Barrel head (front flange) of an injection molding machine

The barrel head is the heavy steel piece fixed to the front of the injection molding machine barrel. It closes the barrel, leads the melt from the bore into the nozzle, and carries the nozzle thread. It goes by several names: barrel end cap, front flange, front head and cannon head; the drawing calls it the barrel front flange (item 2). Because the full injection pressure acts on it and its joints usually seal metal to metal, its faces and bolts must be in good condition or the melt leaks out.

For how it fits with the rest of the plasticizing unit, see the barrel and screw guide.

What the barrel head does

• Closes the barrel. It takes the injection pressure at the front of the bore, the highest pressure in the plasticizing unit.
• Leads the melt to the nozzle. Inside, it tapers from the barrel bore down to the nozzle channel. The taper matches the cone of the screw tip, so that at the end of injection the screw pushes out almost all the melt and little is left to degrade.
• Holds the nozzle. The nozzle screws into a thread in its front, and seals on a face or seat in the head.
• Stays hot. It is heated by the front zone of the barrel or by its own heater band, depending on the machine.

How it is fixed and sealed

On most machines the barrel head is held to the front face of the barrel by a ring of high-strength bolts; some designs screw it into the barrel instead. There is normally no gasket: the seal comes from two clean, flat machined faces clamped together by the bolts, often with a spigot that centers the head in the bore.

That makes the joint sensitive to three things:

• Clean faces. A film of plastic or a burr on either face leaves a gap that melt can enter.
• Even bolt tension. The bolts must be tightened in sequence to the torque in the machine manual. Uneven tightening tilts the head and opens a gap on one side.
• Temperature. The steel expands as it heats. The manual states whether the bolts are tightened cold, hot, or rechecked once the barrel is at temperature.

Signs of a leaking or worn barrel head

• Plastic oozing from the joint between the head and the barrel, often seen under the heater band or the barrel cover.
• Plastic leaking at the nozzle thread.
• Black specks from melt that has been trapped in a gap and degraded.
• A step or erosion where the bore meets the head's inner taper.
• A damaged or stripped nozzle thread.
• Loose, stretched or broken bolts.
• Cracks, which are rare and can follow injecting on plastic that was not fully melted.

The usual causes are plastic left on the faces at assembly, bolts not tightened in sequence or to torque, reused stretched bolts, injecting or turning the screw before the plastic is fully melted, and an over-tightened nozzle.

Removing and refitting the barrel head

1. Heat the barrel to processing temperature, let it soak, and purge it empty.
2. Retract the injection unit and remove the nozzle with its heater band and thermocouple.
3. Remove the heater band from the head, if it has one.
4. Support the head; it is heavy. Loosen the bolts a little at a time in a cross pattern while it is hot, then remove them and the head.
5. Clean the faces of the head and the barrel while warm, with brass tools that will not scratch them.
6. Inspect the faces, the inner taper, the nozzle thread and the bolt threads in the barrel. Replace damaged or stretched bolts with bolts of the specified grade.
7. Refit the head with high-temperature anti-seize on the bolt threads only, and tighten the bolts in sequence to the torque in the manual.
8. Refit the nozzle and heaters, heat to temperature and let the barrel soak, then check the joint for leaks during the first shots.

These steps are also part of changing the check ring and removing the screw, so inspect the head each time it comes off.

What to send for a replacement barrel head

1. The machine brand and model, and the screw diameter.
2. The outside diameter and length of the head, and the diameter and length of any spigot.
3. The number, size and pitch circle of the bolt holes.
4. The nozzle thread and the bore at the barrel end.
5. Photos of both faces and the side, and the OEM part number or a drawing if you have one.

The barrel front flange we supply is made for Haitian MA series machines. Request a quote with the details above.