Identifying and Addressing Common Injection Molding Defects

Injection molding is a versatile manufacturing process used across industries for creating precise plastic components. While it offers many advantages, defects can occur, impacting the quality and performance of the final product. Understanding these defects, their causes, and solutions is key to maintaining product quality and optimizing manufacturing efficiency. Injection molding defects can be costly, but with the right approach, they are preventable. By understanding the root causes and applying effective solutions, manufacturers can produce high-quality, defect-free parts. Regular maintenance, process optimization, and attention to detail are key to achieving long-term success in injection molding operations.

Ejector Marks

Ejector marks (or ejector pin marks) generally occur in plastic injection molded products, caused by mold ejector pins pressing on and damaging the parts during demolding, leading to whitening at the damaged areas. This phenomenon appears on the product as dull or shadowy marks at the ejector pin positions and the corresponding opposite sides.
This defect is not fatal or severe, but if it occurs on the external surface, it can be unacceptable to customers.

1. Product design factor

Unreasonable design, multiple rib positions, thin wall thickness.
Small draft angle.

Solutions

Aim for uniform wall thickness based on product design industry standards.
Increasing the demolding angle.

2. Material factor

Inappropriate raw material selection or lack of additives such as lubricants. Image
Changing materials or add lubricants.

3. Mold factor

Runner: Narrow runners, long sprue, or abrupt turns in runners increase flow resistance and impact molding parameter adjustments. This is a typical factor that leads to ejector pin marks.
Solutions: Adjusting runner placement or increase runner size.
Gate: Inappropriate gate size, form, location, and number. Too small gates can cause excessive flow resistance, generating orientation stress.
Solutions: Adjusting the position and size of the gate.
Ejector pin: Unreasonable ejector pin design, such as the type, arrangement, size, location, and number of ejector pins.
Solutions: Adding ejector pins or replace them with larger ones.
Cavity cooling: Uneven cooling within the mold cavity.
Solutions: Adjusting mold temperature .
Vacuum: The product is in a vacuum state within the mold cavity.
Solutions: Increasing ventilation.
The smoothness of the mold surface: Insufficient polishing of the mold core.
Solutions: Improving polishing precision.
Draft angle: Insufficient draft angle of the mold core.
Solutions: Increasing the draft angle.
Conductivity differences: Conductivity differences between the ejector pin and mold steel materials can cause ejector marks.
Solutions: Adding material at the ejector pin position by 0.05~0.2mm will reduce this effect, which will help in reducing plastic molecular chain tension by creating turbulent flow at the ejector pin; surface texturing on the ejector pin can also help.

Flash

Ejector marks, or ejector pin marks, are common in plastic injection molded products. These occur when mold ejector pins press on the parts during demolding, causing damage that appears as dull or shadowy marks, often accompanied by whitening at the affected areas. While not always a severe defect, ejector marks on visible surfaces are typically unacceptable to customers. Here’s a breakdown of the factors contributing to ejector marks and practical solutions:

1. Product Design Factors

Causes: Issues like thin wall thickness, multiple ribs, or a small draft angle can increase the likelihood of ejector marks.

Solutions:

Aiming for uniform wall thickness in line with industry standards.
Increasing the draft angle to ease demolding.

2. Material factor

Inappropriate raw material selection or lack of additives such as lubricants. Image
Changing materials or add lubricants.

2. Material Factors

Causes: Inappropriate material selection or lack of necessary additives like lubricants can lead to ejector marks.

Solution:

Opting for suitable materials or add lubricants to reduce friction during ejection.

