Text/Meite Polymer Zhong Ming
1. What is wave tracing?
Wave marks are irregularities in form of ring-shaped, helical or cloud-like waves on surface of plastic parts due to improper flow of molten material in mold cavity, mainly in form of waves near sprue. manifests itself in fact that traces of resin flow are engraved on surface of molded product in form of concentric circles centered in direction of gate. The main reason is that temperature distribution of plastic part is uneven or plastic hardens too quickly and melts Turbulent flow near gate, cold material near gate or not enough plastic is added during pressure maintenance step. Wavy flow marks are a common defect in thermoplastic vinyl injection molded products. As shown in Figure 1.
Figure 1. Schematic diagram and physical picture of wave flow traces on products made by injection molding from TPV
Secondly, why do TPV injection molded products have wavy marks?
First of all, melt does not flow well. When low-temperature melt is injected into cavity in solidification state, oscillations occur in injection port and flow channel on one hand: front of melt flow cools on wall of cavity, and it connects with subsequent melt. On other hand , molten material flows over surface of mold cavity and is squeezed out by injected subsequent molten material to form a reverse flow and a flow branch, resulting in annual annular undulating marks on surface of plastic part centered on gate.
Secondly, molten material does not flow well in flow channel. When molten material flows from a narrow cross-section of flow channel into a cavity with a larger cross-sectional area, fluid material easily forms a turbulent flow. , resulting in traces of a spiral wave on surface of plastic part.
In terms of molding operation: (1) If injection speed is too low, modified plastic melt will slowly flow into cavity. Squeeze to form reverse flow and stagnation, thus undulating flow traces are formed. . In this regard, melt flow rate must be increased accordingly. (2) If temperature of mold is low, melt flow rate of modified plastic in mold is slow, and wavy flow is likely to appear. Therefore, it is necessary to increase temperature of melt or reduce flow rate of cooling water to increase temperature of mold.
From figure 2 it can be seen that melt flow front enters cavity and flows relatively quickly, and then flow gradually slows down. Unstable injection speed leads to unstable flow. The wall of mold cavity at low temperature forms a very hard shell, and shell layer is subjected to force of melt flow, sometimes breaking away from surface of cavity, resulting in uneven cooling, and eventually forming corrugated marks on product.
Figure 2. The process of forming wave flow traces during injection molding (the figure is taken from reference book).
(3) If flow rate of modified plastic melt is too high, it is easy to create turbulent flow after entering mold cavity, which is very easy to create wave flow traces. In this regard, download speed can be reduced accordingly, or slow, fast, and slow staggered downloads can be used. (4) If temperature of nozzle is low, modified plastic melt may be cooled, and there will be traces of undulating flow. Therefore, spray rate should be increased accordingly.
(5) The holding pressure in mold cavity has a great influence on eliminating wave flow traces. Therefore, holding time under pressure must be increased accordingly. From fig. 3, it can be seen that after switching speed and pressure, melt flows slowly, and pressure is not enough, and melt remains in position of switching speed and pressure for a long time, forming a solidified layer, and finally forming ripples. Therefore, incorrect switching of speed and pressure, insufficient pressure retention, too low or unstable injection speed can lead to formation of flow wave wakes.
Figure 3. Influence of holding time on wave flow traces in injection molding (picture taken from link).
From point of view of mold: (1) Modified cold plastic material has a great influence on wave flow traces. To prevent influence of modified cold plastic material Large cold material hole to accommodate more cold material. (2) If cross-sectional area of the gate and runner is too small, modified plastic melt cannot flow smoothly in it, and flow rate is slow, which may cause wave flow traces. In this regard, cross-sectional area of the gate and runner must be increased accordingly. (3) The cooling system in mold must ensure that all parts of mold are evenly cooled, otherwise mold temperature will be different due to uneven cooling. Special attention should be paid to design of cooling system. (4) When modified plastic melt enters large cross-section cavity from narrow section of flow channel, modified plastic melt easily forms a turbulent flow, which leads to formation of spiral wave flow traces on surface of modified plastic part. In this regard, gate must be installed in part of thickness of wall or gate must be installed directly from side of wall. The shape of gate is preferably in form of a handle, fan or diaphragm shape.
Regarding materials: Thermoplastic vulcanizate (TPV) is a special type of TPE. It is made in process of dynamic vulcanization. At high temperature and high shear, rubber phase and plastic mix with each other. Gradually crosslink and disintegrate. The result is a "sea island" structure consisting of a large amount of rubber phase dispersed in a small, continuous plastic matrix. Among them, fluidity is determined by plastic phase, and initial value of complex viscosity of TPV is much higher than that of plastic component due to strong rubber network formed by cross-linked rubber particles. Due to strong rubber mesh and molecular entanglement, rheological behavior of TPV has elasticity of rubber at low strain and low frequency, but rubber mesh breaks and deforms at high strain and high frequency, which makes TPV good melt workability. Thus, in order to ensure a constant viscosity ratio of rubber and TPV plastic, plastic phase can better draw rubber phase into a stable product. The viscosity ratio of rubber and plastic can be adjusted by adding oil. As shown in Figure 4, a schematic diagram of TPV injection molded products.
Figure 4. Schematic diagram of surface of TPV injection molded products (Image courtesy of Meite Polymer Co., Ltd.)