Development and application of gravity and low pressure casting coatings
1 Foreword <br> Next page
This article will focus on the development and application of gravity and low pressure casting coatings.
Since the advent of gravity casting and low-pressure casting, people have realized the importance of metal-based coatings. Choose and control the best metal-based coatings, or make a reasonable combination of coatings, like metal design, metal treatment and Like the pouring system, it plays an important role in the production of beautiful castings.
2 The function and characteristics of the coating The basic functions of the metal coating are:
·Protect metal molds and extend mold life;
· Conducive to mold release, prevent bonding and oxide buildup;
· Control the heat flow from the casting alloy to the metal mold.
· Improve the surface finish of castings.
Since the casting of good quality needs to control the whole process of casting the alloy filling metal cavity and solidifying in the metal cavity, the most important function of the metal coating is undoubtedly to make the metal type have a heat insulating layer and control the heat flow from the casting alloy to The transfer in the metal cavity.
The thermal insulation of a coating depends primarily on three key factors:
(1) coating composition;
(2) coating thickness;
(3) Coating porosity.
2.1. Coating composition Metal-based coatings are currently mainly based on water, adding high-temperature binder and refractory filler. The thermal insulation coating contains a thermally insulating mineral mixture, and the filler in the coating should have sufficient refractoriness at the normal temperature of the aluminum alloy. To ensure that it does not undergo chemical changes, it is basically only a physical effect. The most critical coating properties that metal molders need to consider when selecting the coating or coating composition for the specific casting produced are: Thermal conductivity, particle size, lubrication To ensure that the casting is easy to release and smooth, and to ensure that the coating has high thermal insulation to prevent casting casting. The main components of the currently formulated thermal insulation coatings are alumina, talc, mica, diatomaceous earth and titanium oxide. These minerals are highly insulating.
2 . 2 thickness of the coating
Figure 1 shows the relationship between the thickness of the coating of our thermal insulation coating and its adiabatic performance. The figure shows that the development and application of adiabatic gravity and low pressure casting coatings of 300 μm thickness coating is no more than 200 μm coating. The layer is much improved, and since the coating of more than 300 μm is easily peeled off from the metal type, the optimum thickness of the coating should be 150 to 250 μm.
2.3. Porosity of the coating
The formation of coating porosity depends mainly on the evaporation rate of the carrier water in contact with the surface of the metal cavity, that is, depending on the coating method and coating conditions. Generally, the coating is usually carried out at a mold temperature of 170 to 200 °C. Get the best results. Now most of the foundry coatings are used by manual mixing at the production site, and then a part of the dilution is used. Due to many human factors, the proportion of the coating components is often out of balance, sometimes leading to the imbalance of the binder. The metal type insulation performance is uneven and the coating is peeled off. Many foreign plants have set up a central mixing station at the production site, and the paint is diluted in the center mixing station and kept stirring, so that the composition is uniform and the coating is The deposition on the model is uniform.
When coating at a higher metal temperature, due to the strong reaction between the carrier water of the coating and the high temperature metal, a large porosity is usually obtained, and the thermal insulation of the coating is naturally good, but the coating is cast. The adhesion and life of the type are very low, because the excessive metal temperature will cause a strong "rebound", making it difficult for the coating to adhere to the surface of the metal cavity.
On the contrary, although the coating life is higher when the mold temperature is lower, the coating is dense and the heat insulation is poor, and the too low metal coating temperature will form the coating "stacking", when the metal type is heated to the working temperature. When there is a defect in coating cracking and peeling.
3 Development and application of coatings
The metal-based coating products currently developed by Jinkefuske joint venture mainly include: thermal insulation coatings, lubricating/thermal coatings and semi-insulating coatings.
3.1 The thermal insulation coating products trade names are DYCOTE 34, DYCOTE 39 and DY-COTE 140. The formulas of this series are all supplied by Foseco, UK, and processed by domestic raw materials. DYCOTE 34 coating particles are relatively thick, but the thermal insulation performance is good. Durable, ideal for thin section castings and large flat castings, the unique grain shape forms a "zigzag" edge on the mold surface (see Figure 2), which has a very useful function, either "tearing" or "Puncture" An oxide film formed on the surface of molten aluminum to improve the fluidity of the metal, so that the aluminum flowing into the cavity has better meltability and facilitates air escape. When the thickness of the coating is 200 to 280 μm, The instantaneous change in the formation of the air gap is shown in Fig. 3. As can be seen from Fig. 3, due to the thermal insulation of the coating, the air gap tends to be stable in a short period of time, and the formation of the air gap also provides for preventing under-casting and demolding of the casting. Advantageous conditions. Proper and careful application of DYCOTE 34 can prevent castings from being out of stock and cold.
DYCOTE 140 is a lubricated, medium and high temperature insulation coating that is extremely versatile and is commonly used in small or medium section molds such as piston parts, small automotive wheels, motorcycle wheels, etc. This coating is durable and resistant to flaking. Sex.
DYCOTE 39 provides the highest level of smooth and bright casting surface. Due to the large number of hot-filled fillings in DYCOTE 39, such as laminated plates (see Figure 4), the contact area between the coating and the molten metal is increased, thus making the casting The surface gloss is very good. However, this heat-resistant plate shape does not sandwich a large amount of air, so its thermal insulation is lower than that of the angular shape DY-COTE 34. Sometimes, in the position where insulation is required, it should be considered. Used with DYCOTE 34.
The three coatings are arranged in adiabatic manner as DYCOTE 34, DY - 7 5 1COTE 140, DYCOTE 39, and the surface finish of the casting increases in turn.
3. 2 Lubricating / thermal conductive coatings trade name DYCOTE 11 . Based on semi-colloidal graphite, used to help the castings to be stripped. At the same time, DY - COTE 11 is also suitable for the active part of gravity casting, such as core, slide, etc. Lubrication at the place and lubrication of the joint surface of the mold.
3 . 3 semi-adiabatic coating
The trade name of the product is DYCOTE 36. The coating has good release properties and moderate thermal insulation properties.
3.4 Coating performance summary table