In the plastic injection molding process, shrinkage is a factor that cannot be ignored. Shrinkage in injection molding rate refers to the percentage difference between the dimensions of a plastic product at the molding temperature and its dimensions after cooling to room temperature. Understanding the factors that influence the shrinkage in injection molding rate is crucial for optimizing production processes and improving product quality. This article will explore the main factors affecting the shrinkage rate and list the shrinkage in injection molding rates of some commonly used plastics.
The structure of the part and the material used have a significant impact on the shrinkage in injection molding rate. In general, the thicker the part wall, the longer the cooling time, and the greater the shrinkage rate. Specifically:
In general, when the dimensions along the flow direction of the melt differ greatly from the dimensions perpendicular to the flow direction, the shrinkage in injection molding rate also varies significantly. Thick-walled parts often cool unevenly, resulting in varying shrinkage rates in different areas.
Parts farther from the gate tend to have higher shrinkage in injection molding rates due to greater pressure loss, while parts closer to the gate typically have a smaller shrinkage in injection molding rate. Additionally, features such as ribs, holes, and bosses in the structure also affect shrinkage because these areas offer some resistance to shrinkage, resulting in relatively smaller shrinkage rates.
These factors work together to determine the final size and shape of the part, so careful consideration is required during design.
The design of the mold, particularly the cooling circuit and gate configuration, also has an important impact on the shrinkage in injection molding rate. Here are several key factors:
When a small gate is used, the part solidifies before the holding pressure ends, which increases the shrinkage in injection molding rate. An appropriately sized gate can reduce the shrinkage in injection molding rate, especially when the mold temperature distribution is uneven.
An unreasonable cooling circuit can lead to uneven temperatures in the part, resulting in shrinkage differences. The temperature distribution in thin-walled areas of the mold has a particularly significant impact on the shrinkage in injection molding rate. Excessive temperature can cause an increase in the shrinkage in injection molding rate.
Parts of the molded piece that are limited by the mold will experience smaller shrinkage in injection molding rates, while areas without such constraints will have relatively larger shrinkage in injection molding rates.
Optimizing the mold structure and cooling system design can effectively control the shrinkage in injection molding rate.
The shrinkage in injection molding rate is also influenced by multiple factors in the molding process. Here are the main factors:
Higher barrel temperatures can reduce the shrinkage in injection molding rate because they help with pressure transfer. However, when using a small gate, early solidification at the gate may cause the shrinkage rate to increase.
Injection pressure is an important factor affecting shrinkage in injection molding rate. Generally, the higher the injection pressure, the higher the material density, and the lower the shrinkage rate. This is especially true for thermoplastic elastomer materials, where increasing the injection pressure can significantly reduce the shrinkage rate.
In molding conditions, material addition should be appropriate. Insufficient material addition can lead to inadequate pressure, which increases the shrinkage in injection molding rate.
When the mold temperature is too high, the shrinkage in injection molding rate is usually larger. However, for thin-walled parts, high temperature reduces the flow resistance of the melt, thus lowering the shrinkage rate.
Although the molding cycle does not directly relate to the shrinkage rate, speeding up the molding cycle can cause changes in the mold and melt temperatures, indirectly affecting the shrinkage rate.
By making reasonable process adjustments, the shrinkage in injection molding rate can be effectively controlled to ensure product quality and stability.
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