◇ Analysis of Shrinkage Phenomenon
Shrinkage, also known as sink marks or depressions, is a common surface defect in plastic part molding process. It appears as dimple-like or groove-like depressions, which not only affect aesthetic appearance of product but may also affect its performance. Shrinkage is a frequent and difficult-to-eliminate problem in injection molding process.

 

◇ Harm of Shrinkage
Shrinkage causes depressions and unevenness on the surface of plastic parts, severely damaging product's appearance quality. Moreover, obvious shrinkage defects cannot be concealed by simple surface treatment; instead, they may become more prominent due to polishing. For consumer products such as washing machines, refrigerators, and laptops, shrinkage on the surface will reduce product's market appeal and damage brand image. If product is for corporate clients rather than end-users, presence of shrinkage will also affect client's perception of engineer's professionalism, thus damaging company's image.
In addition, shrinkage can affect dimensional accuracy of local areas of plastic part, thereby interfering with product assembly and function. In summary, although shrinkage problem seems minor, its potential harm should not be ignored.

 

◇ Causes of Shrinkage or Voids
Occurrence of shrinkage or voids involves multiple factors, including characteristics of plastic material, design of plastic part, mold structure, and injection molding process parameters. These factors interact with each other and jointly determine quality of final product. In plastic injection molding process, raw material needs to be melted into a molten state. When this molten plastic cools and solidifies, it undergoes a continuous shrinkage process. If thickness of plastic part is uneven or cooling conditions are inconsistent, it will lead to varying degrees of shrinkage. If outer surface of plastic part is not hard enough to withstand tensile force generated by internal shrinkage, surface material will move inward, forming shrinkage depressions on the surface; if outer surface is sufficiently hard, voids will form internally.

 

2.1 ◇ Influence of Plastic Material Shrinkage Rate
If shrinkage rate of plastic material is high, shrinkage is more likely to occur. From data in Table 1, we can observe that for PC/ABS material, if rib wall thickness is designed to be 50% of basic wall thickness, risk of shrinkage is relatively low. However, for PBT material, to reduce risk of shrinkage, rib wall thickness needs to be designed to be 30% or less of basic wall thickness. This indicates that even a slight increase in wall thickness can significantly increase likelihood of shrinkage.
Table 1 Ratio of Rib Thickness to Basic Wall Thickness for Common Plastic Materials

2.2 ◇ Influence of Excessive Local Wall Thickness in Plastic Parts
In plastic parts, if wall thickness of a certain part is significantly greater than other areas, this part will cool down more slowly than thinner areas. As thinner areas solidify while thicker areas continue to cool, uneven shrinkage can occur, potentially leading to shrinkage problems.
2.3 ◇ Influence of Uneven Wall Thickness in Plastic Parts
During production of plastic parts, if there is uneven wall thickness, thinner parts will shrink less due to faster cooling, while thicker parts will shrink more due to slower cooling. This uneven shrinkage often leads to shrinkage problems, affecting the overall quality and appearance of product.

 

2.3.1 ◇ Occurrence of Shrinkage Problems
When wall thickness of a plastic part is uneven, difference in cooling rates between different parts leads to uneven shrinkage. Specifically, thinner areas shrink less due to rapid cooling, while thicker areas shrink more due to slower cooling. This inconsistency in shrinkage often triggers shrinkage problems, thus damaging the overall quality and aesthetic appearance of product.
2.4 ◇ Influence of Distance from Gate
During molding process of plastic parts, area near gate is filled first with molten plastic and begins to cool and solidify, while molten material in areas far from gate may be affected by insufficient filling. Furthermore, holding pressure is relatively lower in areas far from gate, which further exacerbates shrinkage in these areas after cooling, ultimately leading to shrinkage problems.

