Plastic parts are shaped and dimensionally accurate as a function of the temperature of the injection mold during filling, solidification, and production efficiency.
Different kinds of plastics have different mold temperatures scope, enabling plastic melt to fill the cavity with ease.
As long as the mold temperature is reasonable, it ensures that the plastic parts will shrink and warp very little after being demolded, and they will remain stable in size, their mechanical properties and surface appearance will be good, too.
It is necessary to design a temperature adjusting system to control the mold temperature.A mold is typically heated or cooled. If the need arises, both may be desired.Plastic injection molding typically occurs at a temperature of 200 to 300 degrees Celsius, where the melt is formed, cooled, and solidified into the profile.
Injection-molded products are generally removed from the mold at around 60 degrees, which is a result the heat to be transferred back to the mold.The cooling system is designed to move the heat out of the mold in a timely manner, so that the temperature remains within a sensible range.
Conduction, convection, and radiation are the primary methods by which heat is transferred in the mold, and 80% of the heat is carried away by heat conduction.
It is mostly cooling water, including normal temperature water and chilled water, and occasionally beryllium copper, used in the mold for heat conduction.
Moreover, most of the convective heat transfer takes place via a fan, which causes the mold to naturally cool in the air.
Injection molds have their fundamental role as a production tool, which makes it possible to manufacture plastic parts repeatedly and with high dimensional accuracy.
|Barrel Temp (°C)
|Mold Temp (°C)
Furthermore, the heat exchanger ensures the quality of the molding by cooling the mold and preventing it from overheating. This prevents any damage to the molding.
An injection mold’s temperature control system transfers heat continuously from the melt or heats the mold to the desired injection temperature.
Mold temperature affects how the melt flows through the cavity, how quickly the mold cools, and how well the part comes out of the mold, while being required to be contained within the mold.
Mold temperature, part release temperature, and the heat exchange of the mold all affect the productivity of the mold.
A temperature control system is crucial to molds with high precision and mass production demands, and sometimes it is necessary to design a special temperature regulator for each part of the mold.
It is one of the difficulties of mold design to incorporate a temperature control system in this type of injection mold.
Cooling and heating of the mold can be controlled by both systems of mold temperature control.
The mold temperature can be adjusted to enhance the fluidity of plastics with poor viscosity and low flow, such as PC, hard PVC, PSF, PPO, etc.
A mold temperature of 80 to 120 °C should be maintained.In the molds mentioned above, the heat of the melts alone is insufficient to maintain mold temperature if the surface dissipates heat rapidly.Hence, a heating system is essential to allow the mold to heat.
There are molds that have both cooling and heating systems.The cooling system is needed for production in cold areas or for large molds. The heating system must be preheated beforehand for smooth mold operation.
When the temperature of the mold reaches the molding demand, the heating system can be turned off.
If the temperature of the mold is higher after a period of time production.During the molding process, it is necessary to maintain the molding temperature.
It may be necessary to cool the thick wall sections to improve the melt flow and to heat the thin wall sections to improve the melt flow, if the plastic product is large and if its wall thickness is uneven.
Small thin-walled products are typically molded with low mold temperatures, heating devices or cooling mechanisms need not be deployed, and the mold is cooled naturally after molding.
Importance of temperature control
A mold’s temperature directly affects the flow, solidification, quality, the cycle, and the cycle of the casting.
The mold’s temperature should be controlled at 60 degrees Celsius to prevent stress cracking in plastics with good fluidity such as PE, PP, HIPS, ABS, etc.
Plastics with poor fluidity like PC, PPO, PSF, etc. can be worked at higher temperatures, thereby reducing internal stresses.
The mold temperature should be controlled between 80 and 120 °C. For this purpose, a heating system should be installed.
In addition, the cooling process of crystalline plastics (such as PE, PP, POM, PA, PET, etc.) and amorphous plastics (such as PS, HIPS, PVC, PMMA, PC, ABS, polysulfide, etc.) are different.
Cooling through a crystalline plastic’s crystallization zone essentially releases the heat, while at the same time preserving the plastic’s temperature.
Crystalline plastics can only be cooled beyond a certain temperature. Therefore, they require more cooling time than amorphous plastics.
Among the following table is used when no special requirement for the surface quality of the product is to be achieved, the plastic barrel temperature and the mold temperature commonly used (the mold temperature in this table is the temperature of the mold cavity).
Effect on product accuracy
With a high molding temperature, the product will shrink unevenly, resulting in deformed product after demolding. It is also easy to cause flashing and to cause sticking to the cavity and core.
It is possible that the surface will be delaminated if the mold temperature is too low, leading to poor fluidity in the melt.
As a result, uneven shrinkage, internal stress, deformation, cracking, and warpage will result, if the mold temperature is not uniform, the temperature of the solidified product after it is taken out of the mold.
Therefore, the whole product needs to have a balance of cooling.
It has a great effect on shrinkage rates, deformation, stress cracks and surface quality when the cold mold temperature fluctuates.
Effect on injection molding cycle
About 80% of the molding cycle involves cooling.
By controlling the mold temperature properly, the melt will solidify in good time. By opening the mold quickly and removing the finished product in a short period of time, the molding cycle will be shorter.
But if the mold temperature is not properly regulated, the melt may solidify slowly, thereby lowering the mold temperature in a negative way, but it also required the mold to be protected from temperature variations. This also led to an extended injection cycle, thus decreased productivity.
Cooling channel design principles
plastic mold cooling
- A system with as many channels of cooling as possible, and as large of a channel diameter as possible, would be optimal.
There should be 1-2 times (usually 12 mm15 mm) distance between the cooling channels and the mold wall, and the center distance between them should be about 3-5 times their diameters.Generally, the channels are greater than 8mm in diameter.
- The cooling channels should be at the same distance from the mold surface.
The distance between the cooling water hole and the surface of the cavity should be precisely the same everywhere if the wall thickness of the plastic part is uniform.
The cooling should, however, be strengthened at the thickest wall, if the walls of the plastic part are uneven in thickness.
- Improved gate cooling is needed.
- The cooling channel should not be connected to the insert or its joints so that the water does not leak out.
- It is best to avoid the placement of cooling tubes on weld lines in plastic parts.
- The position of the water discharge and inlet joints should be as close as possible to each other, typically on opposite sides of the operator’s side.