Sizing a vertical injection moulding machine for insert moulding
Why the tonnage figure is the last thing to decide, not the first, when the part has an insert in it.
When to change a screw-tip set, and what a worn one looks like
Shot weight variation that behaves like a process problem is often a mechanical one. Here is how to tell.
A worn screw-tip set — the tip, the non-return ring and the seat — produces symptoms that look exactly like a process problem, so it usually gets chased as one for several months first.
The symptoms
Shot weight variation between cycles with no process change. This is the classic one. Material leaks back over the ring during injection, so the actual shot differs from the metered shot.
Cushion instability. The cushion drops away over successive shots, or varies unpredictably.
Short shots appearing intermittently, particularly on thin sections, without a settings change.
Longer recovery time than the machine used to need.
Quality that improves after a purge and then drifts again, which sends everybody back to the process settings.
Confirming it before ordering parts
Run a check with the screw at a fixed position and no injection, and watch whether the screw creeps backwards. Movement means the ring is not sealing.
Also worth eliminating first: a worn check on the hydraulic side, a leaking nozzle, or moisture in the material producing similar variation. Replacing a tip set that was not the fault is an expensive way to learn this.
What to order against
Order against the machine serial number, not the model. Manufacturers revise these parts across production years, and a set that fits the model you think you own may not fit the machine you actually own.
The set is normally supplied as a matched group — tip, ring and seat. Replacing only the ring on a worn seat gives you a short life and the same symptoms again in a few months.
While the barrel is open
Measure the screw flight and barrel bore. If wear is significant, a tip set alone is a temporary repair, and you would rather know that before reassembly than after.
Practical view
On a machine running abrasive or filled material, tip sets are a wear item with a predictable life. Treat them as consumables and hold one set, rather than discovering the need during a production run.