BURN MARKS

 

Burn marks can be defined as small dark brown or black discolorations on the surface of a molded part, usually found at the end of fill or in pockets where gas cannot be vented.

Machine

Excessive Injection Speed or Pressure

Analysis:: Injection speed and pressure determine how fast resin flows into a mold. If either is too high, the resin travels too fast trapping air and gases and not allowng time for venting. Many of these gases are pushed to the edge of the flow front and become compressed to the point that they create shear, burning the flowfront at fill's end, or within certain stages of the injection stage. The burned areas appear as char marks on the molded part, or shorted areas called "gas trap".

Solution: Reducing injection speed or pressure will allow more time for gases or trapped air to escape through normal vent paths.

Excessive Back Pressure

Analysis:: While most materials will benefit from back pressure, there is a limit to the amount needed for a specific material and product. Back pressure is applied to mix materials better, or increasing density and improving flow orientations However, this may introduce air into the melt, which may be too much for venting systems to handle under some conditions. Excess air may be compressed at the vents and auto-ignite, causing burn marks. There is also the possibility that shear from too high a back pressure setting will cause material degradation in the barrel.

Approach:: Use minimum back pressure. All materials benefit from approximately 50-psi back pressure, but some require up to 300 psi. The material Safety Data Sheet (MSDS) is the best source of information regarding proper back pressure windows for material specifics.

Fast Recovery Times

Analysis:: The turning of the screw brings fresh material into the barrel and imparts a certain amount of shear heat to materials through rotational shear friction. The faster a screw turns, the more that friction and shear heat are created. If speeds are set too high, friction will cause materials to overheat and become thermally degraded. This causes small particles of charred material (contamination) to form which are pushed into the melt stream during injection.

Solutions:: Adjust the screw rotation speed. An average speed should be approximately 100 rpm. But, specific materials require specific rotation speeds. Consult the material supplier for the proper speed for a specific resin.

Excessive Barrel Temperatures

Analysis:: When barrel temperatures are set too high, the resin will overheat and undergo thermal degradation. This degraded material will break loose, enter the melt stream, and become molded into the finished part.

Solution: Establish proper barrel temperatures and profile. The material supplier will provide accurate barrel temperature requirements. The profile should have the barrel temperatures increase progressively from rear to front.

MOLD

Improper Sprue Bushing-To-Nozzle Sizing

Analysis:: If the sprue bushing diameter does not match the nozzle opening (or vice-versa) molecular shearing will occur at their junction and some of the material flowing through that area will degrade. The degraded material will enter the melt stream and be molded into the finished part.

Solution: Using bluing dye or thick paper, press the nozzle against the sprue bushing, and check the impression of the openings of each. They should be close to the same and not be off center. Replace the nozzle tip or the sprue bushing if they do not match. Re-center the heating cylinder to the mold if they are off center.

Improper Venting

Explanation: Air is trapped in a closed mold and incoming molten plastic will compress this air until it auto-ignites. This burns the surrounding plastic and results in charred material in the form of burn marks.

Solution: Vent the mold by grinding thin (0.0005''-0.002'') pathways on the shutoff area of the cavity blocks. Vents should take up approximately 30% of the perimeter of the molded part. Vent the runner, too. Any air that is trapped in the runner will be pushed into the part. Blind pockets can be vented using flush core pins or fake ejector pins and grinding a flat down the entire length of the pins.

Undersized Gates

Analysis:: Gates are used to determine the flow of the material into the cavity. They are intended to cause restrictions and impart shearing heat to the plastic, as well as speed controls as the plastic enters the cavity. If gating is too small, restrictions are too great and materials overheat causing degradation. The degraded material appears as burned resin in the finished part.

Solution: Size the gate according to the material supplier's recommendations. The gate should be as thin as possible to minimize cycle time, but as thick as necessary to reduce the tendency to degrade material. Gates should be installed in removable inserts so they can easily be altered or replaced.

MATERIAL

Excessive Use of Regrind

Analysis:: Regrind melts at a lower temperature than virgin, and a regrind/virgin blend must be heated high enough to melt the virgin, which may degrade the regrind. For this reason, regrind use should be minimized if mixed with virgin material. However, regrind by itself can be used successfully utilized by lowering the melt temperature.

Solution: Use 100% regrind, or, if mixing with virgin, limit the amount of regrind to 15% by weight. It may be necessary to use no regrind at all, especially in some medical and electronic products.

OPERATOR

Inconsistent Process Cycle

Explanation: It is possible that the machine operator is the cause of delayed or inconsistent cycles. This will result in excessive residence time and erratic heating of the material in the injection barrel. If such a condition exists, materials may degrade, resulting in locally burned resin.

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Troubleshooting method