Dr. Joseph Greene Copyright 2000 All Rights Reserved

Mold Cooling Introduction Factors which affect Mold Cooling Laying Out Cooling Channels Size and Number of Supply Lines Coolant Requirements for a Mold Dr. Joseph Greene Copyright 2000 All Rights Reserved

Introduction Guidelines for mold cooling Parameters that affect economical mold cooling Plastic materials molded Mold material Molding machines Cycle Time Shape of product Mold Cost Anticipated lifespan of mold Operators Dr. Joseph Greene Copyright 2000 All Rights Reserved

Cool and Injection Mold
Molds shape hot, injected plastic into desired shape Hot plastic cools in mold. Mold is a heat exchanger where heat from plastic is removed by the coolant flowing in mold channels. Parts can be ejected when part is cooled down. Mold and Injection Temperatures for some plastics Dr. Joseph Greene Copyright 2000 All Rights Reserved

Cooling Factors What affects Cooling of Mold? Temperature increase (T) of coolant from in to out Flow rate of coolant Chemical composition & thermal conductivity of coolant Thermal conductivity of mold parts Temperature drop of plastic from injection to ejection Runner system size and layout Type of runner system (hot or cold) Cooling channels layout and size Return lines number and diameter Cooling capacity of chiller Mold close time and related cycle time Dr. Joseph Greene Copyright 2000 All Rights Reserved

Coolant versus Plastic Temperatures
Coolant Temperature Cooling medium is usually water, preferably conditioned so as to minimize corrosion and scale Antifreeze is added when temperatures are low or supply lines are exposed to freezing temperatures. Cooling is usually kept at or above 5C (40F) Plastic Temperature Heat is added to cool plastic granules so that they melt in barrel and then flow as a liquid in mold at melt temp. Heat is extracted from mold during total cycle time, including opening, cooling, closing, and dead times Faster cycle times leaves less time for cooling Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Heat, Temperature, and Energy Energy is capacity to do work. Heat is one form of energy Heat is energy in transit from a hot body to a cold body Temperature is a property of a body that heat is applied Heat is measured in calories or BTUs(British Thermal Units) 1 calorie is the amount of required to raise the temperature of 1 gram of water by 1 degree C. 1 BTU is the amount of energy required to raise the temperature of 1 pound of water 1 degree F You raise the temperature from a low Temperature to a high temperature by adding calories or BTUs Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Thermal Transfer Heat can be transferred by mostly by conduction and convection, and in rare cases radiation. Conduction- the transfer of heat between two materials that are in contact with each other and is proportional to the thermal conductivity, k. Fourier’s Law (Heat flux) Q/A = k (T2-T1)/(L2-L1) Example, A pan on a hot plate will heat up while it is sitting on hot plate and is dependent upon the thermal conductivity of the pan and plate. A pan with high thermal conductivity (copper) will heat up faster than a pan made of lower conductivity (glass). Example, the hot plastic will heat up the mold due to conductivity of the plastic and the mold material. Molds heat mostly due to conduction. Convection- the transfer of heat between two materials that have a fluid motion with at least one. Heat Flux = Q/A = coolant velocity* k-coefficient * (T2-T1)/(L2-L1) Example, A Fan will cool you off due to convection of air Example, Molds are cooled mostly by convection of cooling water in mold that removes heat from mold material. