1. Insufficient melt strength will lead to large cells in the foam sheet and long longitudinal section. Judgment method: go to the back of the three rollers and press the sheet wrapped on the middle roller with your fingers, the melt strength is good and you can feel the elasticity when pressing. If it is difficult to bounce after pressing, it indicates that the melt strength is poor.

2. Too little external slip, the temperature of the 5th zone of the extruder is not easy to control, and it is easy to heat up, resulting in high temperature of the confluence core, large bubbles, string bubbles, yellowing and other problems in the middle of the plate, and the surface of the plate is not smooth.

3. If there is a lot of external slippage, the precipitation will become serious, which is manifested in the structure in the mold and the precipitation of the external slippery on the surface of the plate, and some individual phenomena will also move back and forth on the plate surface irregularly.

4. Insufficient internal sliding, it is difficult to control the thickness of the board surface, which means that the middle of the board is thick and the two sides are thin.

5. There is a lot of internal slippage, and the phenomenon of high temperature of the confluence core is prone to occur.

6. The board surface turns yellow. If the extrusion temperature is too high or the stability is insufficient, the solution is to reduce the processing temperature. If there is no improvement, the formula can be adjusted, and the stabilizer and lubricant can be appropriately increased, which can be changed one by one. It is easy to quickly find the problem and solve the problem as quickly as possible. ;

7. The middle of the plate turns yellow. The main reason is that the temperature of the core of the barrel is related to the high temperature of the mold, and the amount of lubricant is also related, and it is related to the white foaming agent again;

8. The board surface is bent. Uneven material flow or cooling is not in place, the factors that cause uneven material flow are generally large traction fluctuation or unbalanced internal and external lubrication in the formula, and the factors of the machine are easy to eliminate. Great effect while keeping the cooling in place evenly.

9. There are bubbles or bubble layers on the cross section. The reason can be attributed to one point, that is, the melt strength is not enough, and the reasons for the insufficient melt strength are as follows:

9.1. Excessive foaming agent or insufficient foaming regulator or the ratio of the two is not coordinated, or the quality of foaming regulator is a problem.

9.2. Poor plasticization, low processing temperature or excessive lubrication.

10. The thickness of the board surface is uneven. If the discharge is uneven, the opening of the die lip can be adjusted. If the flow rate is too large, the choke rod can be adjusted, and the formula can be adjusted. Generally, the inner lubrication is more thick, the middle is thick, and the outer lubrication is more. The two sides feed faster.

11. Changes in the thickness and texture of the plate that are easy to appear when the shift is handed over. The main reason is related to the mixing. After mixing in the last shift, the interval between mixing is long after the next shift, the mixing tank is well cooled, the first pot of mixing is pre-plasticized, and the mixing with the previous If the difference is formed, when other conditions remain unchanged, it is easy to produce fluctuations, which can be solved by adjusting the traction, processing temperature or through management.

12. The two factors of foam breaking or breaking through the section of foamed plastic sheet:

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

B. Because the surrounding pressure of the melt is too small, the local cells expand and the strength is weakened, and the bubble breaking is formed from the inside to the outside. In production practice, there is no obvious difference between the two effects, and they may exist at the same time. Most of the broken pores are caused by the reduction of melt strength after the uneven expansion of local cells. There are many factors for the low strength of the melt itself and the low pressure around the melt. To sum up, there are mainly the following aspects.

1) First, the molecular weight or polymerization is too low

PVC resin can be generally divided into three types: emulsion PVC, suspension PVC and bulk PVC according to its polymerization method. When producing rigid PVC foam products, if the emulsion method PVC resin is used, a product with uniform cells and a smooth surface can be obtained, but the dimensional stability of the product is difficult to control; and the production cost of the emulsion method PVC resin is high; if the suspension method is used PVC resin, the appearance quality and cell uniformity of the product are slightly worse.

In terms of comprehensive technology, price and performance, it is advisable to mix the two in a certain ratio, and the ratio can be between 80/20-20/80. The cross-sectional area of ​​the sheet is relatively wide, and the extruder head enters the die to a width equivalent to about 1.3 meters. To obtain fully foamed low-density plastic products, the viscosity of the resin used should not be too high. The dispersion and expansion of the bubbles, and the coordinated pressure of the entire cross-section of the sheet, require high melt fluidity. If the viscosity of the PVC resin is too high, the melt fluidity is poor, the flatness of the board surface is difficult to ensure, and the cells are not easy to expand, resulting in a low foaming ratio; on the contrary, if the viscosity of the PVC resin is too low, the melt strength will be low, which is easy to cause rupture. Bubble.

The production of crust foamed plastic sheet should use SC-7 resin, try not to use SC-8 type resin, or use SC-8 type resin mixed with SC-5 type or SC-6 type resin.

2) Poor thermal stability of the melt, improper extrusion temperature setting or control

Good plasticization of the melt is a prerequisite for foamed plastic sheet products. If there is a problem with the heat stabilizer, the extrusion temperature is too high, and the melt is prone to local degradation and will be torn due to the reduction of melt strength and excessive bubbles; the extrusion temperature is too low, the melt is underplasticized, and the strength is very low , the bubble will also burst. During the production of foamed plastic sheets, the stability of the stabilizer must be checked at 180-200 degrees on a regular basis.

