Gas mark is a quality defect that is easy to produce and difficult to eliminate. Gas mark not only affects appearance of product, but also affects strength of product. Relevant domestic technical data does not involve much of mechanism of generating gas mark. When operator encounters specific problems, he cannot quickly find cause of problem. He do not know what process measures to take, problem cannot be solved in time, resulting in a large amount of waste of resources which will cause huge losses to enterprise.
Aiming at defects of gas mark in plastic products, this paper analyzes and studies factors, elaborates mechanism and phenomenon of gas mark. According to characteristics of different gas mark, corresponding countermeasures are formulated. In production process, based on observed phenomena to make an accurate judgment, you can infer cause of gas mark, and then develop a corresponding elimination measures to solve.

 

Gas mark is a defect that appears in flow direction of melt plastic on the surface of product due to interference of gas during filling process. Composition of gas includes water vapor, decomposition gas, and air. Root cause of gas mark is that there is gas. If high temperature molten plastic fluid does not smoothly discharge gas before filling mold, gas will be mixed with plastic melt. After cooling, internal gas forms pores, and external gas generates flow marks, that is, gas mark.
In fact, presence of gas during injection molding manufacturing process is unavoidable and a significant portion remains inside plastic. When in-mold pressure is large enough and gas content does not exceed a certain limit, gas dissolves into plastic in a dispersed state; but when in-mold pressure is not large enough and gas content exceeds a certain limit, gas is released from molten plastic and reaches surface of product to form a gas line.
There are six main reasons for formation of gas mark defects:
1. Injection molding equipment does not match products made by injection moulding;
2. Mold venting groove design is too little or too small;
3. Impact of production process;
4. Plastic melt itself continuously produces gas;
5. Environment humidity of injection molding equipment is large;
6. Plastic melt flow is poor.

Choice of equipment should be reasonable. If sum of weight of product and gate agglomerate is less than 20% of maximum injection amount, remaining raw materials will be decomposed to generate gas due to shear force generated by rotation of screw and heating of barrel. If it is greater than 80% of maximum injection amount, melt may be unevenly heated, and it is difficult to form a precision product. If screw design is reasonable and matches type of plastic being processed, injection molding manufacturing process will be relatively wide; otherwise injection molding manufacturing process may be narrow or even impossible to form.
Different types and sizes of injection molding machines have different nozzle holes, such as 2.5MM, 3.5MM, 5MM, etc., the larger machine, the larger nozzle hole. Nozzle hole is too small, material flows or draws at nozzle, barrel or nozzle has obstacles or burrs, high-speed material passes through friction heat to cause material to decompose to generate gas, and gas may not be discharged to generate a gas mark phenomenon.

Under high temperature and high pressure, low-viscosity molten plastic suddenly passes through narrow gate, and molten material is injected into mold cavity for a short time. It enters cavity with a large free space, forming a high-pressure injection state, entraining air flowing in flow channel and cavity. Filling process of raw material in mold is a process in which gas is discharged. Mold parting surface lacks necessary venting holes or venting holes, clogging, poor position, and no machining gaps such as inserts and thimbles. Air in flow channel and cavity cannot be removed while plastic enters, and it is easily trapped in high-temperature melt. As melt flows, gas escapes from surface of melt to form a flow trace.

If injection molding pressure is too high, injection speed in injection molding is too fast, and gas in mold is too late to be discharged, it will be rapidly compressed, and suddenly a large amount of mechanical energy will accumulate. Gas released from exhaust groove will expand rapidly, and mechanical energy will be converted into heat and released quickly, which can easily cause surface of exhaust groove parts to burn and generate focal spots. In addition, material friction heat is too high, which may cause melt to degrade or change color due to high temperature. At the same time, too fast injection melt flow rate will cause air in cavity to be heated which will cause a yellowing of melt confluence due to degradation.
However, if injection speed in injection molding is too slow, time for plastic products increases, melting time is affected by temperature drop, time required to fill cavity is lengthened, and resistance to flow is increased, which may cause cold joint cracks in molded product, and temperature of raw material will drop drastically. As temperature decreases, viscosity of raw material is increased, and fluidity is deteriorated. On the one hand, gas cannot be separated from molten raw material in time to generate flow mark, on the other hand, rapid cooling of melt front temperature forms a high-viscosity flow body, which hinders forward flow of gas, and is subjected to a large resistance when passing through a narrow region of cavity (such as a thin-walled part), a high-temperature hot gas is easily formed, resulting in over-burning of surface of part.

