Many mold builders and tooling engineers are in pursuit of the perfect tool for trying conformal cooling when in fact, the perfect mold is the one which they are currently struggling with, or the one simulation data determines will be a struggle.
Then there are the processors who slow down mold cycles because of a hot spot in the mold or who decide to process around a quality defect by increasing cooling time, which depletes expected profits or eats up machine capacity. Mold builders and processors must work together to identify molds with which they are constantly struggling and apply additive tooling that will eliminate persistent cycle time and defect issues.
Concerning Cycle Time
Cycle time likes to hide in areas in a mold that we cannot cool effectively, resulting in a hot spot that requires more time to cool the part to the ejection temperature. The difference in cooling rate leads to defects like warp that can create fit, form and function issues, as well as out-of-spec dimensions.
There are tools that shops can use to pinpoint the when and when of cycle time and defects that cause deformation, giving the shop time to create a countermeasure to cool the area effectively.
- Mold simulation. Using simulation software, a designer can carefully review the cooling channels and see where any remaining hot spots will develop. Frequently, designers simulate the mold design and assume that they can achieve proper cooling in the desired areas. However, once the mold design begins, features such as split lines, ejection, venting and mold actions take precedence over cooling. Additive tooling enables shops to integrate critical cooling and any necessary mold functions in areas where conventional cooling lines cannot be milled or drilled.
- Thermal imaging. Shops can use this tool to observe the actual temperature of the molding surface and part temperature at ejection. This powerful tool reveals hot spot locations in existing tooling. It also balances the cooling temperature, determines where additional cooling is required to obtain consistent mold temperatures and cooling rates, and eliminates cycle time and molding defects.
If your shop does not have access to a good thermal imaging camera, rent one for a day at your local tool rental store. Your mold simulation and thermal images will identify the areas where cycle time likes to hide and help you implement cost-effective solutions to eliminate defects and run your molds at the cycles quoted.
Balancing Heat Load to Cooling
Lifters help to remove an undercut in a mold. They are one of the most difficult parts of a mold to cool because they are often mounted on a rod, have minimal space for cooling lines and the surface area forming the plastic is quite high compared to the mass of steel in the lifter.
A common solution for cooling lifters is to use a highly thermally conductive material, but this material type in a high-wear environment does not hold up to the rigors of production molding or filled resins, increasing maintenance costs over the life of the mold.
Cooling Loses Out
Slide actions in molds pose a different set of cooling challenges. While slides offer more access to provide cooling to the outside of a mold, there are more obstacles such as sub inserts, core pins, screws and O-rings that have limited options to route water conventionally. This scenario leaves slide actions under-cooled and ineffective, robbing the mold of precious seconds of cycle time waiting for the part to cool without warp.
Part design drives many mold elements, so design in fixed areas, such as parting lines, cores and sub inserts. Once those details are fixed, then evaluate where cooling is required and add circuits that achieve the most efficiency. It does not take a massively sized circuit to achieve the effective cooling of thin steel areas. In most cases, you can model cooling to fit between fixed components in areas as small as 0.25-inch wide by changing the cooling circuit’s profile without limiting flow. This approach enables the required components to remain in place and adds effective cooling to reduce cycle time to the quoted expectations, and in some cases, much lower.
High Wear Solutions in Steel
A mold’s core is typically the side with more plastic structural features, such as ribs and bosses for added strength and reduced plastic material mass. These core side features have more surface area, which requires more cooling and minimal draft. Additional challenges include features that require extra venting without gas traps or more ejection to effectively demold the plastic part.
For example, consider material savers (standing steel with machined ribs) that demand cooling the standing steel and ejector pins at the bottom of the ribs, which prevents the use of baffles or bubblers. Another example is a round core around which many shops cut a channel and add O-rings to the top and bottom to route water. The part contour can extend beyond the parting line, so this can be an ineffective method for the core side as it leaves no room for cooling in the cores. The insert’s perimeter was cooled but not the standing steel areas.
An alternative is to direct mate the cooling lines in the core block to the cooling lines in the insert, which maintains proper flow while using O-rings to seal the top and bottom. This approach is cost-effective because the cooling circuit covers the entire part surface, reducing cycle time and improving part quality by eliminating warp. These modifications reduced cycle time by 40%.
The goal of every mold builder is to make a mold that produces parts by the most efficient means despite the unique geometry and construction of each mold. The most effective method is to reduce the cooling time to ejection, which is the largest portion of a molding cycle. Use these examples to evaluate the challenges your shop faces and then start identifying critical areas where cycle time hides, so you can implement a solution to deliver molds that efficiently cool parts, lead to satisfied customers and sustainable business.
