investment casting wax part cooling time determination.

by:Keyuan     2020-06-11
1.
A case introduction made by Xiaobian (
Buchmeister, etc. , 2008)
Silicone rubber molding (SRM)
Is the most cost-effective method.
SRMare is usually used for castings of polyurethane, wax and low melting point alloys, even in the food industry (Almond cookies).
In the case of polyurethane, we know exactly when our products are ready (
Aggregation completed).
In the case of wax parts, when our wax parts are completed, there is no simple and accurate way to determine the time (hardened).
The result is the damaged part, which can be fixed in some cases, but usually needs to be cast again.
This resulted in excessive waiting for the wax part to cool in order to ensure that the wax part was properly hardened.
Therefore, as a rapid mold technology, it wastes a lot of time and reduces the efficiency of investment casting.
In order to determine the time required for wax hardening in SRM, we need to consider some parameters.
We can find some useful parameters and equations from thermodynamics.
We have two choices: 1.
One of the commercial software used for casting simulation (
Cosmos, Moldex, Cosmos/Flow or similar more advanced methods (Kovacic et al. , 2005).
In this case, we need to adjust the parameters and get the proper material properties.
Casting can then be simulated.
This program can give us good results, but it takes a lot of time because we need accurate mold CAD models and proper parameter preparation. 2.
The use of simplified equations for experimental selection of parameters.
This paper presents the experiments required to determine the time required for cooling of wax parts, and derives the simplified equation for determining the cooling time from the basic mold and casting parameters. 2.
SRM and cooling time--
The time from casting to when we open the mold depends on several major influences.
Usually we need pre-
Because the wax needs a slower cooling curve to get his shape.
The most common defects of rapid cooling are cracks and excessive material shrinkage.
The mold used in the experiment is pre-heated to70[degrees]C.
The wax used in our experiment is industrial wax from companyMCPHek type WA-70 (MCP-Hek homepage)
In 68-70[degrees]
Recommended casting temperature for C and 80 [degrees]C.
The cooling temperature is 30 [degrees]
C lasts in the oven
The cooling time is from the end of the casting to the time when our wax product in the core reaches 35 [degrees]C. 3.
Several different molds are used for the SRM mold used for the experiment, but for the basic principle demonstration, only one is introduced.
SRM molds are usually block (Fig. 1)
, This shape gives us a uniform thermal distribution from the wax part to SRM to the surface. [
Figure 1 slightly]4.
Measurement and casting equipment temperature measurement using Metrix M-
4560A digital multimeter (Fig. 2)
It is equipped with K-type thermocouple.
Tolerance grade 1 for thermocouple+ or -]1. 5 between -40[degrees]C and375[degrees]C (Wikipedia--Thermocouple).
Check the measurement accuracy with ice beforewater mixture (0[degrees]C)
And boiling water (100[degrees]C)
To obtain the maximum possible measurement accuracy.
Casting wax in vacuum
MCP room 4/01 with wax heater. Vacuum-
The chamber is able to reduce the absolute pressure to 5 mbar where wax is cast to prevent non-
Uniform structure caused by residual air or water vapor.
In order to cool the mold in a controlled environment, an electronically adjusted oven was used with a preset temperature of 30 [degrees]C (With tolerance]+ or -]1[degrees]C).
Use the TFD 128 temperature and air humidity data recorder to ensure control of the surrounding air. [
Figure 2:[
Figure 3 slightly]5.
Combined amount of heat-
The amount of heat connection of Q wax part and silicone rubber mold is the amount caused by temperature change and melting heat: Q = [Q. sub. wax]+ [Q. sub. silicone](1)
The wax heat depends on the wax quality and temperature change (
The wax mold temperature is 80 [degrees]
Temperature is35 at C end of wax core 【degrees]C)[DELTA][T. sub. v]= [T. sub. zv]-[T. sub. kv]= 80[degrees]C-35[degrees]C = 45[degrees]C (2)
The specific gravity of wax, in our case, must also consider fusion heat (QiV = 160 [J/kg]). [Q. sub. wax]= [Q. sub. [DELTA]T]+ [Q. sub. [DELTA]]= [m. sub. v][C. sub. pv][DELTA][T. sub. v]+ [m. sub. v]x 160 [J/kg](3)
The calculation principle of silicone rubber mold heat is the same :[Q. sub. silicone]= [m. sub. s][C. sub. ps][DELTA]T (4)
Because the silicone is preheated to 70 [degrees]
The C-end temperature of silica gel is about 35 [degrees]C: [DELTA][T. sub. s]= [T. sub. zs]-[T. sub. kv]= 70[degrees]C-35[degrees]C = 35[degrees]C (5)
This is the heat that we need to get out of the system in order to cool the wax part to 35 [degrees]C.
