【Abstract】Finite element analysis method was used to simulate and analyze injection molding of ART18Z stent with biodegradable polymer polylactic acid (PLA) as molding material, to explore influence of melt temperature, mold temperature and injection pressure on stent molding process. Numerical results show that increase of melt temperature in plasticizing stage will increase filling rate of scaffold; filling rate of scaffold in filling stage increases with increase of mold temperature and injection pressure. Under premise of ensuring quality of injection molding, injection temperature of 280℃, mold temperature of 90℃ and injection pressure of 150MPa are suitable process parameters. Research results reveal influence of different process parameters on injection molding quality of polymer vascular stents, which can provide guidance for improving optimization of vascular stent injection molding process.

Polymer vascular stents effectively reduce probability of intimal hyperplasia after stent implantation. Characteristics of polymer vascular stents solve long-term safety problems of metal stents and radial retraction of balloon-expandable vascular stents. However, injection molding process of polymer vascular stent, such as melt temperature, injection speed, gate position, etc., will directly affect molding quality of stent. To this end, it is necessary to study injection molding process of polymer vascular stents and obtain influence of injection parameters on molding quality, so as to reasonably optimize design of polymer vascular stents.
Scholars at home and abroad have carried out research on manufacturing technology of metal vascular stents. Sauer et al. used laser processing to process vascular stents and studied effect of laser engraving process parameters on processing quality of stents. Fu et al. used fiber laser cutting technology to study statistical characteristics of surface integrity of fiber laser cutting of nitinol vascular stents. These studies provide a method for processing of vascular stents, but surface quality of metal vascular stents is not high, and heat-affected zone generated after processing seriously affects molding quality of stents.
In terms of polymer materials, research of domestic and foreign scholars mainly focuses on mathematical model of polymer. Makovsha et al. established a one-dimensional thermal linear elastic model of polymers, studied global relationship of thermally responsive shape memory polymers. Hu et al. simulated and verified morphological framework of shape memory polymer materials through simulation, revealing nanoscale morphological structure of polymer unit cells. Su et al. proposed an anisotropic constitutive model of polymer conforming materials, which was verified by experiments. It can be seen that current research mainly focuses on polymer mathematical model and shape memory.
In summary, injection molding process of polymer vascular stents is key direction of future research. In this paper, finite element method is used to simulate and analyze injection process of polymer vascular stents, to study injection molding process of polymer vascular stents under different injection parameters, and to analyze influence of various process parameters on quality of stent injection molding. Study shows that melt temperature, mold temperature and injection pressure have obvious effects on injection molding quality of polymer vascular stents. Increasing melt temperature and injection pressure can effectively improve filling rate, and changing mold temperature can improve injection molding quality. Reasonable injection molding process parameters are obtained by simulation.

Polymer stent is composed of polylactic acid (PLA), which has high biocompatibility and good coronary biodegradation period. Taking bioabsorbable ART18Z stent as an example, an ordinary polymer straight bridge stent based on ART18Z was implanted into a vascular model. Finite element analysis method is widely used in research of vascular stents. Finite element method can effectively avoid a lot of manpower and material resources required for in vitro and in vivo experiments, and effectively shorten research period of drug-eluting stents. Mesh division of ART18Z three-dimensional finite element model of polymer vascular stent and location of injection gate are shown in Figure 1.

 

Figure 1 Mesh division of bracket and gate location

In molding process of polymer vascular stents, quality of injection molding can be improved only when each injection stage is successfully completed. During injection process, flow of molten material in cavity has many uncertain factors, which can lead to quality of injection molding, such as dents, dark lines, sink marks, warpage and incomplete filling. Warpage deformation and filling integrity can have a large impact on structure of stent itself.

During plasticizing stage, material is heated to a molten state and mixed evenly with plastic material of same temperature. Screw pushes material to move and inject into cavity according to a certain injection pressure and speed. In process of vascular stent injection, temperature of molten material is also one of main factors that determine molding quality. Set different melt temperatures and simulate effects of different melt temperatures on injection molding of vascular stents. In order to prevent material from being decomposed and denatured by heat, melt temperature of selected polymer material generally does not exceed 290℃. Set different melt temperatures, as shown in Table 1, and observe molding quality at different temperatures.

