Mold and Structure – Product Sink Marks and Structural Design

 In product development, sink marks are a common and critical issue closely related to structural design. Understanding the phenomenon of sink marks and their relationship with structural design is essential to ensuring product quality and performance.

What Are Sink Marks?

Simply put, sink marks refer to surface depressions or indentations that form on plastic parts during the cooling and solidification phase due to volumetric shrinkage. In injection-molded products, these typically appear as pit-like depressions, usually occurring in thicker sections where volumetric shrinkage is more pronounced. As the molded part cools, it undergoes volumetric contraction, generating inward-pulling forces toward the center of thick sections.

What Problems Do Sink Marks Cause?

Sink marks not only affect the product’s appearance—causing uneven surfaces and reduced gloss—but also diminish its aesthetic appeal and perceived quality. Moreover, they can negatively impact dimensional accuracy and functionality. For high-precision components, sink-induced dimensional deviations may compromise assembly tolerances with mating parts, thereby affecting overall product performance and reliability.

Why Do Sink Marks Occur?

(1) Material Properties

Different plastic materials exhibit varying shrinkage rates. Generally, semi-crystalline plastics (e.g., polyethylene, polypropylene) have higher shrinkage rates because their molecular chains pack more tightly during crystallization, leading to significant volume reduction. In contrast, amorphous plastics (e.g., polystyrene, PVC) have lower shrinkage rates. Additionally, the type and content of additives (e.g., fillers) influence shrinkage—fillers can reduce shrinkage but may also alter other material properties.

(2) Molding Process Parameters

• Melt Temperature: Excessively high melt temperatures reduce melt viscosity and improve flow, but also increase thermal contraction during cooling, promoting sink marks. Conversely, too low a temperature impairs flow, potentially causing incomplete filling and indirectly contributing to sink issues.
• Packing Pressure and Time: Packing pressure compensates for volumetric shrinkage by continuously feeding molten plastic into the cavity during cooling. Insufficient packing pressure or duration fails to offset shrinkage, resulting in sink marks. However, excessive pressure or prolonged packing can induce high internal stresses, compromising part performance.
  
  
  
  
  
• Cooling Time: Inadequate cooling time means the part hasn’t fully solidified before ejection; residual heat allows continued post-mold shrinkage, causing sink marks. Overly long cooling times reduce production efficiency.

(3) Structural Design

• Non-uniform Wall Thickness: Sections with significant thickness variations cool at different rates. Thick areas cool slower and shrink longer than thin areas, creating surface depressions (sink marks) over thick regions. For example, a locally thickened section surrounded by thinner walls is prone to sink marks.
• Poor Rib Design: Ribs enhance strength and rigidity without increasing wall thickness. However, if ribs are too thick, too tall, or unevenly distributed, they create large cooling differentials between the rib and the main body, leading to sink marks near the rib base.
• Abrupt Transitions in Wall Thickness: Sharp changes (rather than smooth transitions) between thick and thin sections cause stress concentration, disrupt melt flow, and impair uniform cooling—increasing sink risk.
  
  
  
  
  

How to Prevent and Mitigate Sink Marks Through Structural Design?

(1) Wall Thickness Design

• Maintain Uniform Wall Thickness: Strive for consistent wall thickness throughout the part. This requires deep understanding of functional requirements and clever structural layout to avoid localized thick or thin zones. Internal support structures can help distribute thickness evenly while meeting strength needs.
• Optimize Nominal Wall Thickness: Balance functionality, material behavior, and processability. Thinner walls reduce material usage, cost, and cooling time—minimizing sink risk—yet must remain thick enough to ensure structural integrity and mold fillability.
• Use Gradual Transitions: Where thickness changes are unavoidable, implement gradual transitions. A transition zone length of 3–5 times the thickness difference helps molten plastic adapt smoothly, reducing stress concentration.
• Apply Generous Radii: Use rounded corners at thickness transitions. A fillet radius ≥ 0.5× the nominal wall thickness effectively relieves stress and improves flow/cooling behavior.
  
