Causes and Solutions for Common Product Defects in Injection Molding

Injection molding is one of the most prevalent manufacturing processes, known for ensuring consistency in complex parts while maintaining cost-effectiveness. However, despite its long-standing application, the process involves intricate aspects such as material selection, product design, mold design, and injection parameters, making it prone to various defects. This article delves into the primary factors influencing injection molding, including product design, molds, materials, and the injection process. It also explores the most common defects encountered in injection molding, providing an analysis of their causes and practical solutions to assist product designers and industry professionals in addressing these challenges.

Injection molding, as one of the most common manufacturing processes, not only ensures the consistency of complex parts but also remains a cost-effective option among various processing methods, making it highly favored by product designers. However, despite its decades of mature application, injection molding science involves multiple disciplines, including materials, product design, mold design, and injection parameters. Additionally, various factors such as temperature, drying, and weather resistance must be carefully considered. As a result, defects in the injection molding process are still prevalent. Without years of accumulated experience, it is often difficult to immediately identify the true cause of complex quality issues, leading to many detours. This article aims to explore the most common injection molding problems, analyze their causes, and provide solutions to assist product designers and injection molding practitioners who may need this guidance.The main injection molding process starts with product design, so defects usually involve the following aspects:

• Product Design
• Molds
• Materials
• Injection Process

Among these, product design is the soul of injection molding. Not all designs can be realized in injection molding, and some suboptimal designs may increase mold and unit costs, elevate production difficulty, and raise defect rates. A good product designer not only makes the product aesthetically pleasing but also understands how to reduce costs and manufacturing difficulty while improving product quality. Molds are like the skeleton, and their quality directly impacts whether a product can be successfully injection molded. Good mold design and molds can reduce the difficulty of injection molding, minimize defects, control precision, and enhance strength. Materials are the lifeblood of injection molding. Selecting the right materials is crucial, and the selection must consider the final use environment, including production strength, lifespan, temperature, humidity, etc. The injection process is akin to muscles; selecting the right machinery, and adjusting the temperature, holding pressure, and cooling time at each stage of injection is like the various functions of muscles, enabling the product to finally take shape with the correct functionality and appearance.Having an initial understanding of the key aspects of injection molding, let us now look at the most common issues encountered in actual injection molding production:

12 Kinds of Main Defects In Injection Molding

Flashing

What are flashings in injection molding?

Flashing occurs when molten plastic is squeezed into the gap between the two halves of a mold during injection molding, usually appearing along the edges of the product and near injection pins.

Causes of Flashing:

Flashing occurs when the pressure inside the mold is too high, or the mold does not close tightly enough, allowing the molten plastic to escape through the gap between the two halves of the mold.

• Product Design:
  1. Overly complex or uneven design on the mold parting line.
• Molds:
  1. Incomplete sealing between the two halves of the mold or an improperly designed parting line.
  2. Poor mold venting, leading to excessive internal pressure.
• Injection Process:
  1. Excessive injection pressure.
  2. Barrel temperature too high.
  3. Insufficient clamping force.
  4. Holding time too long.

How to prevent flashing in injection molding:

Ensure that the mold is fully closed and clamped during injection and check that the mold is properly vented. If flashing occurs, reduce the injection speed and temperature and shorten the holding time. If flashing cannot be eliminated, manual trimming by workers may be necessary, though this will reduce production efficiency.

Short Shots

What are short shots in injection molding?

Short shots occur when the molten plastic does not fully fill the mold cavity, resulting in an incomplete part.

Causes of Short Shots:

Short shots happen when the material flow is insufficient, due to low injection pressure, improper mold design, or inadequate material selection.

• Product Design:
  1. Thin wall sections or complex features that are difficult to fill.
• Molds:
  1. Improperly designed runners or gates that restrict material flow.
  2. Poor cooling system that causes premature solidification.
• Injection Process:
  1. Insufficient injection pressure or speed.
  2. Melt temperature too low.
  3. Incorrect shot size settings.

How to prevent short shots in injection molding:

Increase the injection pressure and speed, and ensure the melt temperature is adequate. Modify the mold design to improve flow paths, such as optimizing runner and gate design, and ensure proper cooling to prevent premature solidification.

Voids

What are voids in injection molding?

Voids are air pockets or empty spaces within the molded part that occur when the material does not completely fill the mold.

Causes of Voids:

Voids form when there is trapped air in the mold or when the material shrinks during cooling without sufficient compensation.

• Product Design:
  1. Thick sections in the design that are prone to void formation.
• Molds:
  1. Inadequate venting in the mold, leading to trapped air.
  2. Poor design that doesn't allow material to flow evenly.
• Injection Process:
  1. Low injection pressure.
  2. Insufficient holding pressure or time.
  3. Rapid cooling leading to shrinkage.

