Using Multiple Cartridge Heaters in One Mold – Spacing and Phasing Tips

A single cartridge heater rarely heats an entire mold uniformly. Larger molds require two, four, or even a dozen heaters working together. But simply adding more heaters introduces new problems: cold spots between heaters, electrical phasing conflicts, and uneven thermal expansion. Proper spacing and phasing turn a collection of heaters into a balanced system.

The spacing rule of thumb.

Heat spreads laterally through solid metal. For a cartridge heater of diameter D, the effective heating radius is roughly 1.5 to 2 times D in steel, and 2.5 to 3 times D in aluminum. This means two 10mm heaters in steel should be spaced no more than 30–40mm center‑to‑center for uniform temperature. Spacing wider than that creates a cold valley between heaters. Spacing closer than 20mm causes overheating and shortens heater life.

The heated length also matters. Heaters should be positioned so their heated zones overlap slightly. If the heated length of each heater is L, place them with L/2 overlap in longer cavities. For example, two 100mm heated length heaters in a 150mm deep hole would overlap by 50mm.

Avoiding the end effect.

Every cartridge heater has cold ends (unheated sections). If two heaters are placed end‑to‑end in a long through‑hole, the gap between their cold ends creates a persistent cold zone. To avoid this, either use a single longer heater or push the heaters so their heated sections overlap. Overlap requires careful dimensioning but works well in practice.

Electrical phasing – a commonly ignored detail.

When multiple heaters run from the same three‑phase supply, how they are connected matters. If all heaters are wired to the same phase, that phase carries the entire current load while other phases sit idle. This unbalances the supply and may trip breakers. Distribute heaters across all three phases as evenly as possible.

For machines with many heaters (six or more), arrange them in three groups. Group A connects to L1‑N, Group B to L2‑N, Group C to L3‑N. If the total load is within the supply rating, this keeps phase currents balanced.

Independent vs. common control.

Should each heater have its own temperature controller or share one controller? For large molds with significant thermal gradients (e.g., a mold that is hotter near the injection point and cooler at the far end), individual control is necessary. For smaller, simpler molds, one controller driving multiple heaters works fine provided the heaters are identical and symmetrically placed.

One controller driving multiple heaters works best when heaters are wired in parallel, not series. Series wiring causes one failed heater to stop all heaters. Parallel wiring allows remaining heaters to continue, though temperature uniformity suffers.

Monitoring heater health in multi‑heater systems.

Detecting a failed heater early prevents scrap parts. Two methods work well. One is to measure current draw per phase and compare to a baseline. A 10% drop indicates one or more heaters failed. The other is to install surface temperature sensors between heater positions. If the temperature between two heaters drops while end temperatures stay normal, one of the heaters has failed.

Real‑world example.

A 300mm long plastic injection mold for automotive parts used six 12mm cartridge heaters, each 150mm heated length, arranged in three pairs. The pairs were spaced 40mm apart center‑to‑center. Heaters were distributed across three phases. The mold achieved ±2°C uniformity across the entire face. Without proper spacing, the same mold showed 15°C variations.

Spacing and phasing are not optional extras. They are core engineering decisions that determine whether multiple heaters work as a team or fight each other.