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AIM Processing Small Plastic Parts Blog

3 Common Plastic Part Defects and How to Avoid Them

[fa icon="calendar"] Jul 20, 2016 1:11:41 PM / by Jon Gelston


When you watch a plastic injection molding production run and see mass quantities of perfect plastic parts being produced, it’s easy to believe that the process is almost effortless. The truth is, however,  there are many, many factors that go into producing that perfect part, and if any of these factors are off by even a fraction, defective parts can result.

Often the person responsible for the design is focused on the dimensional and functional characteristics of the part.  Cosmetic defects can be hard to imagine until the first parts are produced and frustration levels rise.  Some of these defects are easy to correct post-launch, but others pose bigger problems that can derail momentum.

Three common plastic part defects are burn marks, flow lines, and flash. What are they, why do they occur, and how can they be prevented? Keep reading to find out.


Plastic Defect # 1: Burn Marks

Burn marks are discolorations – typically brown or reddish in color – on the surface of a part.  It can also be known as “dieseling”.  There are a number of possible root causes of burn marks, but they all result from air trapped in the tool as plastic rushes in.  When this air is compressed by the pressure of the injected plastic, it becomes superheated and can briefly scorch the plastic in a small spot.  This scorching can turn black parts into a reddish-brown or white parts to a deep brown.

When burn marks are detected, there are typically two areas that should be considered for correction.  First, reduce the injection speed to improve the ability of the tool’s vents to expel the trapped air.  However, relying only on this correction could introduce new defects.  Second, consider whether the size and location of the current vents are adequate for the tool and the material.


Plastic Defect # 2: Flow Lines

Flow lines are lines or streaks visible on the surface of a part. They are typically the same color as the part but are a different tone. Flow lines result from uneven cooling of the plastic within the mold. This can be caused by a poor mold design, or plastic temperatures, mold temperatures, injection speeds, or injection pressures that are too low.

To prevent flow lines, the temperature of the molten plastic can be increased, the temperature of the mold raised, or injection speed and pressure increased. The fundamental fix for flow lines is seldom in the process.  The real key is making sure the mold design is thoroughly reviewed to ensure that material has a smooth and orderly fill.  Changing this after you are in production can be expensive, so take some extra time to consider how the part will fill.


Plastic Defect # 3: Flash

Flash is plastic that escapes from the mold, cools, and remains attached to the part. It is often found near the parting line or the ejector pins. Inadequate clamping force or the use of worn molds can lead to the development of flash. Excessive injection pressure can also cause or aggravate the problem.

To prevent flash, first ensure that molds being used are in good shape. If molds are sound, increase the clamping pressure. If the mold is sound, consider whether the tool is in the right tonnage machine or that tonnage is set properly.  Placing the tool in a machine with too small tonnage could “blow the mold open” when plastic is injected.  Placing the tool in a machine with too large a tonnage could deflect the platens as the tool doesn’t receive the tonnage the machine is capable of.

The Advantage of Experience

We’ve worked with hundreds of customers over the years to ensure they get the flawless parts they need. As you consider your next plastic injection molding project, we’d love to provide insight on the many factors that must be properly managed to produce defect-free parts. Give us a call or use our contact form to reach out to us. We look forward to working with you.  

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Topics: Plastic Injection Molding Defects

Jon Gelston

Written by Jon Gelston