  
  
  
  

(2) Rib Design Optimization

• Control Rib Dimensions: Rib thickness should be 0.5–0.7× the nominal wall thickness. Rib height should be minimized while meeting functional requirements (typically < 3× wall thickness). To prevent sink, the ratio (C−A)/A ≤ 0.04, where:
  
  
  
  
  
  • A = nominal wall thickness
  • B = rib thickness
  • C = diameter of the "three-point circle" at the rib base
    In practice, rib thickness is often fine-tuned through iterative measurement and calculation.
• Distribute Ribs Evenly: Place ribs according to load paths and structural needs, avoiding clustering that causes local overheating and sink marks. On large flat panels, uniformly spaced ribs enhance stiffness while minimizing sink risk.

(3) Boss and "Volcano" Design

• Boss Design: To prevent sink marks at boss bases, reduce material thickness by 0.6 mm (typically 0.5–1.0 mm, adjusted per material and requirements). Apply a 75° draft angle on the outer surface and generous fillets at transitions.
  
  
  
  
  

Conclusion

Sink marks are a critical concern that structural engineers must address proactively. By understanding their root causes and implementing thoughtful design strategies—such as uniform wall thickness, optimized rib geometry, and smooth transitions—engineers can significantly reduce sink mark occurrence and enhance product quality.

Moreover, close collaboration with mold designers and process engineers is vital to holistically address material selection, molding parameters, and manufacturability. In many organizations, dedicated process engineers are unavailable, and mold makers are external partners. Therefore, structural engineers must not only communicate effectively but also continuously accumulate practical experience, refine design practices, and stay informed about industry innovations to effectively tackle sink mark challenges.

Technical Requirements for Successful Two-Shot Molding

Two-shot molding, also known as multi-material or dual-shot molding, is an advanced injection molding process that produces a single, integrated component from two different thermoplastics in a single automated cycle. This technology eliminates secondary assembly operations like bonding, welding, or mechanical fastening, resulting in superior part quality, enhanced functionality, and significant cost savings in high-volume production. However, achieving these benefits hinges on meeting a stringent set of technical requirements spanning material selection, mold design, machine capabilities, and process control.

1. Foundational Requirement: Material Compatibility

The single most critical factor for a successful two-shot application is the chemical and physical compatibility between the two polymer materials.

• Chemical Bonding: The ideal scenario is a permanent molecular bond between the two materials. This requires materials with similar molecular structures and surface energies. A common example is the combination of a rigid substrate (like PC/ABS or PC) with a flexible thermoplastic elastomer (TPE or TPU). The materials must be able to inter-diffuse at the interface during the second shot to create a strong, inseparable bond. Adhesion can often be predicted by comparing the materials' solubility parameters; closer values indicate a higher likelihood of bonding.

• Mechanical Interlock: When a chemical bond is not feasible (e.g., with materials like Polypropylene and Polyamide), the design must incorporate features for mechanical interlocking. This includes undercuts, grooves, holes, and dovetail joints on the first-shot substrate. The second-shot material flows into these features, creating a strong physical lock upon cooling and solidification. While effective, this method is generally less robust than a true chemical bond and requires more sophisticated mold design.

• Thermal Compatibility: The first-shot material must have a significantly higher melting and deflection temperature (HDT) than the second-shot material. When the molten second material is injected onto the first substrate, the substrate's surface must not remelt or deform. A rule of thumb is that the HDT of the first material should be at least 30°C higher than the melt temperature of the second material. Failure to meet this requirement can lead to distortion, warpage, or a compromised bond line.

• Shrinkage Rates: The two materials should have similar volumetric shrinkage rates. Significant differences can induce high internal stresses at the material interface, leading to warpage, curling, or delamination after ejection and during the part's service life.

