Low foaming rigid PVC door and window hollow profile production process

Low foaming rigid PVC door and window hollow profile production process

China's plastic door and window technology has matured through more than ten years of hard work, but there has been no breakthrough progress in the research work on the production of low-foam PVC hollow profiles by extrusion.

Low-foamed rigid PVC hollow profile must have the performance of non-foamed profile, and the core layer of the profile must have a uniform cell structure, and the relative density is above 0.4. The hard non-foaming layer on the skin can be made into a 0.5-1.2mm thick skin layer as needed. This low-foam material has excellent nail-holding properties. It can be nailed, according to data, can be planed, glued, welded, and is not broken or rotted.

The weight per unit length of low-foamed plastic profiles is reduced by 10% to 30% compared to non-foamed profiles. The reduction in cost and price have provided a good condition for accelerating the application of plastic doors and windows.

In order to obtain hard PVC low-foaming profiles with excellent appearance and internal quality, it is necessary to reasonably design the raw material formula, extrusion process, head and styling die structure, extrusion screw, etc., and understand the internal relationship between them.

1 Formulation design

The production of low-foam rigid PVC profiles by extrusion is more difficult than the formulation design of non-foamed profiles. It is far from enough to solve the quality problems of products by proportioning. Although formula design plays a decisive role, it is necessary to master the process characteristics of crust controllable foaming in order to complete the formula design work conveniently. The main points of its design are:

1.1 Selection of blowing agent

Practice shows that the chemical foaming agent amine azodicarboxylate (referred to as AC) is used to produce low foaming hard PVC profiles with good effect. This foaming agent has a relatively narrow and stable decomposition temperature (180 ~ 200 ℃), and the amount of gas generated is 230 ~ 250ml / g. It has good compatibility with PVC tree fingers and excellent dispersibility, without affecting the original performance of the plastic.

The gas released by the AC blowing agent is mainly nitrogen. Nitrogen is not corrosive to mechanical molds, is not easy to burn and explode, and is non-toxic and non-violent exothermic. Nitrogen also has low solubility in PVC resin, low gas permeability, low diffusion rate, and low gas loss. These excellent properties of AC blowing agents provide the most basic conditions for making high-efficiency closed-cell foams.

The amount of blowing agent directly affects the density of the product. Under the same conditions, as the concentration of the foaming agent increases, the gas volume fraction increases, that is, the foaming ratio of the product increases, and the relative density decreases.

Figure 1 shows that under the same conditions, the amount of blowing agent added increases, the foaming point away from the mold lip is unfavorable for foaming (Z = 0 is the mold lip opening). Therefore, the amount of foaming agent should not only ensure the relative density of the desired foam, but also follow the special characteristics of the extrusion foaming process

point.

1.2 Stabilizer

The stabilizer can activate the thermal decomposition of the AC foaming agent and affect the decomposition temperature and the amount of gas generated of the foaming agent. Experiments show that adding the same amount of various stabilizers can promote the reduction of AC decomposition temperature, and the order of its influence is tribasic lead sulfate> dibasic lead phosphite> lead stearate> barium stearate> Calcium stearate. The curve in Figure 2 shows that when the concentration of tribasic lead sulfate is ≥0.5 parts, the decomposition temperature of AC (0.7 parts) is constant at 162 ° C; when the concentration of lead stearate is ≥0.5 parts, the decomposition temperature is constant at 167 ° C. Therefore, it is particularly important to choose which stabilizer in the formulation.

Figure 1 foam hair position

Figure 2 The effect of lead salt concentration on AC decomposition temperature (0.7 parts AC)

The experiment also showed that the concentration of the stabilizer increased and the amount of gas generated by the AC foaming agent increased. It should be pointed out that in the same stabilizer system, the smaller the amount of AC, the higher the decomposition temperature and the lower the gas generation.

1.3PVC resin

The plasticizing temperature of the PVC melt during extrusion must be slightly lower than the decomposition temperature of the AC blowing agent. The lower the average polymerization degree of PVC resin, the lower the processing temperature required for melt plasticization to express viscosity. Therefore, it is appropriate to choose a suspension polymerization polyvinyl chloride resin with a low average polymerization degree of P = 800 (K = 62-65). 

