How to estimate parameters of new die casting machine for quick test?

Time:2020-09-11 09:21:22 / Popularity: / Source:

Purpose: Quickly select die casting process parameters of die casting die under given conditions. Before new die is commissioned and manufactured, its die casting process parameters are calculated in advance. Based on this, in actual commissioning and production, less detours in process parameter settings. Quickly complete mold debugging and produce qualified products.
Cost of die casting mold accounts for a large proportion of cost of die casting parts, and cost of die casting molds is allocated to cost of each die casting parts. This requires us to minimize number of unnecessary mold production times to improve overall life of die casting mold, to minimize allocation of cost of die casting mold to cost of each die casting, and to create greater benefits.
As for how to improve life of mold, we most often think of mold temperature control systems, regular stress relief treatment and surface strengthening of mold forming part, reasonable pouring overflow system, using process parameters such as lower pressure, speed and temperature while meeting product requirements. However, process of debugging and production of new molds is often ignored. If this process is not controlled, it may even be possible that number of mold production times has reached the first stress relief times, but commissioning has not yet been completed, products that meet customer requirements have not been produced. This virtually increases cost of a single die casting. In order to avoid this situation as much as possible and lay foundation for future production, it is very important to quickly determine die casting process parameters of new die casting mold under given conditions.

First, selection of die casting machine

Before mold is manufactured, mold designer should work with mold's die-casting technician to determine die casting machine to be used and determine diameter of pressure chamber.

Second, quickly determine die casting process parameters

die casting process parameters 
Take cold-chamber die-casting machine for aluminum alloy die castings as an example. According to three-dimensional model of mold, metal weight of each mold of product G0 (kg), net weight of product G1 (kg), total weight of slag collecting tank G2 (kg), total projected area of parting surface S (m2) can be obtained, together with rated clamping force T (N) of die casting machine and diameter D1 of pressure chamber that has been determined in advance, which used as basic data for determination of following process parameters.

1. Determination of shot-to-shot ratio Po

Limit specific pressure during shot: P limit = T / S
In formula, T mean die casting machine rated clamping force; S mean total projection area of parting surface.
Po is less than P limit to avoid mold expansion in production, and according to product structure, appearance and internal quality requirements.

2. After injection cylinder of die casting machine is pressurized, pressure P1 is determined. After injection process is completed, force acting on punch and injection cylinder piston is same, that is:

Therefore, die casting machine with real-time control can set P1 directly in its control computer; ordinary die casting machine basically adjusts opening process of booster valve manually, and adjusts nitrogen filling pressure of booster accumulator to complete setting.

3. Determination of shot velocity

(1) Low-speed shot V1 in first stage. Generally it consists of two parts. First is that punch is from stationary to just after pouring. At this time, a slow speed is required. Main reason is to prevent alloy liquid from overflowing from pouring port, which is conducive to gas discharge. Secondly, before metal liquid continues to be filled into inner runner (speed at this time is greater than previous part), mainly to avoid gas entrainment in alloy liquid, and to avoid alloy liquid entering cavity in advance as much as possible.
Reference data: Generally, it can be set to 0.1~0.5m/s; thin-walled parts and exterior decorative parts are 0.25-0.35m /s; high compressive strength parts are 0.15-0.25m/s.
(2) High-speed shot V2 in second stage. When alloy liquid reaches runner, high-speed switching can be performed, so that alloy liquid is filled under high pressure and high speed. Empirical data: High-speed injection speed: more than 2 ~ 4.5m/s, high-speed injection acceleration time t1 is 0.01s, and supercharging time t2 is 0.01s.
(3) Slow down before end of metal filling in third stage. Increasing deceleration action before end of filling can reduce impact of alloy liquid at the end of filling, protect die casting mold, reduce occurrence of flash. However, it should not be set too early, otherwise it will affect filling effect.

4. Selection of important injection speed switching positions

(1) Normally, position of starting point of high-speed injection is at Ⅱ(normal speed switching position), that is, when alloy liquid reaches runner.
(2) For die castings with high surface quality requirements, you can advance switching position between I and II.
(3) If it is desired to reduce local pores of die casting, switching position can be delayed to important part of die casting, that is III, to reduce pores in important part and increase compactness. However, great care should be taken to prevent cold filling of die casting caused by too slow filling speed. This method should not be used when important part of die casting is at the end.
(4) For large die castings and large die casting machines, switching position can be set at about 30% of alloy liquid entering cavity to reduce generation of porosity.
(5) When switching position is below I, volume of air volume is large, and it is not recommended.
Following data calculations are performed based on normal speed switching position for study object.
L0 is low-speed injection stroke, that is, punch stroke at which alloy liquid reaches high-speed injection switching position, L1 is high-speed injection stroke, that is, length of alloy liquid in pressure chamber that is sum of net product weight G1 and total weight of overflow system G2, so L1 can be calculated by:
Density ρ of alloy liquid in above formula, and aluminum alloy liquid can be calculated as 2.65 XlO3kg / m3.
L2 is thickness of material handle (empirical data is 30-50mm).
L = L. + L1 + L2: It can be obtained through measurement after pouring hot pressing chamber.
Based on measured L, calculated L and self-determined values, it can be obtained, switching position of high-speed shot is determined.

5. Related settings for boost pressure

In cold chamber die casting, pressure build-up time indicates response speed of boost pressure. Ordinary die casting machine are implemented by adjusting boost speed and adjusting handwheel. Advanced die casting machine can directly set curve of boost pressure and time on control panel. Starting point of pressurization process can be triggered by position, pressure and speed.
Generally speaking, it is easy to set and adjust by triggering pressurization of setting position. Empirical data of this position setting is: 10 ~ 30mm before the end of the die casting stroke.

6. Setting of pouring temperature and die casting mold temperature

(1) Pouring temperature can be set according to alloy grade and quality requirements of die casting.
(2) Temperature of die casting mold can be controlled at about 1/3 of pouring temperature. Thin-walled and complex-structured die-casting parts can be appropriately raised, but it should be noted that mold should be preheated before starting production, and preheating temperature should be controlled at 150 to 180℃.

7. Setting of holding time and mold holding time

Aluminum alloy die castings are recommended based on wall thickness holding time and mold retention time.
If after above process parameters are set and adjusted according to die casting parts, quality requirements of product have not been met, pouring overflow system on mold needs to modified and adjusted.

Third, conclusion

die casting die 
There are many conditions for producing qualified die casting parts. Above-mentioned selection of die casting process parameters is only one of them, such as design of pouring overflow system of die casting mold, precision of mold manufacturing, state of die casting machine, technical level of die-casting operator, choice of coatings for die casting will affect quality of product. When a problem occurs, it should also be considered from multiple angles and aspects, not limited to a certain aspect, so that problem can be solved quickly.

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