Quality of injection mold cooling system design is a key factor in mold design success, directly impacting quality and production efficiency of plastic products. During injection molding process, cooling time of plastic product in mold cavity accounts for 70% to 80% of the entire molding cycle, cooling speed and uniformity directly affect product's performance. An improperly designed cooling system can lead to extended production cycles and high costs. Furthermore, uneven cooling can cause product warping due to thermal stress, thus affecting product quality.

There are two key criteria for evaluating quality of mold cooling system design: minimizing product cooling time and ensuring uniform cooling across all parts of product. Numerous factors influence cooling system, including geometry of plastic product, cooling medium, flow rate, temperature, cooling water path layout, mold material, melt temperature, mold temperature, and ejection temperature. These factors also involve unsteady thermal cycle interactions between plastic and mold. Experimental testing of effects of different cooling systems on cooling time and product quality is difficult and unrealistic. Traditional cooling system design is largely based on experience, often failing to optimize cooling system for uniform and effective cooling. This results in extended molding cycles and can lead to uneven cooling of parts, resulting in warpage. Computer analysis and simulation are the best methods for achieving this kind of prediction. Moldflow can optimize cooling system design by analyzing cooling system's impact on flow process, optimizing layout of cooling channels and boundary conditions, thereby achieving uniform cooling. This shortens molding cycle, reduces post-molding internal stress, improves product quality, and reduces costs.

During injection molding process, there are four basic modes of heat transfer: forced convection, natural convection, conduction, and radiation. Figure 1 shows heat input and output of an injection mold. Of heat introduced into mold by the plastic, 80% to 95% is conducted through mold metal to cooling water pipe walls, where it is then dissipated into cooling water pipes. Heat conducted to injection molding machine platen and convected away from mold surface only account for 5% to 15% of the total heat and are not significant. Heat radiated into surrounding space is only considered when mold temperature reaches above 85℃. When using a hot runner, heat is also input into mold. In some cases, coolant temperature is significantly higher than ambient temperature. In this case, coolant does not absorb heat from mold but instead inputs heat into it.

 

Figure 1. Heat Input and Output of Injection Molds

Heat accumulation occurs in injection molds. These heat accumulation points can cause temperature variations on mold surface, leading to uneven cooling and warping of molded part. Heat accumulation can occur for two reasons: one is irregular flow of plastic injected into mold, causing variations in heat load. This is often due to improper frictional heat or variations in part wall thickness. The other is mold geometry, such as corners. Cooling is more adequate in mold corners, while cooling is insufficient within corners, resulting in uneven cooling of part. Figure 2 illustrates deformation caused by heat accumulation in mold corners. In this case, mold core side will produce a very steep temperature gradient due to heat accumulation in the corner.

 

Figure 2 Heat accumulation in mold corners and its deformation

Design of cooling system mainly includes layout of cooling water channel and setting of cooling parameters (such as temperature and pressure of coolant).
(1) Physical dimensions and cooling circuit
Setting up: Physical design of cooling system is usually limited by geometric dimensions of mold, position of parting surface, movable mold and ejector pin, so it is impossible to give a strict design rule. For simple products with uniform wall thickness, uniform cooling channel layout can achieve uniform cooling effect. However, most parts have inconsistent wall thickness, and some are designed with ribs, which often lead to heat accumulation. Cooling channel can be placed close to the area with thicker wall and ribs, or additional cooling channels (such as baffles or bubbles) can be added, as shown in Figure 3.

 

Figure 3 Cooling water pipe layout
The farther distance between cooling water hole and cavity, the more uniform temperature on mold forming surface, but the less heat cooling water absorbs, and the longer cooling time. In general, distance between cooling water hole and cavity should be 2 to 3 times diameter of cooling water pipe. After cooling water flows into cooling water pipe from inlet, it absorbs heat of mold along way, water temperature becomes higher and higher, which will gradually reduce cooling capacity. Therefore, temperature difference between inlet and outlet water of cooling water pipe should be as small as possible, not exceeding 3℃, as shown in Figure 4.

 

Figure 4 Temperature difference between inlet and outlet water of cooling water pipe
The longer cooling water pipe, the larger mold area to be cooled. Therefore, (b) in Figure 5 is better than (a). However, the longer cooling water pipe, the greater pressure drop on pipe, the greater temperature difference between inlet and outlet water of cooling water pipe. Therefore, (c) can be used. Optimal distance between cooling water pipes depends on diameter of cooling water hole and wall thickness of plastic part, as shown in Figure 5.