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Thermal Transfer Amount of heat flowing (calories) from one location to another gets larger with Larger Temperature difference in plastic hot to cold Higher thermal conductivity material (Cu or BeCu) Larger cross sectional area of part (Big parts = big heat flow) In mold: Heat from plastic flows into mold, From mold to coolant, From mold to platens and air From coolant to chiller Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Thermal Transfer Temperature Difference Plastic from injection temp (high) to ejection temp (low) Plastic from inside core temp (hot) to mold wall (low) Cooling water from mold inlet (cold) to mold outlet (warmer) Note: for most general purpose molds the temperature difference of coolant from outlet of mold to inlet of mold should be less than 6°C Note: some production molds require 1° to 2° C temperature difference Thermal Conductivity Is a measure of the rate at which a material conducts heat from a hot to cold. Plastics have low thermal conductivity and can insulate a part if the part is too thick and lengthen cooling times Mold material thermal conductivities are in reference books. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Thermal Transfer (Continued) Heat Content To raise temperature of the plastic a certain amount of energy is required to melt it as well as to cool it from melt state. Heat capacity of materials are unique like other properties, e.g. density. Heat removal from the plastic takes place through the walls of the cavity that is in contact with a coolant which takes away heat back to the chiller where it is cooled. Amount of heat required is dependent upon the amount of plastic material in the mold and the type of plastic material. Crystalline plastics require more energy to heat or cool than amorphous. Amorphous regions heat or cool at a constant rate. Crystalline regions require more energy during transition at the Tg. The speed at which the plastic is cooled is dependent upon the conductivity of the mold material, the plastic thermal, and the coolant fluid. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Heat Transfer
Coolant Flow Amount of heat removed is dependent upon the temperature difference between the hot plastic and the cold coolant. Various factors affect the flow of coolant. Pressure drop of coolant in supply and return lines Higher pressures provide higher flow and better heat transfer (70-85psi) Cross sectional area of passages Larger cross section = higher flow rate = higher heat transfer Length of the coolant passages Longer flow length of channels (more passes) = higher heat transfer Viscosity of coolant Lower viscosity = higher flow = better heat transfer Amount of scale and rust in coolant channels Lower scale and rust = better heat transfer Reynolds number Higher Re number = higher flow = better heat transfer Turbulent flow is best with Re # > 4,000 Dr. Joseph Greene Copyright 2000 All Rights Reserved

