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The technical practice of two key links of epc casting

Author : Felix Fan Date : 2018-11-8 11:31:38
 To ensure the quality of epc products, the selection of high quality epc foam beads is a key link.The bubble plastic model of products made by packaging foam will greatly affect the quality of casting products due to uncertain factors such as gas generation and carbon increase.Now with the rapid development of die casting industry in China, the original and auxiliary materials also require more and more specialization.

 

I. general special name of foam bead for epc casting:

(1) hair polystyrene resin beads (EPS) specially used for epc casting;

(2) fatable methyl methacrylate copolymer resin beads (STMMA for short) with styrene;

(3) hair - forming polymethacrylate resin beads {EPMMA for short.

According to the characteristics and requirements of the casting, the user selects the required varieties and specifications to ensure the high quality foam plastic model and epc casting.

 

Selection of raw beads: first, the variety of beads should be selected according to the casting material and quality requirements, and then the specification of beads should be selected according to the casting wall thickness.In the prefoaming process, the foaming rate is 40 ~ 50 times, and the bead diameter will be about 3 times that of the original bead.In order to get a good appearance, 3 beads should be arranged in the direction of the minimum wall thickness in the shape of the minimum wall thickness.Thus, the maximum original bead diameter (mm) selected = the minimum casting thickness (mm) x l/3 x 1/3.For example, to get a casting with a minimum wall thickness of 5mm, you would need to use raw bead material with a diameter of less than 0.55mm.But sometimes for thin - walled parts, especially gray cast iron, the bead diameter is a little smaller.However, for the thick wall parts, the filling of beads is not a problem, and the appearance is of certain strength. If large beads are selected appropriately, ideal casting surface can also be obtained.Prefoaming of original bead: the prefoaming machine used in epc casting usually adopts intermittent prefoaming machine.The intermittent prefoaming machine has the intermittent steam prefoaming machine and the vacuum prefoaming machine two kinds.Intermittent steam pretest bubble machine pretest bubble parameters: EPS pretest temperature 100 ~ 105 ℃;STMMA pretest temperature 105 ~ 115 ℃;EPMMA pretest temperature 120 ~ 130 ℃.When prefoaming, steam entering should not be too concentrated, pressure and flow can not be too large, lest cause agglomeration, non-uniform foam, and even some of the beads overpre-foaming and rupture.When the bead is in contact with water vapor, the mass fraction of the moisture content of the preformed bead is about 10%. Therefore, it must be dried after unloading.

 

The heating medium (steam or oil) of the vacuum prefoaming machine does not contact the bead directly.Therefore, the preheating temperature and time, the degree of vacuum and the time of vacuumization are the key factors influencing the quality of the preforming beads.Generally, the vacuum degree is set as 0.06 ~ 0.08MPa, the vacuum is pumped for 20 ~ 30s, the preheating time is 1 ~ 3min, preferably within 2Min. The preheating temperature is controlled by the steam pressure of the sandwich.

 

2. External and internal quality requirements of bubble shaping appearance:

 

(1) the surface roughness is low, and the beads are well integrated with no obvious protrusions and depressions, no excessive melting, no injury, no oil pollution, and their shape and size meet the requirements of the shape map.

 

(2) the appearance shall be of low density and uniform density in all parts under the premise of ensuring strength, so as to minimize the pyrolysis products, ensure smooth filling of metal liquid and avoid casting defects;No inclusions are allowed in the appearance.

 

(3) the appearance must be dried before applying the coating to reduce moisture and stabilize the size.

 

(4) the appearance shall have certain strength and stiffness when meeting the above requirements, so as to ensure that it will not be damaged or deformed in the process of bonding, coating, drying, transportation, sand packing and other operations.

The drying and maturation of precast bead: general vacuum precast bubble machine not only precast bead multiple is high, bead grain size is even and precast bead is dry.If the pretest pressure steam bubble machine, after the pretest bubble bead high moisture content, need to dry processing, drying air temperature is 25 ℃ ~ 35 ℃, make bead moisture content is below 2%.The pre-foaming bead is in a decompression state, usually to be stored for a certain period of time to dry and mature, so that its stability.

 

EPMMA pretest probability and STMMA bubble bead of curing time is shorter than the EPS of curing time, the best curing temperature is 20 ~ 25 ℃.

 

Iii. Production technology of appearance

 

No matter which kind of bead mold is used, the process is the same.The technological process is as follows: the selection of original beads, prefoaming, drying and maturation of beads, foaming molding, maturation of appearance, combination of appearance.

 

According to the test, the main drawback of EPS appearance is that it is easy to cause bright carbon defects on the surface of cast iron parts and increase carbon on the surface of cast steel parts, while using EPMMA appearance is very effective to solve such defects as carbon addition, wrinkling, black slag and so on.But EPMMA has a lot of steam, about 1.5 times EPS.STMMA is a copolymer of styrene and methacrylic acid.The composition of copolymer can also be adjusted to produce foam plastics with different properties to meet the needs of different casting occasions.According to the properties of raw materials and castings, the principle of choosing the appearance raw materials is as follows:For mild steel castings with high surface carburization requirements, STMMA is preferred. For a few alloy steel castings with high surface carburization requirements, EPMMA can be used.The ductile iron parts with higher performance requirements are sensitive to the carbon black slag involved, which may cause the casting to crack. Therefore, STMMA is also used as the material of the shape.In addition, for thin-walled castings with clean surface requirements, the finest beads, the best foaming rate and STMMA must be used.The size of the beads is chosen according to the thickness of the walls.The thinner the wall thickness is, the smaller the bead size is required.In general, the original bead should not be more than 1/10 of the minimum wall thickness of the casting.