In addition to ensuring that the melt does not degrade during the extrusion process, the stabilizer also has an important function, which is to adjust the decomposition temperature of the blowing agent. If the stabilizer is added too much or too little, the decomposition temperature of the blowing agent is too low or too high, which is not conducive to foaming. The stabilizer should be adjusted according to the grade of resin used. For example, if type 8 resin is used, the plasticizing temperature is low, and the stabilizer should be appropriately increased so that the plasticizing temperature of the material is consistent with the decomposition temperature of the foaming agent. However, if the stabilizer is added too much or the melt temperature is too high, the foaming agent will be decomposed in advance in the extruder, causing the foaming gas to escape from the feeding hole and the vacuum hole. If the stabilizer is added too little or the operating temperature If it is too low, the foaming will be incomplete.

In the extrusion operation, in addition to ensuring good plasticization of the melt, the melt temperature in the extruder must be lower than the decomposition temperature of the foaming agent to prevent the foaming agent from decomposing prematurely in the machine; The body temperature must reach the decomposition temperature region of the foaming agent to facilitate full foaming. The setting and control of the extrusion temperature should also be adjusted in time according to the material of the vacuum hole and the shape of the melt when starting up. Make sure that the material is basically in an orange peel state when passing through the vent hole, and there should be no powder flow at the bottom of the screw; when the melt is extruded from the die, the surface should be smooth and have a certain elasticity, and it should not sag or the cross-section crystallizes rough as soon as the die exits. .

3) Improper addition of foaming agent

The production of foamed plastic sheets generally adopts three different foaming agents: exothermic type, endothermic type or endothermic and exothermic composite balance type. Ammonium azodicarboxylate, also known as AC, is an azo activator. The decomposition temperature of AC foaming agent is high, reaching 232 °C, which is far beyond the processing temperature of PVC, and the decomposition temperature should be lowered when using.

The exothermic foaming agent has a high foaming rate, about 190-260ml/g. It has a fast decomposition rate and a great heat release, but the foaming time is short and the suddenness is strong. Therefore, when the amount of AC foaming agent is too much, the amount of gas generated is too large, which will cause the pressure in the bubble to increase rapidly, the size of the cell will grow too large, the gas will be released rapidly, the cell structure will be destroyed, the cell size will be unevenly distributed, and even open The pore structure will generate larger bubbles and cavities locally.

When producing foamed plastic products, the exothermic foaming agent AC should not be used alone, but should be used in combination with the endothermic foaming agent or a compound chemical foaming agent with heat and exothermic balance. Inorganic foaming agent - sodium bicarbonate (NaHCO3) is an endothermic foaming agent. Although the foaming rate is low, the foaming time is long. Mixing with AC foaming agent can play a complementary and balanced role. The exothermic foaming agent improves the gas generating capacity of the endothermic foaming agent, and the endothermic foaming agent cools the former, stabilizes its decomposition and releases gas in a balanced manner, inhibits the internal overheating degradation of the thick plate, and reduces the precipitation of residues. Has a whitening effect.

Under the premise of not affecting the foaming rate, more endothermic foaming agents can be added appropriately to replace part of the exothermic foaming agents, so as to suppress the foam breaking caused by adding more exothermic foaming agents. 1232 or BLA-616 foaming agent is a balance type of exothermic and endothermic foaming agent. There is no induction period for decomposition, and the decomposition rate is fast. The maximum gas generation can be achieved in about 10 minutes. up to 156mL. Its decomposition temperature is within the range of PVC processing temperature, and it can be used in the dynamic molding process of products with thick dimensions and complex shapes, so as to eliminate foam breakage and ensure the stability of foaming performance.

4) Poor quality or insufficient dosage of processing regulators

During the foaming process of the material, the gas decomposed by the blowing agent forms bubbles in the melt. In these bubbles, there is a tendency for small cells to expand to larger cells. The size and number of bubbles are not only related to the amount of foaming agent added, but also to the polymer melt strength. If the strength is too low, the gas is easy to escape after diffusing to the surface of the melt, and the small bubbles merge with each other to form large bubbles. The long molecular chain of the foaming regulator is entangled and adhered to the molecular chain of PVC to form a certain network structure. On the one hand, it promotes the plasticization of the material, and on the other hand, it improves the strength of the PVC melt, so that the cell wall can withstand the pressure of the gas in the cell during the foaming process, so as not to rupture due to insufficient strength. The foaming regulator can make the product cells smaller and more, the cell structure is more uniform and reasonable, and the density of the foam is greatly reduced. Poor quality or insufficient addition of foaming regulators will result in low foam strength and bubble breakage or bubble stringing.

It should be pointed out that the molecular weight and viscosity of foaming regulators produced by different manufacturers are very different. When foaming products break or bunch bubbles and other methods are ineffective, replace the foaming regulator or increase the dosage appropriately. , often have a noticeable effect. However, adding or replacing a foaming regulator with a larger molecular weight will cause the bubbles in the melt to be unable to expand due to the excessive viscosity, thereby increasing the product density. In addition, due to excessive melt viscosity and poor fluidity, the die discharge will be uneven, which will affect the smoothness of the board surface. Even if the production time is not long, the failure of the mouth blurred material will occur, especially when the production of plates with a thickness of less than 10mm is more likely to occur.

