The cause of crimping and warping of dry composite membrane bag
Oct 27, 2018

The cause of crimping and warping of dry composite membrane bag


1. Crimping and warping after bag making:

1. Bag opening inward roll after bag making:

A).  the heat sealing material has low heat resistance, large shrinkage, and rolls inward;

B).  the tension of the two substrates is inconsistent. If the inner tension is greater than the outer tension, the opening of the finished bag will curl inward.

C).  heat resistance temperature of heat sealing material is lower than that of bag making;

D).  the base film has a lower thermal shrinkage than the inner film, and inner coil is produced after the bag is heated.


2. After making the bag, the bag mouth rolls out:

A) the thermal shrinkage of the outer material is greater than that of the inner material;

B) more residual solvents in the membrane;In particular, it contains more OH roots and is easy to absorb moisture.

C) excessive amount of adhesive coating, incomplete drying, more residual solvents, and tunnel phenomenon in thermal seal position;

D) the heat resistance of the outer material is lower than that of the heat sealing material to produce outer coil;

E) there are still residual stresses after the maturation of the material, especially when the composite film thickness is relatively thin, the base material tension is too large, while the composite material tension is too small, and the finished bag is easier to roll outward.


3. S-type crimp:

A) BOPA film absorbs too much moisture and produces s-type crimp after bag making and heat sealing;

B) the difference between the longitudinal and transverse thermal contraction ratios of BOPA films is too large, and the difference between them should be controlled at 0.5 in general.

C) the thermal shrinkage of BOPA film is too large, resulting in crimp, and the vertical and horizontal shrinkage exceeds 3.0%.(general composite packaging)


2. Non-synchronous contraction produces crimp:

Asynchronous shrinkage of composite membrane is the material of the change of curling and warping, asynchronous contraction have finished bag "curl inward" or "curly" outward two kinds of forms, this state shows that the composite film in addition to the synchronous contraction still exist inside asynchronous contraction (thermal stress and shrinkage difference size and direction of contraction) and curled up.

Generally, the CPP thermal shrinkage rate is not more than 1% in the market. The thermal shrinkage rate of PE film is between 1.4% and 7.2% vertically and between -0.4% and -1% horizontally.

Ordinary BOPA film longitudinal and transverse shrinkage control within 2.5%, the difference control in 0.5%, (160 ℃, 5 nim).


3. Crimp and edge after cooking:

If the product structure is BOPA//CPP, after boiling, cooking bag, after heat treatment is necessary to curl outwards.In the BOPA//PE structure, after heat treatment, the opening of the bag may be outward curly, inward curly or flat.However, when making bags, it is "out". After heat treatment, the opening edge of the bag must be crimped outward.

BOPA cooking longitudinal and transverse thermal shrinkage film (i.e. membrane 0 degrees, 90 degrees of resistance to 121 ℃ 40 nim, thermal shrinkage) control around 5% of crimp rate is relatively less.The difference between the vertical and lateral thermal shrinkage should also be controlled within 0.5%.The indexes of BOPA membrane (UNITIKA) cooking membrane were no more than 2.8% longitudinal and no more than 1.7% transverse.


4. Matters needing attention when purchasing film and entering warehouse and production:

When buying the film into the warehouse, the film should be used under similar processing conditions for thermal shrinkage test, if large deviation is best to adjust the use.Generally, crimping and warping can be concluded as the following:

(l) the thickness of the composite film is inconsistent, resulting in different heat seal temperatures and different shrinkage rates.

(2) if the heat seal temperature is too high or the time is too long, the heat seal part will shrink and wrinkle.

(3) in the longitudinal hot-sealing part, the running track of the composite film is not straight, resulting in contraction and wrinkle.

(4) insufficient cooling of bag-making, especially reflected in the contraction and wrinkle of the heat-sealing part after the final cooling after the heat-sealing of bag-making.

(5) insufficient curing time and more residual solvents will produce air bubbles, delamination or bagging.

(6) the surface substrate film has poor heat resistance and is curled outward.

(7) during the composite process, the tension matching of the composite two substrates is not controlled properly, resulting in the residual stress after its maturation and setting.Especially, the fault is more likely to occur when the composite film thickness is relatively thin, which is manifested as outward roll.

(8) the thermal seal layer, especially PE layer, causes inward curling due to too large shrinkage.

(9) excessive amount of adhesive coating causes external coils.

(10) the tension of base material is not consistent in the composite, resulting in shrinkage and crimp, and which side of the tension is large and crimp to which side.

(11) s-type shrinkage and crimp caused by too large shrinkage of nylon film

(12) the smaller the difference between the thermal shrinkage and the 45 degree diagonal of nylon film, the better the product balance, and the lower the probability of producing warp Angle when the difference is less than 1.5%.

(13) too much residual solvent in the membrane causes outward curl.

(14) the bag making has a tendency to curl outwardly after the substrates are hygroscopic.

(l5) the contraction ratio of the outer layer is too different from that of the inner layer, causing outer volume.The reverse causes an internal curl.

(l6) the heat resistance temperature of the outer material of the bag is lower than the heat seal temperature, resulting in outer curl.


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