Shrinkage marks at thicker positions of plastic mold parts, also known as sink marks, are a common issue in injection molding. They occur due to the non-uniform cooling and solidification of the molten plastic material within the mold cavity. Several factors can contribute to the formation of shrinkage marks in thicker sections of plastic mold parts:
- Material Selection: The choice of plastic material used in injection molding plays a significant role in the occurrence of shrinkage marks. Some plastics have higher shrinkage rates than others, and using a material with high shrinkage in thicker sections can lead to sink marks.
- Injection Parameters: Improper injection parameters, such as high injection speed or excessive injection pressure, can cause uneven cooling and solidification of the plastic material, leading to shrinkage marks.
- Mold Design: The design of the mold itself can influence the occurrence of sink marks. Insufficient cooling channels or improper gating and venting can result in uneven cooling and create shrinkage marks.
- Wall Thickness Variation: Large variations in wall thickness within the same part can cause differential cooling rates, resulting in shrinkage marks at thicker sections.
- Mold Temperature: Controlling the mold temperature is crucial for uniform cooling and minimizing shrinkage marks. If the mold temperature is too low or too high, it can lead to uneven cooling and sink marks.
- Injection Speed and Pressure: Improper control of injection speed and pressure can cause packing-related issues, leading to variations in part density and contributing to shrinkage marks.
- Part Geometry: Complex part geometries with sharp corners or thick-to-thin transitions can exacerbate the formation of shrinkage marks.
- Mold Venting: Inadequate mold venting can trap air inside the mold cavity, resulting in incomplete filling and subsequent sink marks.
- Melt Temperature: The melt temperature of the plastic material can affect its flow behavior and cooling rate, influencing the occurrence of shrinkage marks.
To minimize or eliminate shrinkage marks at thicker positions of plastic mold parts, several measures can be taken:
- Proper Material Selection: Choose a plastic material with a low shrinkage rate for thicker sections of the part.
- Optimize Injection Parameters: Adjust injection speed, pressure, and packing parameters to achieve uniform filling and packing of the mold cavity.
- Mold Design Improvement: Modify the mold design to improve cooling efficiency and ensure uniform cooling across the part.
- Uniform Wall Thickness: Aim for consistent wall thickness throughout the part to promote uniform cooling.
- Mold Venting: Ensure adequate mold venting to allow air to escape during injection molding.
- Controlling Mold Temperature: Maintain the appropriate mold temperature for consistent cooling.
By addressing these factors and optimizing the injection molding process, it is possible to reduce or eliminate shrinkage marks at thicker positions of plastic mold parts, leading to higher-quality finished products.
Shrinkage is more severe at thicker locations of plastic mold parts, such as rib or protrusion, than at adjacent locations, because the cooling rate of thicker areas is much slower than that of surrounding areas. Different cooling rates lead to the formation of concavity at the joint surface, which is known as the shrinkage mark. This kind of defect seriously limits the design and molding of injection mold products for household appliances, especially for large thick wall products such as the inclined case of TV and the display case.
Shrinkage marks may be caused by one or more reasons, including processing method, part geometry, material selection, mold design, etc. The geometry and material selection are usually determined by the raw material supplier and are not easy to change. However, there are many factors related to mold design that may affect the shrinkage of mold manufacturers. Cooling runner design, gate type, and gate size can have multiple effects. For example, small gates such as tubular gates cool much faster than tapered gates. Early cooling at the gate will reduce the filling time of the mold cavity, thus increasing the probability of shrinkage marks. For molding workers, adjusting the processing conditions is a way to solve the shrinkage problem. Filling pressure and time significantly affect shrinkage. After the part is filled, the excess material continues to fill the cavity to compensate for material shrinkage. If the filling stage is too short, the shrinkage will be intensified, and more or larger shrinkage marks will be produced finally. This method itself may not reduce the shrinkage mark to a satisfactory level, but the molding worker can adjust the filling conditions to improve the shrinkage mark.
Shrinkage marks can be a common issue in plastic injection molding, especially in thicker sections of the mold parts. Shrinkage marks are the visible depressions or sink marks that appear on the surface of the molded part. In this article, we will discuss some of the causes of shrinkage marks at thicker positions of plastic mold parts.
- Inadequate Injection Pressure
One of the main causes of shrinkage marks in thicker sections of plastic mold parts is inadequate injection pressure. If the injection pressure is too low, the plastic material may not fill the mold cavity completely, resulting in shrinkage marks in the thicker sections of the part.
- Improper Venting
Another cause of shrinkage marks is improper venting. Venting is the process of allowing air to escape from the mold cavity during the injection molding process. If there is inadequate venting, air may get trapped in the thicker sections of the part, causing shrinkage marks.
- Inadequate Cooling Time
Inadequate cooling time can also cause shrinkage marks in thicker sections of plastic mold parts. If the cooling time is too short, the plastic material may not have enough time to cool and solidify completely, resulting in shrinkage marks.
- Improper Mold Temperature
The mold temperature plays a crucial role in the injection molding process. If the mold temperature is too low, the plastic material may solidify too quickly, causing shrinkage marks in the thicker sections of the part. On the other hand, if the mold temperature is too high, the plastic material may expand, resulting in warpage or other defects.
- Inadequate Gate Size
The gate size is the opening through which the plastic material enters the mold cavity. If the gate size is too small, the plastic material may not flow properly into the thicker sections of the part, resulting in shrinkage marks.
In conclusion, shrinkage marks in thicker sections of plastic mold parts can be caused by a variety of factors, including inadequate injection pressure, improper venting, inadequate cooling time, improper mold temperature, and inadequate gate size. These issues can be addressed by optimizing the injection molding process, adjusting the mold temperature, and ensuring adequate cooling time and venting. A skilled and experienced mold technician can help diagnose and solve these issues to prevent shrinkage marks in thicker sections of plastic mold parts.