A large collection of heat sealing problems for packaging bags, all the solutions you need are here

1、 What to do if incomplete heat sealing causes content leakage or spoilage?

Cause Analysis:

Poor anti-contamination heat sealing performance of the film, mainly due to unsuitable resin used for the inner sealing layer.

Solution:

Use films with good anti-contamination heat sealing properties. Generally, LDPE has moderate anti-contamination heat sealing properties, EVA has good anti-contamination heat sealing properties when the VA content is high, LDPE also has good anti-contamination heat sealing properties, while ionic resins and metallocene polymers have excellent anti-contamination heat sealing properties.

2. What to do if the seal becomes brittle and breaks?

Cause Analysis:

(1) Heat sealing temperature is too high.

(2) Pressure is too high.

(3) Heat sealing time is too long.

(4) The edge of the upper sealer is too sharp or the polytetrafluoroethylene coating is damaged.

(5) The silicone rubber for bottom sealing is too hard.

(6) During the compounding and curing process, some adhesive penetrates into the film. The substrate's toughness (impact resistance) decreases and brittleness increases due to the adhesive penetration.

(7) After cooling and resting, the heat seal strength of plastic composite packaging bags increases, and there is also a tendency to become brittle.

Solution:

(1) Select appropriate processing temperature, pressure, and heat sealing time based on the heat sealing characteristics of the inner sealing layer material.

(2) Improve the surface condition of the upper heat sealing knife to make the sealer surface smooth.

(3) Ensure the polytetrafluoroethylene cloth coating is intact.

(4) Choose a silicone rubber pad of appropriate hardness.

3. What to do if the bag warps after heat sealing?

Cause Analysis:

(1) Inconsistent thickness of the composite film.

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

(3) In the longitudinal heat sealing knife section, the running path of the composite film is not straight.

(4) Insufficient cooling.

(5) Insufficient curing time.

(6) Poor heat resistance of the surface substrate film.

(7) Improper tension matching control of composite substrates during the lamination process leads to residual stress after curing and setting, especially when the composite film is thin, such failures are more prone to occur.

Solution:

(1) Adjust the floating roller tension.

(2) Select an inner sealing substrate with low-temperature heat sealing properties.

(3) Adjust the heat sealing temperature to an appropriate level.

(4) Cool sufficiently.

(5) Cure sufficiently.

(6) Re-select the surface substrate film.

(7) Adjust the tension of each part of the compounding equipment to make the shrinkage rates of the two laminated substrates as equal as possible.

4. What to do if the outer contour of the heat-sealed surface is unclear? (Knife marks are not clear)

Cause Analysis:

(1) Short cooling time.

(2) Poor contact of the cooling plate.

(3) The silicone pad is heated, causing the edge to melt.

(4) Uneven longitudinal heat sealing force.

(5) The edge of the heat sealing knife is uneven or dull.

Solution:

(1) Adjust cooling time.

(2) Adjust the cooling plate.

(3) Replace or adjust the heat sealing knife.

(4) Prevent edge melting caused by overheating of the silicone pad.

5. What to do if the heat seal strength is poor?

Cause Analysis:

(1) The adhesive in the composite film has not yet 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 a problem with the inner sealing layer film.

(5) Poor heat sealing resistance of the ink, leading to reduced composite strength of the film at the heat sealed area.

(6) Dust, powder spray, and other substances adhere to the heat sealing surface.

(7) Low composite strength or excessive decrease in composite strength at the heat seal.

Solution:

(1) Promote adhesive hardening (curing, solidification) through thermal aging to improve the composite strength and heat resistance of the composite film.

(2) Select optimal heat sealing conditions (temperature, time, and pressure) based on the composite film's structure and heat sealing state, or improve the heat sealing method, and cool immediately after heat sealing.

(3) Check the shelf life and storage conditions of the heat sealing layer film, especially if old batch films or heat-treated films are used for the heat sealing layer.

(4) Increase the thickness of the heat sealing layer film.

(5) Change the type and grade of the heat sealing layer film, and use inner sealing layer films with anti-contamination heat sealing properties.

(6) Check if the adhesive grade meets requirements (leaching of low molecular weight substances from the resin can sometimes affect the composition of the adhesive).

(7) Improve the heat sealing performance of the heat sealing layer (substrate), control the content of the release agent, and use mLLDPE film.

(8) Control the monomer content of MDI in the composite adhesive.

(9) Check the degree of heat sealing surface treatment.

(10) Change the ink (it is best to use two-component ink).