 

Foaming molding: the purpose of foaming molding is to fill the loose beads in the mold with a certain shape and size, and then heat them again for the second foaming to form an integral shape consistent with the mold shape and size.The appearance of batch production, usually use molding machine molding.Its foam forming process: preheating.After installation of the mould, preheat to 100 ℃, it guarantee the mold is hot and dry.(2) packing.Generally use the gun to shoot the material, filled with enough foam bead.Vent steam.In general, water steam of 0.1 ~ 0.15mpa is passed in.In order to guarantee the molding quality, after forward ventilation, reverse ventilation once again, and then forward and reverse simultaneously ventilation, so that the steam in the cavity does not flow.In this way, the foaming can be uniform, smooth surface, density light foaming appearance.The time of steam passage is determined by test.(4) cooling.Generally adopt the method of spray cooling mold cooling to 40 ~ 50 ℃.(5) out.For automatic or semi-automatic moulding machines, there are two common methods: mechanical jacking and vacuum sucker moulding.

 

Fourthly, the maturation and steady drying of the appearance

 

The appearance after foaming molding should be dried and stored for a period of time to stabilize it, which is called dryness and maturation of appearance, abbreviated as maturation treatment.Curing natural curing and drying room (chamber temperature control in 45 ~ 60 ℃) in curing (curing).The model was shrunk during drying and curing, and the shrinkage rate of EPS was generally 0.4% ~ 0.7%.The systolic rate of the epmma-looking and stmma-looking was between 0.2% and 0.4%.The curing time is related to the factors such as the appearance material, the thickness of the appearance wall, the curing condition (temperature, humidity, ventilation).As the appearance of the forming process with water and water contact, the appearance of just out of mold contains a lot of water.There are many factors that cause the water content in the appearance, mainly including foaming molding, steam pressure caused by heating, steam circulation time and water supply time when cooling. In general, the water content of the appearance that has just come out of mold is about 6%-8%. Such appearance can never be taken directly to paint.It is very intuitive to say that appearance maturation is the elimination of moisture and stable line shrinkage, which is the purpose of maturation treatment.After aging and drying, the contour contraction is basically stable before the next process is allowed.In order to guarantee the quality of the lost mold casting, the appearance will vary with the thickness of the appearance wall and the drying conditions before bonding and assembly. The drying must be done in a variable manner according to the situation, so that the moisture content in the appearance can be reduced to less than 1% and the linear shrinkage can be stabilized at around 0.45%.It depends on the actual situation.Generally, EPS is forced to mature for 4 to 6 days, and EPMMA and STMMA are forced to mature for 5 to 7 days.I have been to many places, hear that "natural drying 2, 3 days" such extreme unscientific technology concept, we are not against natural drying when conditions permit, but its effect depends on the ambient temperature and humidity, temperature and humidity difference is very big all the year round, and we are such a big regional differences also is very big, plum rains season in the Yangtze river delta where humidity is very big, if only to control the temperature, don't dry, aging is not good.Therefore, the so-called "natural maturation" cannot be generalized, it should follow the scientific laws to analyze and treat the specific problems, and use the scientific technological principles to deal with the variable factors.In order to improve production efficiency and shorten the cycle of maturation drying treatment, reasonable forced drying time can be set according to the production conditions, so as to achieve the purpose of saving energy, improving production efficiency and using rate of drying room.If good control means the shape placed in chamber (600 + 50 ℃ forced drying 6 to 12 h.Thick big piece take upper limit, thin small piece take lower limit, oven humidity should < 40%.It can be determined by comparing the moisture content before and after drying, and the method of measuring the degree of dryness by "test sheet" is generally adopted by production units, which can only be a reference.Conditional unit should perfect test means.

 

The combination and bonding of the appearance: if the more complex foam appearance cannot be formed in a mold, it needs to be divided into pieces. Each piece is formed by the mold separately. People are used to call it the appearance sheet.Then, by the method of bonding, multiple foam sheets were combined into a whole foam look.For small batch production of simple foam appearance, manual bonding is adopted.For the mass production of complex die, the use of automatic bonding machine production, with bonding mold to guarantee the accuracy.

 

Complex models are often composed of piecewise foamy patterns.When two foam molds are joined, the adhesive on the bonding surface always has a certain thickness, usually 0.1-0.3mm.The thickness of the butt joint can be increased appropriately for the thinner foam die.Require the appearance assembly to be gasified quickly with adhesive, no residue.The dosage of adhesive must be thin and uniform when binding the appearance and pouring system. Too much adhesive may lead to incomplete gasification of the appearance, because the residue left after the non-gasification will also affect the casting quality.The foam plastics of the epc casting system should be used correctly. Our opinion is that the material used in the casting system should be consistent with that used in the shaping white parts.As far as I know, many enterprises of epc did not press the special molding machine of foam plastic sheet used in the casting system, but bought the foam plastic molding board in the building materials market, and then pulled back and cut according to the requirement of casting system size.Not only that the strength and beads do not meet the requirements, but that the material requirements for building and casting are different.One of them reduced slag by about 40% after changing the casting system with foam plastics, that is, the foam plastic plates on the building materials used before were not well compressed due to the large size of beads, the materials were drilled when the beads were interbrushed, and the refractory coatings entered the castings with the metal liquid during pouring.In addition, it is strictly required that the bonded operators should stick all parts of the pouring system firmly together, so as to prevent the penetration of the paint, the operator moving, and the cracking of the pouring system leading to the drilling of the paint. Only in this way can the defects of slag inclusion be reduced and the defects of slag inclusion or sand inclusion be eliminated from the source of slag to the greatest extent

 

Therefore, the less adhesive that can be applied on a firm basis, the better.This is one of the basic tasks for the production of perfect epc castings.