Reynolds Number Further experiments show that the three regions do not depend solely on the flow rate, but on Reynolds number = dimensionless combination of the mean fluid velocity, density, viscosity, and diameter Indicated relative importance of inertial (velocity) effects to viscous effects. Laminar Flow Turbulent Flow Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Runner System Hot and Cold runner systems produce different effects on mold cooling Cold runner Plastic in runner is cooled the same way as the mold plates. Cold runner is ejected with part. Runners should be cooled to allow for it to eject. Runners should be as small as possible for minimum cycle time Hot runner Plastic is kept hot from nozzle to cavities. Hot manifold is kept heated and insulated from mold that is cooled Temperature of the Mold Shoe Purpose is to support and hold the cavities, cores, runners Some of the heat travels to mold shoe which must be removed Cavity and core should be at the same temperature Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Laying out Cooling Channels Practical Recommendations Length of Bore Holes be drilled at least 6 mm away from other openings Fig 13.10 Drill Size Use standard, available drill sizes Fitting Size Use standard fitting sizes at the end of a bore. Use NPT and fittings Pipe Threads and Fittings Minimum distance from obstacles is governed by size of wrench ‘O’-Ring fittings Used instead of tapered pipe fittings Less wedging action with O-ring so holes can be closer to walls Less risk of leaking Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Laying out Cooling Channels Practical Recommendations Distance of Cooling Channels Distance from molding surface and from each other, Fig 13.12, are needed to produce even surface temperatures on mold. Distance between channels = B = Diameter of tube Distance from surface = A= B Note:If cooling channels are moved closer to surface, the cooling effect on surface is greater and could have the the channels closer ut may increase tool cost. A B Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Laying out Cooling Channels Practical Recommendations Strength of Mold Material Mold cavity must withstand pressures and forces create by plastic during injection. Can be 20,000 psi. Safe permissible stresses in steel should be less than 10% of the yield stress of the steel. Efficiency of Cooling Channels: Fig 13.14B The surface of the cooling surface can be more active due to: More cooling channel surface near the molding surface Only one side of the channel is in direct contact with the steel of the mold cavity. Separation between the mold plate and cavity can reduce contact area by 50%. Cooling lines should be in contact with the plastic, rather than in adjoining plates. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Series and Parallel Cooling As coolant flows through channel it picks up heat from the mold surface which is heated by the plastic. Flows depend upon pressure differential between in and out ports of cooling channels, length and diameter of channels. Max temperature difference of 5C between in and out channels Coolant flows in cooling lines which can be laid out in series or parallel arrangement as follows. Parallel arrangement (Fig 13.16) Flow of coolant passes through all of the tubes from a common manifold. Series arrangement (Fig 13.15) Preferred Flow of coolant passes through a series of tubes one after another. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Series and Parallel Cooling Cooling efficiency depends not only on the T between the mold and coolant, but also on the amount of coolant flowing in channel (Turbulent is better) The amount of coolant depends on the cross section of the channel (for flow) and the surface area (for contact). For round channels coolant depends upon d2 Flow is cross sectional area time velocity =  r2 (velocity) The amount of heat transfer between coolant and mold walls depends on the surface area of the coolant channels The area is proportional to diameter of tube. Heat transfer = surface area time heat transfer coefficient= 2rh Thus, doubling the size of the tubes has a 4 times increase in flow, but the surface heat transfer only doubles This is why you need many smaller tubes. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Smaller size diameter tubes are more efficient for cooling the molds. Combination of parallel and series layout_Fig 13.17 Example, Number of branches for N1, N2, N3 is 1, 2, and 6. To calculate the size of diameters use the following equation: where D1 is the input line diameter. Combination of parallel & series are used in many molds Plates may be all series cooled Cores and cavities may be series or parallel cooled. Depending upon Quality of cooling required, space availability, strength of mold material, number of cooling circuits available at machine, and number of cooling circuits desired for simplicity of installation. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Baffles in Plates Cooling channels in plates are usually produced by drilling and boring due to typical circular cross section. To achieve flow path for coolant within the plate, the bores must intersect. Occurs in same plane of drilling, but is some cases offsets are required from the center lines of the bores. Important to make sure that the passage from one bore to the other does not restrict the flow and that burrs from drilling can be removed. Fig shows typical end of a plate with plugs at the end of the bores and baffles to direct flow. Baffles are fastened to the rod with pins or screws. Disadvantage of baffles and rods is that their fit in the bore must be loose so that they can be assembled. Rust and sediment can cover bore. Fig shows different method for installing baffles which eliminates need for removal. The baffle is threaded with plug. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Basic Principles Runner and Cavity Plate in Three -Plate Molds Cooling of this plate is a special case, as the plate contains both the runners (with drops to cavities) Important to cool runners and the drops as quickly as the molded products so that they do not delay the ejection. Most cases the drops are thicker than the walls of the molded product and will cool slower. Fig 13.24 Cross section of a three plate mold and location of cooling lines in both runner and cavity retainer plates. The runner must be cooled, especially around the drops. Fig 35. Cross section of a three-plate mold show cooling lines around cavity and runners in one plate. Cooling channels should be as close as practical toward the drops to assure good cooling of the area Dr. Joseph Greene Copyright 2000 All Rights Reserved