3. Mold Design Factors

Runner Design: Narrow runners, long sprue lengths, or sharp turns in the runner system increase flow resistance, which can contribute to ejector marks.
Solution: Adjustment of runner placement or increase the runner size.
Gate Design: An improperly sized or positioned gate can create high flow resistance, causing stress and ejector marks.
Solution: Modify the gate size, position, and number to reduce flow stress.
Ejector Pin Design: Poor ejector pin design—whether in size, number, arrangement, or location—can lead to marks.
Solution: Add more ejector pins or increase their size for better ejection force distribution.
Cavity Cooling: Uneven cooling inside the mold cavity can cause parts to stick or distort during ejection.
Solution: Adjust the cooling system for more consistent mold temperature control.
Vacuum Effect: If the product is under vacuum inside the mold cavity, it can lead to ejector marks.
Solution: Improve mold ventilation to prevent vacuum formation.
Mold Surface Smoothness: Inadequate polishing of the mold surface can increase friction during demolding.
Solution: Enhance polishing precision for smoother surfaces.
Draft Angle: A small draft angle can make demolding more difficult, contributing to ejector marks.
Solution: Increase the draft angle for smoother ejection.
Material Conductivity: Differences in conductivity between the ejector pin and mold materials can cause ejector marks.
Solution: Add 0.05 to 0.2mm of material at the ejector pin position to reduce molecular tension or apply surface texturing to the ejector pin.

Short Moulding

A short moulding occurs when the injection mold doesn’t fill completely, typically leaving thin-walled areas or sections at the end of the flow path incomplete. This happens when the molten plastic solidifies before filling the entire mold cavity, resulting in missing material in the final product.

Key Causes :

Excessive Flow Resistance: The primary reason for short shots is high flow resistance, which prevents the melt from fully flowing through the mold.
Factors Contributing to Short moulding:

Part Wall Thickness: Thin-walled sections are more prone to incomplete filling.
Mold Temperature: Inconsistent or low temperatures can cause premature solidification.
Injection Pressure: Insufficient pressure fails to push the melt through the entire mold.
Melt Temperature: A low melt temperature increases resistance and solidification risks.
Material Composition: Different materials have varying flow characteristics, and improper material selection can lead to short shots.
Prevention:

Optimize Wall Thickness: Ensure part designs have sufficient thickness to allow for smooth material flow.
Increase Mold and Melt Temperatures: Maintaining proper temperatures will reduce the risk of early solidification.
Adjust Injection Pressure: Use appropriate pressure to ensure the melt reaches every part of the mold.
Choose the Right Material: Select materials that are well-suited for the specific mold and product design.

SINK MARKS

Sink marks—also referred to as shrink marks or depressions—are one of the most challenging defects to eliminate in the injection molding process. These surface imperfections create concave, uneven areas that can greatly affect the appearance of a plastic part. When sink marks are too pronounced, they can’t be concealed by surface treatments, and in some cases, glossy finishes may even amplify the defect.

In addition to aesthetic concerns, sink marks can also impact the dimensional accuracy of a part, especially in areas critical for assembly or functionality. Though these imperfections may seem minor, their effect on product quality and customer satisfaction can be significant.

What Causes Sink Marks in Injection Molding?

Sink marks result from shrinkage during the cooling and solidification of molten plastic. Uneven shrinkage, caused by inconsistent wall thickness or poor cooling, can lead to the formation of these defects. If the surface of the part is not rigid enough to withstand the tensile forces caused by internal shrinkage, the outer surface will move inward, creating a sink mark. If the surface is rigid, voids can form internally instead.

Material Factors Contributing to Product Sink Marks In Injection Molding

Inadequate Material Drying: Insufficient drying of the plastic material can cause sink marks during the molding process.

Inconsistent Particle Size: Large or uneven material particles can result in poor melt quality, contributing to sink marks.
Material Shrinkage Rate: Different plastic materials shrink at different rates. Failing to account for this during design can lead to shrinkage issues.

Solution: Ensure proper drying of the material and choose a material with a suitable shrinkage rate. In cases where large material particles are used, refine the material for better melt consistency.

Design Factors Contributing to Product Sink marks

Improper product design is a common cause of sink marks, often due to:

Uneven Wall Thickness: Products with inconsistent thickness are more prone to sink marks.
Excessively Thick Gate Design: Gates that are too thick cause material flow issues and increased shrinkage.

Bone and Rib Design: Failing to account formaterial shrinkage standards can result in excessive thickness in ribs, leading to shrinkage.

Screw Column Thickness: Screw columns that are too thick or improperly designed are vulnerable to sink marks.

Solution: Design with uniform wall thickness and optimize the gate and rib structures to avoid excess material buildup. Ensure all components are designed with standard shrinkage allowances in mind.