How to prevent voids in injection molding:

Improve mold venting and adjust the injection process to increase pressure and holding time. Consider using a material with lower shrinkage or redesigning the part to avoid thick sections where voids are likely to form.

Sink Marks

What are sink marks in injection molding?

Sink marks are depressions or dimples on the surface of the molded part, usually caused by uneven cooling or shrinkage.

Causes of Sink Marks:

Sink marks occur when thick sections of the part cool and shrink unevenly, leading to surface depressions.

• Product Design:
  1. Uneven wall thickness or overly thick sections in the design.
• Molds:
  1. Inadequate cooling in the mold, especially in thick areas.
  2. Poor mold design leading to uneven material flow.
• Injection Process:
  1. Insufficient holding pressure or time.
  2. High melt temperature causing excessive shrinkage.
  3. Rapid cooling causing surface solidification before the core.

How to prevent sink marks in injection molding:

Increase holding pressure and time, and ensure the cooling system in the mold is adequate for thick sections. Consider redesigning the part to eliminate thick sections, and optimize the material selection to minimize shrinkage.

Shrinkage

What is shrinkage in injection molding?

Shrinkage is the reduction in size of the molded part as it cools, which can cause dimensional inaccuracies.

Causes of Shrinkage:

Shrinkage occurs due to the natural contraction of the material as it cools and solidifies.

• Product Design:
  1. Large, thick sections in the design that are prone to shrinkage.
• Molds:
  1. Improper cooling system design leading to uneven shrinkage.
  2. Inadequate venting, causing air traps that lead to shrinkage.
• Injection Process:
  1. Inadequate cooling time.
  2. High mold temperature.
  3. Low holding pressure or time.

How to prevent shrinkage in injection molding:

Extend cooling time and reduce mold temperature to control shrinkage. Increase holding pressure and time to compensate for material contraction, and redesign the part to minimize thick sections that are prone to shrinkage.

Burn Marks

What are burn marks in injection molding?

Burn marks are discolorations or dark spots on the surface of the molded part caused by overheating or trapped gases.

Causes of Burn Marks:

Burn marks are caused by excessive heat or poor venting, which leads to the degradation of the material or burning of trapped air.

• Product Design:
  1. Sharp corners or thin walls that are prone to overheating.
• Molds:
  1. Poor mold venting, leading to gas traps.
  2. Improper gate location causing localized overheating.
• Injection Process:
  1. High injection speed or temperature.
  2. Excessive screw rotation speed causing overheating.
  3. Long residence time in the barrel.

How to prevent burn marks in injection molding:

Improve mold venting and reduce injection speed and temperature. Optimize the gate location to avoid localized overheating and reduce screw rotation speed to minimize heat generation.

Jetting

What is jetting in injection molding?

Jetting is a defect where the molten material does not flow smoothly into the mold, creating visible flow lines or "jets" on the surface of the part.

Causes of Jetting:

Jetting occurs when the melt enters the mold cavity too quickly, causing it to cool unevenly and form visible flow patterns.

• Product Design:
  1. Long, thin sections or complex geometries that are difficult to fill evenly.
• Molds:
  1. Poor gate design or location, causing high-speed entry of the melt.
  2. Inadequate runner system design.
• Injection Process:
  1. Excessive injection speed.
  2. Low melt temperature leading to premature solidification.
  3. Improper flow rate settings.

How to prevent jetting in injection molding:

Reduce injection speed and ensure the melt temperature is adequate to allow smooth flow. Optimize the gate design and location, and adjust the flow rate to ensure even filling of the mold cavity.

Warping

What is warping in injection molding?

Warping is the deformation or twisting of the molded part, usually due to uneven cooling or residual stress.

Causes of Warping:

Warping occurs when different sections of the part cool and contract at different rates, leading to internal stresses that cause the part to deform.

• Product Design:
  1. Uneven wall thickness or asymmetrical design leading to uneven cooling.
• Molds:
  1. Poorly designed cooling system leading to uneven cooling.
  2. Inadequate mold support or clamping.
• Injection Process:
  1. Inadequate cooling time.
  2. High mold temperature.
  3. Incorrect holding pressure settings.

How to prevent warping in injection molding:

Ensure even cooling throughout the mold and increase cooling time if necessary. Reduce mold temperature and optimize holding pressure to prevent internal stresses from forming. Consider redesigning the part to minimize uneven wall thickness.

Weld Lines

What are weld lines in injection molding?

Weld lines are visible lines on the surface of the part where two flow fronts of molten material meet and fail to fully bond.

