Solutions to the Challenges in Extruding Medical PVC Thin-Walled Tubing

I. Problems with Raw Material Drying and Solutions

During the extrusion process of medical PVC thin-walled tubes, the drying stage of the raw materials is of vital importance. Many polymers are sensitive to excessive drying, such as nylon and polycarbonate. Most resin manufacturers will specify the shortest drying time and temperature for their materials, and this is no exception for medical PVC materials. It is necessary to strictly follow these recommendations to ensure that the materials are properly dried before extrusion.

Typically, dry-form drying machines are used in the medical extrusion industry to ensure proper drying. These drying machines must be well maintained and undergo regular cleaning, testing, and calibration to ensure their proper operation. If the maintenance of several drying machines is improper, it may result in insufficient or excessive drying of the raw materials, thereby affecting the extrusion quality. For instance, insufficient drying may cause the raw materials to contain moisture, which evaporates during extrusion and forms bubbles, leading to problems such as pores and rough surfaces in the pipe materials; while excessive drying may cause the material to undergo thermal degradation, affecting the physical properties of the pipe materials.

To address the issue of drying, enterprises should establish a comprehensive maintenance system for the drying machines, and assign dedicated personnel to conduct regular maintenance and inspections. At the same time, in accordance with the recommendations of the resin manufacturers, accurately set the drying time and temperature, and monitor the drying status of the raw materials in real time during the drying process. Additionally, online moisture detection equipment can be used to conduct real-time moisture detection of the dried raw materials, ensuring that the moisture content of the raw materials meets the extrusion requirements.

II. Challenges in Designing Extrusion Molds and Countermeasures

The extrusion die is located at the end of the extruder and is the point where the polymer enters the cooling tank. It forms the initial shape of the tube. Tubing molds typically include a mandrel or tip that forms the inner diameter of the tube, as well as a mold or ring that forms the outer diameter of the tube. The design of the molds and the mandrels plays a crucial role in the extrusion process and the extruder's ability to produce precise dimensions and maintain the appropriate physical properties of the material.

Medical tubes with very thin walls and very small diameters may be difficult to extrude through standard extrusion heads/molds. Usually, the viscosity of these materials in the mold is very high, and the mold gap is very small, so that the extrusion operator has to increase the polymer temperature to reduce the material viscosity and obtain sufficient flow through the mold. However, this approach will greatly change the material properties, leading to problems such as degradation, gelation, black spots or poor residual stress in the tubing.

To address these issues, specially designed heads are usually required to produce high-quality thin-walled pipes. Many custom extruders use high stretch ratios to produce thin-walled pipes with small tolerances and diameters, which can significantly improve dimensional tolerances, increase production line speed, and make tooling (molds and mandrels) easier to manufacture. However, operating at high stretch ratios also leads to significant orientation and residual stress/strain in the finished pipes, affecting the performance of the pipes. Therefore, when designing molds, factors such as stretch ratio, mold and mandrel dimensions, etc. need to be comprehensively considered to balance dimensional accuracy and material performance. For example, by optimizing the flow channel design of the mold to ensure uniform flow of materials in the mold and reduce shear stress; at the same time, using special mold materials and surface treatment processes to improve the wear resistance and corrosion resistance of the mold, and ensuring the surface quality of the pipes.

III. Challenges and Solutions in the Cooling Process Control

The extrusion cooling process is a crucial step in the production of medical PVC thin-walled tubes. Different cooling conditions can lead to significant changes in physical properties and morphological structure. Many polymers are semi-crystalline. When the polymer leaves the mold and cools, rapid cooling/quenching often delays crystallization or completely eliminates it, while slow cooling results in higher crystallinity and/or very large crystal formation. For medical PVC thin-walled tubes, improper control of the cooling process can affect the hardness, flexibility, transparency, and other properties of the tubes.

During the cooling process, common problems include uneven cooling, too fast or too slow cooling speed, etc. Uneven cooling will cause internal stress in the pipe material, making the pipe prone to deformation or rupture during use; too fast cooling speed may cause a hard shell to form on the surface of the pipe material while the interior is not fully cooled, resulting in bubbles or voids inside the pipe material; while too slow cooling speed will affect production efficiency and may also cause uneven crystallinity of the pipe material, thereby affecting its physical properties.

To address these issues, it is necessary to optimize the design and control of the cooling system. For instance, a multi-stage cooling method can be adopted. Firstly, rapid cooling is carried out to initially shape the surface of the pipe material, and then slow cooling is applied to fully cool the interior of the pipe material to reduce internal stress. At the same time, by adjusting parameters such as the temperature, flow rate, and velocity of the cooling medium, precise control of the cooling speed can be achieved. Additionally, a stirring device can be installed in the cooling tank to ensure the uniform flow of the cooling medium and achieve uniform cooling of the pipe material.