Layout and Sizing of Cooling Channels in Mold Components that Contact Molded Products Divide molds into 3 basic groups according to shape Flat products _ Fig 13.28A Both sides of cavity and core are of approx. the same shape Disks, records, trays, lids, test pieces, etc. Cup-shaped products_ Fig 13.28B Most molds. Cavity is a depression into which core enters cups, pails, closures, casssettes, fascias, fenders, bumpers, etc. Combination products_ Fig 13.28C Include some products which for cooling purposes, combine features of both these groups. Spools, disks with hubs, door panels, Dr. Joseph Greene Copyright 2000 All Rights Reserved

Laying out Cooling Channels
Compressibility and Shrinkage Review Hot plastic (expanded) enters the cavity, and then after pressure is released cools, it Shrinks to return to its smaller, cold volume, and Increases in size as the pressure on the plastic decreases. Effect of shrinkage is the greater of these two influences which results in the plastic cooling down and shrinking on core. Flat Products Free to shrink in all directions. If thickness is relatively small, and shrinkage and compressibility are minor, then the cooling efficiency will be the same on both sides and the length of product will reduce Dr. Joseph Greene Copyright 2000 All Rights Reserved

Flat Products Important that both sides of the product are equally well cooled. Fig Uneven cooling for flat product results in warpage For thick parts As part cools and shrinks on core , the cooling efficiency is reduced on cavity side, since it has lost contact with cavity. Smaller the core the more difficult the cooling becomes. Cavity, Core, Insert, Gate Pad, and Stripper Cooling Series Cooling_Fig 13.30A Simple cooling and acceptable. Series-Parallel Cooling_Fig 13.30B Feeder and Return channels must be larger than the branched channels to provide equal flow through each branch. Requires more drilling, but cooling is better than series alone Dr. Joseph Greene Copyright 2000 All Rights Reserved

Flat Products Cavity, Core, Insert, Gate Pad, and Stripper Cooling Series-Parallel Cooling_Fig 13.30B Often used for multi-cavity molds. For very large number of cavities, Feeder channels should be split to provide more even supply to the branches_ Fig 13.31A If feeder channels can be made large, Fig 13.31B is fine. Cooling arrangement of the cavity & core side do not need to be same. Easy to provide parallel cooling for cores, difficult for cavities. Common approach is to use Series or series-parallel cooling for cavities Parallel or series-parallel cooling for cores. OK because cooling of core is more important than cavity. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Flat Products Cavity, Core, Insert, Gate Pad, and Stripper Cooling Parallel Cooling_Fig 13.30C: Typical cooling layout All cavities are equally well cooled, provided branch channels are sized so that the flow to each cavity (or core) is the same. Most efficient but most costly. First choice for high-speed, high-productivity molding ,e.g., thin disposable products, Because of thin wall thickness, the heat must be removed equally fast from both cavity and core to permit fast cycles. Poorest cooled cavity will always control the length of the cycle time. Parallel cooling is the most eficient Restrictors in flow channels Sometimes used to achieve equal flow through a number of parallel branch channels. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Flat Products Restrictors in flow channels Sometimes used to achieve equal flow through a number of parallel branch channels. Can be avoided by properly sizing the flow channels in the first palce. Sealing the passages from plate to plate. O-rings at sealing points. Use standards from manufacturers for groove dimensions and finishes. Flat seals. O rings are located between flat surfaces Fig13.33A_ Three examples of flat O-rings. A.preferred arrangement, B tapered O-ring groove bottom to prevent seal from slipping, and C. Poor arrangement wherein seal may slip into channel. They are compressed as the plates are joined. Circumferential Seals: O rings located between cylindrical surfaces. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Flat Products: Cavities and Cores Flat products have cavities and cores are virtually identical Records, plates, test pieces, etc. Heat must be removed equally between cavity and core. Difficulties include: Gate area is a source of additional heat input. Ejector mechanism requires space that competes with cooling lines. Mounting screws require sealing to prevent leaks. Inserts and/or core pins often make it difficult to provide a symmetrical, equal, and balanced cooling layout. Localized heavy cross sections of the mold product should be discussed with the product designer to discuss warpage isues. Coolant supply to flat cavities and cores can be achieved in three ways. Cooling channels in cavity block piped directly from outside. Cooling channels to cavity block through backing plate. Circular cooling groove around cavity through cavity retainer plate. Fig A, B, and C Dr. Joseph Greene Copyright 2000 All Rights Reserved

Coolant Circuit Layouts in Flat Cavities Mounting flat cavity blocks requires sufficient mounting screws to hold the blocks solidly to the backing plate. Screws should be located so that they will not seriously affect the layout and the symmetry of the cooling channels. If open channels are used (Fig A) (Fig B) O rings are required and prevent coolant leaks through screw hole. Common cooling circuits for mold cavities or cores Concentric cooling circuit (Fig 13.38): Easily produced and cooling is acceptable, but not as good as in spiral arrangements. (Fig 13.39) Spiral cooling Simple cavity block cooling layout for large flat product (Fig 13.40) Cavity cooling for cup-shaped products (Fig 13.42) Cavity inserts in a plate (cavity retainer plate) Modular cavities mounted on backing plate Dr. Joseph Greene Copyright 2000 All Rights Reserved

