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Making Technology of Complex Thin-Walled Part Casting

Posted: 2014-11-20 21:12:38  Hits: 1310
1.1 Mold design
First of all, analyze the parts structure of guide tube, and choose the appropriate parting surface, as Fig.1. Make sure for reasonable die structure, which is also a design process of casting products. Guide tube belongs to the thin wall parts with symmetrical structure. The thickness of its main part is 3 mm, and there is groove in the middle (Fig.1a, Fig.1b). If it is a whole wax models, it can’t take out of the core of the groove, so consider shard production and then bond. Because the part is symmetrical structure, it just needs to design a symmetrical half, and the symmetry plane is located between two grooves and perpendicular to central axis.

It needs to consider investment pattern’s shrinkage rate, profiling material, profiling size, machining requirements, locating mechanism, locking mechanism and die lifter. The mold consists of as the following several parts: ① Molding, including the fluctuation model, core and loose piece etc. ② Loose core. In order to improve the efficiency of mold delivery, reasonable design of the mold core pulling is very important. The mould core pulling structure need to be pulled out easily, and you should use up loose core. ③ Locating mechanism. Ensure that each component is in a fixed position in the process of wax material injection and realize it through setting the pin. ④ Locking mechanism. Fasten mould components together firmly to ensure the accuracy of wax mould. Normal designs contain tighten nut, clamp, eccentric wheel etc, This mould uses tighten nuts.
Manufacture the mould structure as fig.2 after ensuring the above details and the necessary wax injection system.
 

1.2 Pattern material selection and preparation of cerate
Mould material need to be considered its performance of the melting temperature, shrinkage and strength and surface hardness, ash content and welding etc. Compared the performance of selected two pattern materials, we find table1 (The shrinkage test is carried out under the condition of free lingers, not press pouring, so the quantitative value is bigger). A is pattern materials containing half paraffin wax and half stearic acid, which belongs to low temperature wax pattern material. B is modified mold material, with 5% transform polythene, 5% modified rosin, 20% stearic acid, and 70%paraffin wax. The polyethylene is mainly used to improve the strength of the pattern material, the main effect of stearic acid is to improve surface hardness of the mould material, and the effect of rosin is mainly used to reduce pattern material shrinkage rate’s increasing because of the addition of polyethylene. By comparison with the performance of the two kinds of pattern materials in the table, it can be seen that the performance of the modified pattern is better than paraffin wax stearic acid, so the modified pattern material is used in this test.
 

Melt the modified pattern material in the pattern material melt barrels, and stew 20 min after all melting. Then pour into pattern material mixer barrel and start the blender after scooping out the surface bubbles. Add the prepared pattern material scraps when the pattern material becomes pastes, with an amount about 30% of the total quality of pattern material. When the mold material cools down to 45 ℃, pour it into pattern material barrel of injection machine and press pouring into the pattern.
 
1.3 Pattern process
Choose wax injection temperature, injection speed, and pressure maintaining time as factors for the selected modified mould material after getting the cerate under the condition of 5 MPa injection pressure choose three levels to test (table 2). Mold temperature is under the condition of room temperature because it is limited to test conditions and not listed as one of the variables. Wax injection speed is controlled by the ball valve of injection machine cerate channel. Although the form of table 2 and the design of four factors three levels orthogonal experiment are the same, it can’t be carried out in accordance with orthogonal design usually for quantitative analysis since the quality of wax pattern is only qualitative assessment. So, the test is not using the orthogonal experiment method, but according to production experience. Keep the three factors in the second level based on the second level and respectively verify the fourth factor from three levels. For example, when studying the effect of cooling time, keep wax injection temperature 58 ℃, wax injection speed 8 mL/s, holding time 25s, and cooling time 5min, 10min and 15min for three testing in turn.
 

Effect of pattern process parameters on the quality of wax pattern
(1) Pattern temperature. When the wax injection temperature is high, wax material viscosity is smaller, liquidity is good, the wax slurry is easy to produce splash when injection in the mold, and it is easy to produce bubbles in the pattern surface. Thus, it is prefect if the wax injection temperature reduces to less than melting point temperature. For thin-wall parts, when the wax injection temperature is 63 ℃, the contraction of wax models is smaller, but the liquidity of wax material and filling drops significantly. When the temperature is 53 ℃, wax material flow ability drops, and surface of the suppression wax pattern is rough significantly. So for thin-walled wax, the injection temperature should not be too low. The wax pattern has no too much contraction, with good packing when the temperature is 58 ℃.
 
(2) Wax injection speed. The size of wax injection speed can reflect the duration of wax material filling. For wax patterns with fixed structure and size, injection time reduces when wax injection speed increases. For the wax pattern of thin-walled, wax injection time is long when the wax injection speed is 3 mL/s. If the wax injection time is long, decreasing amplitude of wax liquid temperature is large in the process of filling, thin film solidified layer gets thick and wax liquid viscosity gets stronger. In the condition of fixed wax injection pressure, filling capacity gets lower, the corner of mould can't fill well, making the wax mold filling insufficient and get defects. It is easy to have wax liquid flow field defects and reduce the surface quality of wax pattern. When improving wax injection speed to about 8 mL/s, wax liquid filling ability improves significantly, the outline of wax pattern is complete, and defect of surface flow ripple is eliminated. While as a result of too high wax liquid flow rate, wax liquid will splash in mold cavity volume gas when filling. Former injection wax liquid is adhesive on a mold surface. The wax mould surface form a separation layer, which leads to bulge and makes the surface be peeled off easily and finally causes scrapped wax pattern.
 
