Common Injection Molding Defects

Injection molding produces high-quality plastic parts. However, like any manufacturing process, it is susceptible to various defects that can compromise the quality of the final product. Understanding these defects and how to avoid them is crucial. This article highlights common injection molding defects and provides solutions to prevent them.

Sink Marks

Sink marks are depressions or dents on the part’s surface where the material has shrunk. These defects occur when the inner material of the part cools and contracts more than the outer material, creating a visible indentation on the surface. This issue is often seen in thicker sections of a part where the material takes longer to cool.

Causes: Inadequate cooling, excessive wall thickness, insufficient packing pressure.

Solutions

• Ensure uniform wall thickness throughout the part to promote even cooling.
• Incorporate ribs or gussets to maintain strength without increasing wall thickness, helping to reduce cooling time and prevent sink marks.
• Avoid thick sections that cool slower than thinner sections to ensure uniform shrinkage and avoid surface depressions.

Warping

Warping is the distortion or bending of the part after ejection from the mold. This defect occurs when different areas of the part cool at different rates, leading to internal stresses that cause the part to deform. Warping is particularly problematic in parts with large flat areas or uneven wall thicknesses.

Causes: Uneven cooling, residual stress, differential shrinkage.

Solutions

• Design for uniform wall thickness to ensure even cooling and minimize internal stresses.
• Use symmetrical designs to balance the cooling rates and reduce the likelihood of warping.
• Incorporate features that provide structural support to maintain shape and prevent deformation.

Flash

Flash is excess material that extends beyond the intended geometry of the part, forming a thin layer at the parting line of the mold. This occurs when molten material escapes into the space between mold halves, often due to excessive injection pressure or worn mold components.

Causes: Excessive injection pressure, improper clamping force, worn mold.

Solutions

• Ensure proper fit and alignment of mold parts in the design to prevent gaps where material can escape.
• Design appropriate parting lines to minimize gaps and reduce the potential for flash formation.
• Include sufficient draft angles to facilitate clean ejection and prevent material from leaking out of the mold.

Short Shots

Short shots are incomplete filling of the mold cavity, resulting in missing sections of the part. This defect occurs when insufficient material fills the entire mold, leading to incomplete parts. Short shots can be caused by inadequate material supply, low injection pressure, or poor venting.

Causes: Insufficient material, inadequate injection pressure, poor venting.

Solutions

• Design appropriate venting to allow air to escape during filling, ensuring the material can fully occupy the mold cavity.
• Avoid overly complex geometries that may hinder proper flow and lead to short shots.
• Ensure balanced flow paths to promote even mold filling and prevent areas from being starved of material.

Burn Marks

Burn marks are discoloration or dark spots on the part, typically caused by trapped air or excessive heating. These defects occur when the trapped air is compressed and ignited by the high temperatures of the molten material, leading to oxidation and burning.

Causes: Poor venting, high melt temperature, fast injection speed.

Solutions

• Incorporate adequate venting to prevent air entrapment and allow gases to escape from the mold cavity.
• Design flow paths that minimize sharp turns and abrupt changes in direction, reducing the likelihood of air being trapped.
• Avoid designing thin sections that can heat up quickly and cause burns by ensuring uniform thickness.

Jetting

Jetting results in wavy or distorted flow lines on the part surface. This defect occurs when the molten material is injected at high speed into the mold, causing it to snake and create an uneven surface. Jetting is often a result of improper gate location or high injection speed.

Causes: High injection speed, low mold temperature, incorrect gate location.

Solution

• Place gates in locations that promote smooth, laminar flow to prevent the material from jetting and creating flow lines.
• Avoid long, thin sections that can cause jetting by designing more uniform cross-sections.
• Use appropriate gate sizes to control flow rate and pressure, ensuring a smooth filling process.

Weld Lines

Weld lines are visible lines on the part where two flow fronts meet and solidify. These defects occur when the molten material flows around obstacles in the mold and meets on the other side, creating a weak point in the part. Weld lines can compromise the structural integrity and appearance of the part.

Causes: Low melt temperature, slow injection speed, improper gate design.

Solutions

• Position gates to minimize the number and length of weld lines, ensuring that flow fronts meet in less critical areas.
• Design parts with rounded edges and gradual transitions to promote smooth flow and reduce the likelihood of weld lines forming.
• Ribbing or other features strengthen areas prone to weld lines and improve overall part integrity.

Voids

Voids are hollow areas within the part, usually caused by trapped air or shrinkage. These defects occur when insufficient packing pressure fills the mold or when the material cools and contracts, creating gaps inside the part. Voids can weaken the part and affect its functionality.

Causes: Inadequate packing pressure, improper cooling, poor material choice.

Solutions

• Design parts to avoid thick sections where voids are likely to form, ensuring more uniform cooling and shrinkage.
• Include features that promote uniform cooling throughout the part, reducing the likelihood of void formation.
• Ensure proper venting to allow trapped air to escape during filling and packing, preventing voids from forming inside the part.
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