The world’s three advanced mold cooling technology
Advanced mold cooling technology, in addition to recent hot-selling water transport technology, there are many other technologies that are unfamiliar to everyone, but has been put into production. The most representative ones are: pulse cooling technology, CO2 gas cooling technology and nano fluid cooling technology.
This article will focus on pulse cooling technology, CO2 gas cooling technology and conformal cooling technology analysis.
Pulse cooling technology
Pulse cooling technology was first developed by Brunel University in UK, and later Pennsylvania State University of United States conducted an in-depth study of this technology. In recent years, with advancement of technology, pulse cooling technology has been continuously developed and perfected. After long-term technical accumulation, CITO Company of United States has taken lead in designing complete sets of equipment for pulse cooling technology and successfully commercialized it.
Traditional cooling technology mainly controls temperature of mold by controlling temperature of coolant, while pulse cooling technology controls temperature of mold by changing flow rate. Heat generated during injection molding process changes with time, like a pulse signal, fluctuating. According to this feature, pulse cooling technology achieves purpose of adjusting mold temperature by controlling flow rate of coolant to corresponding pulse input. During pulse cooling process, flow rate of coolant is adjusted according to temperature of mold.
pulse cooling
Compared with traditional cooling technology, temperature of pulse-cooled coolant is always low, which ensures a large temperature difference with mold temperature, enhances convective heat transfer between coolant and mold, and greatly improves cooling efficiency of mold. According to statistics, pulse cooling technology is 10%~30% more efficient than ordinary cooling technology, and can significantly improve quality of parts. Pulse cooling equipment is complicated. In order to make cooling uniform, partition cooling is required, and a thermocouple needs to be installed in each area to monitor temperature of mold at any time, which increases difficulty of its application.
CO2 gas cooling technology
C02 gas cooling technology was first developed by injection molding companies in United States. Low temperature CO2 was used as a medium to cool injection mold. Later, many injection molding companies in foreign countries developed C02 cooler.
C02 gas cooling technology uses a low temperature CO2 gas to cool mold. This technology must be used in a mold made of porous material. CO2 gas is introduced into a certain part of mold, shunt is guided, and then other parts of mold are cooled.
CO2 gas cooling
Advantages of CO2 cooling technology:
First, structure of mold is effectively simplified. It does not require a special demolding mechanism. It only needs to open exhaust passage between plastic parts and mold contact wall, and CO2 gas can quickly blow out plastic parts.
In addition, CO2 cooling technology is more efficient than conventional cooling technology. According to structural characteristics of mold, it is divided into different zones, air inlet and exhaust port are respectively set, and then cooled separately. A thermocouple is installed on cavity surface of each area for temperature monitoring, and then controlled according to temperature result of galvanic feedback. Since C02 has a very low temperature (-78℃), temperature of cooling water of mold is generally 8℃, and temperature of most of cavity surface is 100℃ or more. Compared with water, CO2 has a larger surface area than cavity. Temperature difference and cooling efficiency are greatly improved.
Finally, compared with traditional liquid cooling medium, CO2 has good permeability and fluidity in porous metal mold, which can fully cool all parts of mold without leaving a cooling dead angle, avoid warping deformation of plastic parts due to uneven cooling.
Conformal cooling technology
Conformal cooling technology is a new high-efficiency mold cooling technology that has been developed with development of rapid prototyping technology. In 1997, Professor Sachs of Massachusetts Institute of Technology first proposed conformal cooling technology at SFF conference. In 1998, Jacobs in UK conducted a large number of experiments to systematically study effects of conformal cooling technology on quality, performance and production efficiency of products during injection molding. Later, Dalgarno and some scholars used new molding method to prepare conformal cooling water channel based on research results of predecessors.
Design idea of conformal cooling technology is to change cooling water channel with change of cavity structure, and cooling water channel always maintains a certain distance from surface of cavity.
Compared with traditional cooling technology, advantages of conformal cooling technology are mainly reflected in following aspects:
(1) Cooling is more uniform. Traditional cooling channels are limited by processing technology and are generally designed to be straight. Due to irregular surface of cavity, distance between cooling water channel and cavity is different, which results in uneven cooling of mold, and produced product is prone to warpage or poor mechanical properties. However, conformal cooling water channel can be consistent with length of cavity, changes with structural change of cavity to uniformly and efficiently cool plastic parts, thereby greatly improving quality of plastic parts.
