The main reasons for dents and shrinkage marks on injection molded parts are as follows:
I. Material Factors
1. High Material Shrinkage Rate
Different plastic materials have different shrinkage rates. If a material with a high shrinkage rate is selected, it is prone to significant volume shrinkage during the cooling process after injection molding, leading to dents and shrinkage marks.
For example, some crystalline plastics such as polyethylene (PE) and polypropylene (PP) have relatively high shrinkage rates, while non-crystalline plastics such as polystyrene (PS) and polycarbonate (PC) have lower shrinkage rates. When selecting materials, materials with lower shrinkage rates should be chosen according to the specific requirements of the product and the usage environment to reduce the occurrence of dents and shrinkage marks.
2. Poor Flowability
Poor flowability of plastic materials can lead to insufficient filling of the mold cavity during injection molding, especially in areas with thick walls or complex structures, easily resulting in material shortages. During cooling, these areas, lacking sufficient material replenishment, will form dents and shrinkage marks.
Material flowability can be improved by adjusting the material formulation, adding lubricants, or increasing the processing temperature. For example, adding an appropriate amount of flow aid can reduce the material's viscosity, improve its flowability, and make it easier for the material to fill the mold cavity.
II. Mold Factors
1. Inappropriate Gate Size
A gate that is too small will restrict the flow rate and volume of the molten plastic, leading to insufficient cavity filling, especially in areas far from the gate, which can easily result in depressions and shrinkage marks.
A gate that is too large will cause the molten plastic to generate excessive heat at the gate, causing the material to cool and solidify prematurely, affecting subsequent filling and holding pressure processes, and also easily causing depressions and shrinkage marks.
The size and location of the gate should be designed reasonably according to the size, shape, and wall thickness of the injection molded part. Generally speaking, the gate size should be moderate, ensuring smooth flow of the molten plastic while avoiding excessive heat and pressure loss.
2. Uneven Mold Temperature
Uneven mold temperature will cause uneven shrinkage of the injection molded part during cooling, resulting in depressions and shrinkage marks. For example, excessively high local temperatures in the mold will cause the molten plastic in that area to cool slowly and shrink significantly; conversely, excessively low local temperatures will cause the molten plastic in that area to cool too quickly and shrink less.
Mold temperature uniformity can be improved by optimizing the mold's cooling system. For instance, designing appropriate cooling channels within the mold ensures uniform cooling water flow rate and velocity, maintaining consistent temperature throughout the mold. Additionally, heating elements or oil temperature controllers can be used to heat the mold, further enhancing temperature uniformity.
3. Inappropriate Cavity Design
Uneven cavity wall thickness leads to uneven shrinkage of the injection molded part during cooling, resulting in depressions and shrinkage marks. For example, thicker areas shrink more, while thinner areas shrink less, easily causing depressions and shrinkage marks at these thickness variations.
When designing the cavity, strive for uniform wall thickness and avoid excessive thickness variations. Simultaneously, improve the surface finish of the cavity and reduce surface roughness to decrease friction and improve material flowability and filling properties.
III. Process Factors
1. Insufficient Injection Pressure
Insufficient injection pressure leads to incomplete filling of the mold cavity by the molten plastic, especially in areas with thick walls or complex structures, easily resulting in sink marks and depressions.
Increasing the injection pressure appropriately increases the flow rate and filling capacity of the molten plastic, ensuring sufficient cavity filling. However, excessive injection pressure can also cause other problems, such as flash and difficulty in demolding. Therefore, the injection pressure should be adjusted reasonably according to the specific situation.
2. Insufficient Cooling Time
Insufficient cooling time causes the molded part to be too hot at demolding time, before it is fully solidified. This results in continued shrinkage during the cooling process, producing sink marks and depressions.
Appropriately extending the cooling time ensures that the molded part is fully solidified before demolding, reducing shrinkage. However, excessive cooling time will also reduce production efficiency. Therefore, the cooling time should be shortened as much as possible while ensuring product quality.
Sink marks and depressions in injection molded parts are the result of the combined effects of multiple factors, including materials, molds, and processes. In actual production, these factors should be comprehensively analyzed according to the specific circumstances, and corresponding measures should be taken to reduce the occurrence of dents and shrinkage marks and improve the quality of injection molded parts.
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