Thermal Management of Permanent Molds
Click here to see this story as it appears in Modern Casting.
A casting cell in the permanent mold process cycles through five process steps to make a part: mold preparation, metal pouring, solidification, mold opening and casting ejection. The majority of the cycle time is dedicated to solidification. Properly managing the solidification and temperature of the mold is required to achieve the quality and productivity goals of the casting cell.
Casting geometry, material and process determine solidification time. Molding with a metal die or tool, as in permanent mold, is a heat extraction process. The mold’s job is to provide shape information and extract heat.
The rate you can extract heat drives the production rate. High speed sand molding releases heat quickly by producing new molds on a rapid basis. With metallic molds used in high pressure diecasting and permanent mold, a single mold must do all the heat extraction while periodically having hot metal poured or injected into it.
Several factors can affect solidification rate, such as mold materials, alloy type, and the geometry of the casting, but in this article, the focus will be on methods and practices used to control the temperature of the mold when those other variables have already been set.
Mold temperature can be managed using thermal management tools such as inserts, water coolant and forced air lines, mold coatings, and changes to cycle times.
Each casting or mold cycle has an operating window in which to achieve a quality part. In this window, the molten metal has the chance to flow through the gating to completely fill the mold without leaving voids behind because of insufficient metal feeding due to metal in the flow path that solidified too quickly.
The goal of a permanent mold casting operation is to regulate the solidification cycle, and controlling and monitoring the metal mold’s temperature is one facet to achieving that goal.
Permanent molds can be air- or water-cooled. Air-cooling the dies is the simpler method and produces less thermal shock, resulting in the longest mold life, but the capabilities for cooling are limited. In air-cooling, an evaporator removes heat from the mold and a condenser removes heat from the evaporator. Cooling fins and compressed air can enhance cooling rates.
In water-cooled molds, passageways for the lines are drilled in for precise application of high flow-rate coolant. Cooling lines are effective for extracting the casting thermal energy in localized or bulk regions. The heat flux of cooling lines depends on the Reynolds number as well as length, diameter, junction design and cleanliness. Depending on the specific region requirements, lines can be on continuously or controlled to turn off and on by time or mold thermocouple logic.
Water contained in cooling lines is chemically treated to prevent mold and bacterial growth or contamination of waterlines and water stone buildup.
Two categories of mold coatings are used in permanent mold: refractory for altering the rate of heat flow from the metal into the mold, and lubricating for releasing the solidified casting from the mold. Coatings often are applied to preheated mold pieces.
Mold coatings can prevent soldering of the solidifying metal to the mold, minimize thermal shock and control the rate and direction of solidification.
The use of inserts placed in a mold can help improve a poor feed path or direct solidification. Inserts made of materials such as titanium can slow cooling rates and increase the effectiveness of the feeder.
Cooling jackets control the direction of the flow of water for cooling. In most cases, the area must be flat. Jackets are suited for small and thin-wall castings and frequent mold changes. They aid in achieving a more uniform thermal profile, shorter process times, and repeatable ramping. Jackets also work to reduce hot and cold spots on the mold.
Vertical passageways can be sprayed with water from nozzles. The water transforms from liquid to steam and cools the mold, however the cooling lacks uniformity compared to other cooling methods. Another drawback to this method, unless distilled water is used is the buildup of minerals from the evaporated water.
Bubblers are used for targeted cooling where a larger circuit is not practical due to geometry constraints. For example, they may be located at the center of a deep metal core. The fluid comes from the main cooling channel and enters at the bottom of the bubbler, then flows up through an inner tubular device and cascades inside the unit. The fluid then flows down through an outer tubular device and exits back into the main cooling channel. Exiting water carries heat from the bubbler into the main cooling medium.
A cycle time refers to how long each casting is in the mold. Part mass, heavy sections, surface areas and how long a mold is open all affect cycle time. Changes to the casting design to reduce heavy sections or change their locations can aid in solidification. If that’s not possible, some metalcasters may alter the time a mold is open or a casting is extracted from the mold to allow the die to return to its optimal temperature for the start of a cycle.
Controls and Measurements
Thermocouples are necessary to record mold temperature and help alert the metalcaster when a mold is too hot or too cool and may lead to casting defects or misruns. This data is used to monitor the preheat, heater and cooling line timing and can be useful for defining the defect-free process window.
Metalcasters may automate their temperature control of permanent molds. For instance, using PLC logic control, a cell can be set up so that when a thermocouple located at key points registers a temperature that is outside the defined parameter, the part will be automatically rejected, or at a minimum an alarm signal is generated.
Piston Case Study
As the tooling for a part is mocked up in the drafting room, thoughts are initially on the design of the part itself. Part designer, mold designer and casting engineers all must consider the feed path, gating and risering for a quality part that is manufactured efficiently and effectively on the shop floor. However, in many situations, because of the way the part geometry works and other requirements to achieve the necessary mechanical properties, additional mold methods such as forced cooling, coatings, etc., are used for directional solidification. The ultimate goal in casting and tool design is to prevent turbulence in the flow of the metal and avoid leaving places where the solidifying metal is separated from the liquid metal in the feeder.
Two spout ladles are used to fill the permanent mold-cast piston in Figure 1. Casting process simulation proved that gating into the thin skirt wall would feed the hottest metal into the thinnest walls (Fig. 2). No cold shuts were predicted with an initial 65.6F (150C) mold temperature.
The compact design of the gating resulted in a relatively high metal velocity that may result in trapped air defects. Additional top wall stock was considered in the design, as well as cross hatching to aid air evacuation.
With the gating finalized, the mold design must be engineered. In the piston example, an optimized process would achieve a 60-second cycle time, no predicted porosity and a warm-up of four start-up castings before steady production.
A center core and slide water lines were added to the mold to realize the short cycle time through fast cooling (Fig. 3). Multiple angles of the side slide lines were needed to cool the in-gates, as well as at the top crown region.
Extraction tabs were added to the side for a stiff feature to secure the part when extracting it from the mold. The feeder was still in the mushy zone, which is the range between the liquidus and solidus temperatures of the alloy, at extraction and could be stressed at that time or its dimension would be deformed.
The cooling curves in the riser and extraction tabs in Figure 5 show a near-steady state behavior after four castings. Two small regions of 2% porosity were predicted that required further refinement of the water lines and their timing. This included the use of bubblers for targeted cooling in tight regions and high-conductivity mold inserts to act as heat sinks cooling a large mass region.
This article is abstracted from a section of the AFS Institute course, Permanent Mold Thermal Management, which will be held March 28 in Schaumburg, Illinois. You can register for the course here.