(2) High molding efficiency. Mold must be opened to make temperature of all parts of plastic parts below mold opening temperature. Conventional cooling technology makes cooling of plastic parts uneven, cooling time required for some parts is short, while cooling time of some parts is long. In this way, total cooling time is invisibly lengthened. Conformal cooling technology is evenly cooled, and simultaneous cooling can be achieved, which greatly improves molding efficiency.
Studies have shown that conformal cooling technology can increase molding efficiency of mold by more than 30%.
The cooling lines support uniform cooling of injection molds and provide conductive parts without cracking or warping to produce. The injection molding process is heated by injecting resin into mold cavities using heat sinks. The plastic fills a cavity, conforms, and cools the cavity. This cooling system returns a finished component to a solid state but an inconsistent temperature on molds may result in uneven cooling resulting in component defects. Injection mold cooling correctly can reduce defects, repair, reworking, and materials waste.
The Intricacies of Cooling Channel Placement
Precise placement of cooling channels is crucial. It’s a delicate balancing act. Design engineers meticulously analyze the geometry of the part to determine the optimal location for cooling channels.
Straight Line Cooling vs. Conformal Cooling
Two different types of cooling tubes are employed on a mold: straight line and conformal. Straightline cooling is the standard cooling technique in injection mold cooling systems. The cooler line or the channel is cut on a straight line into metal molds to create heat. Side channels or branches intersect with the main channels allowing more surface area for cooling. After forming and molding plastic parts, the cooler fluid flows into the channel. Using these cooling mechanisms, the mold core can be heated out of the injection mold. Generally, straight-line coolants are used to manufacture smaller components. Conformally cool injection molds are commonly used for complex components.
Balancing Act: Cooling Time vs. Cycle Time
The cooling time directly impacts cycle time—the shorter the cooling passage, the better. Engineers work on finding the perfect balance between cooling time and cycle time, optimizing productivity while maintaining quality.
The temperature control system of an injection mold directly impacts the quality of the molded product and the efficiency of production. High temperatures on the mold cavity surface can lead to flash at the parting line and sink marks in thicker sections of the plastic part. Conversely, low temperatures can cause poor filling and weak weld lines. Uneven temperatures across the mold cavity and moving mold surfaces can introduce internal stresses, leading to warping and deformation in the molding process. Thus, the temperature control system, akin to the gating system, is crucial in mold design and warrants significant attention.
Design Principles For Injection Mold Cooling Systems
To enhance the efficiency of the cooling system and ensure uniform temperature distribution across the mold cavity, the following principles should be adhered to:
1. Optimal Cooling Method And Circuit Placement:
During mold design, prioritize the cooling method and circuit placement, ensuring sufficient space for turbulent water flow in the cooling channels. The cooling circuit should meet the molding process requirements, providing ample, uniform, and balanced cooling.
2. Temperature Differential And Flow Dynamics:
Consider the temperature differential at the inlet and outlet and calculate the flow pressure drop to determine the appropriate diameter and length of the cooling channels. Aim for a lower temperature differential (5°C for standard molds, 2°C for precision molds). The length of the cooling circuit should be between 1.2 to 1.5 meters, with a flow speed of 0-1.0 m/s, and the number of bends should not exceed 15. For larger molds, consider multiple independent circuits to increase coolant flow and reduce pressure loss, enhancing heat transfer efficiency. Multiple narrow cooling channels are preferable to a single large diameter channel.
3. Number And Size Of Cooling Channels:
Maximize the number and size of cooling channels, with the diameter depending on the shape of the plastic part and the mold structure. The number, spacing, and proximity of the channels to the molding space significantly impact mold temperature control.
4. Strategic Cooling Near The Gate:
The area near the gate, often in contact with the injection molding machine’s nozzle, tends to have higher temperatures and requires intensified cooling. If necessary, design a separate cooling channel for this area.
5. Avoid Cooling At Weld Lines:
Since weld lines are the coolest areas, avoid placing cooling channels near them to prevent exacerbating weld line defects and reducing the strength of the plastic part at these points.
6. Placement Of Water Inlet And Outlet Connections:
Position these connections on the non-operational side of the mold.
7. Separate Cooling Circuits For Movable And Fixed Molds:
Ensure balanced cooling for both the cavity and the core, with special attention to the cooling efficiency of the core to guarantee uniform cooling and shrinkage of the plastic part.
Key Considerations In Cooling System Design
1. Cooling Methods For Different Molds:
Use rapid cooling for standard molds to shorten the molding cycle and gradual cooling for precision molds, incorporating mold thermometers.
