Mold Design - Temperature Control
When discussing the entire concept of die-casting molds, we cannot neglect the aspect of temperature control. It is a vital aspect of the die-casting industry. As a result, we will be discussing it in this blog.
1.Why Study Mold Temperature Control?
In the whole process of die casting production, we need to control the temperature of the mold. Poor mold temperature control will cause bad problems in production. The main problems include the following.
- Product quality: defects such as cold flow, shrinkage porosity, flow lines, sticking, etc
- Mold life: Unbalanced or high mold temperature will reduce mold life (mainly cracks).
- Production efficiency: Proper mold temperature control can reduce downtime and speed up production, greatly affecting production efficiency.
The role of mold temperature control system:
- Prevent shrinkage cavity: make the mold achieve better thermal balance and improve the sequential solidification conditions of the casting so that the solidification speed of the casting is uniform and conducive to pressure transfer, and the internal quality of the casting is improved.
- Prevent cold isolation: maintain the fluidity of the die-casting alloy when it is filled, have good formability, and improve the surface quality of the casting.
- Prevent the deformation of castings and prevent the biscuit from bursting: improve the productivity of die casting.
- Anti-high temperature difference: reduce the alternating thermal stress of the mold and improve the service life of the mold.
- Stabilize the dimensional accuracy of castings and improve the mechanical properties of castings.
2.History Of Mold Temperature Control:
The cooling water circuit can only be designed by experience at the beginning of die casting. In 1924, someone proposed the electrical stimulation method to simulate the isotherm, and then GM first applied it to the cooling water design. Then, in 1970, a simple mold flow analysis (2D flow analysis) appeared in the field of injection molds, that is, MOLDFLOW.
In the 1980s, casting software such as flow3d and magma soft came out one after another, and the simulation of the temperature field was an important part. However, temperature control of the mold is probably the most empirical part by far.
3.Heat of the mold
The temperature of the mold is not a constant but dynamic equilibrium.
The main sources of mold heat are the heat energy released by the molten metal. In addition, the mechanical energy of the molten metal is also converted into thermal energy.
The heat dissipation of the mold mainly depends on the heat conduction of the cooling water circuit of the mold and the heat radiation transmitted to the environment around the mold. A part of the heat is taken away when the mold release agent is sprayed.
Production is smoother only when heat input and output reach a dynamic balance within a set temperature range.
Problems we often encounter in actual production:
- The mold is locally too cold, which causes the molten metal to be unable to be filled smoothly, resulting in product defects.
- The mold part is too hot to dissipate heat smoothly, which will shorten the life of the mold.
If the mold part is too cold, we need to heat it; if it is too hot, we need to cool it. Make good use of these two methods to solve the problems actually encountered in production.
4.Heating And Cooling Of The Mold
The mold needs to be heated in the following instances
- At the beginning of production, the mold needs to rise from room temperature gradually, and the mold needs to be preheated.
- When the local temperature of the mold is too low, the mold needs to be heated locally.
Preheating of the mold
The preheating temperature of conventional molds has different temperature requirements according to different alloy types, as shown in the figure below.
Common ways of mold preheating:
- At the beginning of the cold mold, use slow injection, and after a dozen or even dozens of times, heat the mold.
- Use gas or natural gas flame to heat the mold.
- Use an electric heating rod to heat the mold.
- Use the mold temperature machine to heat.
How to heat the mold in the cold mold state?
Generally, at the beginning of production, the molten metal is slowly pushed into the mold cavity slowly. Heat the press chamber and sprue sleeve first, then heat the inside of the mold. At this stage, high speed and pressurization cannot be used to prevent the molten metal from entering the fitting gap and blocking the mold.
The order in which the cooling water of the machine and the mold is turned on in sequence:
Punch (plunger) → press chamber → sprue sleeve → inner gate → core and core
What should be done when a large area of cold streak defects occurs in die castings?
First of all, try to shorten the production cycle of die casting as much as possible so that the temperature of the mold rises. It is usually not easy to increase the pouring temperature, and the high pouring temperature will shorten the life of the mold.
What should be done when the die casting has a small area of cold streak defects? One is to open an additional slag bag (overflow) to allow the cold streaks to flow out and increase the mold’s local temperature. The second is to reduce the material on the mold to reduce the loss of heat.
Cooling of the mold
The following are the common ways to cool molds are:
- Water cooling: It is carried out through the cooling water circuit of the mold;
- Air cooling
- Heat pipe
- High heat transfer metal
- Mold temperature machine
This involves the direct cooling of the core with compressed air. It is usually used when the core is relatively small and cannot pass cooling water. The cooling effect of air cooling is very limited. We usually spray release agents and use compressed air to blow off moisture, which is also a way of air cooling.
Heat pipe cooling:
The heat pipe is a closed tube, one end of the tube is put into the mold, and the other end is in contact with cold water. The medium in the heat pipe evaporates when heated, and after entering the condensation area, the heat is dissipated, and the heat is taken away by the cooling water.
As shown in the figure on the left, 1 is the evaporation area, 2 is the condensation area, 3 is the steam, and 4 is the capillary layer.
