Process and machine for wrapping products with stretchable film, and wrapping formed by this process

A machine for wrapping products with stretchable film comprising:

• means for ascertaining length, width, and height dimensions of an article to be wrapped;
• means for feeding a longitudinal segment of the stretchable film to a wrapping station, including at least one reel for feeding the film and means for determining a length dimension of the longitudinal segment to be fed to the wrapping station which is proportionate to at least the length dimension of the article determined by said ascertaining means;
• means for holding, at a product wrapping point, a longitudinal segment of the film, which is extended but not substantially stretched, said holding means comprising
• a fixed front dispenser disposed next to a front fixed folder capable of allowing the film to slide through said front fixed folder only when a specified longitudinal tensile force is exceeded so that the film remains longitudinally extended,
• a rear clamp parallel, complementary and opposite to said dispenser and mounted against a carriage which carries a movable rear folder, and
• a computer which controls a movement of said rear clamp towards and away from said front dispenser wherein a travel distance of said rear clamp is proportionate to at least one of length, width, height dimensions of the product to be wrapped;
• means for transversely pre-stretching a selected intermediate portion of a longitudinal segment of the stretchable film, said pre-stretching means including
• means for selecting a desired length dimension of the intermediate portion to be pre-stretched which is proportionate to the length dimension of the longitudinal segment determined by said determining means such that the desired length dimension of the intermediate portion is also proportionate to the length dimension of the product,
• on each lateral side of the longitudinal segment, a respective variable length clamping means for clamping different lengths of intermediate portions in order to clamp at each lateral side the desired length dimension of the intermediate portion,
• means for actuating the variable length clamping means at each lateral side so that a length of the clamping means substantially equals the selected length dimension of the intermediate portion, and
• means for moving the respective actuated clamping means transversely away from one another to pre-stretch the intermediate portion;
• said variable length clamping means comprising
• opposing clamps disposed to grip the lateral sides of film extended at the wrapping point, each said clamp being disposed next to at least one adjacent said clamp,
• means for rectilinear and self-centering movement of said clamps;
• elastic means for closing said clamps and a centralized operating system for opening said clamps, and
• means for causing said clamps not used for gripping the film by the variable length clamping means to be disposed at a lower height than an assembly comprising said rear folder and said rear clamp;
• means for reducing longitudinal tension in the film, during the pre-stretching phase, with a proportional approach of the rear clamp which holds a rear flap of the film and with a proportional drawing of film from said fixed front dispenser;
• a lifter located at the wrapping point and capable of lifting the product to be wrapped against the film which is held by said holding means and which bears a front of said film on a rear face of the front fixed folder and a rear of said film on a support which allows the film to slide freely, wherein said opposing clamps are able to approach each other to a distance proportionate to the dimensions of the product as the product is lifted, while the longitudinal tension of the film is reduced with a suitable approach of said rear clamp and with a proportionate drawing of film from the feed dispenser;
• movable folding means for executing folding of side flaps onto the bottom of the product, said side flaps being formed by transverse pre-stretching of the film and held by the respective variable length clamping means, the rear folder being made to operate and extend a portion of a rear flap of the film under the product and under a portion of said side flaps, while the film remains fixed to said rear clamp;
• a pusher, mounted on a carriage which also carries the rear folder immediately after the rear folder which pushes the product onto the front fixed folder while a front flap of film is drawn under tension from said fixed front dispenser and extended onto an entire bottom length of the product;
• and
• cutting means for separating the front flap of the wrapping film transversely from a new leading end of the film held by said dispenser.