Temperature Control (Cooling System):
Mold cooling serves to dissipate the heat of the molding quickly and uniformly. Fast cooling is necessary to obtain economical production and uniform cooling is required for product quality. Adequate mold temperature control is essential for consistent molding. The layout of the cooling circuit warrants close attention especially if you consider cooling typically accounts for two thirds of a products cycle time.
Optimal properties of engineering plastics can be achieved only when the right mold temperature is set and maintained during processing. The mold temperature has a substantial effect on:
1. Mechanical properties
2. Shrinkage behavior
3. Warpage
4. Surface quality
5. Cycle time
6. Flow length in thin walled parts
In particular semi-crystalline thermoplastics need to cool down at optimal crystallization rate. Parts with widely varying wall thicknesses are likely to deform because of local differences in the degree of crystallization. Additionally the required cooling time increases rapidly with wall thickness. This calculation is shown in Cooling system equations.
Cooling channel configuration:
In general, the cooling system will be roughly drilled or milled. Rough inner surfaces enhance turbulent flow of coolant, thus providing better heat exchange. Turbulant flow achieves 3 to 5 times as much heat transfer as does non turbulant flow. Cooling channels should be placed close to the mold cavity surface with equal center distances in between (see figures below). The mechanical strength of the mold steel should be considered when designing the cooling system.
1. Cooling system is necessary :
1. To solidify the hot plastic material injected inside the cavity
2. Efficient cooling in necessary:
1. For efficient production ( Less Cycle time )
2. For quality moulding
3. To prevent moulded in-stresses, strains, blisters,warpage, sink mark, poor surface appearance, varying part dimensions etc. on the finished product.
3.In-efficient cooling can cause quality problems in the product such as:
1.Axial and Radial Eccentricity
2.Angular deviation
3.Warpage
4.Surface defects & Flow lines
4.Cooling efficiency:
1. Ability of the cooling medium to carry the heat away from hot plastics
5. Cooling efficiency will depends on:
1. Temperature difference between plastic and coolant
2. Heat travel distance from hot plastic inside the cavity to cooling channel
3. Heat conductivity of mould material
4. Dirt in coolant
5. Rust and Mineral deposits in cooling channels
6. Specific heat of coolant
7. Mould temperature and Coolant temperature
8. Layout of cooling channels
9. Size of cooling channels
10. Methods of cooling systems (Direct, In-direct & embedded copper tubes )
Types of cooling system will depends on:
1. Size and shape of the product
2. Design of the mould
Cooling systems layout in a mould depends on:
1. Part geometry
2. Number of cavities
3. Ejector and Cam systems
4. Part quality
5. Dimensional accuracy
6. Part surface appearance
7. Polymer etc.
Size of the cooling channels dependent on:
1. Rate of cooling
2. Temperature control needed for quality production
Size of the cooling channels ( Based on Mould size )
Cooling channel size should be more for thicker wall section
Other points to be considered:
1. The mould temperature can affect the flow of the material
2. Mould temperature will vary from 750°- 1350°F and it depends on:
Plastic material & size and shape of the product
1. Cooling water to be circulated with the temperature of 40° - 60 ℃ and pressure about 4 - 5 kg/cm2
2. The difference in inlet and outlet water temperatures
< 2° to 5 °C - normal condition
=0.5° to 10 °C - for precision moulding
For effective cooling:
m[Cp(T1-T2) + L] m x a
m2 = ------------------------------------ =--------------------------
K(T3-T4) K(T3-T4)
Where:
m = Weight of plastics material injected per hr. (g)
m2 = Weight of water passed through mould (gm/hr)
a = Total heat content of plasticised polymer (cal/g)
T1=Injection temperature of polymer (℃)
T2=Mould temperature (℃)
T3=Outlet temperature of water (℃)
T4=Inlet temperature of water (®)
Cp = Specific heat capacity of polymer (cal/g ℃)
L = Specific latent heat of fusion of polymer (cal/g)
K = Constant
k = 0.64 for direct cooling
0.5 for indirect cooling
0.1 for embedded copper tubes
Types of Cooling Systems:
Baffle Cooling: Baffles and bubblers are sections of cooling lines that divert the coolant flow into areas that would normally lack cooling, e.g. cores. A baffle is actually a cooling channel drilled perpendicular to a main cooling line, with a blade that separates one cooling passage into two semi-circular channels. The coolant flows in one side of the blade from the main cooling line, turns around the tip to the other side of the baffle, and then flows back to the main cooling line.
Use of Baffles and O - Rings on a cavity having long side walls
Channeling of round flat cores and shallow cavities:
Baffle cooling for long cores:
Use of Bubbler arrangements in Cooling system: A bubbler is similar to a baffle except that the blade is replaced with a small tube. The coolant flows into the bottom of the tube and "bubbles" out of the top, like a fountain. The coolant then flows down around the outside of the tube to continue its flow through the cooling channels.
For slender cores this is the most effective form of cooling. The inner and outer diameters must be adjusted so that the flow resistance in both cross sections is equal. The condition for this is: Inner Diameter / Outer Diameter = 0.7
O Ring fitting details for sealing a curved surface:
Thermal pins
A thermal pin is an alternative to baffles and bubblers. It is a sealed cylinder filled with a fluid. The fluid vaporizes as it draws heat from the tool steel and condenses as it releases the heat to the coolant. The heat transfer efficiency of a thermal pin is almost ten times as greater than a copper tube. For good heat conduction, avoid an air gap between the thermal pin and the, or fill it with a highly conductive sealant.
Cooling of large cores:
For large core diameters (40 mm and larger), a positive transport of coolant must be ensured. This can be done with inserts in which the coolant reaches the tip of the core through a central bore and is led through a spiral to its circumference, and between core and insert helically to the outlet. This design weakens the core significantly.
Cooling of large cores:
Cooling of slender core
Cooling of cylinder cores and other round parts should be done with a double helix, as shown below. The coolant flows to the core tip in one helix and returns in another helix. The wall thickness of the core should be at least 3 mm in this case.
Cooling of slender core:
If the diameter or width is very small (less than 3 mm), only air-cooling is feasible. Air is blown at the cores from the outside during opening or flows through a central hole from inside. This procedure, of course, does not permit maintaining an exact temperature.
Better cooling of slender cores (those measuring less than 5 mm) is accomplished by using inserts made of materials with high thermal conductivity, such as copper or beryllium-copper materials. Such inserts are press-fitted into the core and extend with their base, which has a cross section as large as is feasible, into a cooling channel.
Cooling of slender core with inserts.
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