Analysis and measures to eliminate gas layers created by plastic products

Analysis and measures to eliminate gas layers created by plastic products

Abstract: Factors such as injection molding equipment, molds, material properties, process parameters and environment, as well as causes of air traces, were studied to identify air trace defects in plastic products. air. The characteristics of various air grades were analyzed, and appropriate remedial measures were taken to effectively eliminate such defects.  Keywords: traces of air, exhaust, fluidity

1. Introduction

An air mark is a quality defect that is easy to produce but difficult to fix. Air marks affect not only appearance of product, but also its strength. Relevant domestic technical data little affect mechanism of gas jets, as a result of which operators, when faced with specific problems, cannot quickly find cause of problem and do not know what technological measures to take, problem cannot be solved in time generated in resource A large amount of waste will cause huge loss to company.

Regarding defect of air marks on plastic products, this article analyzes and investigates its causes, details mechanism and phenomenon of air marks, and formulates appropriate countermeasures according to characteristics of various air marks. In manufacturing process, by making accurate judgments based on observed phenomena, cause of air marks can be identified and then appropriate measures can be formulated to eliminate them.

Second, principle of air signs

Air mark is a defect in direction of flow of molten material on surface of product, caused by gas interference during pouring of plastic. Gas components include water vapor, decomposition gas and air. It can be seen that main cause of gas spots is presence of gas. If high temperature molten plastic liquid cannot release gas smoothly before filling mold, gas will mix with plastic melt. Flow, after cooling and molding , inner gas forms pores, and outer gas produces flow traces, that is, traces of air [1].

In fact, presence of gas in injection molding process is inevitable, and a significant part of it remains inside plastic. When pressure in mold is large enough and gas content does not exceed a certain limit, gas is dissolved in plastic in a dispersed state; plastic is released one by one and reaches surface of product, forming air trails.

There are six main causes of gas marking defects: ①The injection molding equipment does not match injection molding product; ②Mold outlet groove design is too small or too small; ③Influence of production process; ④The plastic melt itself constantly generates gas; ⑤ The environment humidity of injection molding equipment is high; ⑥ Thosethe plasticity of melt is poor.

2.1, Influence of injection molding equipment

Equipment selection must be prudent. If sum of mass of product and gate fill is less than 20% of maximum injection volume, remaining raw material will decompose under shear force generated by rotation of gate. screw and barrel heating. Gas is produced. If it exceeds 80% of maximum injection volume, uneven heating of molten material will occur, making it difficult to form precision products. If design of screw is reasonable and according to type of plastic being processed, range of molding processes will be relatively wide; otherwise, range of forming processes may be narrow or even unable to form products.

Different models and sizes of injection molding machines have different nozzle openings, such as 2.5mm, 3.5mm, 5mm, etc. The larger machine, larger nozzle opening. The nozzle orifice is too small, material drips or is drawn into nozzle, barrel or nozzle is obstructed or burred, and frictional heat generated by high-speed material flow causes material to decompose and generate gas, and gas marks will appear if gas is not released in time.

2.2, impact of poor mold exhaust

Under high temperature and high pressure, low viscosity molten plastic suddenly passes through a narrow gate, and time for injecting molten raw material into mold cavity is very short, and it enters cavity with a large free space. , forming a high-pressure injection state, entraining flow channel and air present in cavity. The raw material filling process in mold is also a gas release process. The mold parting surface lacks necessary ventilation holes, or ventilation holes are insufficient, blocked, and position is not good, and there is no processing. gaps such as inserts and thimbles for ventilation. Air in runner and cavity cannot escape when plastic enters, which can easily lead to gas entrapment in high temperature melt.

2.3, production process impact

If injection pressure is too large and injection speed is too high, gas in mold will be drastically compressed before being released in a timely manner, and a large amount of mechanical energy will suddenly be accumulated, and gas released from exhaust groove will rapidly expand and convert mechanical energy into heat energy and quickly release it, it is very easy to burn surface of parts in exhaust groove, resulting in burn spots. In addition, frictional heat of material will be too high, which will easily lead to degradation or discoloration of molten material due to high temperature; This will cause a yellow color whenof molten materials due to degradation.

However, if injection speed is too low, time for injection molded products will increase, time for molten material to fill mold cavity under effect of cooling will be longer, and flow resistance will increase, which will cause cold compound seam marks on molded products. . This will also lead to a significant drop in temperature of raw material. As temperature decreases, viscosity of raw material will increase, and fluidity will deteriorate.

On one hand, gas cannot be separated from molten raw material in time to create flow lines; In a narrow area (for example, in thin-walled part of part), a large resistance arises, which increases internal pressure of melt and easily forms a high-temperature scalding gas, leading to overburning of surface of part.

2.4、 Influence of gas released by plastic melt itself

If injection molding equipment is suitable for production of a particular product, design of plastic mold is reasonable, and exhaust is smooth, but plastic itself constantly releases gas, then plastic melt will still leave traces of air during injection process. and a chill molding process. There are two main types of self-generating gases:

(1) Influence of water vapor

The main source of water vapor is moisture absorbed by raw material when it is wet. During melting process, raw material is heated, and water continuously evaporates and exits. After being injected into mold, water vapor on surface of plastic melt condenses rapidly when it is cold, forming a vapor pattern. If mold is poorly ventilated, hot water vapor will cause surface of part to ignite.

