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.
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.