Mold Factors Contributing to Product Sink Marks

Several mold-related factors can cause sink marks, including:

Small Gate Size: Insufficient pressure due to small or improperly located gates is a major cause of sink marks.
Poor Mold Exhaust: Trapped air in the mold can lead to defects.
Inadequate Cooling: Uneven or insufficient cooling can cause overheating and shrinkage.
Core Misalignment: An unstable or offset mold core can cause biased injection, resulting in sink marks.
Broken Needles or Inserts: Excess thickness from broken inserts can lead to shrinkage.

Solution: Properly size the gate and improve cooling systems within the mold. Ensure exhaust systems are functioning, and align the mold core correctly. Regular maintenance can prevent needle or insert breakage.

Machine Factors Contributing to Product Sink Marks

Insufficient Pressure: Smaller machines or those with worn parts may not provide adequate injection pressure.
Barrel Issues: Ruptured sealing rings or over-molding rings can cause backflow, leading to sink marks.
Inconsistent Barrel Temperature: If sections of the barrel are not heating correctly, the melt quality will suffer.
Machine-Specific Issues: Incompatibility between the machine screw type and the plastic material, or unstable output voltage, can also result in weak injection actions.

Solution: Use properly sized machines and ensure all machine components, such as sealing rings and heating elements, are well-maintained. Monitor the machine’s performance closely and ensure the correct screw type is used for the plastic material being molded.

Preventing Sink Marks: Best Practices
Although sink mark problems may be resolved later through mold design and adjustment of injection molding parameters, it may not always be feasible. However, one thing is certain that it can easily increase the cost of plastic parts, which is not the best case scenario clients look forward to.
Design Considerations: The most effective way to prevent sink marks is during the design phase. Product engineers should:
- Obtain information about material characteristics from suppliers to design with shrinkage in mind.
- Follow industry standards for rib and wall thickness to ensure uniformity.
- Collaborate with mold engineers to determine the correct placement and size of gates, runners, and cooling channels.

Mold Flow Analysis: Mold flow simulation software can predict potential sink marks and other defects during the design phase, allowing engineers to address issues before the mold is manufactured.
Detailed Drawings: Clearly define appearance and dimensional requirements on 2D drawings, so mold engineers are aware of specific concerns related to sink marks. This ensures the design of the mold structure takes these factors into account.
Ongoing Communication: Regular consultation between design engineers, material suppliers, and mold engineers ensures that everyone is aligned in preventing and addressing potential sink mark defects.

WARPAGE

Warpage in injection molding refers to the deformation of a molded product, where its shape deviates from that of the mold cavity. This common defect affects the product’s appearance, functionality, and precision. Warpage typically occurs due to uneven shrinkage during cooling, leading to stress imbalances in the material.
Causes of Warpage in Injection Molding

The causes of warpage can be classified into four main categories: mold-related, machine-related, design-related, and material-related factors. Let’s explore these in detail:

Mold-Related Causes of Warpage: The design and performance of the mold play a significant role in warpage. Some key mold-related causes include:

Uneven Cooling: Inconsistent cooling across the mold water channels leads to varying temperatures in different sections of the mold, causing uneven shrinkage.
Unbalanced Ejection System: If the ejection system doesn’t eject the part uniformly, it can warp during release.
Inadequate Cooling of Raised Mold Cores: Cores that are not cooled properly can cause local overheating, resulting in deformation.
Rough Structural or Guide Positions: Excess roughness in the mold or product’s structure can cause the part to stick and deform during ejection.
Product Sticking to the Mold: If the product sticks to the mold cavity or core, it can warp upon release.
Inadequate Water Flow: Insufficient water flow within the mold can lead to high stress and deformation during cooling.
Thickness Variations: Large differences in material thickness within the product result in varying shrinkage rates, leading to warpage.

Machine-Related Causes of Warpage: Incorrect machine settings or adjustments can also contribute to warpage:

Uneven Mold Temperatures: Different temperatures between the mold cavity and core can result in inconsistent shrinkage.
Incorrect Holding Pressure: Excessive holding pressure can cause the product to bend backward, while insufficient pressure may result in sink marks or incomplete filling.
Too Short Cycle Time: If the part does not have sufficient time to cool and set, it may warp after removal.
Fast Ejection Speed: Ejecting the part too quickly can cause warping, especially if the material hasn’t fully set.
Improper Machine Parameters: Incorrect settings can cause the product to stick to the mold, leading to deformation during removal.
Unstable Molding Conditions: Suboptimal conditions can lead to high internal stresses and warpage.
Improper Fixture Settings: Inadequate fixtures during the setting process may cause the product to warp.
Packaging Issues: If the product is compressed improperly during packaging, it can deform.