 

(2) Special cooling forms

Some special cooling forms, such as those shown in Figure 6, use baffles and bubblers to achieve better cooling effects.

 

Figure 6 Baffle type and nozzle type

(3) Mold material

Use of mold materials with high conductivity (such as BeCu) can increase heat transfer, especially in areas where cooling pipes cannot be arranged. Use of such materials can improve cooling effect.

(4) Cooling parameters

Cooling parameters mainly include coolant flow rate, inlet temperature of cooling pipe, pressure drop of coolant in pipe, type of coolant, etc. Coolant flow rate should be large enough to make Reynolds number greater than 10,000 to ensure a turbulent state. Generally, inlet temperature of cooling pipe should be about 10℃ to 30℃ lower than required mold temperature. Pressure drop of coolant in pipe depends on length, diameter and flow rate of cooling pipe. Types of coolant include tap water, cold water generated by cooling machine, water and oil with antifreeze, etc.

(5) Cooling circuit type

Cooling circuit type is usually divided into parallel and series, as shown in Figure 7.

 

Figure 7 Cooling circuit type

Product is a computer panel, with one mold and one cavity. Plastic is ABS. Cool module of Moldflow is used to optimize cooling system design to achieve uniform and effective cooling, shorten molding cycle, and reduce warping deformation.
1. Cooling water pipe layout
Cooling water pipe layout is shown in Figure 8.

 

Figure 8 Cooling water pipe layout
2. Input process conditions
According to process requirements, material ABS is selected, melt temperature is 240℃, mold temperature is 60℃, and injection time is 2.2s.
3. Analysis and calculation
(1) Original solution
Figure 9 shows cooling effect of cavity. Temperature in circled area is higher, temperature difference between upper and lower surfaces is also larger.

 

Figure 9 Temperature difference distribution of upper and lower surfaces of product
Heat accumulation occurs at the top of part, which is main cause of thermal bending. Therefore, we must modify cooling water pipe or mold structure. Within 17 seconds, plastic has completely cooled, but runner system has cooled less than 45% after 50 seconds. Thick runners require more time to reach a cooling rate of more than 60%. To facilitate mold opening, we must reduce size of runner to shorten cooling time. Figure 10 shows solidification rate after cooling for 50 seconds.

 

Figure 10 Solidification rate distribution of product at 50 seconds

(2) Modified solution

As shown in Figure 11, a cooling insert made of Be-Cu is added to the top of part to reduce temperature in this area and distribute it evenly. In this way, cooling effect of top of part is better and temperature distribution is more uniform than original solution. Figure 12 is cooling effect diagram of modified cavity, which is significantly improved compared to original solution. Figure 13 shows solidification rate of modified solution after cooling for 19.8 seconds. Results show that product has completely solidified after cooling for 19.8 seconds and runner has also solidified by 60%. Therefore, mold can be opened after cooling for 19.8 seconds, and cooling cycle time is greatly shortened.

 

Figure 11: Adding a cooling insert to the top of part

 

Figure 12: Temperature difference distribution between upper and lower surfaces of part

Analyzes volume mesh (tetrahedron) of mold geometry, which can be used for calculating initial or steady-state mold temperature.
Both part mesh and mold mesh must be 3D meshes.

 

- Simulates mold temperature changes throughout the entire molding cycle.
- Provides more accurate filling patterns (for products with large variations in wall thickness).
- Provides more accurate warpage results.
- Comparison of Cool (Steady-State) and Cool (Transient FEM) Analysis
In steady-state analysis, mold temperature is an average value throughout molding cycle, while in transient analysis, mold temperature varies.

 

Transient analysis records mold temperature distribution from start of part injection to steady-state, as well as transient changes in mold temperature within a stable molding cycle.

 

- Transient mold temperature distribution and transient coolant temperature

 

- Transient injection temperature and cooling channel temperature results are also available

 

Moldflow Cooling Analysis module allows you to optimize cooling system of your mold design, including number, location, and size of cooling channels, as well as various cooling process parameters. This optimized solution can reduce temperature difference between moving and fixed molds, shorten production cycle time, and improve production efficiency.