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The Most Comprehensive Types Of Injection Molding Defects

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Injection molding defects are issues that every injection molding practitioner and mold worker will encounter. These injection molding defects are numerous and arise from various causes, possibly involving mold design, injection molding operations, raw material issues, and other factors.

We cannot assume that an injection molding factory with defects is not a good company, because defects are almost inevitable in all injection molding production. The truly good injection molding companies are those that can respond quickly to defects and solve problems effectively.

This article has compiled all types of injection molding defects encountered by FirstMold in over ten years of production experience. It provides a brief explanation of the defect types. If you are an injection molding researcher or mold researcher, you can delve deeper into specific injection molding defect types on the dedicated pages.

If you are a product designer or someone who outsources product production, you should also be aware of the various injection molding defects discussed in this article, as this knowledge will greatly assist you in product acceptance and in setting production outsourcing standards.

What Are Injection Molding Defects

During the molding process of plastic products, due to varying processing equipment, differing molding performances, and a wide variety of raw materials, coupled with the operating conditions of the equipment, the cavity structure of the molds, and the rheological properties of the materials, the intrinsic and external quality of the parts often exhibit various defects.

Many injection molding defects can be resolved by reliably adjusting operating conditions.

FirstMold has summarized the following injection molding defects:

Short shots, sink marks, weld lines, flow lines, poor gloss, voids, black spots, flash, warping, poor demolding, cloudiness, splay marks, burn mark, jetting, silver streaks, burrs, surface delamination, bubbles, discoloration, voids, etc.

The main causes of these defects focus on several factors: mold temperature, runner temperature, injection speed, injection pressure, holding pressure, position, holding time, switchover point, clamping force, cooling time, barrel temperature, material baking temperature, plasticizing speed, mold opening speed, backpressure, etc.

Types Of Injection Molding Defects

1. Short Shots

Short shots refer to incomplete filling at the end of the material flow or partial underfilling in multi-cavity molds, particularly in thin-walled areas or at the end of the flow path. It is a very common injection molding defect.


  1. Excessive flow resistance, causing the melt to stop flowing.
  2. Factors affecting the length of melt flow include: part wall thickness, mold temperature, injection pressure, melt temperature, and material composition.

2. Flash

Flash in injection molded parts refers to the irregular excess material that appears at the edges of the molded product. It usually occurs along the parting lines of the injection mold, and sometimes at locations such as ejector pins and inserts. Injection molding flash is the most frequent injection molding defect that can occur in the injection molding industry.


  1. Insufficient clamping force.
  2. The melt temperature is too high.
  3. Bad Mold design.
  4. Improper control of process conditions.

3. Warpage

Warpage refers to the distortion of the surface of a plastic part that does not form according to the designed shape, typically caused by uneven shrinkage of the molded part.


  1. The barrel temperature is too low; the nozzle temperature is too low.
  2. Melt temperature is too low or/and injection pressure is too high.
  3. Inappropriate holding pressure or holding time.
  4. Improper residence time; improper cycle time.
  5. Significant temperature difference between the core and cavity; the mold temperature is too low.
  6. Excessive difference in thickness between different parts of the mold cavity.
  7. Improper number or location of gates.
  8. Gates, runners, or/and gate sizes are too small or/and too long.
  9. Uneven ejection.

4. Black Specks (Dark Spots)

Black Specks are significant contributors to the injection molding defect rate during normal production, mainly because they affect the product’s appearance, leading to scrapping. Most dark spots are foreign substances, unrelated to the raw materials. However, a minority of dark spots and impurities originate from the raw materials themselves.


Before MoldingAfter Molding
1. Impurities in raw materials during processing lead to black spots.1. Poor mold material, parting surfaces, or forming surfaces shedding iron powder, causing black spots.
2. Impurities from granulation cause black spots.2. Rough ejector pins prone to burning and shedding iron powder, causing black spots.
3. Contamination with color masterbatch or speckled crushed material and scraps.3. Grinding of slide blocks producing iron filings, causing black spots.
4. Mixing of low melting point materials with high melting point particles.4. Leaks inside slide blocks causing rust or other stains; rust and stains ejected by slide block movement can land on the product and form black spots.
5. Impurities introduced during packaging, transportation, and storage.
6. Contamination of raw materials during the feeding process.
7. Carbonization of raw materials.
8. Additives degrade or decompose, causing discoloration.

5. Bubbles

Bubbles: Bubbles in plastic products are often caused by interference from gases during the molding process. This can result in the appearance of splay marks or tiny bubbles on the surface of the product, or the formation of bubbles within the thicker sections of the product. The sources of these gases are primarily due to the presence of moisture, volatile substances, or excessive lubricants in the raw materials, or from degradation gases produced when the plastic is heated for too long at high temperatures. 


  • High moisture content in raw materials.
  • Air trapped in raw materials.
  • Polymer degradation.
  • Contamination of the material.
  • Excessive barrel temperature.
  • Poor melt plasticization.
  • Insufficient injection volume.

6. Burn Marks

Burn marks refer to damage caused to molten plastic due to excessive temperature or prolonged dwell time, which decomposes and releases gases, resulting in distinct brown or silver marks. Causes:

  1. Inability to promptly evacuate air from the mold cavity.
  2. Material degradation: a. Excessively high melt temperature. b. Excessively high screw speed. c. Improper design of the runner system.

7. Glass Fiber Rich Surface (Glass Emergence On The Surface):

To enhance the strength and temperature resistance of products, glass fibers are used to reinforce plastics. While glass fibers contribute to improved performance, their inherent differences from the plastic matrix lead to compatibility issues. Glass fiber emergence, is a direct manifestation of these compatibility issues.