High heat transfer metals:
In the part where the mold forms a hot spot, an alloy with a high heat transfer coefficient is used for indirect cooling—common heat transfer alloys such as beryllium bronze, tungsten-based alloys, etc.
But the disadvantage is that the material is expensive and the cost is too high.
The most common mold cooling method is water cooling. Cooling water is passed through the pipeline channels machined inside the mold to take away the heat of the mold.
5.Importance Of Water Cooling Design
The quality of the waterway arrangement directly affects the molding quality and production cycle (cost) of the product.
- Impact on quality: The water circuit is used to control the temperature of the mold during molding, and the temperature of the mold and its fluctuations have an impact on the shrinkage, deformation, dimensional stability, mechanical strength, stress cracking, and surface quality of the product.
- Impact on production cycle: A molding cycle is mainly composed of four parts, including filling, pressure holding, cooling, and mold opening. The filling time is about 5% of the entire cycle, and the packing and cooling are 80% of the time. Therefore, shortening the cooling time is to improve the molding efficiency.
6.Ways of water cooling
It is suitable for the structure where the forming part is generally not deep. There is no need to cool a certain part separately. It only needs to continuously take away the heat of the mold from the entire forming part.
It is suitable for a single part of deep cavity molds and cores, etc., and the phenomenon that the molding part has only a few deep cavities or a single core pulling of the deep cavity part is more common in die castings.
7.Water Cooling Part
7.1 Hot Spot
7.2 Cooling of sprue sleeve: The wall thickness of the sprue sleeve is thick, and it takes a long time to absorb the heat of the molten metal. It is necessary to strengthen the cooling to improve the production efficiency. It will be matched with the punch here. If the cooling is not in place, it will easily cause the material to run out or be damaged.
7.3 Sprue Spreader cooling: The Sprue Spreader is surrounded by molten metal, and it is difficult to dissipate heat. The Sprue Spreader is usually cooled by a water column-type water channel.
7.4 Cooling of shallower cavities: directly use punched holes to process water passages.
7.5 Deeper cavity: The core is wrapped by molten metal, similar to a diverter cone. The temperature rises quickly, and it is not easy to dissipate heat, so cooling should be strengthened.
Common ones are spacer cooling, water column cooling, and heat pipe cooling.
7.6 Slide core
7.8 Outside the venting area: mainly to prevent the molten aluminum from splashing or generating excessive flash.
8.Design of Cooling Water Pipes
Water column type
9.Design Considerations For Cooling Water Channels
There are a few things that play a huge role in an efficient design in terms of the water channels. So we should have a view on these factors.
- Try to use cooling water channels and nozzles of the same specification to avoid design and manufacturing complexity.
- The diameter of the cooling water channel is generally 6-14mm. Therefore, the effect of using multiple small-diameter water channels is better than that of a single large diameter water channel.
- Generally, the outlet pipe diameter can be slightly larger than that of the inlet pipe, mainly considering the water pressure.
- If it is a point cooling water channel, it can be closer to the cavity wall.
Problem 1: Turn on the cooling water far away from the gate, and the product produced is similar to the cold mold product.
Reason: Not sure if it should be heated or cooled.
Measures: The remote end is prone to defects such as cold streaks and cold partitions. Generally, cooling water should not be passed through, but oil should be passed through to increase the mold temperature.
Problem 2: Turn off the cooling water directly and find the effect is useless.
Reason: The cooling water problem is one-size-fits-all, and the cause is not carefully analyzed.
Measures: Adjust the flow rate of the cooling water temperature
Problem 3: The waterway is blocked.
Such problems are indeed prone to occur,
- If the water quality is too poor or hard water, it is recommended to use soft water as cooling water and add certain preservatives.
- If a scale is produced, add a descaling device and a filter device, which can also be removed and drilled out.
- The pool is easy to rust and is sucked into the pipeline. The stainless steel reservoir is used, and the suction port is a certain distance from the bottom of the pool.
Adopt systematic cooling equipment
Question 4: What is the appropriate temperature difference between the inlet and outlet water?
- This kind of problem is the problem of cooling water system technology. Generally, the inlet and outlet water temperature should be controlled at about 5℃.
- The temperature of the entering water is about 30°C at room temperature, which should be increased in winter.
- If the temperature difference is too large, it is due to adjusting the cooling water flow (if there is a mold temperature monitoring device, the monitoring data should be used as the basis for adjustment).
Question 5: The cooling water circuit is leaking.
- It is generally the problem of the sealing ring.
- The cooling water is too close to the cavity, causing the cavity to crack.
- The sealing ring should be purchased with high-temperature resistance.
- When the production is stopped, the cooling water needs to continue to be turned on and then stop when the cooling is in place.
Let’s check this product information:
- Weight: 2kg for the whole mold
- Wall thickness: 1.8mm
- Machine information: LK 800T
Product problem: The R corner is seriously blistered after heating near the gate.
Case study: Usually, there will be more foaming at the end. This product has no foaming at the end but foaming at the front. It is estimated that the mold temperature at the foaming position is too high, resulting in a thin dense layer, and the gas is easy to bulge after heating.
Solution: In a bid to enhance the cooling of the mold temperature at the defect position, it is recommended to increase the cooling water.