When producing plates with different thicknesses, the dosage of processing regulator should also be different according to the situation. For the same formula, if the thin plate has no bubbles, when the thick plate has bubbles, 0.3-0.5 parts of processing regulator can be added appropriately. The elimination of bubble breakage is the basis, and trace amounts are added. Do not recklessly add more blindly, so as not to increase the density of the product and affect the effective production time.

5) Too much calcium carbonate, too large particle size and poor activity

When the amount of CaCO3 is too large, although more bubble nuclei can be formed, if the amount of calcium carbonate is too large or the particles are too large, the activity is not good, agglomeration is formed, and it is not dispersed in the resin or combined with the resin cross-section. The melt strength is reduced, and the bubbles are easily broken when the bubbles in the melt expand.

The production of foamed plastic sheets must strictly control the dosage, particle size and activity of calcium carbonate. When the amount of calcium carbonate added is too large, the foaming regulator should also be increased accordingly.

6) Improper use of molds

In the production of skinned foamed plastic sheets, the length and compression ratio of the straight section of the die are also different according to the thickness of the product. The thick plate mold has a thick foam layer and has elastic deformation space. Generally, the straight section of the mold design is longer and the compression is relatively large, so as to improve the melting pressure and foaming ratio; the thin plate mold has a thinner foam layer. The elastic space is small and the rigidity is large. Generally, the straight section of the die designed is short and the compression ratio is small to prevent excessive melting pressure.  If it is used improperly, the thin plate is produced, and the thick plate die is misused, the flow resistance of the exit die will increase, and the discharge will be uneven, resulting in uneven surface, reduced melt strength, and even the plate will be broken, and the production cycle will be reduced. Short; if the production of thick plates and the misuse of the thin plate die, the melting pressure of the exit die will be too small, which will increase the cell size and cause bubble breakage.  Before the production of crusted foamed plastic sheets with different thicknesses, the die must be correctly selected.

7) The section of the sheet is foamed or the material is unevenly discharged, and there is a partial lack of material

The cross-section of the sheet is foamed or the material is unevenly discharged, and there are many factors affecting the local lack of material, which run through the whole process of mixing and extrusion. For example, the composition of the formula is unreasonable, the external slip is too small, and the temperature of the 5th zone of the extruder is easily over-temperature, resulting in high temperature of the confluence core, large bubbles, strings of bubbles, yellowing in the middle of the plate, rough surface of the plate and other defects; single-pot mixing Too much material, too low mixing temperature, too short mixing time, and too little internal slip addition will easily lead to uneven dispersion of mixture components.

Poor fluidity and improper adjustment of die temperature or bolts during extrusion production will lead to uneven discharge of the melt from the die and local shortage of material, which will cause the foaming melt to expand from the weak link and break Bubble. Therefore, in the process of mixing and extrusion production, the formula and process operation rules must be implemented, and the corresponding analysis of foam breaking should be carried out, and different methods should be used to deal with them. If the bubble breaker has been fixed at the same position, it means that the melt pressure in this part is too low, and it can be solved by adjusting the die bolt or temperature accordingly. In addition to the above corresponding measures, adjusting the gap difference of each section of the stereotyped template also has a certain effect on eliminating bubbles. If the gap between the first and second sizing plates is too large, the melt will be squeezed more tightly under the pressure of the sizing die without sufficient cooling, resulting in increased density and bubble breakage;

If the gap between the third template and the fourth template is too large, due to sufficient cooling of the melt, there is no room for compression and deformation between the templates, which makes it difficult for the fourth template to be in place and the thickness of the plate increases. Appropriately increasing the gap between the second and third template can effectively prevent the foaming melt from breaking before cooling, on the other hand, the third template can be pressed in place at a certain temperature to prevent the plate from breaking. Thickness is increased.

Secondly, when producing thick plates, appropriately reducing the screw temperature, die oil temperature, cooling water temperature of the first setting device, etc., also has a corresponding effect on eliminating bubble breakage.

13,  Poor dispersion will bring about the phenomenon of sheet surface not smooth

Process temperature control problem: The four problems mentioned above are fundamental problems, basic problems, and deep-seated problems. Compared with the above four problems, the process temperature control is much more intuitive. It is a superficial problem, but poor temperature control will induce the emergence of fundamental problems. If the processing temperature is increased, the material stabilization time will be reduced, and stability problems will occur; the original lubrication balance will be broken, which is generally manifested as insufficient external lubrication, especially in the later period.The amount of slip added; the increase of temperature will also cause the melt strength to decrease, the cells of the foamed sheet to increase, the number of cells to decrease, and the sheet to become brittle and easy to break; the increase of temperature reduces the melt strength and also reduces the melt viscosity. Reducing the shear dispersing ability decreases, and sometimes uneven dispersion occurs for screws with weak dispersing ability.