7/24/2019 Injection Molding Troubleshooting for Common Design Defects

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Injection Molding Troubleshooting for

common design defects 

Common Defects in Injection Molding and Plastic Injection Troubleshooting andSolutions If you have been troubled by a lots of problems and errors during injection molding and

looking for fast ways to troubleshoot them, then do not worry, you are at the right place. We

will discuss about a lot of common errors that occur in injection molding cycle.

1.) Air bubbles in Injection Molded part/Blisters on part surface 

A blister is a raised area on the plastic surface, very similar to the medical condition of the

same name. Many people have no clue as to what's the reason of air bubble in injection 

molded part.It is generally the product of too much heat on the tool or by inadequate cooling

or venting. Depending on the type of tool, you can also find areas where full coverage is not

working. For instance, if the injector has a flow pattern issue, it might not inject all the resin atonce, allowing air bubbles to enter the molten resin. A hot runner tool might also suck air into

the die because of area constriction, slowing the passage of the resin.

Good Solution to blisters: Reduce the local spots with high temperatures by proper cooling in the mold

2.) Burn Marks on the molded part and its surface  Burn marks are generally caused by problems similar to blistering. They

manifest as literal burn marks on the plastic, black

discolorations that resemble scorches. The general cause of burn marks is improper

ventilation for the resin. When this happens, the burns are usually located in the position

farthest away from the ventilation gate. Another cause might be that the resin is gettingtrapped in the injector and heating too long.

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Solution to burn marks on molded parts: 1. Improve air venting to relieve trapped air and gases.

2. Reduce the material residual time in barrel.

3. Reduce the local high temperature spots in mold by proper cooling.

3.) Burrs and Flash appearing on mold's parting line A common problem in many types of machining, burrs or flash appear on injected plastic

 products when extra pieces or scraps are attached to the finished piece. Burrs are usually a

result of dull or inaccurately cut dies or molds. Sharpening or cleaning the dies are typically

the most effective ways to remedy the situation. Mold opening during injection phase due to

excessive pressure is one more resaon.

Solutions to Burrs and Flash problems:1. Reduce the injection pressure.

2. Set clamping force to 100%.

3. Clean the parting line and check for damage or distortion on parting surface.

4.) Embedded contaminates Then there is detritus in the finished product, it means that contaminates have somehow

worked their way into the resin. A full cleaning of the machine may be necessary to find the

source.

5.) Flow marks on injection molding part Flow marks are look like discolored lines or patterns on the finished product.

When they occur, your injector might be functioning at too low a temperature.

The gate might also not be properly ventilating the mold.

Remedy to Flow marks on plastic parts surface: 1. Increase and decrease injection speed to get optimized speed.

2. Increase gate size.

3. Reduce the mold temperature.

6.) Lamination or flaky layer on molding walls 

This occurs when contaminates are introduced into the mold or resin but manifest as shinyflake layers in the wall of the part. These are a sign that purging compound was accidentally

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left in the mold.

Way to remove shiny surface or flakes from manufactured part's surface:

1. Clear the barrel and refill material.

7.) Sink marks opposite to thick walls or bosses Sink marks are small holes or depressions, similar to tiny potholes, in the surface of the piece.

There are a number of reasons they might appear. First, the material might simply be

insufficient for the job. A cause that is easier to remedy is temperature variation, such as too

short cooling time or too high melt and mold temperatures. The mold design can contribute to

these causes, so it might be necessary to start from scratch.

How to remove sink marks from part's surface: 1. Try to increase holding pressure and injection pressure.

2. Increase material temperature.

3. Decrease mold temperature.

4. Increase injection speed.

5. If none of above works, then you need to cut plastic reduce the local section thickness.

8.) Short mold or short shot on thin sections such as ribs, incomplete filling A “short mold” refers to a mold that is not completely filled, leading to deformation or

“shortened,” parts. This occurs when the resin cannot fill the mold due to blockage,

 bottlenecking or injection that is either too fast or too slow.

How to remove short shot and fill part completely: 1. Make arrnagement for proper air venting in mold. Add vent pins with sufficient air vent

allowance on diamter, just enough to let air pass through, but not the molten plastic. This isthe most accurate and suggested solution.

2. Reduce the injection speed and increase the pressure gradually. If no change is noticed till

extreme parameters are reached, then refer to 1.