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Hot plastic goes in cooler plastic comes out
Definition: Mold: 1) A hallow form or matrix into which a plastic material is placed and which imparts to the material its final shape as a finished article. 2) To impart shape to a plastic mass by means of confining cavity or matrix. Heat Exchanger: A device for exchanging heat from one medium to another
Though the principal and costs associated with a mold/mould are in the design construction of the cavities, it has been the authors' experience that cooling design has in the past been left for last, meaning that the cooling lines are not put into the mold design prior to ejection. This topic in itself can lead to a heated debate, and if one has a sense of humor can be fun.
The data, the mold is injected with materials that are heated, soft/melted and must be lowered in temperature to a state of near rigidity prior to be ejected from the mold/mould. Thus the heat from the plastic must be conducted from the cavity to water lines/ cooling lines this involves the transfer through the mold materials.
*Refresher heat transfer:
There are multiple function involved with heat transfer; thermal conductivity, thermal transfer, and the storage of heat. Further heat flows in the direction of hotter to cooler level. Within solid materials this takes place by conduction. Within gas or liquid an additional term call convection is employed as the fluid is moving and or being transported. Another term used is radiation which means that the heat is given up to the surrounding, but as some literature explains below 200C is a minor component though that may be debated.
In the plastic part mold cooling media cycle we than are transferring heat from the plastic to the mold to the cooling media. The issue or question is how much time to take the heat away is the process occupying within the molding cycle, also how much control is there. The issue is that there are interfaces within the system which cause issues. This is the plastic to mold, if shrinkage occurs than a gap exists where heat is no longer transferred at the rate desired as the conductance is broken. The heat is than conducted through the mold where another transfer occurs with cooling media. Note that media and flow are critical as to heat transfer and shall be discussed later.
Since all thermoplastic materials are injected into a mold at a higher temperature than the mold is, than the transfer of heat is from the plastic to the mold. This transfer has to occur for a period of time until the material reaches a set minimum surface temperature for ejection. Basically meaning the part is ridged enough to be ejected without deformation.
In multiple literature and data this cooling function is stated as from 95 to 70% of the overall cycle time. Since overall cycle time is a component of profits than all that can be done should be done to optimize the heat transfer characteristics of a mold. Unfortunately this is not done per numerous reasons, such as cost of mold, it's too hard to do that way, and they get in way of ejector pins and others.
The real issue is placement of cooling lines close enough to the surface of the mold forming areas to affect a good temperature control. It also is important to understand the steels or materials of construction used to build the mold, as the thermal conductance of different steels/materials can have great impact on the rate of cooling of the plastic part.
The above illustration shows that the hottest area is the plastic part/cavity of the mold and heat is flowing to the coolest points the cooling lines. The above also shows that the cooling lines are not uniform and not equal distance from the cavity itself. Thus what than can be determined to happen is that the steel temperature within the mold shall become non uniform, and upon ejection the part itself may not be uniform in temperature, which than would affect its shrinkage rate and possibly lead to warpage.
There is a calculation / equation for thermal conductivity within solid materials. It is based on heat conductivity of the material, transfer area, transfer distance (thickness) and uses the difference in temperature, this than would allow for mathematical calculation of heat conduction. In a basic sense this is how one can calculate cooling, and from a process point is why the steel heats up on faster cycles, or normal cycles above what one sets the cooling media. In a simple sense the mold material can only transfer X amount of heat in Y amount of time, thus placement, distance of fluid lines to cavity wall are critical.
Since the placement of water lines is critical, along with the type of steel used, a rule of thumb to use would be to place a water line from the surface of the cavity or core, at minimum 1.5d and use a pitch between water lines of 3d where d= the diameter of the water line. This rule of thumb is dependent on mold material type, as if a soft material (Aluminum)than deformation may occur, or if highly conductive than bleed through of water line may show on part surface due to cooling rate of plastic material in that area.
SLSILVEY
20012014.02
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