IV. Difficulties in the Dewatering and Traction Stages and Their Solutions

During the extrusion process of medical PVC thin-walled tubes, the sizing and traction sections also encounter many difficulties. The sizing process involves pulling the extruded tube billet into the sizing sleeve, using vacuum to make the outer wall of the tube billet closely adhere to the inner wall of the sizing sleeve, and then cooling and shaping. The existing thin-walled tube extrusion production lines have relatively short sizing sleeves. Due to the thin wall of the thin-walled tubes, if the vacuum suction force is too strong, the tube wall will be punctured; if the vacuum decreases, the tube wall will separate from the sizing sleeve. Especially as the diameter of the thin-walled tubes increases, the required vacuum suction force also increases, making the phenomenon of tube wall puncturing even more likely to occur.

During the traction process, the traction machine uses the tracks to clamp the thin-walled pipe and pull it forward. Due to the easy deformation of the thin-walled pipe, under the action of the clamping tracks, the thin-walled pipe deforms, resulting in insufficient friction between the tracks and the pipe wall, causing slippage or unstable traction speed, and preventing normal production. Especially for large-diameter thin-walled pipes, due to the increase in their own weight, the traction force also increases, making them even more prone to slipping.

To solve the sizing problem, the method of using a longer sizing sleeve can be adopted, along with optimizing the control of vacuum suction. For example, increase the length of the sizing sleeve to 600 - 800mm to enhance the sizing effect; and use an intelligent control system to adjust the vacuum suction in real time according to the size and wall thickness of the pipe material, avoiding the pipe wall from being punctured or detaching from the sizing sleeve. For the traction issue, the design of the traction mechanism can be improved, such as using a transmission device with several axially-driven tubes to advance the pipe material uniformly, and setting pressure plates and clamping rubber blocks on the transmission belt to increase the contact area between the clamping rubber block and the thin-walled pipe, thereby increasing the friction force and ensuring the stable traction of the pipe material. At the same time, control the Shore hardness of the clamping rubber block to be less than 45, and set several slots on its end face to improve the heat dissipation effect, which is beneficial for further shaping of the pipe material and increasing the roughness to facilitate traction.

V. Problems in the Cutting Process and Solutions

After the extrusion of the medical PVC thin-walled pipe is completed, the cutting process also poses certain challenges. During the cutting process, the press foot of the cutting machine first presses the thin-walled pipe tightly, and then performs the cutting. Due to the thin wall of the thin-walled pipe, the press foot is prone to reverse-damage the pipe wall, causing the pipe to be damaged. Especially for large-diameter thin-walled pipes, due to their own weight and larger pipe diameter, this problem is more likely to occur.

To solve the cutting problem, the design of the cutting mechanism can be improved. For instance, several pressure feet controlled by cylinders are installed at the front and rear ends of the cutting mechanism to press and hold the pipe material, reducing the deformation of the pipe material when it is clamped and preventing reverse crushing, thus achieving normal cutting. Generally speaking, 8 pressure feet arranged in a ring configuration provide the best effect. In addition, advanced cutting technologies such as laser cutting can be adopted to avoid direct squeezing of the pipe material by the pressure feet, thereby improving the cutting quality. At the same time, optimizing cutting parameters such as cutting speed and cutting pressure ensures the stability and efficiency of the cutting process.After the extrusion of the medical PVC thin-walled pipe is completed, the cutting process also poses certain challenges. During the cutting process, the press foot of the cutting machine first presses the thin-walled pipe tightly, and then performs the cutting. Due to the thin wall of the thin-walled pipe, the press foot is prone to reverse-damage the pipe wall, causing the pipe to be damaged. Especially for large-diameter thin-walled pipes, due to their own weight and larger pipe diameter, this problem is more likely to occur.

To solve the cutting problem, the design of the cutting mechanism can be improved. For instance, several pressure feet controlled by cylinders are installed at the front and rear ends of the cutting mechanism to press and hold the pipe material, reducing the deformation of the pipe material when it is clamped and preventing reverse crushing, thus achieving normal cutting. Generally speaking, 8 pressure feet arranged in a ring configuration provide the best effect. In addition, advanced cutting technologies such as laser cutting can be adopted to avoid direct squeezing of the pipe material by the pressure feet, thereby improving the cutting quality. At the same time, optimizing cutting parameters such as cutting speed and cutting pressure ensures the stability and efficiency of the cutting process.

In conclusion, the extrusion process of medical PVC thin-walled pipes faces multiple challenges such as raw material drying, mold design, cooling control, sizing and drawing, and cutting. By addressing each issue with appropriate solutions, the extrusion quality and production efficiency of medical PVC thin-walled pipes can be improved, meeting the strict requirements of the medical industry for pipe materials. If you have a project for customizing medical PVC thin-walled pipes, you can contact our sales team.