2. Critical Requirement: Advanced Mold Design and Engineering

The mold is the heart of the two-shot process and its design is exponentially more complex than that of a standard injection mold.

• Mold Architecture: Two-shot molds are typically rotating or shifting molds.

  • Rotating Mold: The most common type, featuring a rotating core plate or a stack. The first cavity is injected, the mold opens, the core rotates 90°, 120°, or 180°, the mold closes again, and the second material is injected over the first shot part now positioned in the second cavity.

  • Shuttle Mold: The core plate shifts laterally to move the substrate from the first cavity to the second.

  • Core-Back Mold: The mold core retracts after the first shot to create a larger cavity for the second shot, allowing for overmolding in the same station. This is less common but useful for certain geometries.

• Cavity and Core Alignment: The alignment between the first and second cavities must be perfect. Any misalignment will result in flash (material leaking into parting lines) or inconsistent wall thickness of the second shot. This demands ultra-precise machining, often requiring hardened tool steels and the use of interlocks and leader pins for exceptional registration.

• Gate Design and Location: Gate location is crucial for both shots. The gate for the first shot must be positioned to allow for a clean, strong substrate and must not interfere with the rotation mechanism or the second-shot cavity. The gate for the second shot is often more critical, as it must ensure the molten material flows optimally over the substrate to achieve a uniform bond and a cosmetically perfect seal line. Hot runner systems are almost always used to provide precise temperature control and eliminate runner waste.

• Cooling Circuitry: Managing the thermal profile is vital. The mold must have independent and highly efficient cooling channels for both cavities. The first-shot cavity often requires more aggressive cooling to ensure the substrate is sufficiently solid before rotation. The second-shot cavity may need different temperature zones to manage the flow and bonding of the second material.

3. Essential Requirement: Specialized Molding Machinery

A standard injection molding machine is insufficient for two-shot work.

• Multiple Injection Units: The machine must be equipped with two (or more) independent injection units. These units can be arranged parallel, at an L-shape, or in a V-shape. Each unit must be capable of independent control over all process parameters: temperature, injection speed, pressure, and screw recovery.

• Clamping Force and Platen Size: The machine must have sufficient clamping force to handle the projected area of the largest cavity (typically the second shot, which might have a larger surface area). The platen must also be large enough to accommodate the complex, often larger, two-shot mold and its rotation mechanism.

• Machine Control System: The machine's PLC must be capable of running a complex, multi-stage cycle with precise synchronization between the injection units, clamp movement, and the mold's rotation or shuttle mechanism. The ability to store and recall multiple, distinct recipes for each material is essential.

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4. Paramount Requirement: Precise Process Control and Validation

The process window for two-shot molding is often narrower than for single-shot molding, demanding a higher level of process control and validation.

• Process Parameter Optimization: Each shot requires a meticulously optimized set of parameters.

  • First Shot: Parameters are set to produce a dimensionally stable, stress-free substrate. Packing pressure and cooling time are critical.

  • Second Shot: This is where the bond is formed. Key parameters include:

    • Melt and Mold Temperature: Higher temperatures generally promote better bonding but must be balanced against the thermal limits of the first shot.

    • Injection Speed: A high injection speed is often desirable to push the material into all microscopic surface pores of the substrate before it starts to cool, enhancing the bond strength.

    • Switchover to Pack/Hold Pressure: A robust packing phase is necessary to compensate for shrinkage and maintain intimate contact at the material interface, preventing delamination.

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• Automation and Robotics: Given the complexity of the cycle, automated part removal is standard. Robots can also be used to transfer the substrate from one machine to another in a "pick-and-place" overmolding cell, which is an alternative to a single multi-shot machine.

• Validation and Testing: Finished parts must undergo rigorous validation. This includes:

  • Bond Strength Testing: Peel tests, pull tests, or shear tests to quantitatively measure the adhesion strength between the two materials.

