The quality of composite packaging bags have great influence on the packaging process, transportation, storage of contents and distribution of products. This article summarizes the causes and countermeasures of five common problems in heat sealing and bag making.

  1. Incomplete heat sealing results in leakage or deterioration of contents

The reason:

The film has poor anti-pollution, mainly because the resin used in the inner seal is not suitable.


Adopt the film with good anti-pollution heat sealing property. Generally speaking, LDPE has medium anti-pollution thermal sealing, EVA has good anti-pollution thermal sealing property when the content of VA is large, LDPE also has good anti-pollution thermal sealing property, ionic resin and metallocene polymer have excellent anti-pollution thermal sealing.

  1. The seal is crisp and broken

The reason:

(1) excessive heat sealing temperature.

(2) too much pressure.

(3) the heat sealing time is too long.

(4) the edge of the upper mouth seal is too sharp, or the polytetrafluoroethylene covered is damaged.

(5) the silicone rubber sealing at the bottom is excellent.

(6) in the process of compounding and curing, part of the adhesive into the inside of the film. As a result of the adhesive penetration, the toughness (impact) decreased, brittleness increased.

(7) after the cooling and placement of plastic composite packaging bags, the heat sealing strength increased, but also the tendency to become brittle.


(1) select the appropriate processing temperature, pressure and heat sealing time according to the heat sealing characteristics of the inner sealing material.

(2) improve the surface state of the upper heat sealing cutter to make the surface of the sealing device smooth.

(3) the polytetrafluoroethylene cloth is covered in good condition.

(4) choose silicone rubber pad with appropriate hardness.

  1. Bag warping after heat sealing

The reason:

(1) the thickness of the composite film is inconsistent.

(2) the heat sealing temperature is too high or the heat sealing time is too long.

(3) in the longitudinal heat-sealing cutter part, the running track of the composite film is not straight.

(4) insufficient cooling.

(5) not enough ripening time.

(6) the surface substrate film has poor heat resistance.

(7) improper tension matching control of composite substrate during the composite process leads to residual stress after curing and shaping, especially when the composite film thickness is relatively thin.


(1) adjust the tension of the floating roller.

(2) choose the inner sealing base material with low temperature heat sealing property.

(3) adjust the heat sealing temperature to the appropriate temperature.

(4) sufficient cooling.

(5) fully mature.

(6) re-select the surface substrate film.

(7) adjust the tension of each part of the composite processing equipment, and try to make the shrinkage rate of the two composite substrates equal.

  1. Unclear outer contour of the heat sealing surface (unclear knife marks)

The reason:

(1) short cooling time.

(2) poor contact of cooling plate.

(3) as the silica gel pad is heated, the edge part melts.

(4) the longitudinal heat sealing force is uneven.

(5) the edge of the heat-sealing knife is irregular and dull.


(1) adjust the cooling time.

(2) adjust the cooling plate.

(3) replace or adjust the heat sealing knife.

(4) prevent edge melting caused by overheating of silica gel pad.

  1. Poor heat sealing strength

The reason:

(1) the adhesive in the composite film has not been fully hardened.

(2) improper heat sealing conditions.

(3) the distance between the heat sealing knife and the cooling knife is too long.

(4) there is something wrong with the inner seal film.

(5) ink poor heat sealing, resulting in heat sealing parts of the composite strength of the film.

(6) dust, powder spraying and other substances adhere to the hot cover.

(7) low composite strength or excessive reduction of composite strength in the heat seal.


(1) promote adhesive hardening (curing and curing) through heat preservation and aging, and improve the composite strength and heat resistance of the composite film.

(2) select the best heat sealing conditions (temperature, time and pressure) or improve the heat sealing method according to the composition of the composite film and the heat sealing state, and immediately cool down after the heat sealing.

(3) check the storage life and storage conditions of the heat-sealing layer film. If the heat-sealing layer USES the old batch number film and the heat-sealing film, pay special attention.

(4) increase the thickness of the thermal seal film.

(5) change the type and grade of thermal seal film and use inner seal film with anti-pollution thermal seal.

(6) check whether the grade of the adhesive meets the requirements (resin in the low molecular material exudation, sometimes will affect the composition of the adhesive).

(7) improve the thermal sealing performance of the heat sealing layer (substrate), control the content of the slipper, use LLDPE film.

(8) control the monomer content of MDI in the compound adhesive.

(9) check the treatment of hot cover.

(10) change the ink (preferably with two liquid ink).

(11) change the printing pattern of the package so that the printed ink part avoids the heat sealing part, or change the heat sealing method. (be careful not to print metal ink on the seal).