(2) Influence of decomposition gas

During melting process of some poor quality plastics, due to exposure to high temperature or high shear forces (too high screw speed, too high back pressure), some volatile phases are released directly from melt, especially when present. Catalytic cracking reactions can occur when trace impurities or moisture. Reasons for formation of plastic decomposition gas: too high a temperature of melt, which leads to decomposition, too high a temperature of drum, or an uncontrolled heating ring of drum. Use a large machine for small products, or amount of cushion is too large, or dead material and dead corners in barrel are decomposed due to long-term heating, or molding cycle is too long, so that melt remains in barrel is too long and causes decomposition; shear of melt in barrel is too large, compression ratio of screw is too large, screw groove is too shallow, etc., plus screw speed is too high, back pressure is too high, etc.

2.5 Environmental impact of injection molding equipmentfood

When injection molding machine is in a humid environment and air humidity is too high, it will condense into water droplets when it encounters a cold mold (such as mold temperature is below dew point), resulting in water traces in mold cavity moulds. Moisture that is evaporated by molten material into water vapor causes traces of gas if gas is not released in time.

2.6, Influence of poor melt flow

Many plastics have poor melt flow. There are two types of impact on gas traces. The first is that gas produced when plastic is melted cannot escape in time and exits from back of barrel. whereby finally it is placed in mold, and other is to prevent plastic melt-wrapped gas from leaking out in time in mold.

Strength Question: If melt flow is too good, resulting in air trapping, wouldn't that be more likely to cause air trails? Or a certain depression? The figure below shows a TPV with relatively good fluidity (hardness approx. 75A, PP/EPDM). Certain concave bands appeared on surface during injection molding:

Analysis and measures to eliminate gas layers created by plastic products

Welcome injection molders, leave a message below and give me advice!

3. Analysis of typical examples

Analysis and measures to eliminate gas layers created by plastic products

Scheme of plastic parts of instrument cover

Analysis and measures to eliminate gas layers created by plastic products

Air-tagged sensor cover image

Instrument lid made by 800T injection molding machine, product drawing number 53A-05171, gross weight 801g, net weight 792g, material class ABS/VK125, mold cycle 65s. .

3.1. Causes of traces of air on lid of appliance

After analysis, main causes of air marks are:

⑴The melt temperature or mold temperature is too low;

⑵The injection speed and pressure are too high;

⑶Back pressure is low;

⑷ Poor mold exhaust;

⑸Insufficient drying of raw materials or decomposition due to overheating;

⑹Gate mold system defective.

3.2 A workable improvement plan

⑴ Properly increase temperature of barrel to facilitate release of gas, but temperature of PC/ABS should not exceed 270°C to prevent pyrolysis and affect performance of PC/ABS.

⑵Reduce injection pressure and injection speed accordingly so that gas has enough time to eject. It should be noted that injection pressure and injection speed are interrelated, and method of changing them simultaneously cannot be adopted when adjusting.Because it is impossible to determine whether cause of air traces is injection overpressure or injection speed is too high. The correct way is to first select one of options to tweak and then see what effect it has before deciding on next step.

⑶Increase back pressure and increase melting speed of material so that air wrapped in raw material and gas generated by volatile phase of raw material itself can completely exit back of barrel.

⑷Improve mold release. To ensure that various gases generated when filling mold with material in injection molding process can be smoothly removed from mold.

⑸The raw materials should be completely dried, recommended temperature is 70℃~80℃, drying time should be at least 4 hours, and moisture protection measures should be taken (especially on rainy days) to prevent influence of water vapor.

Analysis and measures to eliminate gas layers created by plastic products

Qualified instrument cover diagram

Master Accepted Plan: On first section, use medium speed to simply fill chute until adhesive is switched in time. Then, at slow speed and in a small position, second section is filled to form a part near glue inlet. In third stage, 90% of mold cavity is filled quickly to avoid cooling high temperature molten rubber. In fourth stage, cavity is filled slowly to completely remove air from mold cavity, avoiding air entrapment and fire. .

4. Conclusion

Air marks on injection molded products reduce strength of product and affect appearance, so air marks are an important cause of failure of injection molded products. The morphological structure of material in area of ​​the air mark is direct cause of influence on performance of product. In order to eliminate or reduce influence of air mark on performance of product and improve performance of product, morphological structure of air mark must be fully taken into account. Only by correctly understanding relationship between morphological structure of material in air mark zone and performance characteristics of product, it is possible to reasonably select molding material in industrial production. This is of great importance for improving quality of injection molded products and extending service life of injection molded products.


[1] Liu Xiwen: Answers to Complex Problems of Injection Molding Technology, Beijing: Printing Industry Press, 2011. 

[2] Li Haimei: Practical Injection Molding Technology and Mold Design, Beijing: Chemical Industry Press, 2002.

[3] Cui Jiyao: Difficult Problems and Countermeasures in Injection Molding, Beijing: National Defense Industry Publishing House, 2008

Source: Internet, thanks to author of original, very interesting!

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