What are the key factors to consider when choosing the hot runner material and coating?

1 Material composition

The hot runner material should have high thermal conductivity, corrosion resistance, wear resistance, and toughness. The most common materials used for hot runner components are tool steels, such as H13, S7, and 420 stainless steel. These materials have good mechanical properties and can withstand high temperatures and pressures. However, they also have some drawbacks, such as low thermal conductivity, susceptibility to oxidation, and difficulty in machining. Therefore, some hot runner manufacturers offer alternative materials, such as copper alloys, nickel alloys, or beryllium copper, which have higher thermal conductivity, lower coefficient of thermal expansion, and better machinability. However, these materials are more expensive, less resistant to abrasion, and more prone to cracking.

2 Coating type

The hot runner coating is applied to the surface of the material to enhance its properties and reduce its drawbacks. The coating type should be compatible with the material composition and the resin type. The most common coating types are nitriding, chrome plating, titanium nitride, and diamond-like carbon. Nitriding is a heat treatment process that hardens the surface of the steel and improves its wear resistance and corrosion resistance. Chrome plating is an electroplating process that deposits a thin layer of chromium on the steel surface, which increases its hardness, wear resistance, and corrosion resistance. Titanium nitride is a ceramic coating that has high hardness, wear resistance, and thermal conductivity. Diamond-like carbon is a carbon-based coating that has high hardness, wear resistance, and low friction.

3 Resin compatibility

The hot runner material and coating should be compatible with the resin type and grade that will be processed. Some resins have high viscosity, high temperature, high pressure, or high abrasion characteristics, which can cause problems for the hot runner system. For example, resins with high viscosity or high temperature may require higher injection pressure or longer heating time, which can increase the thermal stress and wear on the hot runner components. Resins with high abrasion or corrosive additives, such as glass fibers, carbon fibers, or flame retardants, may erode or damage the hot runner surface or coating. Therefore, the hot runner material and coating should be selected based on the resin compatibility chart provided by the manufacturer or the supplier.

4 Mold design

The hot runner material and coating should also match the mold design and the part geometry. The mold design should consider the hot runner layout, the gate location, the runner size, the cavity number, and the cooling system. The hot runner layout should be balanced and symmetrical, to ensure uniform flow and pressure distribution. The gate location should be optimized to minimize weld lines, sink marks, and flow marks. The runner size should be adequate to avoid excessive pressure drop or heat loss. The cavity number should be compatible with the hot runner capacity and the part size. The cooling system should be efficient and consistent, to prevent thermal expansion or contraction of the hot runner components.

5 Processing conditions

The hot runner material and coating should also withstand the processing conditions, such as the injection speed, the injection pressure, the melt temperature, the mold temperature, and the cycle time. The processing conditions should be optimized to achieve the best part quality and productivity. The injection speed should be fast enough to fill the mold cavity before the resin solidifies, but not too fast to cause shear stress or jetting. The injection pressure should be high enough to overcome the resistance of the hot runner system and the mold cavity, but not too high to cause flash or overpacking. The melt temperature should be within the recommended range for the resin type and grade, to avoid degradation or decomposition. The mold temperature should be controlled to ensure proper cooling and dimensional stability. The cycle time should be minimized to increase efficiency and reduce energy consumption.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?