Who is responsible for optimizing mold cooling and why isn't more emphasis placed on it in design and production? Interestingly enough, for such an important part of the molding cycle (80 percent is cooling time), traditional practice is such that no one takes direct responsibility. The molder assumes that good cooling design is inherent in the tool shop's quotation. Tool shops typically do not prioritize mold cooling, nor are their designers necessarily proficient at heat transfer issues. Their forte is cutting steel and producing a mold as economically as possible. Each assumes that the other party is taking responsibility when, in fact, neither party does. There is a hidden cost associated with this industry-wide disconnect. Poorly cooled parts increase cycle time, scrap and dimensional problems.
Additionally, cooling loads for each fabrication process can differ significantly for similarly sized parts. For instance, a blow-molded part can only be cooled - by the mold steel - on the outer surface. The inner surface of the part is a hollow cavity. Because of this, there is minimal to no cooling on the inner surface. For blow molding, all of the part cooling is in the direction of the outer wall of the part. Comparing an injection molded part of identical thickness to a blown part, cooling takes place on both sides of the part. The injection part will cool much quicker, resulting in shorter cycles. Therefore, the techniques used to cool the parts for each process type must be well planned to ensure competitive advantage.
Blow mold tooling also has a history of using "flood tooling." This practice involves the use of cast molds with large open cavities for water flow. However, these systems do not concentrate cooling water at the critical part locations such as thicker wall sections or tail flash. This technique fails to provide turbulent flow of water for maximum heat transfer. A drilled passageway system in a cut steel tool will allow for optimized flow rates through passageways and selective placement of passageways where cooling is most required. A drilled passageway system is recommended for any system requiring high-performance cooling and definite temperature control.
The important aspects of mold cooling can be summarized under the following five categories:
- Thermal properties of the plastic being molded and the materials of construction for the mold.
- Energy balance from melt preparation to cooling cycle time.
- The influence of coolant flow rate on heat transfer efficiency.
- Mold temperature controller selection.
- Design practices for optimum mold cooling.
First and foremost is to gain an understanding of the thermal properties associated with heat transfer from hot plastic parts to the tool steel and, finally, to the cooling medium. It is not generally recognized that there are significant differences in the heat content of different plastics and the heat transfer rate through different types of mold materials (steels, alloys, etc.).
Plastic Heat Content
The heat content of a plastic is a parameter that is often not considered when sizing mold temperature controllers and designing cooling systems for plastic molds. Every plastic requires a specific amount of energy (per pound) to plasticate the solid resin pellets. Some examples would include:
Likewise, this identical amount of heat energy also must be removed in order to form a stable part. Essentially, energy out must equal energy in. Note that all crystalline materials require almost twice as much heat energy as amorphous resins to plasticate. This is not usually a problem in melt preparation, although feedscrew design will influence this. However, it does imply that for olefinic materials, twice as much heat must be removed from the tool, and often in the same cycle time as for a competitive amorphous resin. The tool, therefore, will require far superior mold cooling for olefinic resins to remain cycle time competitive. This is a critical issue due to the crystallinity of these resins, since slow heat removal will influence crystal growth and affect warpage and dimensional stability of the finished parts.
As many industries down engineer from ABS or PC to resins such as PP, this obviously implies that mold cooling is more critical than ever before.
|THERMAL K OF MOLD STEELS AND ALLOYS (@68xf)|
|Figure 1: Optimum mold cooling|
Conductivity of Typical Mold Steels
In Figure 1, it can be seen that there is a great deal of variation in the thermal conductivity (K) of typical mold materials. K is the rate at which heat can travel (or transfer) through the material. The higher the value, the more effectively heat is transferred. The units simply indicate a measurable quantity of heat per unit time - all other properties being equal.
Copper is an excellent heat transfer material (10 times P20), as is aluminum. However, both are soft materials and are not used for large-scale production tools. Titanium is a hard metal with a very low thermal K value. This poor heat transfer characteristic allows titanium to be effectively used for insulator plates in hot runner systems. If maximum heat transfer is required in a critical area, beryllium copper alloy is best, combining excellent heat transfer and hardness.
Water and Heat Transfer
Without doubt, the most critical - and totally within our control - aspect of mold cooling is coolant flow rate. Recall from the thermal K chart that water (standing) is 50 times less effective than P20 steel. Therefore, water is the limiting factor in heat transfer. However, flowing water has significantly better heat transfer due to turbulent flow. Turbulent flow allows for mixing of the coolant and sweeping the heat from the cooling passageways. Turbulent flow can be calculated from the Reynolds (Re) number. This is a unitless value based on the passageway diameter, the coolant velocity and the viscosity of the cooling media. A value greater than 5,000 implies turbulent flow and excellent heat transfer. The more turbulent the flow, the better the heat transfer.
Examining the formula shows that for a given existing tool, the pipe diameter cannot be changed, the coolant remains the same and, therefore, only the coolant velocity can be altered to positively influence Re number. Velocity is GPM. Increasing GPM will greatly improve both heat transfer from the steel to the coolant and also improve temperature difference (delta T) across the mold temperature controller. The golden rule for optimum cooling is to maximize GPM.
The end result is that turbulent flow improves all aspects of heat transfer. Therefore, since turbulent flow requires high coolant flow rates, GPM should be maxed out at all times.
If this is so obvious, why is GPM not measured directly? Why is it not recorded for SQC and prototype tool trials? Why is it not found on most mold temperature controllers if it is the most important and only influencing parameter for mold cooling? As a moldmaker, what can be done to ensure coolant flow is not restricted by tool design?