Design-Related Causes of Warpage: The design of the part itself can contribute significantly to warpage:

Thickness Variations: Uneven wall thickness in the product can cause different shrinkage rates, resulting in warpage.
Reinforcement Rib Design: Incorrectly designed reinforcement ribs can lead to shrinkage-related warping, particularly in box-shaped products made from materials like PP and PE.
Gate Design: Poor gate positioning or style can cause uneven filling, leading to warpage due to inconsistent material flow and stress.
Product Structure: A poorly designed structure can create imbalances in shrinkage and increase the likelihood of warpage.
Cavity and Core Parting Lines: Inaccurate parting line design can lead to misalignment during molding, resulting in warpage.
Shrinkage Value Settings: Failing to account for proper shrinkage values during design can lead to excessive deformation in the final product.

Material-Related Causes of Warpage:The properties of the material used in the molding process can also impact warpage:

High Shrinkage Rates: Some materials naturally have higher shrinkage rates, which can lead to warpage. Modifying the material or switching to a lower-shrinkage alternative can help mitigate this.
Molecular Orientation Differences: Shrinkage rates can vary depending on the direction of material flow. Differences in molecular orientation during the molding process can lead to uneven shrinkage and deformation.

Avoiding warpage mainly starts with product design and mold design, as detailed below:

Through thoughtful design, advanced manufacturing techniques, and gradual changes in wall thickness, effective prevention of deformation due to uneven cooling shrinkage can be achieved, thereby enhancing the stability and reliability of the product.
Adding reinforcing ribs in thin-walled or large flat areas enhances the overall rigidity and stability of plastic parts, ensuring that the final product maintains its intended shape and functionality.
Leveraging the mechanical properties of curved structures can be a game-changer in reducing stress concentration. Curved designs distribute stress more evenly, lowering the likelihood of warpage and providing consumers with products that are both reliable and aesthetically appealing.
Proper arrangement of gate position, number, and size is crucial to ensuring uniform filling of molten plastic into the mold cavity and avoiding internal stress caused by uneven filling.
Ensuring smooth gas discharge and uniform mold cooling, leads to high-quality products and increased production benefits. Select plastic materials with low shrinkage rates, good flowability, and stable mechanical properties based on the intended use and process requirements.
Carefully adjusting key injection molding parameters, such as injection speed, pressure, holding pressure time, and cooling time, can significantly impact product quality. Employing rhetorical techniques in design can enhance expression effectiveness, leading to better communication of the product’s intended use.

Silver Streaks

Silver streaks, also known as splay marks, are a common defect in injection molding, often appearing as flower-like spray patterns near the gate or bright V-shaped lines when viewed under light. Here’s a breakdown of the causes and solutions:

Mold-Related Factors

Small Gate Size: A small gate can restrict material flow, leading to streaks.
Poor Mold Venting: Trapped gas in the mold causes splay marks.
Sharp Corners in Gate/Runner Design: These can cause flow disruptions and residual plastic build-up.
Residual Plastic in Dead Ends: Often seen in hot runners, it can cause material degradation.
Hot Runner Issues: Malfunctioning temperature sensors or blockages in hot runners result in overheating and streaks.
Water Leaks in Molds: Water contamination in the plastic can cause streaking.
Nozzle Tip Caps: Improperly capped nozzle tips can create uneven material flow.

Remedies:

Optimize gate size and design.
Ensure proper mold venting and clean parting line vents every four hours.
Inspect and maintain hot runners and water systems regularly.

Machine Setting Factors

Incorrect Back Pressure: Low back pressure causes moisture streaks, while high back pressure causes thermal degradation.
Fast Injection Speed: High speeds can trap air and moisture, leading to streaks.
Improper Cooling Time: Insufficient cooling leads to unformed material causing streaking.
Nozzle Temperature Issues: Incorrect nozzle temperature causes inconsistent flow and streaking.

Remedies:

Adjust back pressure and injection speed based on material flow needs.
Ensure correct cooling times and maintain appropriate barrel and nozzle temperatures.

