Common Problems And Solutions During Casting Film Production (Part One)

The casting process is a relatively common film production process. The film produced by this casting process is non-stretch and non-directional, which is commonly used in the composite film and stretch film applications. In general, cast film are polyolefin film, specifically divided into polypropylene (CPP) and polyethylene film (CPE film) etc..CPP is made from the copolymerization with melt flow rate of 5-12 or the homopolymerization of PP as raw materials, which is characterized by high transparency, good flatness, good rigidity, damp proof, oil resistance, high temperature resistance and good smoothness, these characteristics make it widely used in the inner layer substrate of cooking resistant and vacuum aluminum plating film substrate.

CPE adopts PE whose melt index is about 2 as production raw materials, its has soft, high smoothness, good moisture resistance, good transparency and good heat sealing performance characteristics, widely used in the production of composite membrane lining material.

Belowed are some common products quality and production operation problems, and the corresponding solutions.

1.Plastic Film Lateral Thickness Is Not Uniform

Nonuniform discharge amount of die is the fundamental reason for uneven transverse thickness film, the following specific factors and some solving methods as followed:

(1) Improper adjustment of die lip gap, need partial adjustment or all re-adjustment;

(2) Uneven lateral temperature distribution of T-Die, to check the heater whether is failure;

(3) There are debris in the mold cavity, which causes the logistics disorder and uneven material , need to do regularly clean up several times every year;

(4) The melt pressure fluctuation is too high or too large, it will cause instable extrusion to affect the thickness uniformity , to improve such situation by adjusting the temperature, extrusion speed and raw materials adjustment;

(5) Air knife position or direction is inappropriate or the uneven wind ,which causes uneven cooling so that the film thickness is not uniform;

(6) The recycling material is added too much or is not uniform, generally,adding a return rate should be less than 15-20%, then even to join.

2.The Stretch Film Has Crystal Point

Crystal Point is due to a small amount of high molecular weight resin had failed to plasticization. Therefore,static mixer, other improved shear and melt pressure method can be used in the equipment.

The following conditions are easy to produce grain:

(1) The barrel temperature is not enough or too high, low temperature, raw materials can not be completely plasticized; high temperature,small melt pressure and small shear force are bad for polyolefin plastics plasticizing;

(2) The melt pressure is small, shear is also small, bad plasticating, may be appropriate to increase screen mesh layers and numbers ;

(3) Low extrusion speed,so that shear is not enough;

(4) The poor raw materials, to choose appropriate grades raw materials, but also pay attention to whether the material is mixed with other great difference material.

3.Film Roll With Storm

The fundamental reason for film roll with storm lies in the uneven thickness, even very small, accumulated may also cause crack obviously, affecting the appearance and use. Can be solved from the aspects :

(1) For uneven thickness, find out the influence factors, then carefully adjusted;

(2) The winding tension is too large, the film roll too tight;

(3) The melt pressure instability cause uneven thickness;

(4) Air knife position or direction is inappropriate or the uneven wind ,which causes uneven cooling so that the film thickness is not uniform;

(5) The membrane becomes shrinkage, to adjust from the raw materials production process and film storage .

4.Bad Film Transparency

Polyolefin is the crystalline polymer, the key to improve transparency is to reduce the crystallinity degree and make small spherulites, the main factor is the resin material. The production process adjustment can only to improve cast film transparency in a certain extent.

Measures are as follows:

(1) Reduce cooling roller temperature to make better quenching effect;

(2) Shorten the distance between T-die and cooling roller, reduce the crystallization;

(3) Slow down the extrusion speed to make melting film cooling enough in cooling roller.

5.White Spots On Film Surface

The main spots are mainly caused by the additive of raw material, low molecular weight resin and the dust which is condensed on the cooling roller in the proceed of production, accumulated after a certain number ,constantly took away to form as membrane spots ,it is easy to make while producing thicker (≧0.06mm) film.

Because it is caused by the precipitates in the resin, it cannot fundamentally eliminated , main adjustment measures:

(1) To selected less resin precipitates grades;

(2) Stop machine to clean the cooling roller after a certain time working;

(3) Increase the roller temperature, the precipitation is difficult to condense on the roller, avoid obvious white spot;

(4) To make the biggest air knife and cleaning roller pressure after producing every several volumes film, ues the film tightly on the roller to put precipitate condensation away, then remove this film, white spot can be greatly alleviated, but also to avoid frequent stop;

(5) Put the knife pressure larger when start to production , take the precipitates appeared gradually away, then gradually reduce the air knife pressure, until the cleaning roller;

(6) The roller quenching cooling is not uniform, use non corrosive cleaner to remove limescale;

(7) Adjust cleaning roller pressure to be small.

6.Bad Film Smoothness

Smoothness is an important requirement for the actual use of the resin film, generally, add smoothing agent in resin to ensure good. But during production, the smooth performance is not good, can be analyzed from the following aspects:

(1) Low crystallinity of raw materials, such as some copolymer;

(2) The processing temperature and the cooling temperature is high, the membrane viscosity is increased.

(3) Too much additives in raw materials causes the membrane between the paste tightly;

(4) The distance from T-die to cooling roller is too large;

(5) Too fast extrusion speed so that the cooling is not sufficient and generate friction heat.

(Source: Innovative Film Solutions)