Selection of Injection Mold Parting Lines and Defect Prevention

I. Overview of Injection Mold Parting Lines

1.1 Definition and Function of Parting Lines

The parting line (PL) of an injection mold refers to the surface where the mold separates during the opening and closing process—specifically, the contact surface between the cavity (A plate) and the core (B plate). It holds a central position in the injection molding process, serving as the critical interface for part ejection, venting, and sealing.

From an ejection perspective, the parting line determines the direction and method of demolding, ensuring the product can be removed smoothly from the mold without damage caused by difficult ejection. Regarding venting, molten plastic injected into the mold cavity must displace internal air and gases; a rational parting line design ensures these gases escape smoothly, preventing defects like bubbles and burn marks. For sealing, high-pressure melt is injected into the cavity; the sealing performance of the parting line is vital. Poor sealing leads to melt leakage (flash), compromising product quality.

1.2 Importance of Parting Line Selection

The selection of the parting line significantly impacts injection mold design and production efficiency; it is often the decisive factor in the success or failure of a mold design.

For smooth ejection, an unreasonable parting line may cause difficulties in demolding or leave visible parting marks on the product, affecting aesthetics and functionality. In terms of cavity venting, improper design prevents gas from escaping, leading to internal bubbles, surface burn marks, short shots, and severely reduced yield rates. Regarding high-pressure melt sealing, unsuitable parting lines result in poor sealing performance. Under high pressure, the melt easily escapes to form flash (spew), increasing post-processing workload and potentially causing dimensional deviations or even scrap. Thus, the rationality of parting line selection directly determines whether fatal defects like flash and sink marks can be avoided, ensuring product quality and production efficiency.

II. Core Principles of Injection Mold Parting Lines

2.1 Root Cause of Flash Generation Due to Improper Parting Line Selection

Improper selection of the parting line readily causes flash (spew/burrs). If the parting line passes through thin walls, corners, or thick-walled sections of the product, high-pressure molten plastic encounters less resistance and can force open the mold seam, directly leaking material to form flash. When the parting line involves large surface drops or narrow sealing surfaces, the clamping force may fail to keep the mold tightly closed, leading to misalignment during cycling and subsequent burrs. Furthermore, if the parting line obstructs venting locations, trapped gas cannot escape. Under the pressure of the melt, this gas exerts additional pressure on the mold seam, indirectly exacerbating flash formation and severely affecting quality and appearance.

2.2 Root Cause of Sink Marks Due to Improper Parting Line Selection

An unreasonable parting line position severely restricts the uniform layout of cooling channels, causing inconsistent cooling speeds between thick and thin wall sections. Thick sections cool slowly; if internal melt shrinkage lacks timely compensation, the surface collapses, forming sink marks. If the parting line design obstructs the packing (holding) phase—causing conflicts between the gate location and the parting line—the melt cannot replenish the product effectively during packing, leading to surface sink marks. Additionally, incorrect splitting of undercut parting lines can trap air locally. This trapped gas creates internal voids that manifest as hidden sink marks, drastically reducing product quality and performance.

III. Four Common Types of Parting Lines + Application Scenarios

3.1 Flat Parting Line

Flat parting lines are the simplest in structure and lowest in machining cost, making them widely used in injection molds. They are particularly suitable for flat panel products, such as flat electronic housings and thin-walled articles without undercuts. For these products, a flat parting line provides a stable molding environment, ensuring dimensional accuracy and surface quality. However, they carry risks; in thick-walled areas, high melt pressure can cause flash if the design is inadequate. Therefore, when using flat parting lines, the wall thickness must be carefully considered, and sealing structures optimized to minimize flash.

3.2 Stepped Parting Line

Stepped parting lines feature a stair-like design that cleverly bypasses thick-walled sections of the product. They are ideal for housing products with height variations, such as appliance casings with concave-convex structures. Their advantage lies in increasing the sealing area via the stepped structure, better containing high-pressure melt within the cavity and significantly reducing flash. Moreover, stepped lines optimize mold venting, allowing smoother gas escape and further enhancing quality. However, machining stepped parting lines is more complex and demands higher manufacturing precision.

3.3 Contoured (Conformal) Parting Line

Contoured parting lines follow the external profile of the product and are suitable for curved or irregularly shaped plastic parts. Products like mobile phone or laptop housings often have complex curves; contoured parting lines fit the shape closely, ensuring aesthetic appearance and dimensional accuracy. However, they present challenges: high machining difficulty requiring precision equipment, resulting in higher costs. Additionally, sealing is harder to control; improper design or machining can lead to poor sealing and flash. Thus, careful consideration of structure and material properties is essential during design.

