During injection molding, if the injection speed of the melt at the gate is too fast, a snake-shaped pattern will be generated on the surface of the plastic part (in front of the side gate). The formation principle is shown in Figure 3-48, and specific products are shown in Figures 3-49.
Snake-like patterns often appear when the mold uses a side gate. When molten plastic flows at high speed through narrow areas such as nozzles, runners, and gates, it suddenly enters a relatively wide, open area. The molten material then meanders along the flow direction like a snake, rapidly cooling upon contact with the mold surface, as shown in Figure 3-51. Because this portion of material cannot effectively fuse with the resin subsequently entering the cavity, noticeable patterns form on the product. Under specific conditions, the melt initially exits the nozzle at a relatively low temperature. Before contacting the cavity surface, the melt viscosity becomes very high, resulting in a snake-like flow. As the hotter melt continues to enter the cavity, the initial melt is forced deeper into the mold, leaving the aforementioned snake-like pattern.
Figure 3-51 Snake pattern generated by Moldflow simulation
The causes and solutions for serpentine patterns on plastic parts are shown in the table below.
| Original Analysis (Common Faults) | Solution / Fix Method |
|---|---|
| ① The nozzle position is incorrect (directly facing the temperature control probe) | ① Change the nozzle position (move to the side) |
| ② The material temperature is too high | ② Reduce the barrel temperature setting |
| ③ The injection speed is too fast (near the gate area) | ③ Reduce the injection speed (especially near the gate) |
| ④ The gate size is too small or uneven (side gate) | ④ Enlarge the gate or change to a central gate (or optimize gate proximity to the thick wall area) |
| ⑤ Poor plastic flowability (FPI too high) | ⑤ Use plastic with better flowability |
Tiger stripes and solutions
Tiger-skin pattern refers to a defect resembling tiger skin on large plastic parts. It's commonly found on dashboards, bumpers, door panels, and other large-area plastic parts with long flow paths. It's also known as tiger-skin spots, as shown in Figure 3-53.
Polymer materials are viscoelastic; they shrink under pressure and expand upon regaining their original volume when the pressure is released. When the polymer melt is extruded through a die, the cross-sectional area of the extrudate is larger than the die exit cross-sectional area.
This phenomenon is called die expansion. It was first observed by the American biologist Barus in 1893, hence the name "Barus effect."
During injection molding, when the plastic melt passes through a small gate, it encounters significant resistance at the gate, causing substantial volume shrinkage in the runner. Once it passes through the gate, it immediately expands, resulting in an expansion jump along the melt flow front, which manifests as a tiger-skin pattern.
Tiger-skin pattern on the plastic part (Figure 3-53)
Similarly, during melt flow, if the part is thin, the cavity clearance is small, and the mold temperature is low, the part structure may cause flow difficulties or an excessively long flow path, leading to increased resistance at the melt front. This results in a significant slowdown or stagnation of melt flow, causing a poor gloss appearance in the filled area. However, as hotter melt continues to flow in from the gate, the rubber system absorbs and stores energy. When the energy accumulates to a certain level, it overcomes the resistance at the melt front, causing the melt to expand rapidly and jump forward. The newly filled area will then have a better gloss.
The more rubber elastomer in the plastic, the more likely this phenomenon is to occur. Materials with poor toughness rarely exhibit tiger-stripe patterns. For example, reinforced materials, non-toughened nylon, and PBT rarely show tiger-stripe patterns during molding, while ABS, HIPS, and PP materials with added EPDM, POE, or other rubber components are very prone to tiger-stripe defects.
Figure 3-54 shows a schematic diagram illustrating the formation mechanism of tiger stripes.
Eliminating or reducing the tiger-skin pattern mainly involves addressing the molding process, such as increasing the material temperature, increasing the mold temperature, and reducing the injection rate.