It is highly recommended that all mold temperature controllers on critical jobs include either a built-in or aftermarket flow meter.
Temperature Controller Selection
The amount of heat to be removed from the mold differs depending on the resin being processed. Additionally, the rate at which the heat can be removed also varies based on the materials of construction of the mold. Therefore, the sizing of a mold temperature controller must consider all of these variables or it may be undersized, resulting in excessive cycle times.
Energy in will always equal energy out. If the cooling system or mold cooling design is not adequate, the energy will still find a way out. However, this is usually via too high a mold temperature controller delta T across the tool, or the part is de-molded with too much retained heat, or the cycle must be extended to allow enough time to remove all of the heat. The challenge is to get all of the energy out under the right conditions.
Cooling Line Placement
Consider the actual part design when placing cooling lines in the cavity and core steel. All too often, the line placement is made after all other design issues, and there are usually no options left to optimize cooling through good line placement. Anticipate these issues early in the design. If the part has a thicker section, then consider placing the line slightly closer to the wall or placing two smaller diameter lines in place of one. Cooling of deep cores is always a challenge. As the part cools, it will shrink onto the core and pull away from the cavity. Therefore, about 80 percent of the cooling is from the core steel. Yet the core has the smallest surface to volume ratio (compared to the cavity), and the ability to get adequate cooling water in this confined space is typically extremely difficult. This is why most cores operate quite hot.
When cooling becomes a challenge for simple passageways, there are other options. Hard-to-cool locations like cores can be cooled with baffles, bubblers and heat pipes. However, be cautious as there are many different designs for each option and many simply represent the lowest cost standard item that a tool shop provides in a low bid. It is best to specify the design and not rely on tool shop expertise in cooling. Many tool shops know very little of optimizing mold cooling, yet most molders assume it is a given that a tool shop supplies a perfectly cooled mold. Recent surveys of several reputable tool shops have confirmed that mold cooling is the last thing to be considered and often only standard practice rules are all that are used.
Baffles and bubblers are very similar in design and intent. Both take cooling water from a local cooling passage and deliver it to a hard-to-reach location such as in a core. In a baffle, the water flows into a drilled passageway into the center of a core. The passageway is split in half with a steel baffle, which allows the water to flow in on one side and back on the other. The baffle does not reach the end of the passageway, thereby allowing the water to cross over. A good design ensures that the smallest cross sectional area is in the baffle half. This maximizes local velocity and, therefore, turbulence.
When plumbing a tool, parallel flow is preferred over series flow. Series flow goes in at one end and travels through the entire tool before exiting. This design results in maximum pressure drop and a large delta T across the tool - non-uniform temperature across part and potential warpage. Parallel flow minimizes delta T, thereby ensuring uniform temperature across the tool. Pressure drop is low across a tool with parallel flow.
Practical Mold Design
It has been well established that GPM - or local coolant wall velocity - is the most important factor in optimizing mold cooling. So what is holding back maximizing GPM? The answer is pressure drop. Every unnecessary restriction in the flowpath reduces GPM. Every hose connection, elbow, reducer, kinked hose, excessive hose length, etc. - all contribute to pressure loss, and, therefore, reduce the GPM. Enough restrictions and flow drops to near zero. Once flow is such that there is no longer turbulence, heat transfer drops significantly. To balance energy out with energy in, the return cooling water temperature rises. This increase causes part dimensional instabilities due to excessive thermal variation across the part.
The more the pressure drops, the larger the requirements for pump horsepower in the mold temperature controller just to keep the flow rate consistent. Conversely, if restrictions can be eliminated from an existing system, the pump can now supply more GPM (this is free heat transfer). This is equivalent to an aerodynamic car getting better gas mileage.
The biggest misconception in understanding available GPM from mold temperature controllers is that only the pump curves are provided by the supplier. A system curve for the tool is never provided by the tool shop. For example, a 40 GPM pump rated at 25 psi does not imply that it can deliver 40 GPM. The pressure drop is not known for the tool and all the hose connections. This can be easily determined and, in fact, this data should be supplied by all tool shops upon delivery of the tool to the molder. The pump is the engine and the tool is the car body. The two have to be matched in order to determine the resulting performance. A small engine in a large, heavy car will not perform. Likewise, an undersized mold temperature controller pump will not develop turbulent flow in a large, high-restriction tool. The tool characteristic curve must be matched to the pump curve (see Figure 2).
Since restrictions affect GPM, if a tool is connected to a good mold temperature controller one day, and another with different hose diameters and lengths the next, GPM will vary day to day. Turbulence varies, heat transfer varies and the cooling rate is different - finally affecting part quality day to day. For this reason, mold temperature controllers must be connected consistently day after day.
Again, since restrictions should be minimized to ensure maximum GPM, a good rule is that the minimum restrictions should only be in the cavity and the core. These are where the turbulence should be maximum - which is what the small restriction will do. There is no point in having turbulence - which does consume pump horsepower - in a location that does not need heat transfer, such as coupling, reducer, etc.
|OPTIMUM MOLD COOLING DESIGN RECOOMENDATIONS|
Flow Rate (GPM) Determination
Minimum GPM = 3.5 x pipe I.D. (for good Re #). Also consider the GPM required to remove total heat load. Whichever is greater must be available from thermolator.
Each line requires 1.75 GPM for good Re #.
10 lines x 1.75 = 17.5 GPM.
The heat load also must be determined:
Design all passages for turbulent flow.
Know the heat load for the part.
Provide the minimum GPM to the molder.
Provide a "tool characteristic" curve.