3.4 Angled Parting Line

Angled parting lines are used in conjunction with mechanisms like angled lifters and slides, suitable for parts with snap-fits. For electronic accessories or household items requiring snap-fit connections, angled parting lines are common. The key is the angle design; unreasonable angles inevitably lead to flash and ejector marks (ejector whitening). Excessive or insufficient angles can cause uneven forces during ejection, resulting in defects. Precise calculation of the angle is necessary to ensure smooth ejection while maintaining sealing integrity.

IV. Methods for Selecting Parting Lines to Avoid Flash

4.1 General Selection Principles

• Avoid Appearance Surfaces: A fundamental rule is to keep the parting line away from visible surfaces, ensuring any potential flash does not affect aesthetics.

• Stay Away from Thick Sections: The parting line should be distant from the thickest sections to allow cooling channels full coverage, balancing cooling rates and reducing sink marks.

• Ensure Sufficient Seal Width: Standard molds require at least 8mm of sealing width; high-pressure products need 12–15mm. Adequate width enhances sealing, resisting melt pressure and preventing leakage.

• Prioritize Venting: Reserve space for vents along the parting line to alleviate trapped gas, reducing both sink marks and flash.

4.2 One-to-One Selection Schemes for Different Products

V. Methods for Selecting Parting Lines to Avoid Sink Marks

5.1 Formation Mechanism of Sink Marks

During cooling, plastic melt undergoes volumetric shrinkage. When cooling rates are uneven, sink marks occur. Sharp corners dissipate heat quickly and harden first. Thick sections near the center cool slowest. As the rigid outer layer solidifies, the still-shrinking interior pulls the weaker surface inward, creating a depression. Material shrinkage rates, part geometry, and mold temperature all influence this process.

5.2 Parting Line and Gating System Design

Rational design improves sink mark issues. The parting line position should facilitate uniform cooling channel layouts. For thick sections, offset parting can bring water lines closer to the heat concentration. The gating system (gate location, size, shape) must ensure smooth, balanced filling to minimize pressure loss, allowing effective packing compensation. For large parts, multi-point gating can shorten flow distances and balance cooling.

5.3 Parting Line Venting Design

Effective venting via the parting line reduces sink marks. Trapped gas creates reverse pressure, causing internal voids that appear as sink marks. Properly designed vents provide escape routes for gas. Vent dimensions and locations must be calculated based on part geometry and process parameters to ensure venting without causing flash. Good venting results in denser parts with fewer voids and surface depressions.

VI. Case Studies

6.1 Case Study 1: Avoiding Flash

In automotive interior trim production (high appearance requirements, complex structure):

• Strategy: The parting line was placed on non-visible inner sides and bottom surfaces.

• Execution: Thick sections were bypassed to allow uniform cooling. Sealing width was increased to 15mm to resist melt pressure. Vents were added to relieve trapped gas.

• Result: Flash was effectively eliminated, ensuring high quality and aesthetics.

6.2 Case Study 2: Avoiding Sink Marks

In mobile phone housing production (thin-wall, high precision):

• Strategy: The parting line was positioned to allow uniform cooling.

• Execution: Offset parting was used near thick bosses to bring cooling lines closer. Gate design ensured balanced filling and effective packing pressure transfer.

• Result: Surface sink marks were prevented, maintaining dimensional accuracy and appearance.

VII. Q&A: Temporary Remedies for Existing Molds

7.1 Flash Remedies

• Lapping/Polishing: Grind the parting surface to reduce gaps.

• Increase Clamping Force: Boost machine tonnage to keep molds tightly closed.

• Optimize Injection Speed: Reduce speed to lower peak melt pressure.

• Add Auxiliary Vents: Cut additional vents to release trapped gas pressure.

7.2 Sink Mark Remedies

• Adjust Packing Parameters: Increase holding pressure and time.

• Optimize Cooling Time: Extend cooling to ensure thorough solidification.

• Local Rib Thickening: Modify design (if possible) to add material in sinking areas.

7.3 Important Note

These remedies treat symptoms, not the root cause. For new products, priority must be given to optimizing the parting line design at the initial stage to fundamentally prevent defects.