1.4 other main additives

In order to improve the impact resistance of the product, and have a stabilizing effect on light and heat, to meet the requirements of outdoor use, CPE modifier should be added to the formula, and ACR can be preferably used if possible.

Light calcium carbonate requires a small particle size, which is used as a nucleating agent and as a filler to be added to the formula.

2 Process design

2.1 low foaming rigid PVC profile extrusion process

Formulation weighing → high speed stirring → low speed cold mixing → cone twin screw extrusion → die forming die → dry vacuum cooling shaping → wet vacuum cooling shaping → traction → sawing → profile

2.2 Mixing process

After the formulation of the hard PVC low-foam formulation, controlling the process conditions of high-speed stirring and low-speed mixing and cooling, such as the feeding amount, feeding order, mixing temperature and mixing time, is the key to ensuring the quality of the dry-mixed powder. The following points can not be ignored in the mixing process.

(1) When the volume of the mixture is 50% to 70% of the volume of the high-speed mixer, the feeding amount is the optimal value, the material is well turned, and the heating time is short.

(2) Stabilizers and internal lubricants should be added early to help gel and homogenize the PVC mixture.

(3) The external lubricant should not be added early in the high-speed stirring. If added prematurely, the outer layer of PVC resin particles is covered with an external lubricant to reduce the attraction between its molecules, reducing the performance density, uniformity and gelation rate.

(4) In order to prevent the thermal decomposition of the foaming agent, it is not appropriate to add the foaming agent during high-speed mixing, and it can be added during low-speed cooling and mixing.

(5) The processing aid ACR easily adsorbs the stabilizer, which affects the stability of the plastic. The stabilizer will hinder the effect of the CPE additives and PVC resin, and affect the function. Therefore, CPE or ACP additives should not be added together with stabilizers.

(6) When cooling and mixing, the material will absorb the moisture in the air, so it should not be too cool.

In summary, it is necessary to formulate a stricter hot and cold mixing process. The technological process is as follows.

Put the PVC resin into a high mixer and stir to warm up to 60 ℃ → put in a stabilizer, stir and warm up to 90 ℃ → put in internal lubricants, processing aids, fillers, colorants → put in external lubricants → continue to stir and heat up to about 120 ℃ 、 Unloading → Put into a low-speed mixing cooler, add foaming agent and stir → stir and cool to about 50 ℃ → discharge

2.3 Extrusion process

Extrusion of low-foaming rigid PVC has complicated process conditions. It is due to different factors such as the screw structure of the extruder, the structure of the head and the shaping die, the types of main and auxiliary materials and their proportions, and the process parameters have changed considerably. Moreover, the controllable skinning foaming process has its unique characteristics.

(1) The material must be plasticized in the extruder before foaming.

(2) The melt must be foamed off the mold, and the die pressure has the strongest influence on the expansion ratio.

(3) Delay the foaming time of mold release, the skin is thick; the mold release speed is fast, and the skin is thin.

There are four main factors that affect the foaming process, namely extrusion temperature, extrusion residence time, extrusion rate and extrusion pressure.

In order to ensure that the material is plasticized uniformly in the extruder, the melt is dense, and the best foaming state is reached, it is important to control the extrusion temperature, see Table 1 for experimental data.

Table 1 The influence of extrusion temperature on foaming state

Extrusion temperature ℃

Average bubble diameter, mm

Number of gas in 1cm3 foam, × 104

Foam density g / cm3

180

0.09

8.2

0.95

185

0.05

334

0.75

190

0.09

131

0.48

195

0.27

4.6

0.50

200

0.80

0.171

0.52

It is known from the table above that when the extrusion temperature is low, the gas-containing melt forms large bubbles due to uneven mixing, less nucleation and uneven dispersion, and the foaming density is large; when the temperature rises to a certain value, the materials are mixed uniformly , Good plasticization, gas solubility increases in the melt and the number of nucleation increases, thus obtaining a foam with a small pore diameter and a small density; when the temperature continues to increase, due to the decrease in viscoelasticity of the material, the cells can change due to the penetration Large and gas diffuses from the melt to the external surface to increase the density of the foam; when the temperature exceeds a certain value, the extruded melt collapses quickly because it cannot withstand the expansion force of the internal gas.