 

2.5 ◇ Influence of Filler Flow Direction Changes
If gate design is unreasonable, causing molten plastic to flow from thin-walled areas to thick-walled areas, molten material will cool prematurely when flowing through thin-walled areas due to contact with lower-temperature mold cavity. This can lead to shrinkage problems in thick-walled areas due to insufficient filling, greatly increasing risk of shrinkage.
2.6 ◇ Uneven Cooling Problems
During injection molding process, if mold's cooling channel design is improper, resulting in uneven cooling, areas with faster cooling rates will solidify earlier and have smaller shrinkage rates; while areas with slower cooling rates will solidify later and have larger shrinkage rates, leading to shrinkage problems.

 

2.7 ◇ Shrinkage Caused by Insufficient Injection Pressure and Holding Pressure
If injection pressure is not controlled properly or holding time is insufficient during injection molding process, when plastic part shrinks, there will not be enough material to fill these shrinking areas, ultimately leading to shrinkage.
2.8 ◇ Shrinkage Caused by Improper Adjustment of Injection Volume
In injection molding process of screw-type injection molding machines, a certain amount of molten plastic, usually about 5 mm, needs to be retained between screw head and nozzle at the end of injection. This acts as a buffer. If this buffer amount is set to zero, and screw is pushed all the way to end when adjusting injection volume, then screw will not be able to advance further during holding pressure stage, thus preventing normal holding pressure resulting in shrinkage.

3.1 ◇ Prevention is Key, Avoid Reactive Responses
Measures should be taken to prevent shrinkage problems before they occur, avoiding a reactive and rushed response. Once shrinkage defects actually occur, modifications may be needed in various aspects, including plastic part design, mold gating, and runners. This not only increases product costs and extends development cycle but may also be impossible to implement effectively due to limitations in product and mold structure, or cost considerations. While adjusting machine parameters can temporarily alleviate problem, it is often difficult to provide a permanent solution.

 

3.2 ◇ Differentiating Shrinkage Problems for Different Plastic Parts
During product design, different areas of plastic parts have different requirements regarding shrinkage. Therefore, shrinkage problems cannot be treated uniformly. For example, for exterior surface of a product, such as the A-surface, aesthetics are often more important. Therefore, during design, it is necessary to control wall thickness of reinforcing ribs and even eliminate root fillets to reduce shrinkage marks. For internal structural components, strength is often more critical, so wall thickness of reinforcing ribs can be appropriately increased, and fillets can be added at roots to improve structural stability.
3.3 ◇ Comprehensive Consideration
To effectively prevent shrinkage problems in plastic parts, a comprehensive approach is essential. Addressing problem from a single perspective or direction is often insufficient to completely solve it.

 

◇ Holistic Consideration of Shrinkage Problems
When dealing with shrinkage problems in plastic parts, a comprehensive approach from multiple dimensions is necessary. A single method or means is often insufficient to completely solve this complex phenomenon.
◇ Plastic Part Design is Crucial
Although adjusting mold design and injection molding parameters may solve shrinkage problems to some extent, this approach often increases cost of plastic parts, which we must strive to avoid. For example, for plastic parts such as light guides, improper design leading to excessively thick walls in certain areas may result in shrinkage during injection molding.

 

◇ Light Guide and its Gate and Runner Design
For example, a light guide, as a lightweight product, weighs only 0.6 grams. However, in production process, design of runners plays a crucial role. Runners not only affect molding cycle but also have a profound impact on material and processing costs. Therefore, optimizing design of gates and runners is particularly important in production of light guides.
◇ Team Collaboration
Preventing shrinkage defects requires close collaboration among product design engineers, mold engineers, injection molding engineers, and plastic raw material suppliers. During plastic part design phase, product design engineers need to obtain information about material characteristics, including shrinkage properties, from plastic raw material suppliers. These suppliers typically possess extensive knowledge and experience and can provide solutions for various defects.
3.4 ◇ Utilizing Mold Flow Analysis Software
In development process of plastic parts, mold flow analysis software such as Moldflow has become an important tool for predicting and resolving shrinkage defects. Through this software, designers can predict extent of shrinkage early in design phase, allowing them to take appropriate optimization measures. If shrinkage problems exceed appearance requirements, design team can promptly adjust plastic part design, such as optimizing wall thickness or improving structure to reduce risk of shrinkage.