Causes of Weld Lines:

Weld lines occur when two or more flow fronts converge and do not fully bond due to insufficient temperature or pressure.

• Product Design:
  1. Multiple gates or complex flow paths that cause multiple flow fronts to converge.
• Molds:
  1. Poor runner or gate design leading to weak weld lines.
  2. Inadequate mold temperature control.
• Injection Process:
  1. Low melt temperature.
  2. Insufficient injection pressure.
  3. Slow injection speed.

How to prevent weld lines in injection molding:

Increase melt temperature and injection pressure to ensure better bonding at the weld lines. Optimize gate location and runner design to minimize the number of flow fronts. Adjust injection speed to ensure even flow throughout

Discoloration

What is discoloration in injection molding?

Discoloration refers to unwanted changes in the color of the molded part, which can appear as streaks, patches, or complete color changes.

Causes of Discoloration:

Discoloration occurs due to several factors, such as material degradation, contamination, or improper processing conditions.

• Product Design:
  1. Use of multiple colors or materials that are difficult to process uniformly.
  2. Design features that cause uneven flow or cooling, leading to localized overheating.
• Molds:
  1. Residual material from previous runs left in the mold, contaminating the current batch.
  2. Poorly designed or maintained gates, causing uneven material distribution.
• Injection Process:
  1. High barrel temperature causing thermal degradation of the material.
  2. Excessive screw rotation speed or back pressure leading to material shear and color changes.
  3. Inconsistent material feed or improper mixing of colorants.

How to prevent discoloration in injection molding:

Ensure thorough cleaning of the mold and equipment between runs, particularly when changing colors or materials. Optimize processing conditions, such as reducing barrel temperature and screw rotation speed, to prevent material degradation. Ensure consistent material feed and proper mixing of colorants to maintain uniformity.

Flow Marks

What are flow marks in injection molding?

Flow marks are visible streaks, lines, or patterns on the surface of the molded part, caused by irregular flow of the molten material during injection.

Causes of Flow Marks:

Flow marks occur when the molten material does not flow smoothly into the mold cavity, often due to variations in speed, temperature, or pressure.

• Product Design:
  1. Abrupt changes in wall thickness or complex geometries that disrupt the flow of the material.
  2. Long, thin sections that are difficult to fill uniformly.
• Molds:
  1. Poor gate location or design, causing uneven flow distribution.
  2. Insufficient or improper venting, leading to air pockets that disturb material flow.
• Injection Process:
  1. Excessive or insufficient injection speed, causing flow patterns.
  2. Low melt temperature causing the material to solidify prematurely, creating flow lines.
  3. Inconsistent injection pressure leading to irregular flow.

How to prevent flow marks in injection molding:

Optimize gate design and location to ensure smooth and even flow into the mold cavity. Adjust injection speed, pressure, and melt temperature to ensure consistent flow and avoid premature solidification. Ensure proper venting in the mold to prevent air traps that could disrupt material flow.

Delamination

What is delamination in injection molding?

Delamination is a defect where the molded part shows layers or flakes on the surface, indicating poor adhesion between layers of the material.

Causes of Delamination:

Delamination occurs when there is poor bonding between layers of material, often due to contamination, incompatible materials, or improper processing.

• Product Design:
  1. Use of multiple materials or coatings that are not compatible, leading to poor interlayer adhesion.
  2. Sharp corners or complex geometries that cause stress concentrations, leading to layer separation.
• Molds:
  1. Contaminants on the mold surface preventing proper adhesion.
  2. Poor surface finish or incorrect mold temperature leading to inadequate bonding of layers.
• Injection Process:
  1. Low melt temperature causing incomplete fusion of layers.
  2. Insufficient injection pressure leading to poor material fusion.
  3. Inconsistent material feed, leading to contamination or incompatible material layers.

How to prevent delamination in injection molding:

Ensure that materials used are compatible and properly mixed, with no contamination. Clean the mold thoroughly and maintain an appropriate surface finish to promote good adhesion. Optimize melt temperature and injection pressure to ensure proper bonding between layers, and maintain consistent material feed to avoid introducing contaminants or incompatible materials.

Injection molding is a process that demands both technical expertise and hands-on production experience. The quality of the final product is influenced right from the product design stage, followed by crucial decisions in mold design and material selection. At IdealPro, we have been dedicated to injection molding for over 30 years, backed by a seasoned technical team that has encountered and resolved countless molding challenges. We are committed to supporting our clients from the initial product design all the way through to successful mass production, ensuring a product that meets and exceeds expectations. Feel free to consult with our team at any time! IdealPro - Shaping your ideas with precision and quality!