The screw tip, also called the screw head, is the hardened, pointed part screwed into the front of the injection molding machine screw. It guides the melt into the front of the barrel and, on most machines, carries the non-return valve: the check ring slides on it and seals against the seat ring behind it. It is a wear part, replaced together with the ring and seat, and it has to come off whenever the valve is inspected or replaced.
The screw tip and check ring are item 5 in the drawing. For how the valve opens and seals, see the check ring guide; for the rest of the plasticizing unit, see the barrel and screw guide.
Where the screw tip sits and what it does
- Thread. The tip's shank screws into the front end of the screw. On a ring-type valve it passes through the check ring and the seat ring, and its shoulder clamps the seat ring against the front face of the screw.
- Flutes and wings. The front part of the tip has wings with flutes between them. The wings stop the check ring in its open position, and the melt flows forward through the flutes.
- Cone. The pointed front is shaped to match the inside of the barrel head, so that at the end of injection the screw pushes out almost all the melt and little is left behind to degrade.
Types of screw tip
| Type | How it works | Typical use |
|---|---|---|
| Ring-type (sliding ring) | A check ring slides on the tip and seals on the seat ring | Most materials; the standard design |
| Ball check | A ball inside the tip lifts off its seat to let melt through and is pushed back onto it to seal | Some machines and applications; more restriction to flow than a ring |
| Free-flow (smear) tip | No valve; the melt can flow back during injection | Heat-sensitive materials such as rigid PVC |
Tips are also made in different steels and with hard coatings for abrasive or corrosive materials. Fit the type and material the screw was designed for, unless the machine maker or the screw supplier recommends a change.
Signs of a worn or damaged screw tip
- The cushion varies or runs to zero, usually together with a worn check ring.
- The wings are worn or rounded, or the shoulder that holds the seat ring is damaged.
- Black specks from degraded plastic trapped between the tip, the seat ring and the screw face.
- A cracked or broken tip: a sudden loss of cushion, metal in parts, or a fragment blocking the nozzle. Stop the machine at once.
- A seized or damaged thread found when you try to remove it.
The usual causes are turning or injecting before the barrel has heated through, abrasive and corrosive materials, a tip that was not tightened to the specified torque, and mismatched parts from different sets.
How to remove a screw tip
- Heat the barrel to processing temperature, let it soak, and purge it empty.
- Retract the injection unit and remove the nozzle and the barrel head.
- In manual mode, at low speed and pressure, push the screw forward until the tip is exposed.
- Make sure the screw cannot turn while you loosen the tip; follow the method in the machine manual.
- Fit a screw head wrench that matches the tip. The wrenches we supply come in 3-prong, 4-prong and 6-prong versions for different tip designs.
- Remove the tip while it is hot. Most screw tips have a left-hand thread, so check before applying force.
- Slide off the check ring and the seat ring, and clean all parts while they are warm with brass tools.
If the tip will not move, keep it at temperature longer rather than forcing it. Do not heat a hardened tip with a torch: the heat can soften it and damage the screw thread. On some machines it is easier to pull the screw first and remove the tip on the bench, as described in how to remove the screw; do it before the tip cools.
Refitting the screw tip
- Clean the thread in the screw and the face the seat ring sits against. Any plastic left there stops the parts seating and can let the tip loosen.
- Fit a matching set: seat ring, check ring and tip. Make sure the check ring slides freely on the tip.
- Coat the thread with a high-temperature anti-seize compound. Keep it off the sealing faces.
- Tighten the tip to the torque in the machine manual, with the barrel at the temperature the manual specifies.
- Refit the barrel head and the nozzle, heat the barrel and let it soak before turning the screw.
What to send for a replacement screw tip
- The machine model and the screw diameter.
- The thread size and hand, and the overall length of the tip.
- The type (ring, ball or free-flow) and the number of flutes.
- The outside diameter, inside diameter and length of the check ring and seat ring, if you can measure them.
- Photos of the tip, ring and seat, and the materials you run.
Request a quote with these details. The tip, ring and seat are quoted as a set, because their sealing faces must match.
Updated by the IMSPARES technical team.