2. Minimize Use Of Sealing Rings:
Design cooling circuits with dual straight-through paths for easier maintenance. Ensure leak-proof seals and check for water leakage at the seals and nozzles.
3. Directional Cooling For Specific Materials:
For materials like PE with significant shrinkage, orient the cooling channels along the shrinkage direction to prevent deformation. Align the channels with the layout of the mold cavity.
4. Cooling Circuit Configuration:
For molds with a single inlet and outlet, use a series connection for the cooling channels. For parallel connections, ensure each circuit has a flow control device and flow meter to maintain uniform cooling conditions.
5. Enhancing Cooling In Challenging Areas:
In areas where cooling is less effective or structurally limited, consider using materials with high thermal conductivity, like beryllium copper or copper alloys, or a heat-conducting rod structure. Provide cooling for cores, inserts, and sliders as necessary.
6. Color-Coding And Labeling:
Mark the water inlet with red and the outlet with blue. Label the cooling water inlets and outlets on the movable and fixed mold plates with “IN” and “OUT” in English, and group the water channels accordingly.
In injection molding, the largest part of the manufacturing cycle is cooling. The cooling phase of the manufacturing cycle is about 60-70 percent of the cycle, and reducing this by a small amount will allow your production operations to produce more products in less time.
To ensure that a minimum cooling time is achievable, you must first have the correct injection mold cooling system design and the best cooling method.
Cooling method of injection mold
There are two standard methods for cooling systems: air cooling or water cooling.
• Air-cooled molds are not often used because they take a long time to reduce the heat in the injection mold, dissipating through heat transfer to the surrounding air. If the surrounding environment of the injection molding machine and the mold itself is kept cold, it will increase the heat released into the air. It may also require additional operating costs to cool the space.
• Fluid cooling molds are the primary source of cooling, with glycol and water being the most commonly used fluid mixtures. Water provides cooling as it flows through the mold, taking heat away from the mold. Glycol prevents corrosion from forming in the mold’s cooling pipe and helps the mold maintain a stable temperature during manufacturing.
Cooling system design
When designing a new injection mold cooling system, several issues need to be addressed to maximize cooling and reduce cycle time:
• All cooling channels in the mold must be close to the thickest part being formed.
• If the cooling channels in the mold are greater than 8 mm, they should remain the same diameter in the mold.
• Do not have a large cooling channel inside the mold, it is better to add several smaller channels to evenly distribute the coolant.
• When designing molds, use conductive materials to improve cooling. This will help heat transfer from the part as it cools in the mold.
• Make sure both halves of the injection mold are fully cooled. Only cooling half or part of the injection mold increases the chance of partial warping as it cools.
Air cooling system
The use of an air cooling system involves an evaporator to remove heat from the injection system. An air-cooled condenser is then used to remove the heat from the evaporator. What you can expect from the air cooling system is an intake fan to direct the cooler air to the mold, and an exhaust fan to remove the hot air from the mold. The air cooling system transfers heat from the flowing water in the injection molding machine assembly line to the air around the assembly line in the cooler. Because air doesn’t transmit heat the way water does, this fan-cooled air typically consumes 10 percent more electricity.
An air cooling system will discharge hot air into your plant. Therefore, it is best to install an air-cooled refrigerator outside the building or where there is no air conditioning. As airflow increases, the air cooling system becomes dusty and must be regularly maintained and cleaned to maintain a high level of performance. In terms of space, an air-cooled chiller can technically occupy any open or flat space, requiring less total space than a water-cooled unit requiring a cooling tower.
Water cooling system
A water cooling system, also known as a hot press, works by transferring frozen water through pipes running through the mold cavity and outside the runner gate. These waterlines will be closest to the outside of the molded product surface to ensure uniform cooling and prevent warping of the product surface. The water in the cooling line is chemically treated to prevent mold and bacteria from growing or contaminating the line.
The two types of flow that pass through these waterlines are laminar and turbulent, and they describe the way water flows. Laminar flow describes the flow of water through a straight line, so the water in the center of the flow does not touch the inner surface of the line. Turbulent cooling is more efficient because more surface area of water is in contact with the heated cavity.
The water cooling system leaves condensate outside the mold. This is because the temperature of the mold is lower than the dew point of the surrounding air. In warmer or humid climates, it is recommended to raise the temperature of frozen water when using a water-cooling system to avoid condensation water remaining on the outside of the mold.
An external cooling source, such as a tower or an evaporative condenser, is required for a water-cooled refrigerator. The cooling tower pumps the warm water into the chiller.