  1. Glass fibers have much poorer flowability compared to plastic. As plastic flows in the mold, it moves from the middle layers forward, rolling outward, so the materials with the best flowability reach the front, while those with poor flowability remain on the mold surface.
  2. Glass fibers promote crystallization, and materials like PP and PA are crystalline. They crystallize and cool quickly; if cooling is rapid, it becomes difficult for the resin to fully cover and contain the glass fibers, leading to their emergence.

8. Discoloration/Off Color:

Discoloration refers to variations in the color of a product’s surface due to multiple factors, including differences in material properties and processing conditions. Such color variations can make products particularly unacceptable if clarity is critical. Factors affecting color include the base color of the raw resin, colorants (masterbatches or pigments), compatibility of colorants with the resin, injection molding processes, the injection molding machine, and the mold.


  1. The greater the difference in base colors of the raw resin, the more significant the product color variation.
  2. The base color, thermal stability, dispersibility, and covering capability of the colorants.
  3. Compatibility of the colorants with the raw resin.
  4. Changes in injection temperature can directly affect the material temperature, thereby causing changes in product color.
  5. Poor mold venting can cause the plastic to be adiabatically compressed and react violently with oxygen.
  6. The size of the injection molding machine’s nozzle can affect the product’s color by influencing the material shear rate.
  7. Other factors like storage time, impurities in the raw materials (moisture, scraps, etc.), and additives.

9. Brittleness

Brittleness in molded parts refers to certain areas of a part becoming prone to cracking or breaking. It is primarily due to material degradation, which causes the molecular chains in polymers to break, reducing their molecular weight and thus deteriorating the overall physical properties of the polymer.


  1. Inappropriate drying conditions.
  2. Incorrect injection molding temperature settings.
  3. Poorly designed gate and runner systems.
  4. Inappropriate screw design.
  5. Low weld line strength.
  6. Excessive use of recycled materials.

10. Silver Streaks

Silver streaks are fine, elongated, and groove-like injection molding defects that appear perpendicular to the direction of the main stress on polymers. These defects include surface bubbles and internal voids, and they can cause inconsistencies in the surface finish of injection molded parts, resulting in a silver-like appearance.


  1. High moisture content in the raw materials.
  2. Air trapped in the raw materials.
  3. Polymer degradation due to: a) Material contamination. b) Excessive barrel temperature. c) Insufficient injection volume.

11. Jetting (Jet Lines):

Jetting occurs when molten material flows at high speed through narrow areas such as the nozzle, runner, or gate and then suddenly enters a more open and relatively wider area. The melt bends and snakes along the flow direction, and upon contacting the mold surface, it cools rapidly. If this portion of the material does not merge well with the resin that subsequently enters the cavity, jet lines can form on the product.


The main cause is the excessively high injection speed of the resin starting at the gate. If the plastic enters the gate without encountering any obstacles and travels a long distance before contacting the main mold area, it cools rapidly, resulting in jetting.

12. Weld Lines

Weld lines refer to surface injection molding defects that occur where two flow fronts meet and weld together, also known as knit lines or meld lines. In most cases, weld lines are the locations on an injection molded part where optical properties and mechanical strength are weakest. Notches or discoloration may appear along these lines.


  1. Insufficient temperature and pressure at the junction of the melt flows make it difficult to fill the edges and corners of the flow fronts. On smooth surfaces, notches along the weld lines can be clearly seen, while on textured surfaces, there may be a gloss difference at the edges of the lines.
  2. Because the joining of the melt flows is not homogeneous, this can lead to the formation of weak points.
  3. If plastics containing additives (such as colorants) are used, the additives can align neatly near the weld lines due to the flow direction, causing more noticeable color deviations near the weld lines.

13. Sink Marks

Sink marks are localized depressions on the surface of a product, typically occurring where there are variations in wall thickness, such as at ribs, bosses, or internal grids. If shrinkage is not properly accommodated, it can lead to voids.


  1. Material Issues:
    • Excessive material shrinkage.
  2. Mold Issues:
    • Improper product design with excessive or uneven wall thickness.
    • Gates are too small or runners too narrow or shallow, causing premature cooling of the melt during filling.
    • Uneven mold cooling.
  3. Molding Process:
    • Insufficient injection pressure and inadequate compensation for shrinkage.
    • Too fast injection speed, insufficient injection and holding times, and gate not solidified by the end of the holding pressure.
    • The melt temperature is too high, which can lead to sink marks in areas with thicker walls or ribs.

14. Flow Marks

Flow marks refer to wavy surface defects near the gate, characterized by resin flow traces that form concentric circles centered around the gate direction, imprinting on the surface of the molded product.


  1. The melt temperature is too low.
  2. The mold temperature is too low.
  3. The injection speed is too low.
  4. Injection pressure is too low.
  5. Runner and gate sizes are too small.

15. Ejector Pin Marks (Ejected Mark)

Ejector pin marks generally refer to the visible signs on the surface of the finished product caused by the ejector pins, such as whitening or protrusions, as well as dark marks or shadows (without any protrusions or indentations) that appear at the position directly opposite the ejector pins.


  1. Unreasonable product design, including the design of the product shape.
  2. Unreasonable mold design, including the gating system, gate design, ejector system, mold cooling system, and mold venting system.
  3. Unreasonable machine parameters, including injection parameters, holding pressure parameters, mold temperature parameters, material temperature parameters, ejection parameters, and clamping force parameters.
  4. Unreasonable influence of materials, including the processing parameters of the materials.

Wrapping Up

The injection molding defects listed above are common in injection molding production. In actual production applications, many other injection molding defects may occur. We will analyze and address these common injection molding defects one by one in the future, so please stay tuned.

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