(11) Change the printing pattern of the packaging bag so that the part with ink avoids the heat sealing part, or change the heat sealing method (pay special attention not to print metallic ink on the sealing part).

6. What should I do if there are bubbles or uneven surfaces at the heat sealing part?

Cause Analysis:

(1) When heat sealing on materials with hygroscopic properties such as nylon film, moisture absorption will cause bubbles.

(2) The heat sealing knife or heat sealing silicone pad is uneven.

(3) Insufficient heat sealing pressure.

(4) When using a single-component adhesive, such as chloroprene rubber adhesive, the heat sealing surface is prone to uneven small pits during heat sealing bag making.

Solution:

(1) Properly store raw materials and semi-finished products to prevent moisture absorption.

(2) Check or adjust the heat sealing knife and heat sealing silicone pad.

(3) Change the adhesive.

7. What should I do if the dry lamination product has poor heat sealing?

Cause Analysis:

(1) Excessive corona treatment of the heat sealing substrate film leads to local breakdown.

(2) The content of release agent in the film is too high.

(3) Poor heat sealability of the substrate itself or too high content of recycled materials.

For commonly used blown film substrates, their heat sealability is affected not only by the resin grade but also by the extrusion temperature, blow-up ratio and draw ratio, cooling line, and recycled materials used.

The heat sealability of blown film will decrease with the increase of extrusion temperature and oxidation degree.

A certain blow-up ratio and draw ratio are beneficial to improving the heat sealing strength, but because film blowing and drawing actually cause the molecules entering the highly elastic state to be oriented, accelerating and promoting their crystallization, resulting in an increase in the oriented strength of the film while reducing the heat sealability, that is, the heat sealing strength. Therefore, excessive blow-up ratio and draw ratio are not suitable.

A high cooling line indicates that the blowing is carried out in the liquid state, and the blowing and drawing do not produce molecular orientation, only making the film thinner, and will not orient the freely moving polymer molecules in the liquid state, and the produced film is close to cast film, with good heat sealability.

Conversely, if the cooling line is too low, the blowing is carried out in the highly elastic state of the polymer, and the blowing not only thins the film, but also causes the molecules to be oriented under the action of the blowing force, and promotes the crystallization of the polymer, resulting in poor heat sealing performance.

Recycled materials used in the film have undergone multiple thermal degradations, and their heat sealing temperature has been greatly increased.

(4) The adhesive has not yet fully cured.

Solution:

(1) Check the shelf life and storage conditions of the film, and pay special attention when using old batches, films with high release agent content, or heat-treated films.

(2) Increase the thickness of the film.

(3) Check the type or grade of the adhesive.

(4) Check the degree of treatment of the heat sealing surface, and replace it in time if any problems are found.

8. What should I do if the lubricity is poor?

Cause Analysis:

(1) The curing temperature is too high, and the release agent loses its effect at high temperature.

(2) The adhesive penetrates to the inner surface, affecting the friction coefficient of the substrate film surface, especially when the film is thin and the corona treatment is excessive.

(3) The substrate film itself has poor lubricity.

Solution:

(1) Strictly control the curing conditions.

(2) Check the type or grade of the adhesive.

(3) Change the substrate or use powder spraying.

9. What should I do if the laminating plate is agglomerated and it is difficult to collect materials?

Cause Analysis:

Quality problems of film and adhesive.

Solution:

Powder sprinkling during bag making.

10. What should I do if the bag making length and its related deviations exceed the tolerance?

Cause Analysis:

(1) The length setting system error of the bag making machine is too large.

The length setting methods for bag making can be broadly classified into two categories: mechanical length setting and electrical length setting. Mechanical length setting uses an overrunning clutch plus a pneumatic or voltage fine-tuning system for length setting. Electrical length setting uses a stepping motor or AC servo motor for length setting. Both types of length setting systems consist of a heat sealing, cutting linkage system and a film feeding system, which are coordinated through a photoelectric eye. If there is an error in the linkage or coordination, it will cause dimensional deviations related to the bag making length.

(2) Inaccurate photoelectric eye tracking.

When the photoelectric eye cannot find or cannot continuously and stably find the cursor, that is, when the tracking is inaccurate, the bag making length control cannot be discussed. There are several reasons for inaccurate tracking:

① Photoelectric eye problem: Improper adjustment method and working state selection of the photoelectric eye.

First, it is necessary to confirm whether the photoelectric eye is properly adjusted, whether the reflection and transmission selection of the photoelectric eye are correct, and then confirm whether it is leading edge control or trailing edge control, that is, the so-called polarity, and finally confirm the tracking method, such as: scanning type (that is, first complete the length setting, and then scan the color mark), fixed length positioning type (that is, check whether there is a color mark after completing the length setting, whether it is over or not in place).

