What are the bugs when Start-up of U-PVC Profile Production, how to fix them?

1. U-PVC Production Flow

In china market, Conical Double-Screw Plastic Extruder is popular to be used.

2.Typical parameters setting(Below is typical setting by KraussMaffei φ75 parallel twin-screw extruder produces single-cavity BR60 profiles)

3.Start-up preparation

3.1 Preparation

Mix the materials per formula, leave them to mature for 24 hours after the mixing, discharge static electricity, and send them to the hopper.

3.2 Mold installation and inspection

3.3 Start heating and constant temperature First, the first stage of heating of the extruder barrel is carried out. The temperature is usually set to 130℃. After the temperature reaches the constant temperature for 20 minutes, adjust the barrel temperature to the normal extrusion temperature and continue to raise the temperature; the die temperature is usually set it to 195°C. When the temperature of the die head rises to 145°C, tighten the die head twice and continue to heat up. When the barrel temperature and die temperature both reach the set temperature, keep the temperature for about 10 minutes, and then start the extruder.

3.4 Start-up extrusion traction and shaping

4. Common problems and debugging method

4.1 Adhesion of inner ribs to outer wall

(1) When starting the machine, the puncture and deflation of the parison are not in place, and the puncture and deflation are continued.

(2) Reduce the process temperature, thereby reducing the melt temperature and viscosity, and improve the cooling effect of the inner ribs and increase their strength.

(3) Check the reasonableness of the distance between the inner rib and the outer wall.

(4) Check the rationality of the gradient design between the shaping mold and the shaping block.

4.2 Uneven discharge

Uneven discharge can easily lead to profile bending, unqualified wall thickness, excessive meter weight, unqualified internal cavity function size, etc. From the perspective of mold adjustment, the solution is usually:

(1) Check the uniformity of the cavity of the flat section of the die and the gap between the die forming plate and the preformed plate. The gap difference is not more than 0.03mm.

(2) Check whether there are dead corners, steps or pastes in the runner, and check whether the cavity is polished to a mirror surface (Ra ≤0.02mm).

(3) Check whether the center position of the diversion cone of the die is reasonable.

(4) To adjust the local discharge speed of the die, two different methods can be adopted in the compression section and the steady flow section (support plate), "block flow" or "drain flow" (without changing the gap between the straight section of the die). For the area where the material is discharged quickly, a baffle can be set, or the cavity can be inlaid and welded with the same material as the mold body to achieve the purpose of "current limiting"; for the area where the material is slow, a partial extension can be used Measures such as compressing the length of the section (ie shortening the length of the straight section), increasing the size of the diversion section cavity or adjusting the angle of the diversion cone to achieve the purpose of "draining".

(5) For slight uneven discharge, it can be solved by adjusting the mold temperature. For areas with slow discharge, increase the temperature of the heating plate corresponding to the mold surface; for areas with fast discharge, reduce the temperature of the heating plate corresponding to the mold surface.

4.3 There are shrinkage marks on the surface of the profile

The shrinkage marks on the surface of the profile are usually produced on the wall corresponding to the inner rib, the small boss, or the "T"-shaped intersection of the two walls, as shown in Figure 2, at the 1, 2, 3, 4, and 5 of the visible surface A .

The causes and solutions for shrinkage marks are as follows:

(1) Reason: There is a speed difference between the inner rib and the outer wall, and the extrusion speed of the inner rib lags behind the outer wall.

Solution: When repairing the mold, increase the supply of inner ribs, and round the corners of the core at the intersection of the inner ribs and the outer wall (R 0.3 ~ 1) to eliminate the sharp point stagnation effect.

(2) Reason: The thickness of the inner ribs is too large (the thickness of the inner ribs is about 60% of the outer wall), which leads to insufficient cooling and "post-shrinkage" to produce shrinkage marks.

Solution: Reduce the discharge of the inner ribs by reducing the cross-sectional area of the inner ribs, lowering the melt temperature and flow rate at the inner ribs, and enhance the cooling capacity of the shaping system or reduce the traction speed.

(3) Reason: The vacuum adsorption force is weak, and the shaping system is not well sealed.

Solution: ①Set up a vacuum adsorption hole at the corresponding part of the shrinkage mark; ②Install a sealing strip or water injection seal between each section of the shaping mold to improve the vacuum shaping ability.

(4) Reason: The core supply cavity and groove are not polished in place, causing the internal ribs or small protrusions to lag behind.

Solution: Polish the corresponding parts to the mirror surface requirements.

(5) Reason: There are more internal ribs.

Solution: When designing the cross-section, design grooves on easy-to-shrink parts to cover shrinkage marks, such as grooves on door panels. 

4.4 There are bright lines and dark bands (lines) on the surface of the profile

(1) The internal ribs are discharged too slowly to form a dark band.

Solution: repair the die head to make the internal rib discharge and the large surface converge; repair the fillet at the junction of the internal rib and the large surface of the mold core insert to about R 0.5; in the setting mold facing the internal rib The position increases the degree of vacuum.

(2) The inner ribs are discharged too quickly to form bright bands. .

Solution: repair the die head to make the internal rib discharge and the large surface converge; reduce the vacuum at the corresponding internal rib of the shaping die.

(3) Improper setting of vacuum adsorption system.

Solution: reduce the diameter of the air hole, or move the air hole away from the profile. Install a strong vacuum and strong cooling device at the entrance of the shaping mold, such as adding a cooling plate, cutting a special-shaped water tank for water supply, etc., which can appropriately alleviate the above-mentioned defects.

 (4) The position of the shaping block in the water tank is not accurate. During the process of pulling the profile, it locally rubs with the profile, resulting in bright lines and dark bands.

If you want to know more, you are welcome to contact with us anytime, we are 7/24 available for technical support.---by Jiangxi Yuefeng Group Polymer Co.,Ltd, Sophie:0086-153 6785 2031.