Injection Molding Solutions For Automotive Parts

Solutions for Surface Dents (Sink Marks / Depression) in Injection Molded Automotive Parts

Automotive injection-molded parts have strict requirements for surface quality and dimensional stability, especially for exterior and interior A-surface components. During production, surface sink marks (shrinkage defects) frequently occur, typically concentrated at areas such as ribs, screw bosses, and locally thickened protrusions. These defects not only affect the visual appearance but, in severe cases, can lead to poor assembly fit, exaggerated paint defects, and ultimately result in product rejection. The root cause of sink marks lies in uneven shrinkage between thick and thin regions during the melt cooling phase, where continued contraction in thicker sections pulls on the surface layer, creating depressions. Factors contributing to this defect include product design, mold construction, injection molding process, and raw materials. Improvement should follow a systematic approach-first adjusting the process parameters, then modifying the mold, and finally optimizing the product structure, progressing from easier to more difficult solutions. Below is a comprehensive review of corrective measures.

 

ABS+PC Plastic Storage Box

 

I. Adjustment of Injection Molding Process

 

1. Pressure and Pressure Maintenance

Increase the holding pressure:During the holding pressure stage, material is continuously added to the cavity to counteract shrinkage. For automotive parts, it is recommended to rely mainly on holding pressure rather than solely on injection pressure.

Extend the holding pressure time：It is ineffective to apply pressure until the gate freezes and becomes completely sealed.

Segmented pressure retention：First-stage high-pressure filling, second-stage low-pressure to prevent excessive stress (to avoid white spots and warping in the exterior parts).

Misconception: Simply increasing the injection pressure. After the mold cavity is filled during the injection process, the pressure is difficult to be transmitted to the thick-walled areas, and the improvement is limited.

 

2. temperature control

Reduce the material temperature appropriately：The melt temperature should not be too high as it will increase the shrinkage rate; it should also not be too low to avoid poor welding and short shots.

die temperature

✅ Appearance part:just enoughincrease the mold temperature, the cooling of the melt surface becomes slower, which is beneficial for pressure transmission and reduces wrinkles

⚠️ If the mold temperature is too high, it will prolong the cooling period and increase the total shrinkage. It is necessary to find the balance point.

Reduce the temperature of the nozzle and the hot runner system,Reduce the post-processing shrinkage caused by the continuous thermal expansion of the melt.

 

3. Speed and changing positions

- During the filling process, reduce the injection speed in the later stage to prevent premature freezing of the thick section and prevent the pressurized melt from entering;

- Shorten the V/P switching position and switch to the pressurized state as soon as possible. Do not wait until the filling is completely completed before switching.

 

4. cooling time

Properly extend the cooling time to allow the part to fully solidify before removing it; if the removal is too early, the plastic part will be at a high temperature and soft, which makes it prone to subsequent shrinkage and depression.

 

5. backpressure

Slightly increase the back pressure, make the melt denser, reduce internal voids, slightly improve the shrinkage marks. However, it should not be too high as otherwise the temperature rise will be too large.

 

 

II. Optimization of Gate, Runner and Mold Structure

 

The gate should be positioned as close as possible to the depression BOSS For the columns and thick ribs, apply the glue nearby to ensure that the holding pressure reaches the thick-walled area directly; applying glue at a long distance will result in significant pressure loss.

 

Increase the gate diameter / Shorten the freezing time of the gate The small gate quickly solidifies, making it impossible to maintain pressure for continuous feeding; for large automotive parts, fan-shaped gates, submerged gates, and hot runner gates are commonly used.

 

Optimize exhaust system In the thick-walled area, trapped air will form depressions and bubbles; at the ends of the reinforcing strips and in the thick sections, venting grooves should be provided.

 

Optimization of cooling water systemThe thick-walled area increases the water channels, with the water channels close to the cavity, reducing the temperature difference between the thick and thin areas and avoiding excessive local overheating and contraction; the BOSS column is given priority for processingWater-blocking sheet, fountain cooling.

 

Avoid local heat accumulationThe sharp corners and thick sections of the mold core are prone to accumulate heat. Therefore, it is necessary to use beryllium copper inserts to accelerate heat dissipation.

 

III. Product Structure Design

 

The most common cause of automotive plastic parts defects: sudden change in wall thickness.

1. Uniform wall thickness, with the difference in wall thickness controlled to be less than 20%;

2. Rib thickness ≤ 0.6 to 0.7 times the main wall thickness (exceeding this point, the back of the rib is prone to depression);

3. BOSS column:

o Do not make it a solid thick column;

o Make the root of the column recessed or hollowed out (steal meat);

o Increase the radius at the root and reduce the local thickness;

4. Thick protrusions, thickened areas directly steal material from the back (empty out), eliminating thick meat at the root from the source.

Industry experience: For appearance A parts, if there are thick ribs or BOSS on the back, stealing meat alone is difficult to completely eliminate shrinkage marks when adjusting the machine.

 

Automotive front-end frame

 

IV. Adjustment of Material Direction

 

1. Under the same conditions: Crystalline materials (PP, PA, POM) shrink much more than non-crystalline materials (ABS, PC), and are more prone to indentation;

2. Adding glass fibers and mineral fillers can reduce the molding shrinkage rate and alleviate the indentation;

3. Selecting a low-shrinkage grade;

4. Excessive recycled material ratio leads to fluctuations in melt density and unstable shrink marks. Strictly control the blending ratio.

 

V. Rapid Investigation Sequence

 

Confirm whether the indentation position corresponds to the back ribs / columns / thick sections → Determine if it is a typical thick-wall indentation

 

Increase the holding pressure time and raise the holding pressure for testing (priority)

Fine-tune the mold temperature and material temperature, optimize the V/P switching point

Check if the gate distance and size are sufficient

Check if the exhaust at the thick position is poor

 

If the above methods are ineffective: Evaluate mold modification (increasing the gate size, adding insert cooling) or product meat theft

- Indentation (indentation): The surface is concave inward, without holes, on the outer side of the thick section;

- Vacuum bubble: An internal cavity in the molded part, severely when the surface collapses, mostly caused by insufficient holding pressure + excessive wall thickness. The handling approach is the same.