Theoretically speaking, there are two main factors in the production of broken or penetrating bubbles in the cross-section of PVC foam board.
One is due to the melt itself local strength is too low, the broken bubble is formed from the outside to the inside.
The second is due to the melt around the pressure is small, the local bubble hole expansion occurs, the strength is weakened, the broken bubble is formed from the inside to the outside.
In the actual production process of PVC foam board, there is no obvious difference between the two roles, there may exist at the same time, and most of the broken holes are caused by the reduction of the melt strength after the uneven expansion of the local bubble hole. There are many factors for the low strength of the melt itself and the small pressure around the melt, in summary, there are mainly the following aspects.
Molecular weight or polymerization is too low
PVC resin can be generally divided into three types according to its polymerization method: emulsion PVC, and suspension PVC, and native PVC.
In the production of PVC foam board, if you use the emulsion method PVC resin can get uniform bubble pore, smooth surface products, but the dimensional stability of the products is difficult to control. Moreover, the cost of emulsion PVC resin production is too high.
If the suspension method PVC resin is used, the product’s appearance quality and bubble pore uniformity effect will be poor
The production of PVC Celuka foam board should use SC-7 sex resin, and try not to use SC-8 type resin, or use SC-8 type resin mixed with SC-5 type or SC-6 type resin.
Poor thermal stability of the melt, improper extrusion temperature setting or control
Good plasticization of the melt is a prerequisite for the production of PVC foam boards. If there is a problem with the heat stabilizer, the extrusion temperature is too high, the melt is prone to local degradation and will be torn due to the reduced strength of the melt and excessive bubbles.
Extrusion temperature is too low, the melt under-plasticized, the strength is very low, the same will appear broken bubble. Foam plastic sheet production, the stabilizer must be regularly 180 ~ 200 degrees of stability testing, does not meet the stability requirements of the raw materials shall not be used or adjusted by the dose, after passing the test and then used.
Too much calcium carbonate addition, too large particle size, bad activity.
In the production of PVC foam board.When CaCO3 is added too much, although more bubble nuclei can be formed, but if calcium carbonate is added in too many doses or the particle size is too large, it will form agglomerates, disperse in the resin or poorly combined with the resin cross-section, resulting in lower melt strength and easy bubble breaking when the bubbles in the melt are expanded.