② Cursor printing or spacing problems

a. Unreasonable cursor design, excessively narrow usable scanning area, and poor contrast between the cursor and surrounding area.

b. Variation in cursor spacing.

Due to the influence of printing and lamination processes causing stretching and shrinkage, the actual distance between cursors has changed relative to the original set dimensions. The actual input dimensions must be changed accordingly based on the actual data. Uneven stretching caused by the lamination process, frequent large adjustments in color registration during the printing process, and large temperature fluctuations causing unevenness and stretching will also directly affect the error related to the bag length.

In addition, uneven thickness of the composite film bagging substrate and ruffling will also have a certain impact, and is one of the main reasons for the deviation in the alignment of the finished bag. Generally speaking, composite films processed using dry lamination have a relatively higher yield of qualified bags.

③ Film feeding resistance:

When the film feeding resistance is too high (affected by the pressure tightness of the pressure film ruler and the friction coefficient of the composite film), it may cause the film to not feed properly or the cursor to deform, thus affecting the scanning effect and causing deviations in the bag length.

④ Pre-adjusted film feeding length:

If the pre-adjusted film feeding length (when adjusting the blank bag) has too large an error from the actual bagging size, and the scanning area does not cover the cursor, it can directly lead to inaccurate tracking.

(3) Influence of plastic composite film bagging processing speed

When the bagging processing speed is faster, the deviation related to length control increases significantly. The main reason is that under the action of high-speed inertia, the movement of the cutting knife and the feed roller is affected.

Since the degree of influence on the movement of the heat sealing knife, cutting knife, and feed roller is different, this leads to an increase in deviations such as the bottom margin. However, for bagging machines with higher manufacturing precision, it is possible to form a quantitative system error, which can be basically stabilized after adjustment.

In addition, when the film feeding speed increases and decreases too quickly, due to inertia, it is also easy to cause a large film feeding error.

(4) Operational factors

① Insufficient pressure setting of the film feeding rubber roller pressure cylinder.

② Improper tension setting in each section. Especially for thinner composite films, excessive tension may cause changes in spacing, so the tension should be set carefully.

Solution:

(1) Adjust the length setting system.

(2) Adjust the photoelectric eye working status.

(3) Check for and adjust any cursor printing or spacing problems.

(4) Check whether the pressure tightness of the pressure film ruler and the friction coefficient of the composite film are appropriate.

(5) The pre-adjusted film feeding length (when adjusting the blank bag) should be as accurate as possible to the actual bagging size.

(6) Reduce the bagging speed, and the adjustment of the bagging speed should be carried out slowly.

(7) Adjust the pressure of the film feeding rubber roller, the pressure of the cylinder, and the tension of each part.

11. What to do if the bag width deviates or the seal is not tight? (Center seal, side seal)

Cause Analysis:

(1) Uneven thickness of the composite film.

(2) The size of the specification plate is out of tolerance.

(3) Improper tightness of the film wrapping the specification plate. The tighter the wrap, the greater the resistance, and it will also affect the forming width of the bag. If it is too loose, the seal is likely to be not tight.

Solution:

(1) Adjust the specification plate.

(2) Adjust or replace the composite film.

(3) Adjust the tightness of the film wrapping the specification plate. For thinner films, lateral shrinkage will occur when tightened, clamping the mold plate, so the mold plate size should be smaller than the standard size. For thicker films, due to the influence of thickness, the mold plate size should also be smaller than the standard size.

12. What to do if the top and bottom are not aligned properly?

Cause Analysis:

(1) Uneven thickness of the composite film, with ruffling.

(2) The tension of the double floating rollers is too small.

(3) Some rollers rotate unevenly.

Solution:

(1) Adjust the tension of the double floating rollers.

(2) Check and adjust the relevant guide rollers (such as compensation rollers, deviation adjustment rollers, etc.).

13. What to do if the cutting line is not neat?

Cause Analysis:

(1) The cutting knife is worn.

(2) Insufficient pressure of the cutting knife pressure spring.

(3) The upper knife angle is not adjusted well.

(4) There are foreign objects on the cutting knife.

(5) The cutting knife is bent.

(6) Poor cooling effect.

(7) The resin used has too high viscosity and strength.

Solution:

(1) Replace or adjust the cutting knife.

(2) Adjust the pressure of the cutting knife pressure spring.

(3) Adjust the angle of the upper blade.

(4) Enhance the cooling effect.

(5) Adjust the base material resin formula.

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