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.

1. Analysis and elimination of gas mark in plastic products to help
you make accurate judgments
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.
 Principle of gas mark
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

2. 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.
 01 Impact of injection molding equipment
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.
 02 Effect of poor mold exhaust
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.
 03 Production process impact
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

3. 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.
 04 Influence of gas generated by plastic melt itself
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:
(1) Influence of water vapor
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.
(2) Effect of decomposition gas
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.
 05 Environmental impact of injection molding equipment
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.
 06 Poor melt flow
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.
 Typical case analysis

4. 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.
 01 Reason why meter cover appears gas mark
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.
 02 Feasible improvement
(1) Appropriately increase temperature of barrel to make gas escape easily, but temperature of

5. 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.
 Conclusion
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.