Industrial Career Editorial

Working in Injection Molding? Learn 80+ Common Defect Types (NG) Found in Production

A practical guide for production operators, quality control inspectors, setters, technicians, and fresh graduates who want to understand product defects in the injection molding process.

Why Many New Operators Are Surprised When They First Start Working in Injection Molding

Many people assume that working in an injection molding department simply involves loading materials, pressing machine buttons, and waiting for finished products to come out. In reality, the process is far more complex.

In the plastic manufacturing industry, product quality is a top priority. A product that looks good at first glance does not necessarily meet quality control standards. Even a very minor defect can cause a product to be rejected by a customer.

That is why injection molding operators must understand the different types of NG (Not Good) defects that commonly occur during production.

This knowledge is essential because almost every company in the molding, automotive, electronics, packaging, household products, and medical product industries uses NG terminology as part of its daily operations.

The Reality of Working in Injection Molding

Injection molding operator inspecting plastic product quality beside an injection molding machine

During job interviews for injection molding production operator positions, recruiters often ask whether applicants are familiar with plastic product defects or NG conditions.

The reason is simple. Operators who can identify defects quickly help companies reduce scrap, save materials, and maintain customer satisfaction.

Some automotive manufacturers even have very strict PPM (Parts Per Million) targets, meaning that even minor defects must not reach the customer.

Key Insight:

The faster an operator can identify NG defects, the greater their opportunities to develop into a Team Leader, Quality Inspector, Setter, Technician, or Production Supervisor.

What Does NG Mean in Injection Molding?

Examples of good and defective plastic parts for production quality inspection

NG stands for Not Good. It refers to products that do not meet the quality standards established by the company or its customers.

NG defects can be caused by:

Unstable machine parameters
Material-related problems
Mold damage
Improper operator handling
Incomplete cooling
Material contamination
Packing process issues

Common Types of NG Defects Found in Production

1. Material-Related Defects

Plastic resin pellets used in the injection molding production process

Black Spot
Burn Mark
Contamination
Material Mix
Foreign Particle
Yellowing
Discoloration
Silver Streak
Splay Mark
Moisture Mark

2. Incomplete Material Filling Defects

Short shot defect caused by incomplete filling of the mold cavity during injection molding

Short Shot
Under Fill
Incomplete Fill
Flow Hesitation
Flow Line
Jetting
Cold Slug
Cold Mark

3. Excessive Pressure-Related Defects

Example of flash along the edge of an injection molded plastic part

Flash
Burr
Over Pack
Over Flow
Excess Resin
Parting Flash

4. Cooling-Related Defects

Plastic parts showing warpage and deformation after the cooling process

Warpage
Bending
Twist
Distortion
Sink Mark
Shrinkage
Uneven Shrinkage
Cooling Mark

5. Surface Defects

Visual inspection of plastic product surface defects using an inspection light

Scratch
Dent
Crack
Hair Line
Pin Hole
Bubble
Blister
Wrinkle
Gloss Difference
Surface Damage

6. Dimensional Defects

Quality control inspector measuring plastic product dimensions using a digital caliper

Oversize
Undersize
Dimension Out
Gap NG
Height NG
Thickness NG
Length NG
Width NG

7. Gate Area Defects

Excessively long gate vestige on an injection molded plastic part

Gate Long
Gate White
Gate Crack
Gate Broken
Gate Sharp
Gate Dent
Gate Stringing
Gate Deform

8. Weld Line and Material Flow Defects

Example of a weld line on the surface of an injection molded plastic component

Weld Line
Weak Weld
Flow Mark
Resin Line
Melt Line
Knit Line

9. Mold Damage-Related Defects

Technician performing injection mold maintenance in a tooling workshop

Mold Damage
Insert Damage
Core Damage
Cavity Damage
Ejector Mark
Ejector White
Drag Mark
Tooling Mark

10. Handling-Related Defects

Operator wearing gloves while handling and visually inspecting a plastic product

Finger Mark
Oil Stain
Dirty Part
Dust Mark
Packing Damage
Broken Part
Missing Part
Mix Product

11. Assembly and Functional Defects

Functional inspection of plastic components before the assembly process

Assembly NG
Fit NG
Loose Fit
Tight Fit
Clip Broken
Hook Damage
Functional NG
Lock Failure

12. Other Common Defects

Color Mismatch
White Spot
Black Dot
Air Trap
Vacuum Void
Void
Chipping
Peeling
Layer Separation
Resin Leak
Marking NG
Printing NG
Stamping NG
Label NG
Barcode NG
Identification NG

When all the categories above are counted, the list contains more than 80 types of NG defects commonly encountered in modern injection molding production environments.

“In many injection molding companies, operators who are familiar with different types of NG defects are often trusted with important customer products sooner than operators who focus only on running the machine.”

Common Mistakes Made by New Operators

Assuming that every product coming out of the machine automatically meets quality standards.
Failing to compare products with the approved master sample.
Not understanding the customer’s visual inspection standards.
Ignoring minor defects.
Failing to report recurring NG defects.

How to Learn NG Defects Faster

If you are new to injection molding, start by learning the most frequently encountered defects, such as Short Shot, Flash, Sink Mark, Weld Line, Gate Long, Scratch, Black Spot, and Warpage.

Ask your team leader or quality control inspector for permission to examine both OK samples and NG samples. This approach is much more effective than simply memorizing defect names from work documents.

The more real products you inspect, the faster you will develop the ability to recognize manufacturing defects.

Conclusion

Working in injection molding involves much more than operating production machines. One of the key skills that distinguishes an average operator from an operator with strong career potential is the ability to recognize different types of NG defects.

Understanding product defects prepares you to handle quality control inspections, customer audits, and potential career advancement opportunities in the future.

For fresh graduates and job seekers interested in entering the plastic manufacturing industry, understanding injection molding NG terminology is valuable preparation before starting work.

Key Takeaways

NG refers to products that fail to meet established quality standards.
Short Shot, Flash, Sink Mark, Gate Long, Scratch, and Weld Line are among the most common defects.
Understanding NG defects can increase your professional value in the manufacturing industry.
Learning from actual OK and NG samples is one of the most effective ways to understand product quality.