Gas Marks on Injection Molded Part: Detail Analysis and Remedies

  • 2023.02.14
  • Flying Tiger

A gas mark is a form of a manufacturing defect that can be easily formed on the products' surface but is difficult to remove. This molding defect can affect the quality of manufacturing output, especially the strength and appearance of the product. However, finding the cause and fixing it can be challenging for many businesses. With the right support at hand, such as an industrial-grade dehumidifying dryer, this error can be effectively recognized and averted.

Gas Marks Defined

A gas mark is a fault that grows in the direction that melt plastic flows on a product's surface resulting from gas interference during the filling process.
Air, water vapor, and decomposition gas are all components of gas, and the presence of gas is the primary cause of this mark. Gas will mix with plastic melt if the high-temperature molten plastic fluid does not smoothly discharge gas before entering the mold. After cooling, internal gas develops pores, and external gas produces gas marks.

Leading Causes of Gas Marks Defects in Injection Molding

Improper Choice of Injection Molding Equipment

The remainder of the raw materials will transform into gas due to the shear force produced by the rotation of the screw and heating of the barrel when the combined weight of the gate agglomerate. It can also happen when the product is less than 20% of the maximum injection amount. If this value exceeds 80% of the maximum injection amount, the melt heating process might become inconsistent, making the material melt unevenly and thus challenging to shape a particular product. Lastly, an appropriate screw design that is suitable to the type of processing polymers contributes to a smooth injection molding process. Otherwise, the manufacturing process may be too tight or even impossible to get shape.

Poor Mold Exhaust 

When the injection molding pressure is too high, the injection speed is too fast, or the gas discharge time is too late can swiftly compress the gas in the mold and suddenly build up much mechanical energy. Rapid gas expansion and the quick conversion of mechanical energy to heat can easily cause the surface of the components of the exhaust groove to burn and create focal points.

Another potential reason for gas mark defects is when there is excessive heat from material friction. This could lead to material degradation or discoloration. Additionally, an excessively fast injection melt flow rate can heat the air in the cavity, which will degrade, making the melt confluence turn yellowing.

Factors of Production Process

Injection molding items in general, take longer to produce. Because melting time is affected by temperature changes, cavity filling time acquires longer, flow resistance rises, potentially leading to cold joint cracks in the finished product, and the raw material temperature drops steeply. Moreover, when the temperature drops, raw material fluidity declines, and viscosity rises. The rapid cooling of the melt in front makes it hard for the flow to pass through the thin-walled part. Last, the gas that cannot be eliminated from molten raw material in time can leave gas mark defects.

Poor Plastic Melt Flow 

Plastic has different traits of melt flow. Some can have naturally slow flow, and some flow poorly by other factors. Poor melt flow traps the gas generated by plastic during the melt phase inside the mold. As a result, gas is eventually poured into the mold or back into the cylinder when it should not happen.

Residual Moisture in Plastic Particles 

This can refer to dampness on the surface of non-hygroscopic pellets or moisture inside hygroscopic polymers. Beware that gas is the root cause of gas mark defects, while air, water vapor, and decomposition gas can all lead to excessive gas during production. 

Key Preventive Measures to Avoid Gas Mark Defects 

As we have discussed the relationship between gas markings' morphological structure and factors that create asymmetry between molecules, the following part will discuss how to prevent this imbalance.

Eliminate Moisture in Materials before Processing

Once the material is dried sufficiently and adequately, venting or exhausting the mold and barrel is minimized. At the same time, when properly dried, air and moisture will be restricted from entering the barrel and inside the mold, preventing the occurrence of gas marks defects. To conduct effective and efficient plastic resin drying, selecting the right industrial-grade dehumidifying dryer is necessary. At Flying Tiger, we offer a range of dehumidifying dryers, including the Three-In-One Dehumidifying Dryer, which features the combined design of a dehumidifying dryer, hopper dryer, and autoloader. Follow the link to learn more. 

Reconsider Types and Sizes of Nozzles and Barrels

Besides reconsidering the types and sizes of nozzles and barrels, properly raise the temperature of the barrel to allow gas to escape freely. However, the temperature of the PC/ABS plastic should stay below 270°C; because PC/ABS plastic's decomposition and performance lowering start when the temperature exceeds the range.

Ensure Sufficient Venting on Mold Parting Surface

Air and gas produced from raw materials, with the latter from the volatile phase of the material, can be sufficiently released from the back of the cylinder when raising back pressure and melting speed. Ameliorating the mold exhaust is the next step to ensure that all gases produced during the injection molding process (when pouring materials into molds) are released from the mold smoothly. 

Reduce Injection Molding Pressure and Speed

Lower the injection molding pressure and speed precisely to ensure removing all the gas for an appropriate time set. Be noted that injection molding pressure and injection speed are complementary. Adjusting both parameters at once is not recommended since it is unclear to identify the cause of gas marks, whether the reason is from excessive injection molding pressure or excessive injection speed. The recommended troubleshooting procedure is to choose one of the parameters to alter, observe the result, and then determine what to do next.

Improve Plastic Melt Flow by Raising Mold Temperature

Keeping the raw materials completely dry can also improve the plastic melt flow. One tested solution is to bake polymers hot enough and long enough according to your manufacturer's or technician's advice. Also, remember to adjust the mold temperature so as to match the ideal range of material types.

Control Temperature Better During Filling Process

The morphological structure of the gas-marked area can directly pose a negative impact on product performance. This is why temperature control during the filling process is key to preventing gas marks: If the high-temperature molten plastic fluid does not smoothly discharge the gas before filling the mold, the gas will be mixed with the melt and then generates gas marks after cooling. A mold temperature controller will prevent this from happening by ensuring proper venting.  

Reduce Injection Molding Pressure and Speed can help prevent gas marks.

Final Thoughts: How to Resolve Gas Mark Defects?

By accurately comprehending the relationship between the morphological structure of the material in the defect area and the product properties, processors and manufacturers can choose compatible molding materials. This is crucial for enhancing the quality and life cycle of plastic items.

On the one hand, the temperature is the primary element impacting the link in the morphological structure of materials in gas-marked zones. To address most issues with air marks at their source, consider using a mold temperature controller that will speed up and stabilize the production progress while improving ventilation. 

On the other hand, consider introducing a dehumidifying dryer from our Dryer & Crystallizer Series. As discussed earlier, unwanted moisture in polymers is a potential culprit of gas marks. Proper drying and dehumidifying processes, therefore, are key to ensuring consistency in output quality. 

Gas marks have a detrimental effect on the finished surface as well as the strength and structure of molded items. Businesses can avoid common molding flaws like gas mark defects by better achieving the temperature control of mold or melt and subsequent melt fluidity.

Flying Tiger is a leading company having experience in providing industrial solutions for thousands of businesses in injection molding plastic manufacturing. Consult with Flying Tiger now to have tailored solutions that best suit your business problems.


Article Update: 2023.02.14
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