Gas mark in injection molding is a common and challenging defect in the injection molding process. Understanding what gas marks are, their negative effects, causes, and how to avoid them is important to achieve high-quality injection molding parts.
What Is Gas Mark in Injection Molding?
Gas mark in injection molding refers to surface defects that appear as streaks, lines, or burn-like marks on the finished plastic parts. The appearance will be different depending on the material, mold design, and process parameters. These marks form when gases such as trapped air, water vapor, or decomposition gases fail to escape the mold cavity during the injection and cooling phases. The gas marks are often found running in the direction of the plastic flow, visibly disturbing the aesthetic finish of the molded parts.
If the trapped gas remains inside the part, it leads to voids in injection molding, which are internal cavities that weaken structural strength. By contrast, when the gas is forced to the surface, it creates gas marks, appearing as burn-like streaks that mainly affect appearance.
Negative Effects of Gas Marks on Products
The presence of gas marks in injection molded parts has significant implications beyond surface appearance. The primary negative effects include:
- Appearance Defects: Gas marks degrade the visual quality of the product by creating unsightly lines or discoloration. This consequence is critical in industries such as automotive, consumer electronics, and medical devices, where aesthetic appeal and surface finish are important.
- Reduced Mechanical Strength: Gas trapped inside the molded parts can create voids or pores beneath the surface, weakening the structural integrity of the product. This can lead to cracks, reduced durability, or premature failure under stress.
- Increased Production Costs: Parts exhibiting gas marks often must be scrapped or reworked, adding material waste and labor expenses. Extended cycle times might also result from the need to adjust processing parameters or improve mold venting.
- Impact on Brand Reputation: Repeated quality issues due to gas marks can harm a manufacturer’s reputation, especially when supplying to industries with strict quality standards.
Main Causes of Gas Mark Defects in Injection Molding
Gas mark in injection molding is caused by a conspiracy of factors. Here are the main causes of gas marks.
Poor Mold Exhaust and Venting
Insufficient or poorly designed mold venting systems trap gas inside the mold cavity during injection. When vents are too small, clogged, or incorrectly positioned, they prevent the smooth escape of air and gases as molten plastic fills the cavity. This trapped gas compresses and may burn against mold surfaces, leaving visible gas marks.
Residual Moisture in Raw Materials
Moisture retained in hygroscopic plastics evaporates during melting and introduces water vapor into the mold cavity. This vapor becomes trapped gas, resulting in gas marks. Overheated resins may also decompose, releasing volatile gases that exacerbate defects.
Improper Process Parameters
High injection pressure or speeds force the melt into the mold too quickly, trapping air. Excessive shear rates increase melt temperature, leading to burns. Low back pressure fails to homogenize the melt, allowing gas bubbles to persist, causing gas mark injection molding.
Mold Design Flaws
Incorrect gate placement creates turbulent flow, trapping gases. Uneven wall thicknesses cause inconsistent cooling, forming voids that appear as gas marks in injection molding. Sharp transitions or thin sections increase defect risks.
Environmental and Equipment Factors
High humidity in the production environment increases resin moisture content, contributing to gas marks in injection moulding. Undersized injection machines lack sufficient clamp force for large molds, disrupting fill stability. Worn screws or barrels reduce melt uniformity, promoting defects.
How to Avoid Gas Mark Defects in Injection Molding？
Preventing gas mark injection molding defects requires a comprehensive approach.
Dry Raw Materials Thoroughly
Hygroscopic resins must be dried before processing. Use desiccant dryers to reduce moisture content below 0.1%. Select low-volatility additives to minimize decomposition gases.
Optimize Mold Venting
Ensure molds have sufficient venting channels, typically 0.02-0.05mm deep, along parting lines or at cavity ends. Additional vents or porous inserts improve gas escape, minimizing gas mark injection molding defects.
Adjust Process Parameters
Reduce injection speed to allow gases to vent. Increase back pressure for better melt homogenization, reducing trapped bubbles. Maintain barrel temperatures and mold temperatures at proper temperatures to balance flow and cooling, avoiding gas marks in injection molding.
Improve Mold Design
Use multiple gates or optimized gate locations to ensure laminar flow, reducing turbulence that traps gases. Design uniform wall thicknesses to prevent uneven cooling, which contributes to gas mark in injection moulding.
Control Environmental Conditions
Maintain low humidity in storage and production areas using dehumidifiers. This prevents moisture absorption in resins.
Implement Advanced Techniques
Use vacuum-assisted molding to evacuate air from the cavity before injection. Select resins with high melt flow indices for smoother filling.
Maintain Equipment
Regularly clean vents and calibrate injection machines to ensure precision. Routine maintenance prevents equipment-related issues that cause gas mark injection molding.
Zhongde Professional Solutions for Gas Mark
Zhongde delivers targeted solutions for gas mark injection molding through advanced mold design and stringent quality control. The precise mold design, finely tuned process parameters, and optimized material handling guaranteed high quality injection molding products free from gas marks.