  • Dimensional Inspection: Checking for warpage and ensuring critical dimensions are held, as internal stresses can cause post-molding deformation.

  • Environmental Testing: Exposing parts to thermal cycling, humidity, and UV light to ensure long-term reliability of the bond.

How to Resolve Short Shot Problems

 

Understanding Short Shot in Injection Molding: Causes, Analysis, and Solutions

Injection molding is one of the most widely used manufacturing processes for producing plastic components with high precision and efficiency. However, like any complex manufacturing process, it is not immune to defects. Among the most common and troublesome defects is the short shot problem. A short shot occurs when the molten plastic does not completely fill the mold cavity, leaving voids, unfilled sections, or incomplete parts. This defect not only leads to rejected parts but also increases production costs, delays, and material waste. In this article, we will provide a comprehensive analysis of the short shot issue, explore its main causes, and outline practical solutions to resolve and prevent it.

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1. What Is a Short Shot?

A short shot refers to an incomplete injection-molded part where the molten plastic fails to reach and fill all areas of the mold cavity. As a result, the finished part appears incomplete, with missing features, voids, or uneven surfaces. In severe cases, the part may not even resemble the intended geometry, rendering it completely unusable.

Short shots are critical issues because they affect product functionality, aesthetics, and mechanical properties. For industries such as automotive, electronics, and medical devices—where accuracy and strength are paramount—short shots can cause significant quality control failures and customer dissatisfaction.

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2. Main Causes of Short Shot in Injection Molding

The reasons for short shots can be grouped into four broad categories: material-related issues, machine-related factors, mold design problems, and process parameter settings.

2.1 Material-Related Causes

1. Poor Flowability of Resin
   Certain plastic materials, particularly those with high viscosity, struggle to flow into thin or complex mold sections. If the material cannot maintain adequate flow before cooling, it results in incomplete filling.

2. Moisture or Contamination in Material
   Moisture content in hygroscopic resins (e.g., nylon, PET) produces steam bubbles during injection, which can obstruct flow and prevent complete cavity filling.

3. Improper Material Selection
   If the resin chosen does not match the product’s wall thickness or part design, filling difficulties may arise. For example, highly crystalline plastics shrink quickly and may solidify before the cavity is fully filled.

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2.2 Machine-Related Causes

1. Insufficient Injection Pressure
   The injection molding machine may not generate enough pressure to push molten plastic to the far ends of the mold. This often occurs when the machine is under-capacity for the part size.

2. Inadequate Injection Speed
   Low injection speeds mean the material cools before reaching thin or deep sections of the cavity, causing incomplete filling.

3. Improper Plasticizing Capacity
   If the machine’s screw or barrel cannot melt and convey enough resin for a full shot, the mold cavity will remain partially unfilled.

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2.3 Mold Design Causes

1. Poor Gate and Runner Design
   Small gates or long, narrow runners create high flow resistance, reducing the ability of molten plastic to reach the entire cavity.

2. Insufficient Venting
   Trapped air and gases act as barriers, preventing molten resin from advancing. Without proper vents, air pockets cause short shots in specific regions.

3. Complex Part Geometry
   Sharp corners, thin walls, and long flow paths increase flow resistance and cooling rates, making complete filling difficult.

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2.4 Process Parameter Causes

1. Low Melt Temperature
   If the melt temperature is too low, the resin viscosity increases, reducing its flowability into the cavity.

2. Low Mold Temperature
   A cold mold accelerates premature solidification, particularly in thin sections, leading to incomplete parts.

3. Insufficient Shot Size
   Setting the shot volume too low means that there is not enough resin to fill the cavity completely.

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3. How to Resolve Short Shot Problems

Resolving short shots requires a systematic approach. The solution depends on the root cause, so manufacturers typically analyze the problem using scientific molding techniques, material data, and mold flow simulations. Below are common corrective actions:

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3.1 Material Solutions

• Improve Flowability: Select a resin with lower viscosity or add flow-enhancing additives.