Design-Related Factors

Non-Smooth Gate Design: Improper gate design leads to flow issues and streaking.
Gate/Runner Size: Too small dimensions lead to turbulent flow and streak marks.
Improper Gate Type or Location: Incorrect placement or size of the gate leads to incomplete filling and splay marks.

Remedies:

Optimize gate and runner dimensions and choose appropriate gate types (pin, fan, round, etc.).
Ensure smooth flow through the gates and balance wall thickness in design.

Material-Related Factors

Moisture Content: Moisture in the plastic before molding can cause hydrolysis, resulting in moisture splay marks.
Thermal Degradation: Overheating of the resin can release gases, causing thermal silver streaks.
Contaminants: Impurities in the material cause streaks during molding.

Remedies:

Control moisture content to under 0.03% and ensure materials are adequately dried.
Avoid overheating the resin by controlling barrel temperatures and reducing residence time in the machine.

Types of Silver Streaks

Moisture Splay Marks: These occur when plastic undergoes hydrolysis in the barrel due to moisture in the material before molding, or when adequately dried material absorbs moisture while sitting in the hopper without effective insulation.

Remedy –Ensure proper drying of materials and verify moisture levels before molding. The drying process conditions must be strictly managed.

Thermal Silver Streaks: These occur due to resin overheating during the molding process, generating gases such as carbon dioxide, which cause silver streaks on the surface of the molded parts. These are typically identifiable by their appearance, which does not follow a specific pattern and sometimes resemble a comet. Common causes include:
Excessively high barrel temperatures.
Residual plastic in dead ends in the barrel or nozzle.
Prolonged residence time in the barrel.
Reduced molecular weight of the resin due to excessive use of regrind, thereby compromising the impact strength and making the material too brittle for use.
The back pressure is too high.

REMEDY-Appropriate measures should be taken based on the cause of degradation like Lower barrel temperatures, clean dead spots, and minimize the resin’s residence time. If these measures are ineffective, consider using a smaller-capacity injection molding machine for production.

Structural silver streaks: Structural silver streaks occur due to poor design of the molded parts, such as significant variations in wall thickness or abrupt changes in section. These design flaws cause the molten plastic to expand or contract suddenly during the filling process, allowing air to mix with the molten material inside the mold cavity. While these streaks primarily affect the appearance of the part, they have minimal impact on its strength and impact resistance.

REMEDY Structural silver streaks can typically be resolved by adjusting the injection speed. For smaller sectional changes, reducing the speed allows smoother filling of the mold cavity and helps prevent air from mixing with the molten plastic, thus avoiding streaks. However, a slower injection speed may lead to short shots, requiring adjustments in other parameters like mold and nozzle temperatures. For larger sectional changes, increasing both the injection speed and pressure can help force air out from the parting surface. If these adjustments do not fix the issue, it may be necessary to revise the part’s structural design and improve the mold’s venting system.

Gate and Runner Design Splay Marks: These are caused by the irrational design of the gating system or local blockages. Their causes and solutions are as follows:

1) The sprue’s cone angle is designed to be too large: If the sprue’s cone angle is too large, it causes the material to detach from the cone wall at the beginning of injection, creating a gap. As the mold cavity fills, air gets mixed into the material flow, resulting in silver streaks. These streak marks typically follow the direction of injection.

Remedy: Start by performing a dry cycle to eliminate the possibility of resin hydrolysis and degradation. Then, check the sprue cone angle for accuracy. While adjusting the injection speed may reduce runner silver streaks, the primary solution is to modify or replace the sprue bushing to reduce the sprue cone angle.

Gate and Runner Design Splay Marks:

These are caused by the irrational design of the gating system or local blockages. Their causes and solutions are as follows:

Remedy: Start by performing a dry cycle to eliminate the possibility of resin hydrolysis and degradation. Then, check the sprue cone angle for accuracy. While adjusting the injection speed may reduce runner silver streaks, the primary solution is to modify or replace the sprue bushing to reduce the sprue cone angle.

2) Irrational gate design: If the gate’s cross-sectional area is too small, it can cause turbulent flow or jetting as the material passes through, mixing air with the molten plastic and creating silver streaks near the gate. These streaks typically appear in a radial pattern centered around the gate.