Table 1 Process parameter table

Material is pushed into cavity until cavity is filled with melt material, a process that, although over a short period of time, still has a large impact on quality of plastic forming. This stage mainly focuses on influence of mold temperature and injection speed on melt flow of cavity filling. Improper selection of parameters will lead to flash, cracks, bubbles and injection dissatisfaction.
Mold temperature is temperature of surface of mold cavity that plastic part contacts, and this parameter directly affects flowability of melt and quality of plastic part after molding. Mold temperature field and mold temperature are standards for measuring mold temperature. Uneven distribution of mold temperature field will lead to warpage deformation and uneven shrinkage of injection molded parts. If mold temperature is too high, cooling time will be prolonged, resulting in large crystal particles and not easy to fall off. Therefore, law of plastic forming under different mold temperatures was simulated, and appropriate mold temperature was obtained. Parameters are shown in Table 2.

Table 2 Process parameter table
Injection pressure controls injection speed, speed at which material is injected into mold is injection speed, and flow speed of molten material in cavity is controlled by injection speed. Melt flow increases temperature of molten polymer material, which aids flow of melt in cavity, but can also cause problems such as difficulty in demolding. If injection pressure is too large, residual stress and surface stress of solidified part will increase, and injection rate will affect flow rate of melt, which in turn affects internal orientation distribution of solidified part, resulting in uneven shrinkage and warpage deformation. Therefore, appropriate injection pressure should be selected to ensure smooth flow of melt in cavity and quality of plastic molding. Injection pressure parameters are shown in Table 3.

Table 3 Process parameter table

Effect of different melt temperatures on filling effect is shown in Figure 2. When mold temperature is 80 ℃, injection pressure is 130 MPa, and injection speed is constant, line graph of filling rate and melt temperature is shown in Figure 3. Analysis shows that filling rate shows an upward trend with increase of melt temperature. When temperature increased from 270 ℃ to 280 ℃, filling rate increased significantly. As melt temperature continues to increase, filling rate increases slowly. Because melt temperature is too high, polymer material will be decomposed, resulting in defects such as difficulty in demolding. Therefore, in order to avoid deformation of material due to thermal decomposition, 280 ℃ was selected as a reasonable melt temperature, used in subsequent analysis and simulation.

 

Fig. 2 Filling simulation diagrams at different melt temperatures
a——melt temperature 230℃ b——melt temperature 240℃
c——melt temperature 250℃ d——melt temperature 260℃
e——melt temperature 270℃ f——melt temperature 280℃

At a melt temperature of 280℃ and an injection pressure of 130MPa, analysis results are shown in Figures 3 and 4. It was found that injection molding process of polymer vascular stent is very sensitive to change of mold temperature, and filling effect is obviously improved by increasing mold temperature. When mold temperature is from 80 ℃ to 90 ℃, filling rate of stent is significantly improved. Taking into account that temperature is too high will lead to difficulty in demolding, mold temperature is the most optimal temperature of 90 ℃.

 

Fig.3 Effect of melt temperature on filling rate

 

Figure 4 Filling simulation diagram at different mold temperatures
a——Mold temperature 50℃ b——Mold temperature 60℃
c——Mold temperature 70℃ d——Mold temperature 80℃
e——Mold temperature 90℃ f——Mold temperature 100℃

Polymer vascular stents are thin-walled structures and therefore require high injection pressures. Injection pressure generally depends on nature of injection material and complexity of molding structure. On premise of ensuring filling quality, problems such as flash and flash caused by excessive injection pressure should be avoided. At the same time, proper injection pressure can ensure integrity of cavity filling. Melt temperature was set at 280℃ and mold temperature was set at 90℃, and results shown in Fig. 5 were obtained. Figure 6 and Figure 7 show effect of injection pressure on filling rate.

 

Figure 5 Effect of mold temperature on filling rate

Fig. 6 Filling simulation diagram under different injection pressures
a——Injection pressure 120MPa b——Injection pressure 130MPa
c——Injection pressure 140MPa d——Injection pressure 150MPa
e——Injection pressure 160MPa f——Injection pressure 170MPa

 

Figure 7 Influence of injection pressure on filling rate

In this paper, an injection molding process model of degradable polymer vascular stents is established. Considering influence of plasticizing stage and filling stage on injection molding quality, finite element method was used to simulate and analyze influence of melt temperature, mold temperature and injection pressure on filling rate of degradable polymer vascular stent during injection molding, solving problem of degradable polymer vascular stent. Due to difficulty of filling in injection molding process of polymer vascular stents, degradable polymer vascular stents have high molding quality under conditions of melt temperature of 280 ℃, mold temperature of 90 ℃ and injection pressure of 150 MPa, as shown in Figure 7. Optimum filling rate is obtained as shown in Figure 8. Simulation results determine appropriate process parameters for injection molding of degradable polymer vascular stent, which greatly simplifies complicated and tedious experiments, has a certain reference value for selection of injection process parameters and improvement of injection molding quality.

 

Figure 8 Optimum fill rate