• Dry Materials Properly: For hygroscopic plastics, use dryers to eliminate moisture before molding.

• Use Compatible Grades: Choose material grades designed for thin-wall or long-flow applications.

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3.2 Machine Solutions

• Increase Injection Pressure and Speed: Ensures the molten plastic reaches the extremities of the mold cavity before cooling.

• Upgrade Machine Capacity: Use a machine with adequate injection unit size and plasticizing capacity for the part dimensions.

• Optimize Screw Design: Employ a screw with proper compression ratio to ensure consistent melt quality.

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3.3 Mold Design Solutions

• Enlarge Gates and Runners: Wider gates and balanced runner systems reduce flow resistance and allow smoother filling.

• Add or Improve Venting: Place vents at critical areas where air is trapped, ensuring smooth resin flow.

• Redesign Problematic Geometry: Increase wall thickness in challenging areas or modify corners and ribs for easier flow.

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3.4 Process Parameter Solutions

• Raise Melt Temperature: Higher melt temperatures reduce viscosity, allowing resin to fill thin sections more easily.

• Increase Mold Temperature: Warmer molds slow down solidification, promoting better filling.

• Adjust Shot Size: Ensure sufficient material is injected by setting an appropriate shot volume.

• Optimize Packing Pressure and Time: Apply adequate packing to compensate for shrinkage and ensure the cavity is fully filled.

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4. Preventive Measures and Best Practices

To avoid recurring short shot issues, manufacturers should implement preventive practices during design and production:

1. Use Mold Flow Analysis: Computer simulations predict filling patterns, pressure distribution, and air trap locations before tool manufacturing.

2. Design for Manufacturability (DFM): Collaborate between product designers and mold engineers to ensure geometry supports complete filling.

3. Material Handling Discipline: Proper drying, storage, and handling of resins prevent contamination and moisture problems.

4. Scientific Molding Approach: Establish robust process windows by conducting design of experiments (DOE) on melt temperature, pressure, speed, and cooling time.

5. Regular Maintenance: Keep gates, runners, and vents free of blockages, and ensure mold alignment is maintained.

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5. Conclusion

Short shot is one of the most common yet preventable defects in injection molding. It results from a combination of material characteristics, machine limitations, mold design flaws, or improper process parameters. By carefully analyzing the underlying causes, manufacturers can take targeted corrective actions—such as adjusting temperatures, improving venting, optimizing gate design, or selecting more suitable materials.

Ultimately, preventing short shots requires a proactive approach that blends good mold design, proper machine selection, robust process settings, and effective material handling. With these measures in place, manufacturers can minimize defects, reduce costs, and consistently deliver high-quality molded parts to meet customer demands.

Solution to mold expansion deformation in injection molding

 Poor venting and mold plate deformation have an impact on the product. We proposed an idea to control the distribution of injection pressure by changing the number and distribution of gates, and introduced a method to eliminate mold plate deformation in injection molds by pre-setting a compressed gas space through an example.

In the production of injection-molded products, it is often encountered that after prolonged use, the mold plate deforms, causing defects such as flash and burrs on the product, leading to non-conformity. Typically, such issues are addressed through major repairs or scrapping. However, for products with less stringent dimensional requirements, this solution is not cost-effective. This article proposes a simple and practical solution for a common type of mold deformation known as mold plate expansion.

I. Impact of Poor Venting and Mold Plate Deformation on the Product

Before the plastic melt fills the mold cavity, the cavity is filled with air. During the injection process, the plastic melt also generates a large amount of gas. During the filling process, these gases are entirely expelled from the mold cavity. The pathways for gas expulsion are roughly as follows:

① Gaps between mold inserts and ejector pin gaps;
② The mold parting line;
③ Specially designed vent holes and vent slots.