Since the wax does not move in the mold after the end of the casting process, the heat is transmitted through conduction.
The ambient air temperature remains unchanged (
Or we assume in the equation). [DELTA]T = [T. sub. zv]+ [T. sub. zs]/2 -[T. zub. z]= 80[degrees]C +70[degrees]C/2 -30[degrees]C = 45[degrees]C (6)6.
Our starting point is a simplified heat transfer equation :[DELTA]Q/[DELTA]t = -K x A x [DELTA]T/[DELTA]x [? ? ][DELTA]t = Q/Kx A x ([DELTA]T/[DELTA]x])(7)
Because of simplification and some unknown parameters (
Material and geometric parameters)
We cannot calculate the exact time and need the parameter Kso approximation: K [? ? ][k. sub. B](8)Parameter [k. sub. B]
Experimental setup by measuring the cooling time. [k. sub. B]
All unknown parameters are given and the effects of all parameters are represented as linear.
Therefore, the required cooling temperature is: t = Q /[k. sub. B]x A x ([DELTA]T//[DELTA]x)(9)Where is: t--
Cooling time of wax parts Center, Q--
The combined thermal energy of our system ,[k. sub. B]--
Correction factor,--
[Outside area of silicone mold]DELTA]T--
[Average temperature difference of wax/mold/surrounding systemDELTA]x--
Distance from the center of the wax (
On the Bushes)
Shell and mold.
As we can see from Figure 3, 35degrees]
Reach C in 178 minutes. Parameter [k. sub. B]
Equation 9: derivative [k. sub. B]= Q/t x A x ([DELTA]T/[DELTA]x)(10)
What is our cooling time? 178 min); kB is: [k. sub. B]
= 47285,16 J/10680 s x 0,0658 [m. sup. 2]x(45[degrees]C/0,0548 m)= 0,082 [J/ms[degrees]C](11)7.
Conclusion investment casting is one of the indirect rapid manufacturing/processing processes (Badida et al. , 2006)
The key part of us is the wax core.
The wax piece must be absolutely perfect as this is the only way to get the part that meets the accuracy (Brezocnik and others. , 2004)and shape.
In addition to vacuum and vacuum casting, we also need to establish slow and uniform cooling (
Prevent specific cooling defects such as microcracks).
On the other hand, this is a small serial production program, we want to shorten the cooling time and speed up the production.
It is difficult to predict cooling times based solely on experience, as commercially available software is both expensive and time consuming, resulting in a relatively large number of inappropriate wax parts.
These parts can be reused as raw materials, but this will waste a lot of time.
The calculations and measurements proposed are only part of the study, as some unused factors still need to be considered in order to obtain better and more accurate final results.
The biggest error is caused by the shape of the wax part, resulting in errors around [+ or -]
15% between simple and complex shapes (Valentan et al. , 2008).
In order to calculate the time more accurately [additional experiments are being carried out]+ or -]
Between 5 and 7% are expected. 8.
References Badida, M. ; Hricova, B. & Vargova, J. (2006).
TMT 2006, Barcelona, eco-design and new product development, 10 international research/expert meetings: \"Trends in the development of machinery and related technologies\"
11-Spain, March-
September 15, 2006. Buchmeister, B. ; Pavlinjek, J. ; Palcic, I. & Polajnar, A(2008).
Whip effect in supply chain
Progress in Production Engineering and Management, Volume: Phase 3: Page 1: 45-55, Mar. 2008. Brezocnik, M. , Kovacic, M. & Ficko, M. (2004).
Journal of surface roughness material processing technology predicted by Genetic Programming, Volume: issue 157 special issue: SpIss. SI Pages:28-36, Dec. 2004.
Kovacic M, Brezocnik M, Pahole I, etc (2005).
Evolution programming of CNC machine tools
Journal of Material Processing Technology Volume: Page 164: 1379-1387, May 2005. Valentan, B. ; Brajlih, T. ; Drstvensek, I. & Balic, J. (2008).
Basic solution for shape complexity evaluation of STL data. J. Achiev. Mater. Manuf. Eng.
Volume: 26, iss. 1, Pages: 73-80, Jan. 2008.
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