Summary of knowledge on pyrolysis of foam in lost foam casting process
1. Factors affecting the pyrolysis rate of lost foam foam
Material properties
Foam material: Different types of foam, such as polystyrene (EPS), polymethyl methacrylate (PMMA), etc., have different pyrolysis rates due to different chemical structures and chemical bond energies. Generally speaking, EPS has relatively good thermal stability and a slower pyrolysis rate; PMMA has a relatively low pyrolysis temperature and may have a faster pyrolysis rate.
Density: The higher the density of the foam, the more material content per unit volume, and more heat needs to be absorbed during pyrolysis, and the pyrolysis rate is usually slower.
Pyrolysis conditions
Heating temperature: Temperature is the key factor affecting the pyrolysis rate. According to the Arrhenius equation, as the temperature increases, the reaction rate constant increases, and the pyrolysis rate increases.
Heating rate: The faster the heating rate, the more heat the foam absorbs in a short period of time, the pyrolysis reaction may occur more quickly, and the pyrolysis rate increases. However, too fast a heating rate may cause uneven heat transfer inside the foam, affecting the uniformity of pyrolysis.
Pyrolysis time: As the pyrolysis time increases, the foam continues to be heated, the pyrolysis reaction continues, the degree of pyrolysis deepens, and the pyrolysis rate is not constant throughout the process. Generally, the initial pyrolysis rate is faster. As the reaction proceeds, the unreacted substances decrease, and the pyrolysis rate gradually slows down.
Atmosphere: In an inert atmosphere (such as nitrogen and argon), foam pyrolysis is mainly a decomposition reaction induced by heat; in an oxidizing atmosphere (such as air), in addition to pyrolysis, oxidation reactions may also occur, which accelerates the pyrolysis rate, and the pyrolysis products may be different.
Mold and process factors
Mold structure: The shape, size and wall thickness of the mold will affect the efficiency of heat transfer to the foam. If the mold structure is complex and the heat transfer is uneven, the pyrolysis rate of different parts of the foam will be different.
Coating properties: The coating applied to the foam surface can play a protective and heat-insulating role. If the thermal conductivity of the coating is poor, it will slow down the speed of heat transfer to the foam, thereby reducing the pyrolysis rate.
Casting process parameters: such as negative pressure, pouring speed, etc. Negative pressure affects the discharge of foam pyrolysis products. Appropriate negative pressure can discharge pyrolysis products in time, which is conducive to the pyrolysis reaction; pouring speed that is too fast or too slow will affect the interaction between the metal liquid and the foam, and then affect the pyrolysis speed.
2. How to reduce harmful gas emissions during the pyrolysis of lost foam foam?
Choose suitable foam materials
Use low-pollution foam materials, such as expandable polylactic acid (E-PLA) and other biodegradable materials, whose pyrolysis products are relatively environmentally friendly and have low harmful gas emissions.
Optimize the foam formula, and improve the pyrolysis performance of the foam and reduce the generation of harmful gases by adding some environmentally friendly additives, such as antioxidants, thermal stabilizers, etc.
Optimize the pyrolysis process
Control the pyrolysis temperature and heating rate, avoid excessively high temperatures and excessively fast heating, so as to reduce incomplete combustion and harmful gas generation caused by overheating.
The segmented pyrolysis process is adopted to control the pyrolysis conditions at different temperature stages, so that the foam pyrolysis is more complete and orderly, and the emission of harmful gases is reduced.
Improving the pyrolysis environment
Using inert gas protection, nitrogen, argon and other inert gases are introduced during the pyrolysis process to reduce the oxygen content, inhibit the oxidation reaction, and reduce the generation of harmful gases.
Optimizing the structural design of the pyrolysis furnace to make the temperature distribution in the pyrolysis furnace uniform, the foam pyrolysis is sufficient, and it is convenient to discharge the pyrolysis products, avoiding the generation of harmful gases caused by secondary reactions caused by local overheating and product accumulation.
Post-processing technology
Install efficient tail gas treatment equipment, such as activated carbon adsorption device, catalytic combustion device, wet scrubber, etc., to purify the tail gas generated by pyrolysis and remove harmful gases and particulate matter.
Recycling the pyrolysis products, and recycling some recyclable materials generated by pyrolysis, such as styrene, not only reduces resource waste, but also reduces harmful gas emissions.
3. What are the main effects of the gas generated by the pyrolysis of lost foam foam on the flow of molten metal?
Change the flow pattern of molten metal
Produce turbulence: Pyrolysis gas forms bubbles in the molten metal. The presence of these bubbles will interfere with the normal flow of the molten metal, causing the flow of the molten metal to become turbulent and form turbulence. For example, during the filling process, the originally relatively stable molten metal flow will have local eddies and fluctuations due to the disturbance of bubbles.
Change the flow direction: The pressure generated by the gas will exert a force on the molten metal, causing the flow direction of the molten metal to change. Especially in molds with complex shapes, pyrolysis gas may cause the flow direction of the molten metal in some narrow channels or corners to deviate from the original path, affecting the filling effect of the molten metal on the mold cavity.
Affect the filling capacity of the molten metal
Increase the flow resistance: The gas film or bubbles formed by the pyrolysis gas in the molten metal will increase the friction between the molten metal and the mold wall, and will also increase the viscous resistance inside the molten metal, resulting in an increase in the flow resistance of the molten metal. This requires the molten metal to overcome greater resistance during the filling process, thereby reducing its filling capacity and possibly causing defects such as insufficient filling and cold shut in the casting.
Reduce the filling speed: Due to the increase in flow resistance and the obstruction of gas to the molten metal, the filling speed of the molten metal will be significantly reduced. Especially for some thin-walled castings or castings with complex structures, the reduction in filling speed may cause the molten metal to solidify before it completely fills the cavity, affecting the integrity and quality of the casting.
Causes the molten metal to be entrained and gas entrapped
Gas entrapment: The escape of pyrolysis gas will drive the surrounding molten metal to form a vortex, causing the gas to be entrained in the molten metal. These entrained gases may form defects such as pores and shrinkage holes during the solidification of the molten metal, reducing the density and mechanical properties of the casting.
Gas entrapment phenomenon: The interaction between gas and molten metal may cause part of the gas to be enclosed by the molten metal, forming a gas entrapment phenomenon. Gas entrapment not only affects the appearance quality of the casting, but may also become a crack source, causing crack propagation when the casting is under load, reducing the reliability of the casting.