According to the characteristics of the conical twin screw, there are the following body temperatures for reference. First section: 150 ° C; second section: 175 ° C; third section: 180 ° C; continuous section 170 ° C.

In order to prevent the decomposition gas of the AC blowing agent from escaping, the vent hole of the extruder should be blocked.

In our actual operation, if the foam density screw rotation speed is found to increase, it means that the extrusion temperature is slightly lower (or just); if the extrudate does not foam at all, it means that the extrusion temperature is lower than the foaming agent Decomposition temperature.

Controlling the extrusion rate can control the length of the low-pressure supersaturation zone of the melt from the die. It is one of the keys to the advantages and disadvantages of foam molding. When the low-pressure gas supersaturation zone is too short, the material foams close to the die, and the surface is broken by radial shearing force to form a rough skin; when the low-pressure gas supersaturation zone is too long, the product is prone to wavy surface. Under normal circumstances, the extrusion rate is proportional to the screw speed. When the screw speed increases, the low-saturated gas supersaturation zone extends, and the foam tends to be flat and smooth. In actual production, the low-pressure gas supersaturation zone is generally controlled at 10 ~ 30mm.

The retention time of AC foam extrusion in the extrusion process also directly affects the density and cell size of the foam. It can be seen from the experimental data in Table 2 that when the residence time increases, the number of bubbles increases, and then decreases once it reaches the maximum value; and when the residence time is longer, the foam density becomes smaller and the bubbles become thicker.

The control of the process parameters of the machine head is the most important, which is mainly reflected in the following points.

(1) The temperature of the machine head is too high, the foaming point is far away from the die lip, resulting in premature gas generation, and the melt is prone to pulsation. At the same time, the gas is torn by the melt flow along the mold wall and the foam surface is rough .

Table 2 The effect of the residence time of extrusion on the foaming state

Retention time in the rheometer barrel, min

Average bubble diameter, mm

Number of bubbles in 1cm3 foam, × 104

Foam density g / cm3

1.0

0.09

8.2

0.93

3.0

0.08

27.4

0.86

5.0

0.05

334

0.75

7.0

0.04

1120

0.60

10.0

0.09

134

0.47

Note: blowing agent AC: 0.7 parts extrusion pressure: 19MPa

Extrusion temperature 180 ℃ Die size: 10.2 × 21mm

Residence time: 8mim Lead angle: 90 °

(2) If the temperature of the machine head is low, the pressure at the die lip will increase, and the foaming point will move to the die lip, which is beneficial to the foaming away from the die.

(3) The die lip pressure plays a decisive role in the product's foaming ratio. When the die lip pressure decreases, the product's foaming ratio increases. In actual operation, we control the die lip pressure by controlling the traction speed and cooling setting temperature. If traction and cooling are blocked, foam extrusion is extremely difficult. The expansion ratio is generally controlled at 1.7 to 1.8.

In short, during the extrusion process, it must be ensured that the gas is still dissolved in the extruder and the head. Practice shows that in the extruder, the foaming agent decomposition gas accounts for more than 50%, and the decomposition gas in the head accounts for about 40%. At this time, the obtained cell structure is ideal. Therefore, it is better to set the temperature of the feed part of the head to 165 ° C and the die temperature to 150 ° C.

3 The choice of extruder

As we all know, the tapered counter-rotating twin screw extruder has low shear heat, low material temperature, small temperature distribution range, easy to adjust the material temperature, low temperature extrusion, suitable for processing hard PVC products with poor thermal stability and fluidity, and more Suitable for processing hard PVC low foam products.

This type of extruder has a large surface area of screw and barrel in the feeding area, good heat transfer, fast material heating, easy compression, high conveying efficiency, and can directly process dry-mixed powder without granulation, and the dispersion and mixing effect is good.

The use of conical counter-rotating twin screw extruder is easy to operate, the process is stable, and the product quality is good. The domestic SJSZ-55 conical anisotropic twin-screw extruder can meet the requirements of producing low-foam rigid PVC doors and windows shaped materials.

