Several Problems in Domestic Lost Foam Casting

1 Production manufacturing problems of plastic molds

Plastic mold production is the most important more critical in FMc technology

The model materials must meet the following requirements:

(1) The density should be small, the rigidity should be good, there should be a certain compressive strength.

(2) It can withstand mechanical processing manufacture, it is easy to get a smooth outer surface.

(3) The vaporization temperature should be low.

(4) When it interacts with liquid metal is destroyed, there should be less impurities, less gas generation, harmless to the human body.

EPs, which are more commonly used in manufacturing, have lower cost than other model materials, mature production processing technology a wide range of raw materials. However, this type of feature is specific to the packaging industry. For FMC technology, the current domestic EPS model is still far the requirements. The specific manifestations are:

(1) The EPS model the packaging industry to FMC has uneven internal thickness, the bead model lacks a unified standard specification;

(2) The enterprise lacks the standard specifications experience in the processing technology of manufacturing FMC plastic molds, the production rate of plastic molds produced is very low, the product quality fluctuates greatly;

(3) The surface of the plastic mold is rough. It is difficult to produce plastic molds with surface quality similar to that of wax molds using EPS foaming technology in the packaging industry.

(4) High cost. The market price of EPs used in the packaging industry is about o. 02～o. 05 yuan/g, its pricing method is based on the quality of the product, which must cause manufacturers to want to increase the density of the model, this is fundamentally contrary to the requirements of FMC. Therefore, the current enterprises have changed the pricing of FMC plastic molds to piece-by-piece, which is about O.D. 12～o. 15 yuan/g, the cost is greatly increased.

(5) Molding foaming is basically foamed with water vapor, so when pouring high-quality castings, the plastic mold must be dried. In this regard, domestic processing technology is far developed countries abroad.

In order to reduce the amount of gas residues of plastic molds, many domestic scientific research institutions recommend using EPMMA-EPS EPMMA raw materials to replace EPs, but this has been very effective in a short time. The reasons are: ①The cost is expensive the quality The price ratio is lower than EPs, it is difficult for ordinary iron casting manufacturers to adopt it; ②The molding foaming temperature is high (about 125℃), the best foaming method should be hot air foaming. This processing technology is used by many domestic enterprises Immature, difficult to control product quality, lack of foaming equipment.

Because of the above reasons, the lost foam casting industry is calling: Standardize the EPS raw materials production process for FMc method in China as soon as possible.

2 paint problem

FMC coatings include surface finish coatings refractory coatings.

2.1 Surface smooth coating

The purpose of the surface smooth coating is to improve the surface smoothness of the model. Generally, it is an organic film-forming coating such as polyethanol dissolved in hot water + alcohol.

The coating method is generally dip coating (mainly for small parts) spraying (medium large parts). After dipping, it is easy to produce defects such as flow marks uneven coating thickness under the model. Therefore, even if the processing technology is simple, it does save labor. Therefore, the author recommends spraying.

The surface finish coating can reasonably effectively eliminate the surface texture of FMc castings.

Even if the surface finish coating can partially improve the surface quality of castings, it will increase a lot of labor in actual operation, especially suitable for the production of large quantities of small parts.

Improving the surface finish of plastic molds is the fundamental way to obtain smooth surface castings.

2.2 Refractory coating

The role of refractory coatings is to avoid mechanical thermal chemical adhesion of sand castings, to improve the stiffness of the plastic model. The types of refractory coatings include water-based coatings, alcohol-based coatings, resin coatings. Water-based coatings are generally thicker (0.5 ~ 2mm), their strength is higher than that of alcohol-based coatings. Because water-based coatings are generally very thick, in order to improve their air permeability, a small amount of foaming agent is generally added to the water-based coatings. After the coating is dried, the foaming agent will produce a large number of capillary pores in the coating. It can even be clearly observed against the lighting. Water-based coatings must be manually dried.

Both alcohol-based coatings resin coatings are quick-drying coatings. There is no foaming agent in the coating, the permeability of the coating is mainly formed by the volatilization of the binder solvent. In addition, the quick-drying coating is thinner than the water-based coating, so the air permeability is better. Water-based paint has a different style quick-drying paint.

It should be noted that in water-based coatings, capillary suction is formed for liquid metal due to capillary pores, which makes the coating collapsible. It is precisely because of this that many lost foam manufacturers give up water-based coatings choose alcohol-based coatings.

It must be emphasized that two kinds of prejudice have been formed in the company. One is to overemphasize the role of coatings such as the quality of air permeability; the other is that alcohol-based coatings are convenient fast but expensive. In fact, the objective fact is the case. A lot of practice shows that whether the gas in the mold can be quickly eliminated is a comprehensive practical effect between the thickness of the coating, the degree of vacuum, the particle size of the molding sand. Specific to each kind of coating, it can be felt that the air permeability of water-based coatings (without foaming agent) quick-drying coatings under the same thickness is almost the same, it is impossible to have a significant impact on the FMC process. On the one hand, although the cost of raw materials in alcohol-based coatings is high, the coating is thinner, it is as easy to clean as specialized drying, so its average variable cost is lower than water-based coatings.

Water-based coatings contain a large number of capillary pores, which make the castings easy to form needles the coating is difficult to eliminate, but the thickness of the water-based coatings can increase the coating strength; alcohol-based coatings have anti-sticking sand needle punching functions Good, but its strength is low. In view of this, this article suggests; for thick castings, important parts parts, the combination of these two coatings will have a very good practical effect, that is, the inner layer uses alcohol-based coatings (≤0.5mm) outside Layer water-based paint.