Core Cooling Flat Products Cooling method is same for cavities, but need to make room for ejector pins which must pass through cooling lines. Ejector pins must be sealed against coolant with O-rings, similar to screws. (Fig 13.34) Spot cooling is sometimes required, as in opposite gate. Bubblers can be used with baffles. (Fig 13.52 Cup-shaped products Small products Choice of cooling is restricted and is often done with bubblers. Some bubblers require baffles for better cooling. Medium size products Those whose core is too big to be cooled with a simple bubbler. Depth of draw is generally one-half to double the diameter of the open end. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Cup-shaped products Medium size products General rules Area opposite the gate must always be properly cooled. Cooling supply for the core should be directed first against the gate area before being channeled to cool the rest of the core or use a bubbler. Provide adequate cooling for rest of the core. Cores for Large Products No clear distinction between medium and large products. All methods used with medium-sized products can be applied to large Large products typically have heavier walls and slower cycle times. Coolant flows spirally in concentric grooves with connecting slots and baffles from the center toward the rim of the product, Venting is provided with vent pins Many large cores are cross drilled for cooling and must have a bubbler opposite the gate. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Gate Pad Cooling Gate pad is an insert in cavity, surrounding the gate area. Area is especially noted for cooling: In hot runners, the plastic is kept hot up to the gate. With open gates, the gate must freeze as the product cools down If not properly cooled, gate may not freeze of and drool will result. In valve gates, the tip of the valve where it shuts off from the gate must be cooled so that the portion of the product opposite gate will have time to solidify. The valve pin is cooled by contact with the cooled gate pad. In 3-plate molds, the heat content of heavy drop must be removed quickly so that runners and drops care cold enough for ejection. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Gate Pad Cooling Why have gate pad inserts? If a witness line is permissible Wear of the gate, especially in molding glass-filled systems. Different materials can be selected for higher cooling rates. Venting is easily done with a circular vent slot around gate. Differential cooling is possible and eliminate separate gate pad cooling circuits by properly designing the size of passages or permanent restrictors. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Size and Number of Supply Lines (hoses) Need to calculate the amount of coolant per hour is required (Sec 13.6). Amount of coolant per hour is proportional to amount of plastic processed in the mold per hour. Have manifold be apart of the mold plates to supply the various channels of the core and cavity Advantages Minimize number of hose connections to one in and one out. Mold operators can not alter the cooling flow pattern. Automatic mold installation. Have fewer hoses. Improved accessibility for air hoses and electrical connections. Lower actual cost and upkeep since the number of small hoses and joints is less. Dr. Joseph Greene Copyright 2000 All Rights Reserved

Coolant Requirements for a Mold (heat exchanger) Calculation of Heat Input Estimated cycle time depends on Shape of product, wall thickness, method of ejection, machine characteristics (clamp speed, time, pressure), type of plastic Estimate efficiency of cooling depends on: Design and cooling layout, mold materials, temperatures of material, mold, and plant. Example, Mass of plastic will be heated from Tr to Ti. Subsequently, in the mold, it will be cooled from Ti to Te. Ti = Tinject and Te=Temp eject Mass of plastic (Ms) shot size, per cycle equals Mass of each product multiplied by number of cavities. Ms = Mp x n Mass per hour (Mh) = Mass of shot divided by Cycle time is Mh = Ms (3600 sec/hr)/tc) Dr. Joseph Greene Copyright 2000 All Rights Reserved

Coolant Requirements for a Mold Specific Heat (Cp) Ratio of the heat required to raise the temperature of a certain mass of the given material by one degree C to that required to raise the temperature of a similar mass of water by 1C. Specific heat ratioof water is 1. Specific heat ratio of most plastics is between 0.25 & (Sec 22.11) Can be expressed in either BTU/lb-F or cal/g-C Example, Product made from PS (Cp = 0,34) weighs 35 g and molded in 16-cavity mold at 6 second cycle. Shot size = 560 g Shots/hour = 600 Mass molded/hour = 336,00 g/hour Calories/hour = 114,240 cal/hour for 1C rise = heat = 478,300 J/hour Total energy = 123 MJ/hour = 34.17kW Dr. Joseph Greene Copyright 2000 All Rights Reserved

Dr. Joseph Greene Copyright 2000 All Rights Reserved

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