Injection Molding Burn Marks: Causes and How To Avoid Them

The injection molding process can be complicated. If you rush your project or do not pay close attention to detail, you may end up with defects. These defects can harm the quality and appearance of your final product. One of the most common defects in plastic injection molding is burn marks.

The article will discuss injection molding burn marks, how to detect them, what causes them, and how to avoid them. 

What Are Injection Molding Burn Marks?

Injection molding burn marks occur when air, resin, or gas traps and overheats during molding. This overheating creates visible marks on the surface of the finished plastic product. Typically, these marks appear as dark streaks or discolorations.
They can indicate different problems in the injection molding process. These issues may include too much heat, poor venting, or high injection speeds.

In injection molding, molten plastic flows into a mold cavity. If the cavity does not have enough venting for trapped air to escape, the compressed air will heat up.

This happens because of the incoming molten plastic. The heated, compressed air then scorches the plastic material, resulting in burn marks. Sometimes, the raw material may overheat due to incorrect temperature settings or prolonged time in the barrel, leading to molding defects.

Manufacturers can adjust process settings to reduce burn marks in injection molding. They can lower the resin melt flow and mold temperatures. They can also optimize the injection rate and use the right nozzle size. Improving venting mechanisms can help as well.

Proper mold design and well-placed vents can also prevent burn marks on plastic components.  Periodic maintenance produces high-quality, defect-free parts.

How To Detect Burn Marks?

The best method for detecting burn marks will depend on their cause and location. Some burn marks are apparent, but others may require a keen eye. 

Visual Inspection

Visual inspection remains the most straightforward method for detecting burn marks. After the parts are ejected, operators and quality control personnel examine the plastic parts immediately. They look for any dark streaks, patches, or discolorations on the part's surface. Remember that burn marks can sometimes be subtle and require close examination to catch.

Tactile Inspection

Sometimes, burn marks may alter the texture of the plastic part's surface. Operators running their fingers over the surface can often feel areas where the plastic may have burned, which can then be verified more precisely.

UV Light Inspection

Some burn marks might not be easily visible under regular lighting conditions but can become evident under ultraviolet light. Placing the part under UV light can make these marks stand out for easier identification.

Microscopic Analysis

Basic inspections won't always catch internal or very subtle burn marks. To find these, you may need to use a high-powered microscope. Microscopic analysis lets you closely examine the plastic's surface. This helps identify small or early burn marks.

Common Causes of Injection Molding Burn Marks

Many reasons exist for your finished projects to have injection molding burn marks. From rushed jobs to inexperienced injection molders, quality can break down at any point in the process. Some of the most common causes of burn marks from injection molding machines include the following. 

High Injection Speed

When molten material flows into the mold too quickly, trapped air and gases cannot escape. This creates a high-pressure environment. The compressed air in this small space heats up dramatically due to the incoming molten plastic. Such rapid flow may even cause turbulence, further hindering the proper escape of trapped air or gases.

This heated, compressed air scorches the plastic material, which is already sensitive to temperature variations, resulting in burn marks. A high injection speed raises shear stress on the material. This can lead to its degradation and make it more prone to burns.

Excessive Mold and Material Temperatures

Maintaining the correct temperature is essential in injection molding. If the mold or the plastic material becomes too hot, the resin can break down chemically.

A hotter mold can make the material less thick, causing it to flow too quickly into the channels. This can also worsen problems linked to high injection speed.

This rapid flow can accentuate the problem of a gas trap not having enough time to escape, thereby increasing the risk of burn marks. High temperatures can be harmful when working with temperature-sensitive materials like PVC. PVC can break down quickly and is very prone to burns.

Resin Degradation

Resins are sensitive to multiple factors, including heat, shear forces, and certain chemicals.

High barrel temperatures or leaving the resin in the barrel too long can cause it to break down. Likewise, too much shear force from the machine parts, like a worn screw and barrel, can damage the resin. Once the resin degrades, it loses its original thermal and flow properties. A degraded resin doesn't process the same way as an intact one, and this altered state makes the material much more prone to developing burn marks.

Avoid Manufacturing Mistakes & Prevent Errors With the Right Partner

The best way to avoid defects and errors in your injection molding project is to work with a skilled company. Choose a dedicated plastic injection molding company like Crescent Industries. Our engineers use advanced techniques and simulation software to ensure your finished products are of the highest quality.