Injection molding nozzle filter: types, use and cleaning

An injection molding nozzle filter, or filter nozzle, is a nozzle with a filter insert in its melt channel that catches contamination before it reaches the mold. The melt from the barrel has to pass through narrow slots or a screen, which hold back metal particles, unmelted pellets and other foreign matter. It is used mainly with regrind and recycled material and to protect hot runner nozzles and small gates from blocking.

A filter nozzle takes the place of the standard nozzle, item 1 in the drawing. Sizing the tip radius and orifice to the mold works as for any nozzle; see the nozzle guide.

How a filter nozzle works

In the most common design, a cylindrical insert sits in the nozzle body. Its surface has two sets of grooves running along it: inlet grooves, open at the rear and closed at the front, and outlet grooves, closed at the rear and open at the front. The melt enters the inlet grooves and can reach the outlet grooves only by flowing across the narrow gap between the insert and the body. Particles larger than the gap stay in the inlet grooves.

Screen designs use a perforated plate or a mesh held in the nozzle or in an adapter behind it. In both designs, the smaller the gap or hole, the more the filter catches and the higher the pressure loss.

Types of nozzle filter

When to use a nozzle filter

• Regrind or recycled material that may contain metal, paper, dust or other foreign matter.
• Molds with hot runners, valve gates or very small gates, where one particle can block a gate.
• Parts where a particle in the surface or a thin wall would be a reject.

A filter adds a pressure loss and some shear heat, needs regular cleaning, and gives melt another place to hang up. For heat-sensitive materials, use the coarsest filter that does the job, and ask the material supplier. It is better to keep contamination out at the source: clean regrind, covered hoppers, and a hopper magnet to catch loose iron and steel.

Signs of a blocked or worn filter

• Injection pressure rises at the same speed setting, or the machine reaches its pressure limit.
• Fill time gets longer, and short shots appear.
• Burn marks or black specks, from shear heat and from material held in the filter.
• Contamination passes into the parts again, which points to a worn or damaged insert.
• Plastic leaks at the joint between the insert and the nozzle body.

A blocked filter looks like a blocked nozzle. Our article on nozzle drooling, stringing and blockages covers the other causes of high pressure at the nozzle, and short shots covers incomplete parts.

Cleaning and replacing a filter nozzle

1. Purge the barrel and keep the nozzle at processing temperature.
2. Retract the injection unit and remove the nozzle hot, as described on the nozzle page, wearing heat-resistant gloves and a face shield.
3. Take out the insert or screen following the maker's instructions.
4. Remove the plastic and the trapped particles with brass tools while the parts are warm, and finish with the cleaning method the nozzle maker recommends. Do not heat hardened inserts with a torch, which can soften them.
5. Look at what the filter caught: metal points to wear in the machine or the regrind grinder, unmelted pellets to a melting problem.
6. Inspect the slots, edges or screen. Replace the insert if the gaps have worn wider or the edges are chipped.
7. Reassemble with high-temperature anti-seize on the threads, refit the nozzle, and record the injection pressure with the clean filter as your reference for the next cleaning.

What to send for a quote

1. The machine brand and model, and the nozzle thread and length.
2. The tip radius and orifice to suit the mold's sprue bushing.
3. The filter type and, if you know it, the slot or mesh size.
4. The material you run and what kind of contamination you need to catch.
5. Photos of the old nozzle and insert, and the OEM part number if you have it.

The injection nozzles we supply include filter types. Request a quote with the details above.