(3) Pressure maintaining time. Maintain the pressure for a certain period of time after wax injection. In the case of wax injection, pressure and temperature must be fixed, for a holding time of 20s. If time is too short and the wax material temperature is still high after the internal pressure’s eliminating, solidification shrinkage of wax mould becomes bigger, resistance capacity of shrinkage strain get weaker, and mould gets a little deformation. So the holding time should be kept to make the wax mold strength enough to resist shrinkage deformation. When the holding time is 30s, because of too long holding time, the wax mould closes to the core after cooling contraction. It is difficult for drawing, and even easy to damage the mold. Besides, wax material in wax injection hole has already curdled after pressuring for a certain period of time because the hole of the mould is small. At this time, it has no feeding effect for internal wax mold shrinkage if you continue to maintain pressure. Therefore, the holding time need to be moderate. When the holding time is 25s, the thin wall wax mould almost has no distortion, and it is easy to deliver mould.
 
(4) Cooling time. Better cooling time of wax mould is 5 min, but wax mould still has shrink, which affects the size precision after cooling. In order to make the size of the thin-walled package more stable, extend the cooling time of wax pattern in the mould. The size of wax pattern change is very small in 10 min, and the size is almost the same as in 15 min. To improve the work efficiency, keep cooling time 10 min and we can get stable wax mould size.
For thin wall parts, under the condition of 5 MPa wax injection pressure, we can identify the best mould process - taking the second level of various factors - after doing many trials comparing and overall considering the influence of various process parameters on the surface roughness of wax
 
1.4 Preparation of wax mould
Mould process: checking the mould→wax injection→press pattern→cooling→delivery→cleaning→finishing. Use mold knife to remove fins on the wax mold parting surface and other wax injection remains. Use repair wax to fill the defects and smooth finish. Clean the qualified wax patterns in the purge tank to remove parting agent, then use compressed air to blow away residual moisture on the surface of the wax pattern. Install and weld wax pattern at last. Weld two symmetrical wax moulds together, which needs a high accuracy. When welding, if wax pattern’s two symmetry parts of the upper and lower welding position are not allowed, maybe it will affect the precision of the wax moulds, making the two parts of upper and lower not be in the same center axis. Therefore, when we design an auxiliary positioning tool, its diameter should be equal to the inner diameter of wax moulds, the length should be greater than or equal to two-thirds of the wax pattern height. Set in wax pattern symmetrical between two parts when welding, so that two parts of upper and lower will be in the same center axis to meet the requirement of precision positioning. For the wax models and the casting rods, its accuracy is not high, thus it only needs to be welded firmly. Finish installed and welded wax pattern with pouring system (see Fig.3).
 
                           

1.5 Wax pattern defect analysis
Analyze reasons of four kinds of wax models defects most commonly occurring in the test (Fig.4). The major reason of cold insulation or misrun (Fig. 4 a) is that the air in the cavity can not be ruled out very well when pressing wax pattern. Mould material injection temperature is too low, and wax liquid’s liquidity is not good. Solutions are to appropriately improve the injection temperature of the mold, improve the pattern material’s liquidity, and clean up the air flow every time to avoid congestion. The axial crack defects in the wax pattern in the testing can be seen in Fig.4b. The main reason of this kind of defects is that the mould temperature is so low, which makes the pattern material temperature cool down too fast. Other reasons include to small parting agent or uneven daub. Solutions are to appropriately improve mould temperature, increase the dosage of the parting agent and evenly daub. The bulge defects in the experiment are shown in Fig. 4c. The reason is too high wax injection rate, bad profiling breathing, too short cooling time of investment in the mould, or too high room temperature. Solutions are adjusting valve and reducing the wax injection rate. Lengthen the time suitably and accelerate cooling in the cold water immediately after delivery to avoid the defects. Wax pattern has the fluxion of defects as shown in Fig. 4d. The main reason is that the parting agent dosage is too much, or daub is uneven. If the wax injection temperature is low, the solution is to improve wax injection temperature appropriately, reduce the amount of parting agent and daub thin and evenly, to avoid the flow lines.
 
 

2. Summary
Design suitable moulds through the analysis of casting structure and considering the factors of die joint and profiling size. Comprehensively survey wax injection temperature, injection speed, holding time and cooling time impact on the quality of wax pattern, and get the good pattern process. We can get the good quality of the pattern, when the wax injection pressure is 5 MPa, the holding time is 25 s, the wax injection temperature is 58 ℃, injection speed of wax is 8 mL/s, and the cooling time is 10 min.
 
The ratio of the third layer and after paste powder liquid, (refractory clay 50% + mortar powder 50%): water glass=11.3~1.4, and 8-12 earthen bowl sand. Use the crystal aluminum chloride solution, with 6.7% Al2O3, 30 min of hardening time, and 40 min of drying time after hardening. Place the module overnight after finishing shell. Use hot water to dewax and dry for 1 day, then conduct roasting and pouring.
 
Production practice shows that sodium silicate and special precision casting powder has a good solubility, slurry flow smoothly, uniform investment pattern starching attachment, good smearing and sanding operability. Deep hole and slot without adhering sand, to achieve stable quality, good shell pie collapsibility after pouring, which is suitable for 5 ~ 10 kg of casting. Use mold material with low temperature to pour 1.4408 austenitic stainless steel casting, the casting surface roughness will be Ra 6.3, meeting a composite technology level of silica sol - water with high dimensional accuracy.

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