天美影院

Injection molding defects are common issues that may arise during the injection molding process. Among them, sink marks in injection molding are a very common defect.

For product designers, structural designers, and design companies, understanding sink marks and other injection molding defects helps them improve design methods and inspect finished products or parts from injection molding factories (refer to injection molded part acceptance standards).

For mold factories and injection molding factories, analyzing sink marks in injection molding and solving this defect perfectly is fundamental. Every mold and injection molding practitioner has the responsibility and obligation to fully master all knowledge of injection molding defects.

Below is a common injection molding defect chart and detailed information about them. Interested friends are welcome to click for further understanding:

What is a Sink Mark in Injection Molding?

Sink marks, shrink marks, or depressions on the surface of plastic parts are defects that occur frequently and are very difficult to eliminate during the injection molding process.

Harms Caused by Sink Marks

Sink marks cause the surface of plastic parts to be concave and uneven, severely affecting the appearance. Sink marks in injection molding that are too obvious cannot be concealed by surface treatment, and glossy surface treatments will even magnify the defects.

For consumer products such as washing machines, refrigerators, and laptops, if end users see numerous sink marks on the surface, with uneven surfaces and concave irregularities, they will perceive the product’s quality as poor, seriously affecting the brand image.

If the product is not directly facing end users but rather enterprise customers, seeing numerous sink marks may lead them to perceive engineers as unprofessional and affect the company’s image.

Additionally, sink marks can also affect the dimensions of specific areas of plastic parts, impacting assembly and corresponding functional implementation.

In summary, sink marks may seem minor, but their potential harm includes the loss of users and customers, as well as affecting assembly, etc., which should not be underestimated.

Why Do Sink Marks Occur?

Plastic material needs to be melted into a molten state during injection molding, and the molten plastic needs to continuously shrink during the cooling and solidification process. If the thickness of the plastic part is inconsistent or cooling is uneven, different degrees of shrinkage can occur.

If the outer surface of the plastic part is not sufficiently rigid to resist the tensile force generated by internal shrinkage, the surface material will move inward, resulting in sink marks on the surface without compensation in subsequent processes. If the outer surface of the plastic part is sufficiently rigid, voids will form internally.

Specific factors contributing to sink marks in products include plastic materials, product design, mold structure, and injection molding process parameters, among others.

1. Material Factors Contributing to Product Sink Marks In Injection Molding

When the shrinkage rate of the plastic material is high, sink marks are more likely to occur. From Table 1, it can be seen that for PC/ABS, when the reinforcement rib thickness is designed to be 50% of the basic wall thickness, the likelihood of sink marks is relatively low. In contrast, for PBT, sink marks are less likely to occur only when the reinforcement rib thickness is designed to be 30% of the basic wall thickness. This means that even a slightly thicker reinforcement rib thickness greatly increases the likelihood of sink marks.

In addition to the shrinkage rate, other material factors contributing to sink marks in injection-molded parts include:

1. Sink marks caused by inadequate drying of the material.
2. Sink marks caused by excessively large or uneven material particles resulting in substandard melt quality.

Tips: Click the hyperlink to obtain more detailed information on plastic shrinkage.

2. Design Factors Contributing to Product Sink marks

Regarding sink mark phenomena resulting from product design, we will focus on discussing wall thickness.

Areas with locally excessive wall thickness cool more slowly than areas with thinner walls. Areas with thinner walls have already cooled and solidified, while areas with excessively thick walls have not fully cooled and solidified. As the excessively thick-walled areas continue to cool and solidify, sink mark occurs.

Areas with locally excessive wall thickness are most likely to occur at the root of reinforcement ribs, pillar roots, connections between reinforcement ribs, connections between reinforcement ribs and walls, and connections between pillars and walls.

Additionally, uneven wall thickness results in faster cooling and less shrinkage in thin-walled areas, while slower cooling and greater shrinkage occur in thick-walled areas, leading to sink marks.

In summary, improper design leading to sink marks in injection-molded parts is caused by the following:

1. Uneven wall thickness due to unbalanced product design.
2. Excessive thickness of gate design.
3. Inappropriate design of bone position without consideration of material shrinkage standards resulting in excessive thickness and subsequent shrinkage.
4. Excessive thickness of screw column positions not designed according to standard plastic product screw column designs.

3. Mold Factors Contributing to Product Sink Marks

The reasons for mold-induced sink marks in injection molding are more complex and mainly include:

1. Small inlet size causes insufficient pressure, especially in molds for PC materials, glass fiber-reinforced materials, and thermally conductive materials.
2. Poor mold exhaust leads to trapped air.
3. Inadequate cooling of the mold causes excessive heat and overheating shrinkage.
4. Mold core offset or unstable mold core causing biased injection and extrusion.
5. Sink marks are caused by mold sticking.
6. Broken needles or mold inserts cause excessive thickness and subsequent shrinkage.
7. The small size of the mold gate or hot runner causes uneven filling.
8. Blockage of foreign objects in the hot runner causing uneven filling.
9. Hot runner plate cracking and leaking.
10. Improper inlet position not entering from the thickest part causing insufficient pressure and subsequent sink marks.
11. Unreasonable product layout causing insufficient injection pressure reaching distant areas resulting in shrinkage. (For reasonable product layouts, please refer to injection mold flow design.)
12. Improper cooling water channel design with insufficient channels leads to excessive mold temperature during production.
13. Improper selection of material shrinkage values during mold design.

