Influence of Hardening Rate of Moulding Sands with Chemical Binders on Bridges Structure and Strength Properties

摘要

Modern technology of the foundry core production applies the so-called 'cold-box' process, in which the moulding sand hardening (binding with a chemical binder) is obtained by means of the active gas blowing through the thickened compound in the core box.The rate of the hardened moulding sand thickness growing up, depends on several factors.The results of testing the hardening of a few moulding sands prepared of high-silica sands of various average grain sizes: dL=0.24; 0.28; 0.34 mm, are presented in the hereby paper.The phenol-formaldehyde resin,of a resol type (2,5％), as well as water glass R-145 (5％) were applied as binders.The cores hardening was realized using CO2, pressed into a core box at pressures of: 0.25; 0.50 and 1.0 atmosphere.The pathway of the hardening process was controlled by means of the ultrasound technique, using the method patented by AGH, University of Science and Technology in Cracow, Poland.The hardening process rate, including chemical reactions, is characteristic for each binder-hardener system and depends on the concentration of reagents and the process temperature.Hardening process proceedings of moulding sands prepared with using water glass and phenol-formaldehyde resin are very similar, however their binding process rates are different.Investigations of the structure of grain binding, it means building of the so-called binder bridges in the tested moulding sands, indicate that it depends on the moulding sand hardening rate.Binder bridges in the tested moulding sands were observed by means of the scanning microscope, SEM.It was found that in case of the phenolformaldehyde resin, regardless of the applied pressure of the hardening gas, cracks in the bridges structure can be noticed, especially at the grain-binder boundary.Cracks are smaller for the moulding sands hardened by a gas of a lower pressure (lower binding rates) than for the moulding sands hardened by a gas of a higher pressure (higher binding rates).In both ways of gas pulsation (gas-interruption and gas-air) bridges without microcracks were obtained, which was unavailable at a continuous supply of the hardening gas.The strength of the moulding sand with water glass at a continuous supply of the hardening gas is low or very low.The reason of such low strength is the linear shrinkage of a binder, of the order of a dozen or so percent, which is the main reason of stresses.The formed bridges have a lot of discontinuities (microcracks).An application of pulsations in a system gas-interruption(1 second-gas, 5 seconds-interruption in gas supplying) causes that a binder much slower changes from the plastic into elastic state and due to that shrinkage stresses of a binder undergo-to a certain degree-the relaxation realized by plastic deformations of bridges.As a result there are much less cracks in the bridges structures and the hardened moulding sand is characterized by a higher strength.