple wiring, and flexible voltage options (220V or 380V). However, due to material and processing limitations, the following points must be carefully considered during mold design:

1. Cold Ends of Heating Tubes
   Heating tubes typically have relatively long cold ends at both ends, which do not generate heat. This must be accounted for when calculating effective heating length.

2. Surface Load Limitation
   The power density of the heating section should not exceed 10 W/cm.
   For example, a 30 cm heating tube should have a maximum power of 300 W. Exceeding this limit results in excessive surface load, which accelerates oxidation and corrosion of the steel tube, increasing the risk of short circuits and premature failure.

3. High-Temperature Applications
   For mold designs requiring temperatures above 250°C, the use of standard heating tubes becomes challenging. In practice, heating tubes can reach temperatures as high as 420°C, but such applications demand very high-quality heating elements and frequent inspection of circuit continuity and insulation.

   Under these extreme conditions, heating tubes, terminal blocks, copper wires, and steel sheets are prone to oxidation, which may cause disconnection or failure. Special protective treatments for electrical transmission components are required to prevent exposure to air and to extend service life.

Soldering Iron Core as a Heating Element

Soldering iron cores are also commonly used as mold heating elements. Their main characteristics include:

• High power density per unit length
  (e.g., a 10 mm diameter, 80 mm long core can deliver approximately 150 W)

• Good durability and safety

• Low likelihood of electrical breakdown

• Can be embedded by drilling blind holes in the mold steel

However, soldering iron cores also have limitations:

• Difficult to customize for specific mold designs

• Fragile and easily damaged during replacement

• Require comprehensive electrical protection, including fuses, air switches, and other safety devices

During operation, the working environment must be kept clean, dry, and well insulated. Regular inspection of electrical components is essential to prevent potential safety hazards.

Heating Pipe Installation and Drilling Design

From a heat transfer perspective, heating pipes should be installed as close as possible to the mold surface to allow heat to reach the cavity quickly and efficiently.

In practice, the actual contact area between the heating tube and the mold steel is limited. Heat transfer primarily occurs through radiation, with conduction playing a secondary role. For this reason, most heating tubes used in molds are coated with infrared radiation–enhancing materials to improve heat transfer efficiency.

Additionally, limiting the heating power density to approximately 10 W/cm not only improves temperature uniformity but also significantly extends the service life of the heating elements.