4 Low foam hollow profile head design

The design of the rigid PVC low foam profile extrusion head is more complicated. In addition to the Balas effect and the speed distribution rearrangement, the melt separation characteristics also have melt expansion. The interaction between various factors is also large, which brings some special features to the design of low foam profile molds, and it is difficult to calculate in theory. At present, the design basis is still experience and trial mold, and the cost of trial mold accounts for 20% to 50% of the head cost.

The basic structure of the low foam head is similar to the non-foam head, and the design principles are basically the same. In addition, the following design specifications should be mastered.

(1) Because the melt contains volatile gas, the nozzle flow path must not be sharply expanded, otherwise the gas will suddenly expand here, causing unevenness in product thickness and density. The flow path of the machine head should be gradually reduced to the die lip, and stagnation areas and "dead corners" should also be avoided in the flow path. The design of the head runner is shown in Figure 3.

(2) Due to the low viscosity of the gas-containing melt, which easily flows through the machine head, the design of the machine head structure should make the melt form a certain pressure. The head pressure is generally 10 ～ 30MPa.

(3) The foam head core mold bracket is easier to leave flow marks than the non-foam head core mold bracket, so the number of brackets should be as small as possible. Its location is also as far away as possible from the location of the desired appearance surface.

(4) It is very important to determine the length of the shaped section of the die. Generally, the length is shorter, so as to reduce the pressure at the die and initiate foaming.

(5) On the basis of empirical data, considering the characteristics of large expansion rate and large shape change, there is an empirical formula for the size of the low foam hard PVC head. The recommendations are as follows:

1) Height dimension of die section

H = (0.5 ～ 0.65) H1

In the formula: H-die section height dimension

H1-profile nominal height dimension

2) The width of the die section

W = (0.77 ～ 0.9) W1

In the formula: W-die section width dimension

Nominal dimension of W1-profile section width

3) Die gap size

T = 0.5h

In the formula: T-die gap size

h-profile wall thickness nominal size

Note: The above coefficients are all small values, so that there is a mold repair margin. The aspect ratio is W1 / H1 = 3 ～ 15, and it needs to be corrected if it is not listed here.

4) Size of die shape section

L = (5 ～ 10) T

In the formula: L-die length

T-die gap

Figure 3 The relationship between the runner section of the die and the product quality

A: die lip exit; B: die lip entrance; C: connection site; D: die entrance

5) In order to obtain a uniform and dense foam and eliminate the joint caused by the diverter bracket, the compression ratio of the machine head is (10-15): 1.

5 Design of cooling mold

The setting cooling mold is set at 10 to 300 mm away from the mouth mold, and it is adjusted according to the changes in product quality requirements and traction speed. The contact between the outer surface of the product extruded from the die and the inner surface of the cooler setting mold suppresses foaming and forms a non-foamed hard skin layer. The cooling speed affects the thickness of the skin layer.

Low-foam stereotypes require higher cooling and use more advanced
The combined use of dry vacuum setting and wet vacuum setting can not only ensure the skinning foam and accuracy of the product, but also ensure that the profile does not generate internal stress, while strengthening the cooling effect and increasing the molding speed.

At present, most of the factories in China that produce non-foamed profiles are using the dry vacuum setting method. Spray-type vacuum shaping technology has not been promoted due to certain difficulties. The principle of this wet vacuum setting method is to supply spray cooling water in a closed water tank, and the air and cooling water are pumped out of the tank by a water ring vacuum pump to decompress the water tank. Install a number of plate-shaped shaping modules in the water tank to shape the profile foam. After passing through the water tank under reduced pressure, due to the effect of the atmospheric pressure inside the foamed profile, the shape is maintained in the expansion direction and cooled. Due to the poor thermal conductivity of the foamed profile, the cooling water tank is longer, generally longer than 2m, and the cooling water is required to be below 15 ℃. See Figure 4 for details.

Figure 4 Cross section schematic of wet vacuum shaping cooling mold

1. Spray nozzle; 2. Vacuum box; 3. Stereotyped block; Aluminum base

The surface of the profile can be decorated by printing, hot stamping, painting, laminating, embossing, metal coating, co-extrusion and other surface processing techniques.