4. Machine Factors Contributing to Product Sink Marks

1. Small machines cause insufficient pressure.
2. Wear of small parts (e.g., barrel components) or rupture of sealing/molding rings may cause backflow, contributing to plastic shrinkage.
3. Oil leakage in the injection cylinder causes unstable and insufficient pressure.
4. Damage to oil pump blades causing insufficient pressure.
5. Abnormal barrel temperature with a section not heating, causing substandard melt quality and subsequent sink marks.
6. Poor cooling effect of the barrel cooling water causing backflow.
7. Incompatibility between machine screw type and plastic, such as using a standard screw for PC materials.
8. Unstable output voltage causing weak injection action.

Injection Molding Sink Mark Defects

1. Prevention is Key for Sink Mark Issues

Don’t wait until sink mark defects actually occur to hastily solve them. While this may resolve the problem, it could also mean modifying plastic part designs, modifying mold gates, runners, etc., which not only increases product costs but also extends product development cycles.

Of course, there’s a greater possibility that due to structural and mold structural limitations or excessive costs, modifications cannot be made, and the issue can only be addressed through adjustments to the machine, which often cannot fundamentally solve the problem. The end result is compromise from all parties, accepting the status quo of shrinkage.

2. Attitude Towards Sink Mark: Different Approaches for Different Plastic Parts

Generally, a product includes A, B, C, and D surfaces.

• A-surface: The front surface of the product frequently seen by users.
• B-surface: The side surface of the product.
• C-surface: The bottom surface of the product.
• D-surface: Non-external surface of the product, such as internal surfaces and structural components.

Clearly, different areas of plastic parts have different requirements for sink marks injection molding. Therefore, in design, one cannot take a uniform approach, especially when there’s a conflict between appearance and strength. For example, to enhance the strength of a plastic part, the thickness of reinforcement ribs may be slightly increased, or fillets may be added to the base of reinforcement ribs. However, this inevitably leads to sink marks. Therefore, for critical appearance surfaces like the A-surface, the thickness of reinforcement ribs should be reduced, or fillets may even be omitted entirely. For internal structural components, strength is paramount, so the thickness of reinforcement ribs may need to be slightly increased, while adding fillets to the base.

3. Comprehensive Consideration

Preventing sink mark issues requires comprehensive consideration. Addressing the issue from a single direction cannot fundamentally prevent sink mark problems.

4. Starting from Plastic Part Design is the Best Solution

Although sink mark problems may be resolved through later mold design and adjustment of injection molding parameters, it may not always be the case. One thing is certain: it can easily increase the cost of plastic parts, which should be avoided.

For example, in the case of the light guide plate shown in the figure, sink marks occurred during injection molding due to improper plastic part design, with locally excessive wall thickness. To address the sink mark problem, the mold was modified to increase the gate and runner diameter, which somewhat helped with sink mark but not significantly. This, however, resulted in an even larger runner, increasing the molding cycle even further. The light guide plate weighs 0.6 grams, while the runner weighs as much as 22.0 grams, with a cooling time of approximately 45 seconds. Production based on such data would result in extremely high material and processing costs.

5. Team Collaboration

Preventing sink mark defects relies on sincere cooperation among product design engineers, mold engineers, injection molding engineers, plastic material suppliers, and other departments as well as suppliers.

During plastic part design, product design engineers should first seek information about the characteristics of plastic materials from plastic material suppliers, including shrinkage characteristics. Generally, plastic material suppliers have rich knowledge and skills to address various defects.

During specific plastic part design, product design engineers need to follow specific rules and guidelines for detailed plastic part design, such as adhering to guidelines that reinforcement rib thickness should not be too thick. Whenever there are any doubts or uncertainties about the design, product design engineers should seek advice from mold engineers, injection molding engineers, plastic material suppliers, etc., in a timely manner. And during design reviews, invite all parties to participate.

Once plastic parts are designed in 3D software, product design engineers should accurately define the appearance requirements of plastic parts in 2D drawings, informing mold engineers of specific sink mark requirements. This way, mold engineers can consider the design of the mold structure based on this information, including the size, position, and number of gates, the size of runners, and the structure of cooling water channels.

During mold trials, all parties mentioned above should jointly address potential sink mark issues on-site.

6. Use of Mold Flow Analysis Software

In predicting and addressing sink mark defects, mold flow analysis software is a very useful tool.

Based on existing plastic parts and mold designs, using mold flow analysis software (such as Moldflow) can predict the magnitude of the sink mark. If sink mark exceeds appearance requirements, optimization of plastic part design (e.g., hollowing out where the wall is thick) and mold design (e.g., adjusting the position and size of gates) can be requested.

In Summary

The probability of sink marks occurring in injection molding is equivalent to flash. Preventing and addressing injection molding sink mark issues requires a complete understanding of material shrinkage characteristics. While material shrinkage rate data can be obtained online, the actual shrinkage rate for the same material used in different parts of products may vary.

天美影院 will compile a comprehensive table of “Actual Shrinkage Rates of Different Plastic Products” based on years of injection molding production experience. Stay tuned for this information.