The speed at which the molten material is injected into the mold of a plastic injection machine has a significant impact on the surface smoothness of plastic products. Injecting too fast or too slow can cause changes in surface smoothness. Below is a detailed introduction to the influence of melt injection speed on surface smoothness:

1. Injection speed too fast:
Flow Marks: Excessive injection speed can cause uneven flow of molten plastic, especially in larger surface areas of the mold. This uneven flow may leave obvious flow patterns or ripple marks, affecting surface smoothness and causing uneven marks on the surface of the product.
Surface burning or blackening: When the injection speed is too fast, the residence time of the molten plastic is short, which may cause the plastic to overheat and produce too much gas or volatile substances. These gases cannot be discharged in time and may form burnt or blackened marks on the surface of the product.
Excessive pressure: Excessive injection speed can cause the pressure inside the mold cavity to be too high, which may lead to excessive contact between the mold surface and the melt, resulting in fine defects on the surface and reducing surface smoothness.

2. Moderate injection speed:
Uniform flow, smooth surface: Moderate injection speed helps the melt flow smoothly in the mold, and the filling process is uniform, which can reduce the phenomenon of flow lines and surface unevenness, thereby achieving a smooth surface effect.
Reduce bubble generation: Appropriate injection speed can ensure smooth gas discharge, avoid surface bubbles or voids caused by gas retention, and improve surface smoothness.
Maintain mold details: Moderate injection speed helps to fully fill the complex structure inside the mold with plastic, maintain fine texture and surface details, and enhance the smoothness and accuracy of the finished product.

3. Slow injection speed:
Uneven cooling and rough surface: If the injection speed is too slow and the melt flow is slow, it may cause some areas of the mold to cool first, resulting in an uneven cooling process and surface roughness or cooling marks.
Surface defects: Due to prolonged plastic filling time, the plastic may come into contact with the cold mold for a long time, resulting in surface defects such as uneven surfaces or unavoidable burrs.
Shrinkage effect: When the injection speed is slow, the residence time of the plastic in the mold increases, which may cause rapid or uneven shrinkage during cooling, further affecting the surface flatness and smoothness.

4. Gas emissions and bubbles:
Excessive injection speed: High speed injection may cause air to be unable to be smoothly expelled, resulting in bubbles that may appear on the surface and affect smoothness.
Slow injection speed: When the injection speed is too slow, it may cause incomplete mold filling or delayed gas emission, and may also form bubbles or voids on the surface, affecting surface smoothness.

5. Coordination between cooling speed and mold design:
Fast injection+fast cooling: If the injection speed is too fast but the cooling speed is too fast, it may cause the plastic surface to lack sufficient time for smooth cooling, resulting in surface defects.
Slow injection+slow cooling: The combination of too slow injection and cooling process may cause the surface to be exposed to the cooling process for a long time, resulting in uneven cooling and surface roughness.

6. Liquidity and material properties:
Low fluidity materials: For plastic materials with poor fluidity, excessive injection speed may not fully fill the mold with molten material, resulting in voids or uneven flow, leading to poor surface smoothness.
High fluidity materials: High fluidity materials can maintain good surface quality at faster injection speeds, but if the speed is too fast, it may cause surface damage due to excessive pressure.