Remedy: To eliminate these silver streaks, enlarge the gate or modify its cross-sectional shape. Additionally, reducing the injection speed during this process can further help in preventing the formation of silver streaks.

Cold material at the nozzle tip can cause blockages in the gating system during injection, leading to silver streak formation. This occurs similarly to streaks caused by a gate that is too small. Signs of this issue can include cold material traces within the gating system.

- Remedy: To address this, enlarge the cold slug well in the mold and raise the nozzle temperature. This will help prevent blockages and reduce the occurrence of silver streaks.
- Pulsation silver streaks:Pulsation silver streaks occur due to uneven material feeding during the retraction of the pre-plasticizing screw, which resembles a pulsing effect. This results in air entering the barrel and being injected into the mold cavity along with the molten material, forming irregular silver streaks. These streaks can appear randomly and are often accompanied by short shots, sink marks, or internal bubbles.
- Remedy: Eliminating pulsation silver streaks requires addressing the root cause of the pulsation. This may involve checking for malfunctioning automatic control instruments or issues with the barrel heating device. Any abnormal temperature fluctuations should be corrected, and instruments or circuits should be inspected to ensure proper operation. Additionally, adjusting the temperature settings may help resolve the issue.
- Trapped air silver streaks: These occur when gases trapped during the material-filling process form visible streaks, often near weld lines. These streaks are typically localized around the weld lines, while other areas remain unaffected.
- Remedy: To eliminate trapped air silver streaks, adjustments may be needed in the mold design or processing parameters. This can include changing the gate location and type, adding effective venting slots, or modifying the structure of the plastic part—though these changes often require significant modifications to the mold. During production or trial molding, process conditions can be adjusted, such as fine-tuning injection pressure and speed or altering the temperature differential between stationary and moving molds, to address this issue effectively.

FLOW MARKS

Flow marks are broadly defined as irregular patterns that appear on the surface of a molded part, caused by inconsistencies in the flow speed of molten plastic within the mold cavity. These patterns are often observed as concentric, spiral, or cloud-like waves centered around the gate.

As illustrated in the following diagram, several types of flow marks can occur depending on the injection speed of the plastic. The main types include wave flow marks and jetting marks, which are the primary focus for addressing flow-related defects.

Causes of Wave Flow Marks

Small Gate and Runner Cross-Sectional Area: A small cross-sectional area increases filling resistance, hindering the smooth flow of the melt and leading to wave marks. Solution: Enlarge the gate and runner cross-sectional area.
Improper Design of Cold Slug Wells: Cold material entering the cavity can create wave marks. Solution: Place large cold slug traps at the ends of the main and sub-runners.
Improper Design of the Cooling System: A poorly designed cooling system can cause uneven cooling, resulting in wave marks. Solution: Re-evaluate and improve the cooling system design.
Improper Design of Gate Location and Shape: Incorrect gate placement and shape can create turbulence as the melt transitions from a narrow runner to a wider cavity. Solution: Design gates in thicker sections or directly on the side wall, ideally in a fan or film shape.
Volatile Gases Produced During Plasticization: Certain materials, like ABS, release volatile gases at high temperatures, causing cloud-like wave flow marks. Solution: Lower the processing temperature if possible.
Poor Fluidity: Resins like PC may exhibit poor fluidity, leading to flow marks. Solution: Use resins with good fluidity and ensure consistency in fluidity across batches.
Insufficient Lubricant in the Material: A lack of lubricant can reduce fluidity, resulting in flow marks. Solution: Increase the amount of lubricant additives appropriately.
Holding Pressure Time Too Short: Insufficient holding pressure time can lead to surface flow marks. Solution: Extend the holding pressure time during the injection process.
Mold Temperature Too Low: Low mold temperatures slow the melt flow, contributing to wave marks. Solution: Reduce the cooling water flow and ensure the mold heating system is functioning properly.
Improper Injection Speed:

Too Fast: Excessively fast injection can cause turbulence, leading to flow marks. Solution: Reduce the injection speed or use a slow-fast-slow graded injection technique.
Too Slow: Conversely, slow injection speeds can lead to backflow and stagnation, creating flow marks. Solution: Increase the injection speed as needed.

Identifying and Addressing Common Injection Molding Defects