In injection mold design, hot runner and cold runner are the two major runner systems. Their primary difference lies in the temperature control of the molten plastic within the runner, which directly impacts production efficiency, material costs, and product quality.
Cold Runner: The runner is not heated. After injection, the melt cools and solidifies in the runner and is ejected together with the part. Advantages: Simple mold structure, low manufacturing cost, easy maintenance, suitable for small-batch, multi-variety production, and heat-sensitive materials. Disadvantages: Generates significant runner waste (often 20–50% of part weight), longer cycle time (runner must cool), requires additional trimming/grinding of waste, increasing labor and energy costs.
Hot Runner: The runner, manifold, and nozzles are heated to keep the melt in a constant molten/flowable state, injecting directly into the cavity with no solidified runner waste. Advantages: Near-zero runner waste, cycle time reduced by 30–50% (thin-wall parts can be <5 seconds), minimal gate marks, more stable part quality (fewer weld lines and sink marks), ideal for high-volume, high-precision production. Disadvantages: Higher initial mold cost (complex heating system), more demanding maintenance, slightly higher energy consumption, not suitable for certain heat-sensitive plastics.
Selection Recommendation: Choose cold runner for small-batch or cost-sensitive projects; strongly recommend hot runner for large-volume, precision parts, or high-value materials — it can significantly reduce scrap rates and overall costs in the long run.