Five factors affecting the quality of PVC foamed sheet
In the extrusion process of PVC foamed sheet, the problems can be classified into 5 types: stability, melt strength, lubrication and dispersion. These five kinds of problems, especially the first three kinds, will restrict and influence each other.
1. Lack of stability, will affect the entire plate, plate surface yellow, foamed sheet Brittleness
2. Lack of melt strength will lead to foamed sheet bubble large, Long Longitudinal section bubble.
To judge whether the melt strength is insufficient, the most direct way is to go to the back of three rollers with a finger pressure package in the roll on the plate, melt strength good pressure can feel elastic. If it is hard to spring up after pressing, the melt strength is poor. Because of the difference of screw structure and cooling method, it is difficult to judge whether the temperature is reasonable or not. In order to obtain the uniform foaming products in the foam pipe, also need to ensure that the PVC material has good melt strength. So the quality and type of foaming agent is crucial, of course, with a good yellow and white foaming agent.
Lubricant is divided into external lubricant and internal lubricant, external lubrication is conducive to demoulding, good for the smooth surface of the sheet, too little external lubrication, the temperature in zone 5 of the extruder is difficult to control and easy to heat up, which will lead to high temperature of the confluence core The surface of the board is not smooth because of the big bubble, string bubble, yellow, etc. in the middle of the board. The surface of the board is slippery outside, which will become serious. The structure inside the mould and the surface of the board is slippery outside It will also show up as some individual phenomenon moving back and forth on the board irregularly. The inner sliding is good for the fluidity of plastics and melt, and the inner sliding is difficult to control the thickness of the plate, which is characterized by the thin middle of the plate, the inner sliding and the high temperature of the combined core.
4. Poor dispersion will lead to the surface of the plate is not smooth
Process temperature control problem: the above four issues are fundamental, is the foundation, is a deep-seated problem. Relative to the above four issues, the process temperature control is much more intuitive, is a surface problem, but the temperature control is not good, will induce the emergence of fundamental problems. When the processing temperature is raised, the stability time of the material will be reduced and the stability problem will appear, and the original lubrication balance will be broken The increase of temperature also leads to the decrease of melt strength, the increase of foam cell number, the decrease of foam cell number and the Brittle fracture of foamed sheet For the screw that disperses the ability not strong, sometimes can appear disperses the non-uniformity.
The more foaming agent, the better the foam, the lower the density, the lighter?
In the actual production we met many PVC foam products customers, products appear big bubble hole or weight down, asked a variety of experts friends, got the answer they believe: This is less blowing agent, did not rise You got ta put more foam in it. It's GONNA build up. The results were even worse, so together they agreed on an alternative, increasing the amount of the foam regulator, and things got better until the increased amount of the foam regulator was enough to not cause any problems.
The yellow blowing agent AC is most often used in PVC products. When the products are added with AC, they decompose under the heat and produce a large amount of heat. The products expand and the melt strength decreases until they can not contain the small bubbles produced by the decomposition of the blowing agent AC A large number of small bubbles coalesce into larger ones, hindering the reduction in density. For this reason we must add foam regulator for reinforcement, to increase melt strength, so that the uniform distribution of bubbles, to reduce the density.
And some people say, well, that's what the first paragraph was about. No, here we introduce a concept, specific surface area, refers to the total area per unit mass material. There are two kinds: outer surface area and inner surface area. The greater the specific surface area, the lower the density, and the lighter the mass.
When a certain amount of AC and foaming agent added, the formation of countless uniform size of small bubbles, bubble density uniform, when the maximum surface area. When the amount of AC exceeds the melt strength enhancement range of the foaming regulator, small bubbles will cluster, many small bubbles will form a big bubble, then the specific surface area will drop, the density is naturally large If the cut surface even appears bubble hole, even more in the middle and two layers.
So how do we adjust to problems like bubble holes? I think that those who have read the above text carefully already have the answer. First, increase the blowing regulator or reduce the amount of blowing agent, one of the movers, in order to facilitate the search for the reason, cost considerations, of course the latter is the best policy, that is, to reduce the amount of blowing agent AC If the effect is better, continue to reduce the normal. Therefore, the first paragraph of increasing the amount of blowing agent is certainly not correct.
In addition, in order to get better performance in the production process, it is recommended to add white endothermic blowing agent to offset the excess heat released in the process of AC decomposition.
In actual production, the reasons for the bubble often more, there are technological reasons for the composition, we work with good technology and formula adjustment, will soon be able to make even foam, light weight, good mechanical properties of the products. Specific problems need to be analyzed in detail.