Changing colour on a hot runner mold takes longer than on a cold runner tool. Melt stays inside the heated manifold between shots. Old colour hides in dead spots around the manifold. Purging removes it step by step. A rushed change leaves streaks and wastes material.
Why Hot Runner Colour Changes Are Difficult
A hot runner keeps plastic molten from the nozzle to the gate. The manifold has channels, corners, and heater wells. Each creates low-flow areas where colour lingers. Glass-filled or flame-retardant resins degrade in these spots. Black-to-white changes are the hardest. White shows every trace of the previous colour.
Manifold design affects purge time. A well-balanced manifold with streamlined channels purges faster. Sharp corners and oversized channels trap material. Valve-gated systems add cylinders and pins that need cleaning. Knowing your manifold layout helps you plan the purge sequence.
Preparation Before the Colour Change
Good preparation cuts purge time in half. Schedule the change from dark to light colours when possible. Light-to-dark changes need fewer purge shots. Check that enough new resin is dried and ready. Confirm the next job’s melt temperature and drying requirements. A cold hopper stops the whole line.
Inspect the hot runner controller before starting. All zones must reach setpoint and hold steady. A weak heater leaves a cold spot that traps old colour. Check thermocouples for correct readings. Record the current process parameters as a baseline. You will return to them after the change.
Step-by-Step Purging Procedure
The following procedure works for most hot runner molds. It follows the original shop practice: raise temperatures, purge the machine, then step temperatures back down. Each step has a reason. Follow them in order for the shortest change.
- Increase mold temperature by 15 °C. Warmer steel keeps the skin layer open. Colour releases from the cavity wall more easily. Do not exceed the resin’s safe limit.
- Increase manifold and nozzle temperature by 50 °C. Hotter melt flows faster through dead spots. Old colour is dragged out with the new material. Watch for degradation with heat-sensitive resins.
- Retract the molding machine nozzle from the sprue. This separates the machine barrel from the hot runner. Purge the barrel independently without pushing old colour back. Keep the mold closed and heated.
- Auto purge the molding machine as per your standard practice. Run the screw at high back pressure. Extrude until the melt shows the new colour. This cleans the barrel and screw before reconnecting.
- Hand purge using standard material. Feed purging compound or natural resin by hand. Push it through until streaks disappear. Manual purging reaches spots that auto purge misses.
- Re-start the normal cycle and run 6 shots. Reconnect the nozzle and resume molding. These shots flush the manifold and gates. Check each part for colour streaks.
- Lower manifold and nozzle temperature by 20 °C and run 1 shot. Cooling in stages prevents thermal shock. The single shot shows how much old colour remains. Inspect the part carefully.
- Lower manifold and nozzle temperature by another 20 °C and run 1 shot. The melt viscosity rises toward normal. Streaks should fade with each step. Keep the mold temperature raised for now.
- Lower manifold and nozzle temperature by 10 °C and run 1 shot. You are now close to the setpoint. The part should show clean new colour. If streaks persist, repeat the purge.
- Lower mold temperature by 15 °C back to the process setpoint. The mold returns to normal production conditions. Cycle time stabilizes as steel cools. Verify dimensions on the first good parts.
- The new colour is now ready for production. Run a final inspection on colour match. Compare parts against the approved standard. Document the purge time for the next change.
Purging Compounds and Materials
Commercial purging compounds speed up colour changes. Chemical grades foam slightly and scrub the flow channels. Mechanical grades use glass fiber or minerals to scour walls. Natural resin also works but needs more shots. Match the compound to your resin family.
Never mix incompatible resins during purging. Check the melt temperature overlap first. Purging with the wrong material causes carbonized specks. When in doubt, use the new production resin. It costs more but avoids contamination.
Common Problems During Colour Change
Streaks after purging point to dead spots. Increase the temperature step and purge again. Black specks indicate degraded material in the manifold. Disassemble and clean the manifold if specks persist. Splay marks mean the new resin needs drying.
Long purge times often come from oversized manifolds. A manifold designed for large shots purges slowly on small parts. Consider a smaller manifold for frequent colour changes. Record purge shots per colour pair. The data guides future scheduling.
Reducing Colour Change Time
Plan production to minimize changes. Group jobs by colour family and resin type. Run light colours before dark colours. Standardize on fewer resin grades across products. Each avoided change saves material and machine time.
Train operators on the full procedure. A written purge sheet at the machine prevents skipped steps. Track purge time and scrap per change. Review the numbers monthly with the team. Small improvements compound over hundreds of changes.
