System for using synchronous secondaries of a linear motor to biaxially draw plastic films

Apparatus for continuously drawing a web of material in a tenter frame by propelling individual carriages with tenter clips attached thereto at predetermined speeds in endless paths defined by a pair of guide tracks positioned opposite each other equidistant from a centerline between the tracks, using a linear motor, such tenter frame including:

• a pair of endless tracks each having first portions defining a transport section for transporting said web without stretching, second portions defining a section for stretching said web, and third portions defining a section for stabilizing said web;
• said tracks also having fourth portions not associated with said web which connect the third portions to the first portions of the tracks;
• a plurality of carriages guided by the tracks, said carriages having tenter clips attached thereto for gripping the edges of the web during its movement through the first, second and third portions of the tenter frame;
• said linear motor including
• a primary positioned adjacent each track of said pair of tracks, each primary including a plurality of groups of coils with the groups of coils in one primary sized to match opposed groups of coils in the other primary and with each of the opposed groups of coils being electrically joined and defining a single control zone;
• a synchronous secondary attached to each of the carriages;
• means for simultaneously developing matching waveforms to the opposed groups of coils in each control zone, said waveforms for a control zone having specific predetermined frequency and phase characteristics for developing matching traveling electromagnetic waves in the opposed groups of coils in said control zone, wherein the speeds of the matching traveling electromagnetic waves developed in the opposed groups of coils in each control zone varies,
• whereby the synchronous secondaries attached to the carriages adjacent a control zone synchronously engage the matching traveling electromagnetic waves developed in the opposed groups of coils in said control zone to propel the carriages in opposed pairs in symmetry through said control zone from a first speed in one control zone to a higher speed in another control zone, to stretch the web.

A process for continuously drawing a web of film by propelling individual carriages having tenter clips attached thereto at predetermined speeds in endless elongated paths defined by a pair of carriage guide tracks positioned opposite each other equidistant from a center line between the tracks, using a linear motor, including the steps of:

• positioning a linear motor primary adjacent each track, each primary including a plurality of groups of coils with the groups of coils in one primary sized to match opposed groups of coils in the other primary and with each of the opposed groups of coils being electrically joined and defining a single control zone;
• providing a synchronous secondary attached to each of the carriages;
• simultaneously developing matching waveforms to the opposed groups of coils in each control zone, such waveforms for a control zone having specific predetermined frequency and phase characteristics for developing matching traveling electromagnetic waves in the opposed groups of coils in each control zone,
• synchronously engaging the secondaries attached to a pair of opposed carriages with the matching electromagnetic waves developed in a control zone at the entrance of the tenter frame;
• gripping opposite edges of the film with symmetrically opposed pairs of clips attached to the carriages;
• propelling an opposed pair of carriages, one pair at a time, completely through a control zone and coordinating the waveform characteristics to that control zone in a predetermined manner with the waveform characteristics to each adjacent control zone, whereby such coordinating includes simultaneously controlling the waveform developing to all of the control zones,
• whereby the waveform developing and coordinating results in the continuous propelling of a pair of carriages with attached secondaries synchronously through each control zone and from one control zone to the next control zone.

A process of drawing a web of film in a tenter frame with independently propelled tenter clips including the steps of propelling such clips in opposed loops in paired symmetry using synchronous linear motors wherein synchronous secondaries are attached to the clips;

• positioning a linear motor primary adjacent each loop;
• developing in each linear motor primary a plurality of electromagnetic waves that are coordinated in time in a precisely predetermined manner, with the electromagnetic waves on one loop matching the respective, opposed electromagnetic waves in the other loop;
• synchronously engaging the synchronous secondaries attached to the carriages with the electromagnetic waves to propel the carriages throughout the endless loops;
• grasping the edges of the film with the tenter clips;
• and, drawing the film.