Plastic Bag Making Machine Automation

The machine layout depends on the product: on bottom-seal machines (garbage bags, bags on a roll) the seal runs across the film and bags can be perforated and wound into a roll; on side-seal machines (T-shirt, courier, zip bags) a hot knife separates two bags and seals their sides at the same time; on wicket machines bags are punched and stacked onto a wire wicket. Courier bags add tape application, flap folding and strip feeding. The common core is the same: unwinder, tension dancer, servo feed, sealing and cutting station, stacking.

On intermittent machines the film stops every cycle: the servo feed roll advances one bag length with an S-profile, the sealing jaw comes down while the film is stopped, applies heat for a set time and lifts, and the knife cuts. Because the unwinder unwinds continuously, a dancer in between buffers the difference between intermittent feed and continuous unwinding and sets the unwinder speed. On continuous machines the film never stops; a rotary sealing drum or a sealing jaw moving with the film is tied to film speed with an electronic cam.

On printed film the bag length has to equal the print repeat. The photocell captures the position where the print mark arrives each cycle (touch probe); the controller adds the difference between the expected and measured position to the next feed length. This corrects film stretch, winding tension and small differences in print repeat before they build up. On plain film the length comes directly from the servo axis position, or from a measuring wheel if slip has to be excluded.

What usually limits cycle rate is sealing time: thick film and wide seals need longer contact. Feed axis acceleration and jerk limits are chosen for film stretch and slip; a feed that is too hard causes length errors on thin film and roll slip on thick film. When the sealing jaw is driven by a crank or servo, its down and up profile is cammed to the same master axis as the feed, so sealing time can be held constant when speed changes.

In the ONX architecture the feed roll, sealing jaw and auxiliary axes (flap folding, tape feed, wicket arm) are tied to one virtual master axis with CODESYS SoftMotion; each station runs from cam profiles defined against this master. The photocell is wired to a fast capture (touch probe) input on the drive or I/O module, and registration correction is applied to the next cycle's length.

The unwinder dancer is read as an analog input and coupled to the unwinder AC drive speed by PID; sealing heaters are held by temperature control. Bag length, sealing time and temperatures, registration settings and counters are managed from recipes; product changes are made by selection on the screen.

The architecture on this page is built on the servo feed, registration correction and electronic cam platform ONX Control uses in its rotary knife, horizontal and vertical packaging applications. We work out your machine's station layout and cycle target together in a meeting.

A typical picture on bag machines: a feed driven by a mechanical cam or by a drive tied to a single control card with no spare parts, registration adjusted by hand, mechanical adjustment at every product change and old stepper motors that limit cycle rate. In a retrofit the frame, seal bars and knives are kept; feed and sealing motion move to servo axes and cam profiles in software.