Hard PVC door and window profile extrusion molding process

First, the production process

The production process of hard PVC door and window profiles mainly includes two kinds of single-screw extrusion molding process and twin-screw extrusion molding process, and the raw materials for extrusion of the two processes are mixed powders prepared according to a certain formula in the previous process. .

At present, most of the extrusion of hard PVC door and window profiles uses a conical screw extrusion line.

(I) Single-screw extrusion molding The single-screw extrusion molding process is especially suitable for the production of small-volume, small-size profiles.

The process flow is as follows:

Mixed powder→Single-screw extrusion granulation→Single-screw extrusion molding→Fixing→Traction→Cutting→ Turning table→Finished product

(II) Twin-screw extrusion molding The twin-screw extrusion molding process can be directly formed by powder, and has a large production capacity, and is particularly suitable for the production of large-volume conventional profiles and large-sized profiles.

The process flow is as follows:

Mixed powder→ Twin-screw extrusion molding→Finishing→Traction→Cutting→ Turning table→ Finished product

Second, the twin-screw extruder extrusion hard PVC door and window profiles control points

The extrusion process of hard PVC door and window profile can be briefly described as follows:

The modified PVC mixture is screwed in the barrel, and under the action of internal friction heat and electric heating, the material gradually becomes a molten viscous flow state, and the material spirals in the direction of the head under the flow of the rotating screw.

After entering the die, the mold is subjected to split compression molding under the high temperature and high pressure through the die cavity of the die, and the parison which approximates the cross section of the product is extruded, and then the parison is vacuum cooled and shaped by the shaping die to meet the product design requirements.

Molding temperature, setting cooling, screw speed, traction speed, feeding speed, etc. are all important factors affecting the quality of profiled products.

It is now described as follows:

(1) Temperature control

After the PVC mixed powder enters the extruder, it is compacted, conveyed, melted, homogenized, and extruded at a lower temperature.

In order to obtain high-quality, high-yield profiles, the temperature of each section needs to be repeatedly adjusted and accurately controlled so that the material is always in the melting temperature and decomposition temperature range during the extrusion molding process.

To properly set the temperature, you need to fully consider and guide the factors related to the material forming temperature.

1 Factors affecting material processing temperature

1) Formula and raw material quality

The formula that has the greatest influence on the processing temperature is the formulation, and the mixture of different raw materials has different plasticizing temperatures.

2) Extrusion speed

Under normal production process conditions, increasing the extrusion speed will cause the material to heat up, so the heating temperature should be appropriately reduced.

3) Cooling water

4) Room temperature

Correspondence between material temperature, display and set temperature The display temperature of the extruder is the temperature of the barrel, the head and the die, not the actual temperature of the material.

When the heater is heated outside the temperature control point of the screw barrel, the machine head, and the mold, the material temperature is actually lower than the display temperature;

When the heater stops heating outside the temperature control points such as the screw barrel, the machine head, and the die, the material temperature may be equal to or higher than the display temperature.

The correspondence between material temperature, display temperature and set temperature under different extrusion conditions is the basis and basis for setting and controlling the extrusion temperature.

2 Temperature control points

The profile extrusion temperature control is mainly carried out around the set temperature.

Since the conical twin-screw extruder has the functions of temperature self-control and manual cooling control, the general production state can automatically control the set temperature.

When a certain temperature control fails, the manual cooling control can also be used to control the temperature within the set temperature boundary.

1) Preheating

The extruder is preheated prior to extrusion.

First, adjust the temperature to 140 ° C, after the temperature rises to 140 ° C, keep warm for more than 1 hour, and then raise the temperature to the required temperature for production, heat 5-10 minutes to start extrusion production.

2) Twin screw extruder temperature control

1 The feed section temperature should be high.

Twin-screw extrusion is almost exhaust-type, that is, there is a vent hole in the fuselage section, and a vacuum suction device is provided for sucking out the vinyl chloride monomer and moisture in the mixture, which requires the material to be materialized. The inside of the cylinder is sent to the vent hole and must be uniformly plasticized and coated on the surface of the groove without being sucked out by the vacuum.

2 The temperature of the head connection sleeve should be moderate.

If the temperature is too high, the material will enter the mold smoothly, but the shape stability of the product will be poor, and even the running materials, bubbles, yellowing or even blackening of the product will be caused.

3 head and die temperature should be high.

In order to obtain better appearance and mechanical properties of the profile, and to reduce the expansion of the melt die, the temperature of the control body is generally low, and the temperature of the machine head and the die is relatively high.

(2) Vacuum degree control and cooling setting

During the extrusion process, the PVC profile is completely softened just after exiting the mold. After entering the vacuum setting die, the profiled material is tightly adsorbed on the shaping cavity model by the action of the negative pressure, and the cooling of the chilled water in the model is different. After the profile is taken out of the vacuum sizing die, the desired shape and size can be obtained.

Generally, the degree of vacuum should be greater than 67 lPa. However, if the degree of vacuum is too large, the load on the tractor will increase, and at the same time, the product will be delayed or even hindered from smoothly entering the vacuum setting die, resulting in blockage of the accumulation between the die and the vacuum setting.

In addition, it will reduce the output and shorten the service life of the vacuum pump; if the vacuum is too small, the suction will be insufficient, resulting in severe deformation or no molding, and the appearance quality and dimensional accuracy of the product cannot be guaranteed.

The cooling water of the vacuum sizing die is usually flowed from the rear of the sizing sleeve, and the front part flows out, so that the direction of the water flow and the advancement of the profile are reversed, which makes the profile cooling more moderate, the internal stress is smaller, and the front end temperature of the sizing sleeve is lower. High, the profile is easy to absorb.

Hard PVC profile cooling water is required to be below 20 ° C, preferably 15-20 ° C.

Because hard PVC profiles are often asymmetrical products;

Several vacuum sizing dies can be used to cool the sizing, which avoids the bending deformation of the meta-rules.

(3) Controlling the adjustment of screw speed and traction speed

Screw speed is an important process parameter for extrusion rate, throughput and product quality, and is generally considered based on the shape and size of the die, the capacity of the cooling device, and the like.

The rotation speed is too low, and the extrusion efficiency is not high; however, if the rotation speed is too high, the shear rate is increased, the melt release mold is expanded, and the surface is deteriorated.

After the screw speed is increased, the traction speed is also increased accordingly. If the traction speed is too fast or too slow, the product will be too thin or too thick, especially when driving the traction product. If the traction speed is too fast, the product will be broken. If it is too slow, it will cause blockage between the die and the shaping die.

Experience shows that the hard PVC profile twin-screw extrusion, screw speed is 15-25r / min, which is less than 1mm wall thickness, 20-25r / min; wall thickness is greater than 2mm, the speed is 15-20r / Min.

Similarly, if the wall thickness is less than 1 mm, the traction speed may be 3.0-4.5 r/min, the wall thickness is 2-5 mm, and the traction speed should be 0.5-3 r/mm.

(IV) Feeding speed The twin-screw extruder is equipped with a metering and feeding device, which can be matched with the screw speed. Adjusting the screw feed amount ensures the built-in head pressure.

In the twin-screw extrusion operation, the screw is not completely filled with material. The shear rate, molding temperature, and pressure distribution are determined by controlling the full state of the material in the channel. The amount of extrusion is controlled by the amount of feed.

Generally, the metering screw speed is 1.5-2.5 times the screw speed of the extruder.