Four problems and five common solutions to be considered by production of the PVC foam profile

Four problems and five common solutions to be considered by production of the PVC foam profile.

 

In extrusion process of PVC foam profile, problems are encountered ,can be basically classified into four situations. One is stability ; second  is melt strength ;  third is lubrication ; and  fourth is uniform dispersion .

These four types of problems, especially the first three types ,there will be mutually restrictive and cross-influential. It is sometimes difficult to distinguish them. It is necessary to observe and analyze the situation and find the root cause of the problem before it can be solved.

1.Insufficient stability will affect the entire profile surface, the surface will be yellow, and the foamed sheet will be brittle.

 2.Insufficient melt strength will cause the foam cells to have large pores and very long longitudinal /machine directional.

To determine whether the melt strength is insufficient, the most direct way is to press the plate wrapped on the middle roll with your fingers after it extrude out. When the melt strength is good, it has a feel of elasticity. If it is difficult to pop up after pressing, it indicates that the melt strength is poor.

Sometimes, the screw structure and cooling method are quite different, it is difficult to judge whether the temperature is reasonable. In order to obtain foamed uniform products in foamed profile, it is also necessary to ensure that PVC materials have good melt strength. Therefore, the quality and type of the foaming regulator is of utmost importance. , the ADZ yellow and white Bi carbonates foaming agent should be in the right ratio.

3.Lubricants are divided into external lubricants and internal lubricants. The external lubricant is good for demoulding, which is good for the surface finish of the sheet. If external lubricant  is too little. The temperature in the barrel of the extruder is not easy to control and can easy to heat up, which will cause the core high temperature, hence there are  large bubbles in the middle of the plate, stringing, yellowing,  and the surface of the plate is not smooth;

 Too much lubricant, slipage will become serious, the structure shown in the mold and the precipitation of the slide surface of the plate will also show. Internal lubricant is good for plasticization and melt fluidity. Insufficient internal lubricant ,make it difficult to control the thickness of the board surface, which is shown as the middle thickness of the plate ,and is thin on both sides. The more internal lubricant, the higher the temperature of the core.

4.Uniform dispersion, good dispersion at the high speed mixer is critical, and it is important to note the blowing agent should add at the last 45 seconds before discharging the batch. The main reason caused uniformity is related to the mixing. After the last shift, the interval between the mixing is long after the next shift. The mixing barrel is cooled . The first pot is pre-plasticized, which is the same as the previous mixing. Differences are formed, and when other conditions remain unchanged, fluctuations are likely to occur, which can be adjusted by adjusting the traction, processing temperature.

 

Five common possible solutions

1 . Surface turns yellow.

If the extrusion temperature is too high or the stability is insufficient, the solution is to reduce the processing temperature. If the improvement does not occur, the formulation should be adjusted, and the stabilizer and lubricant can be added appropriately. The adjustment can be done one at a time.

2. Yellow in the middle of the profile.

The main reason is that the temperature of the core of the barrel is high and need to consider  the amount of lubricant, also blowing agent.

3. The profile curved.

Uneven flow or inadequate cooling, the factors causing uneven material flow are generally large traction fluctuations or unbalanced internal and external lubrication in the formula. The factors of the machine are easy to eliminate. Formula adjustment ,i.e. adjust internal lubrication if it is too little external lubrication .Sometimes, can achieve good results, by ensuring that the cooling is evenly in place.

4. Blisters or blisters appear on the cross section.

The reason can be attributed to the melt strength is not enough, the reasons for the lack of melt strength are:

- Excessive foaming agent or insufficient foam regulator or inconsistent ratio of the two, or quality problems of the foam regulator;

- Poor plasticization, low processing temperature or excessive lubrication.

The two reasons for bubbles issues.

4.1.  the local strength of the melt itself is too low, the bubble breaking , formed from outside to inside;

4.2.  the pressure around the melt is too small, local cell expansion occurs, the strength is weakened, and the bubble breaking.

In the production practice, there is no obvious difference between the two reasons, which may exist at the same time. Most of the broken holes are caused by the uneven expansion of the foam cells and the weaken of  melt strength. The strength of the melt itself is too low and the pressure around the melt is too small.

5. Uneven thickness of the profile surface.

Uneven material output, adjustable die lip opening, if the flow rate is too large, adjustable choke bar,

 Sometimes need to adjust the ratio of internal and external lubricants.