Plastic injection molding companies can make strong, high-quality parts. This process is useful for creating new designs for both small and large production runs. Prototype injection molding helps speed up the design of new products and reduce costs by testing new part versions. However, if inexperienced engineers or technicians handle the process, it can lead to more defects in the plastic parts. This can result in defective or scrap products, extra tooling costs, and unhappy customers.

This article will look into the most common defects in the injection molding process.

Burn Marks

Burn marks are black or rust-colored spots or unfilled areas on plastic injection molded parts. Usually, small burn marks do not affect the strength of the plastic part. However, if these burn marks or discolorations become more noticeable and keep happening, they cause the material to degrade. Most importantly, burn marks ruin the look of products, making parts that need to look good unusable.

Causes

Burn marks are black or rust-colored spots or unfilled areas on plastic injection molded parts. Usually, small burn marks do not affect the strength of the plastic part. However, if these burn marks or discolorations become more noticeable and keep happening, they cause the material to degrade. Most importantly, burn marks ruin the look of products, making parts that need to look good unusable.

Now that you know the real cause and fix, let’s look at some main reasons for burn marks in the injection molding process. First, molding at a higher temperature than the plastic resin can handle or using plastic that hasn’t been properly dried can cause problems. High temperatures and wet material lead to more off-gassing of the plastic, making poor mold venting conditions worse. Another issue is injecting the molten plastic too quickly into the mold. While filling a mold fast is usually good for reducing molding times and getting nice finishes, a poorly designed venting system can stop a technician from properly processing.

Main Cause: Trapped air and gases in the mold cavity that cannot escape as the plastic is injected.

Contributing Conditions :

• Molding at too-high plastic temperatures
• Molding with wet material
• Injecting/Filling the mold faster than the venting design allows

Flow Lines

You can easily identify plastic flow lines on your injection molded parts because they look like rings, light streaks, wavy lines, or uneven patterns on the surface. These flow lines don’t affect the part’s strength, but they do make the part look bad. Customers notice flow lines quickly, and for parts that need to look good, flow lines are a big problem.

Causes:

Flow lines happen when the molten plastic in the mold changes speed or pressure. As the plastic flows into different parts of the mold, like pockets, ribs, corners, and holes, it can slow down or speed up. This causes the flow lines. Poorly designed gate locations or gate types, incorrect temperatures for the plastic or mold, or incorrect fill speeds can also cause flow lines.

Main Cause: Molten plastic in the mold changes speed or pressure as the mold fills.

Contributing Causes:

• Poorly designed or chosen gate type or location
• Incorrect temperatures for plastic and mold
• Too fast or too slow cavity fill speed

Warping

Warping happens when the plastic part cools after being taken out of the mold. This can make the surfaces uneven like they have been bent or twisted. You can see this problem when you measure the part, put it together, or lay it flat.

Causes

Warping usually happens because the plastic cools at different rates in different parts, or it shrinks unevenly. If the part has walls of different thicknesses, it can also cause warping. Some plastics shrink more in certain directions, adding stress that leads to warping.

Main Causes:

• Non-uniform cooling rates
• Non-uniform shrink rates

Contributing Causes:

• Different wall thicknesses
• Choice of plastic with uneven shrink values

Sink Marks

Sink marks are small, noticeable dents on the surface of plastic items. These dents are often round and appear where the plastic material drops or recedes. They are usually easy to see but sometimes require you to tilt or move the item under different lighting to spot them. If you notice a dent on a plastic part, it’s likely a sink mark.

Why Sink Marks Happen

Sink marks occur when the wall thickness at the sink is greater than the surrounding walls. After molding, the thicker plastic shrinks more, leaving a dent. These marks are not desirable on plastic parts that need a smooth appearance. Fortunately, sink marks can be predicted and avoided by designing the parts properly.

Main Cause

A thicker mass of plastic at intersecting walls shrinks more than the thinner surrounding areas.

Solution

Design intersecting wall thicknesses to be 66% of the main wall thickness.

Other Factors

• High resin molding temperature
• Poor design or blockage of the sprue’s orifice
• Rapid cooling
• Low injection molding pressure

Weld Lines

Lastly, weld lines often show up as streaks or lines on the surface of a plastic molded part. Weld lines are also called knit lines. This issue happens when two or more plastic flow fronts meet at a point when the mold cavity is filled. They are called weld lines because the two flow fronts try to “weld” or “knit” together under injection pressure. If molding conditions are not ideal, a visible line appears where the weld happened.

Causes

Molding temperatures and injection fill speeds play a role here. Usually, molding at too low of plastic or mold temperatures for a given resin can cause weld lines. Also, choosing a poor gate location to inject the plastic into the mold can cause weld lines. There are software tools available to plastic part and mold designers that can predict the location of potential weld lines. This predictive mold flow software can help designers choose the proper gate locations to avoid these defects before the mold is made.

Main Causes:

• Molding with temperatures that are too low
• Poor choice of gating location