Abstract: This article introduces the nylon tube extrusion process of automotive nylon smooth tubes. Analyzes the factors affecting the quality of nylon tubes, and also summarizes some defects in the production process and their causes.
Since Citroen used nylon (PA11) single-ply fuel lines on its DS models in the 1950s. After that, more and more car manufacturers have chosen plastic tubes to replace metal ones. Plastic tubes have obviously advantages over metal tubes, such as 1) Flexible, easy to form and install. 2) High corrosion resistance and good resistance to petro. 3) Easier to connect. 4) Cost-effective.
Nylon (PA) tube widely used in automobiles for oil pipes, water pipes, air pipes, vacuum booster pipes, and other products. Single-ply automotive tubing mainly uses PA11 and PA12 as the primary raw materials. These two materials have the advantages such as oil resistance, chemical resistance, wear resistance, pressure resistance, high and low-temperature resistance, flame retardant, lightweight, etc.
One of the main differences between PA11 and PA12 is the melting point. The melting point of PA11 is about 190℃, which is about 10℃ higher than that of PA12. So, PA11 can work in a higher environment.
The following is an example of a smooth nylon tube extrusion process with an outer diameter of 10mm and a wall thickness of 1mm.
1. Raw materials, equipment, and devices of nylon tube extrusion process
1.1 Raw Material: PA11 resin, Arkema, France
1.2 Equipment and devices:
Single Screw Extruder: Screw diameter 50mm, L/D: 24:1, heating power 15kw, driving power: 15kw, screw speed 10-150r/min.
Automatic feeding machine
Mouth die: Inner diameter 17.6mm
Core die: Outer diameter 13.6mm
Sizing sleeve: Inner diameter 10.64mm
Vacuum sizing tank: Length-2300mm
Submerged water cooling tank: Length-7800mm
2. Production process and factors affecting the quality
2.1 Nylon Tube Extrusion Process
The following is the production process of PA11 smooth tube:
Raw material drying – Vacuum feeding – Extruder melting – Mouth die extrusion – Sizing sleeve vacuum sizing – Water tank cooling – Automatic wall thickness measurement and adjustment – Flame treatment – Spray code (if required) – Cutting – Packaging
2.2 Nylon Tube Extrusion Process Conditions
Temperature setting basically: Area 1-175℃, Area 2- 220℃, Area 3-235℃, Area 4-235℃, connection body-230℃, Head- 220℃, Mouth die-220℃.
Screw speed: 133r/min, traction speed: 30m/min, vacuum sizing tank vacuum degree: 13.5kPa, traction roller clamping force: 0.3MPa.
2.3 Process Control Essentials
PA11 can form hydrogen bonds with water molecules due to the hydrophilic nature of the amide groups in it. Therefore PA11 has a large water absorption rate. Generally, the raw resin is usually dry granules and packed in moisture-proof bags. Before opening the package, make sure that the bag temperature reaches the ambient temperature so that water vapor does not condense onto the cold resin particles.
Freshly opened bags of resin do not need to be dried and can be used directly in production. Drying is only required if the original packaging is damaged or has been left open for more than 2 hours. In this case, the resin should be dried at 80℃ for 4-8 hours to bring the moisture content down to less than 0.1%. Prolonged drying will decolorize the resin particles. Moreover, the resin-containing plasticizer may cause loss of plasticizer during the drying process. Therefore, once the package is opened but the unused resins need to be tied as tightly as possible to the bag.
2). Feed Material
The temperature of the feed zone is 60-90°C. The setting temperature of the first heating zone should be 20°C higher than the melting point of the resin. The temperature needs gradually increased from the first heating zone to the head part because it helps to increase the hardness of the melt when it leaves the mold mouth.
When the set temperature is reached, adjust the bolt on the mouth mold fixing ring. The end of the bolt will move the mold and change the position of the mold to adjust the gap between the mold and the core mold. This will make the circumferential gap equal and ensure uniform wall thickness of the tube.
The function of the central through-hole of the mandrel is to maintain communication with the atmosphere and to achieve the outer diameter sizing.
Start the extruder and tractor, increase the speed to traction speed 6m/min. and pull the extrudate into thin winding through the cooling system and haul off, and press down the traction roller. At the same time, turn on the cooling water switch and adjust the traction speed after the material enters the traction machine, and turn on the vacuum system for sizing operation. Adjust the wall thickness and outer diameter of the tube to pass.
4) Sizing & Cooling
The material should be sized and cooled immediately after extrusion out of the die. The tube blanks are first cooled by air to prevent sticking to the sizing sleeve arms and then enter the vacuum sizing sleeve. The distance between the sizing sleeve and the mouth die is determined according to the outer diameter of the pipe and the traction speed, generally 10-100mm.
The vacuum sizing device ensures that the products have precise structural dimensions. And the principle of the vacuum sink is that the top of the sink is fastened with a gland and sealing ring to form a sealed state.
The sizing sleeve is a single-walled metal tube with many through-holes evenly distributed on the tube wall, fixed at the feed end of the sink and below the water surface. The sizing sleeve is generally choose brass and beryllium copper with high thermal conductivity as material. The vacuum acts on the tube embryo through the hole of the sizing sleeve. It uses the pressure difference between the inner and outer wall of the tube embryo, to make the outer wall of the tube embryo adhere to the inner wall of the sizing sleeve and achieve cooling and sizing. Another important point, the end of the vacuum chamber must be reliably sealed in order to produce dimensionally accurate tubes at high speed.
The tube leaving the vacuum sizing water tank is not completely cooled, to prevent deformation needs to enter the submerged cooling water tank for further cooling. The direction of cooling water flow is opposite to the direction of tube movement, which facilitates gradual cooling of the tube and also reduces internal stress.
5) Wall thickness detection and adjustment
Wall thickness detection and traction speed adjustment constitute a closed-loop control system. In another word, traction speed will be adjusted automatically according to the difference between the measured value and the set value instantly. For the purpose of ensure the wall thickness is up to standard and save raw material to reduce cost.
6) Haul Off
The belt haul-off machine is mainly suitable for thin-walled, small diameter tubes and fast production speed. Using two flat belts moving at the same speed, the product receives an evenly distributed pressure. For this reason, JF haul off machines use one motor and two inverters to ensure the same speed of the belts.
It is important to realize, contact pressure should be as low as possible to prevent tube deformation. The stability of the haul-off speed affects the accuracy of the tube extrusion wall thickness.
7) Cutting or Coiling
Due to the small diameter of the tube, it is more suitable to use flying knife to cut. When the extrusion speed is greater than 2m/min, a duplex winding machine should be selected.
3. Analysis of Quality Defects and Causes in Nylon Tube Extrusion Process.
The following are the quality defects in the extrusion line of PA11 smooth tubes for automobiles:
There are many reasons for defects in the extrusion production process of PA nylon tubes for automobiles. For example, raw materials, equipment and process and many other aspects. To ensure the quality of products, we should master the principle of extrusion and be familiar with the performance of equipment and dies. It is also important to be able to eliminate the factors that cause product quality defects in a timely manner.