In precision injection molding, maintaining dimensional accuracy and surface integrity is paramount. One of the most common aesthetic and structural challenges engineers face is the appearance of sink marks (surface depressions) and vacuum voids (internal holes).
From a polymer physics standpoint, the root cause of these defects is volumetric shrinkage during the cooling phase. As molten plastic transitions to a solid-state, its density increases, causing it to shrink. If this shrinkage is not properly compensated for by packing additional material into the cavity, depressions form on the outer surface, or voids develop internally.
Because the precise location of the defect points to different root causes, troubleshooting requires a targeted approach. Below is a scientific analysis and actionable solutions based on defect location.
1. Sink Marks Far from the Gate or at the Last-to-Cool Thick Sections
Root Cause Analysis
When sink marks or voids occur in heavy-walled sections or at areas furthest from the injection gate (the end of fill), it is typically caused by a drastic drop in cavity pressure.
As the plastic melt travels through the cavity, its temperature drops, causing an increase in effective viscosity. This high viscosity amplifies pressure loss along the flow path. Consequently, the packing pressure cannot effectively reach these remote or thick-walled zones, resulting in insufficient material compensation during the volumetric shrinkage phase.
Technical Solutions
- Increase Packing Pressure & Time: Elevate the holding/packing pressure to force more material into the far ends of the cavity before the gate freezes.
- Optimize Injection Speed: Increase the initial injection rate. Faster filling maintains higher melt temperatures and reduces effective viscosity, allowing pressure to transfer more efficiently to the end of the fill.
- Adjust Melt/Mold Temperature: Moderately increase the melt or mold temperature to improve flow length and pressure transmission, provided it does not excessively prolong the overall cycle time.
- Modify Product/Mold Design: If the issue persists, review the wall thickness. Implement a thickness ratio where the nominal wall transit smoothly to thick sections, or relocate/add gates closer to the thick-walled zone.
2. Sink Marks or Voids Located Near the Gate Area
Root Cause Analysis
Counterintuitively, surface depressions can also manifest directly adjacent to the gate. This phenomenon is almost exclusively driven by premature gate freezing failure or backflow.
Ideally, the holding pressure must be maintained until the gate solidifies (freezes), sealing the polymer inside the cavity. If the gate does not freeze properly, the pressurized melt inside the cavity will actually backflow into the runner system once the holding pressure drops. The primary drivers for delayed gate freezing are elevated temperatures (which lower effective viscosity) or a premature cut-off of the packing profile. Excessively high mold temperatures around the gate area also severely delay solidification.
Technical Solutions
- Extend Holding (Dwell) Time: Ensure that the packing/holding time strictly exceeds the gate freeze time. Perform a gate freeze study (weight-measurement test) to determine the exact solidification point.
- Optimize Thermal Management: Lower the mold temperature specifically near the gate area by optimizing the cooling channel layout. Lowering the overall melt temperature slightly can also accelerate freezing.
- Adjust Packing Pressure Profile: Maintain a stable holding pressure curve to counteract backflow until the gate is completely solid.
- Enlarge Gate Size: If the gate geometry is too restricted, it may cause localized shear heating, keeping the core molten for too long. Optimizing the gate cross-section can help achieve balanced solidification.
3. Crucial Engineering Note: Differentiating Voids from Gas Bubbles
In troubleshooting, vacuum voids (holes) and gas bubbles are frequently misdiagnosed because they can look identical to the naked eye. However, their root causes and solutions are completely opposite:
| Feature | Vacuum Voids (Holes) | Gas Bubbles (Blisters) |
| Internal Physics | Contains a vacuum (negative pressure). | Contains trapped gas/air (positive pressure). |
| Root Cause | Caused purely by localized volumetric shrinkage in thick sections pulling the material outward. | Caused by trapped air, volatiles, or degraded resin gases that cannot escape the cavity. |
| Diagnostic Test | The Heat Test: Gently heat the defective area with a heat gun. If the surface sinks further, it is a void. | If the surface swells or blisters upward upon heating, it is a gas bubble. |
| Primary Solution | Increase packing pressure, extend cooling, or reduce wall thickness. | Improve mold venting, dry the material thoroughly, or reduce injection speed to prevent air entrapment. |
Partner with a Precision Molding Expert
Resolving shrinkage defects requires a deep understanding of scientific molding principles. At China Mold Maker, we utilize advanced Moldflow simulation analysis during the DFM phase to accurately predict shrinkage behavior, optimize gate placement, and design high-efficiency cooling channels before steel cutting even begins.
Contact our engineering team today to optimize your part design and eliminate molding defects from your production line.