If injection molding equipment is suitable for production of a certain product, plastic mold is designed reasonably and exhaust gas is smooth. However plastic itself continuously generates gas, and plastic melt still generates gas lines during injection molding and cooling injection molding manufacturing process. There are two main types of gas produced by itself:

Source of water vapor is mainly moisture absorbed by raw material when it is wet. During melting process, raw material is heated, and water is continuously evaporated and escaped. After being injected into mold, water vapor on the surface of plastic melt condenses rapidly and forms a vapor pattern on the surface of part. If mold is poorly ventilated, hot water vapor will cause surface of part to burn.

In some poor quality plastics, due to high temperature or high shear (excessive screw speed, excessive back pressure), some volatile phases escape directly from melt; especially when it contains trace impurities or traces of moisture, catalytic cracking reactions may occur. Cause of decomposition gas of plastic: melt temperature is too high, causing decomposition; cylinder temperature is too high, or heating coil of cylinder is out of control. Small products use large machines, or amount of cushions is too large, or dead ends and dead corners in the barrel are decomposed due to long-term heat, or molding cycle is too long, so that residence time of melt in barrel is too long to cause decomposition. Shear of melt in barrel is too large, compression ratio of screw is too large, groove is too shallow, screw speed is too large, and back pressure is too large.

When injection molding machine is in a humid environment, humidity of air is too high, water droplets that are condensed into water when encountering a cold mold (such as mold temperature is lower than dew point) cause a trace amount of moisture in the cavity of mold. This moisture is vaporized by melt to form water vapor, and gas will not be discharged when it is discharged.

Many plastics have property of poor melt flow, and there are two types of effects on gas mark. One is that gas generated by plastic in melt phase cannot escape in time and is discharged from back of cylinder, causing it to be finally brought into mold; the other is gas trapped in plastic melt in the mold does not escape in time.

 

Instrument cover plastic parts

 

Gas-marked instrument cover

This instrument cover is produced in an 800T injection molding machine, product number 53A-05171, gross weight 801g, net weight 792g, material grade ABS/VK125, molding cycle 65s.

Main reasons for formation of gas lines are:
(1) Melt temperature or mold temperature is too low;
(2) Injection speed in injection molding and pressure are too high;
(3) Back pressure is low;
(4) Mold is poorly exhausted;
(5) Insufficient drying of raw materials or decomposition of superheat;
(6) Mold casting system is defective.

(1) Appropriately increase temperature of barrel to make gas escape easily, but temperature of PC/ABS plastic should not exceed 270℃ to prevent decomposition of high temperature and affect performance of PC/ABS plastic.
(2) Properly reduce injection molding pressure and injection speed in injection molding so that gas is discharged for a sufficient period of time. It should be noted that injection molding pressure and injection speed in injection molding are interrelated, and method of changing both at the same time cannot be adopted because adjustment is not sure whether cause of air mark is excessive injection molding pressure or excessive injection speed in injection molding. Correct way is to first select one of parameters to adjust, see effect, and then decide next step.
(3) Increasing back pressure and increasing melt speed, air generated in raw material and gas generated in volatile phase of raw material can be sufficiently discharged from rear portion of cylinder.
(4) Improve exhaust of mold. It ensures that all kinds of gases generated when materials are filled into mold during injection molding manufacturing process are smoothly discharged from mold.
(5) Raw materials should be fully dry. It is recommended to be 70℃ ~ 80℃, baking time is not less than 4h, and moisture-proof measures (especially in rainy days) are taken to prevent influence of water vapor.

 

Gas mark on plastic products will reduce strength of product and affect appearance, so gas mark is an important reason for failure of plastic products. Morphological structure of gas-marked area material is direct cause of performance of product. To eliminate or reduce influence of gas mark on performance of product and improve performance of product, morphological structure of gas mark formed by different materials should be fully considered. Only by correctly understanding relationship between morphological structure of material in gas mark area and properties of product can molding material be reasonably selected in industrial production. This is of great significance for improving quality of plastic products and extending service life of plastic products.