When the mold has poor venting, as the plastic melt continuously fills the mold cavity, the gas inside the cavity is gradually compressed. The greater the degree of compression, the stronger the resistance to the melt's advance.

During the flow process, the plastic melt experiences energy loss, and its temperature decreases, resulting in reduced fluidity. Combined with the obstruction of compressed gas, the consequences are the following two aspects: First, the melt is insufficient to break through the blockage of compressed gas, forcing it to stop advancing, causing short shots or burning of the product. Second, the melt breaks through the blockage of compressed gas, but due to excessive pressure (especially in multi-point gate molds), mold expansion occurs.

After prolonged use, molds (especially multi-point gate molds) are most likely to experience expansion at the center gate, which is directly subjected to the injection pressure of the injection machine screw. This is one of the main factors leading to product non-conformity.

II. Causes and Countermeasures for Mold Deformation Due to Expansion

1. Mold Example

This example involves an abalone tray mold with an outer diameter of 500 mm, uniformly distributed with hundreds of small holes of equal diameter, all through-holes. The product shape is shown in Figure 1, and the mold gating system is shown in Figure 2.

Due to the mold's long service life (5 years) and high production volume (300,000 pieces), expansion occurred around the center gate of the 5-point gating system under injection pressure, causing flash on the through-holes of the product. This resulted in a through-hole rate of only 70%, severely affecting the product's functionality. The non-through-hole areas were concentrated around the center gate.

2. Cause Analysis

The flow distance ratio leads to uneven pressure distribution. Since the mold uses a central 5-point gate, according to the die formula:

ΔP = jL (1)
Where:
ΔP — Pressure drop in the die
j — Die constant
L — Die length

From Equation (1), it can be seen that the pressure drop at the gate is proportional to the flow distance. From this, it can be deduced that the pressure at the center gate during molding, P_center, is greater than the pressure at other runner gates, P_branch, i.e., P_center > P_branch. Therefore, it can be concluded that excessive pressure at the center gate is the root cause of mold expansion.

The molding process of the 5-point gate mold is shown in Figure 3. The center gate fills first, then expands outward. To ensure the product is fully filled, the center part of the product must withstand excessive packing pressure.

Figure 3: Molding Process of a 5-Point Gate Mold

3. Solutions to Avoid Uneven Pressure and Resulting Issues

The simplest solution to the above problem is to block the center gate. As shown in Figures 1 and 2, after blocking the center gate, the ΔP values at the four gates become consistent, and there is no longer uneven pressure distribution. However, a new issue arises: the product is prone to forming a burn mark at the center point after molding, which is unacceptable for the product. Clearly, the problem has not been fundamentally resolved, as shown in Figure 4. Accordingly, we conducted a sample test on the modified mold and found that the molded product left a burn mark with a diameter of 3–8 mm.

Figure 4: Molding Process of a 4-Point Gate Mold

4. Pre-Set Compressed Gas Space

Based on the above experiments and analysis, we adopted the method of pre-setting a compressed gas space to resolve the issue. The specific method is shown in Figure 5.

(a) (b)

1. Aluminum Core

2. Pre-Set Compressed Gas Space

Figure 5: Pre-Set Compressed Gas Space
a — Before Modification
b — After Modification

In the original center gate cavity, using the upper half's diameter and taper as a reference, a truncated cone-shaped aluminum core with a length half of the original cavity length was made to seal the upper half of the center gate. The lower half was drilled and reamed to a straight hole with a diameter of 6 mm.

In this way, during the injection process, the gas not fully expelled from the center area is compressed into the pre-set compressed gas space under the pressure of the melt. Even some of the melt at the junction at the bottom is pressed into it, forming a tapered protrusion about 5 mm in height, with a diameter similar to the breakpoint mark left by the original center gate. This does not affect the product's appearance, as shown in Figure 5.

5. Schematic Diagram of Using a Pre-Set Compressed Gas Space

1. Compressed Gas

2. Melt Flow Direction