Injection compression molding (icm) is an advanced form of traditional injection molding. It can increase the injection length/wall thickness ratio of injection molded parts. And it can use smaller clamping force and injection pressure. Besides, it will reduce material internal stress and improve processing productivity.
Injection compression molding is suitable for products made of various thermoplastic engineering plastics, such as large-size curved parts, thin-walled and miniaturized parts, optical lenses, and parts with good anti-attack characteristics.
Compared with the traditional injection molding process, the main feature of injection compression molding is that its mold cavity space can be automatically adjusted according to different requirements. For example, it can close the mold guide part before the material is injected into the cavity. While the cavity space is expanded to twice the finished wall thickness of the part. In addition, according to different operation methods, the size of the cavity space can be controlled during or after the injection of the material, so as to match the injection process. So the polymer maintains an appropriate pressure state and compensates for the shrinkage of the material.
According to the geometry of the injection molded parts, surface quality requirements, and different injection molding equipment conditions, there are four types of injection shrinkage protection.
They are: seq-icm, sim-icm, breath-icm
Sequential icm (seq-icm)
In the sequential injection compression molding, the injection operation and the push-fit of the mold cavity are performed sequentially. At the beginning, the mold guiding part is slightly closed. And there is a cavity space about twice the part wall thickness. When the resin is injected into the mold cavity, the movable part of the mold is pushed until it is completely closed. Also, the polymer is compressed in the cavity. During this process, there will be a moment when the polymer flow pauses and stops from the completion of injection to the beginning of compression. Therefore, it may form a streamline trace on the surface of the part. The visibility depends on the color of the polymer material and the texture structure and material types of the molded parts.
Crank rod type equipment can be used in this icm.
Simultaneous icm (sim-icm)
Same as the sequential icm, the mold guiding part of the sim-icm is slightly closed at the beginning. The difference is that when the material is injected into the cavity, the mold starts to push and press. During the joint movement of the extrusion screw and the mold cavity, there may be a delay of s2 or s2. The polymer has always maintained a stable flow state in the front. So it will not appear pause as the seq-icm process and streamline traces on the surface.
The above two methods both leave a large cavity space at the beginning of the operation. When the molten polymer is injected into the cavity without encountering directional pressure, it may first flow into the lower side of the cavity due to gravity. Besides, it may cause undesirable bubbles due to being temporarily under pressure. Moreover, the larger the wall thicknessof part, the larger the cavity space. While the extension of the injection length will also increase the time period for the complete close of the mold, which may aggravate the above phenomenon.
With breath-icm, the mold is completely closed at the beginning of injection. Therefore, the polymer will remain under pressure once injected. This overcomes the potential problems that may occur in the above two methods. When the polymer is injected into the cavity, the mold gradually opens to form a larger cavity space. And the polymer in the cavity is always kept under a certain pressure. When the material is close to the full cavity, the mold has begun to push back until it is completely closed. So the polymer is further compressed and reaches the required thickness of the part. The injection pressure transmitted by the polymer injected into the cavity or a preset movement program of the injection molding machine can realize the movement between the expansion cavities of the above mold.
Local compression icm (select-/com-icm)
When using com-icm or line pressure icm, the mold will be completely closed. There is a built-in line indenter that presses from a certain part of the cavity to the cavity when or after the injection. Thus, the larger solid part of the part is locally compressed and thinned.
This local comoression can be controlled by the injection molding equipment or a preset built-in line program in the separate hydraulic device.
Design of injection parts and molds
Injection compression molding is suitable for the parts with a curved appearance, such as laptop computer shells, car tailgates, car dashboards, and flat car fenders. It is necessary to select the inlet of the injected part and the position of the flow channel. So that it can achieve a good effect of filling the cavity. Some commercial injection molding filling procedures can be used to detect pushing force and injection pressure. We can also adopt some standard rules formulated for plastics, such as the thickness ratio of stiffener/wall thickness, and some combination techniques.
Pay attention to the guiding blade rail, guiding core and cavity of the mold. There are tight tolerances to prevent polymer leakage from overflowing the cavity. There must be a nozzle with a check switch to prevent the polymer from flowing back into the injection molding machine. It is also possible to install a thermal injection nozzle with a check valve on the mold instead of the above nozzle.
For parts with through holes, insert the lock nails fixed on one side of the mold into the other side and have a good sliding fit. So it can prevent the mold cavity movement from forcing the pins to loosen or get stuck. In addition, the cavity pressure in the icm injection molding process is lower than that of traditional injection molding. Thus, the mold structure doesn’t have to be as solid and bulky as traditional injection molding.
Injection molding equipment
The icm’s pushing force clamping and the movement of the feeding screw are different from the corresponding operations of traditional injection molding. Therefore, we msut add some software functions to the injection molding machine. To obtain simultaneous movement of molds and screws like sim-icm and breath-icm, we must increase the fluid flow rate of the hydraulic injection molding machine. What’ more, when using hydraulic main injection molding equipment for seq-icm, the hydraulic valve used for mold clamping in traditional injection molding can be used to realize the pushing movement of the mold.
Most hydraulic injection molding equipment can be used for injection compression molding of large parts. But the closing movement of the cavity should be controlled by a pre-programmed pressure program. Otherwise,, there may be some troubles.
Pay attention to keeping the linear movement of the mold cavity. Because the non-linear mold movement will have a temporary effect of polymer flow. And that will result in some strange appearance on the surface of the part.
The icm can have longer injection length, lower locking force and injection pressure than traditional injection molding. So it can use smaller equipment to produce large parts.
Tests on noryl gtx964 show that under the same conditions of wall thickness and part geometry, icm can reduce the clamping force by 75% and the injection pressure by 30% compared with traditional injection molding. The test of using sim-icm to make car body panels shows that when the mold adopts the central pouring channel and the part wall thickness is 1.5mm, the injection length can be increased by 200% compared with traditional injection molding.
Moreover, the reduction of the clamping force largely depends on when the mold is closed. Too fast or too slow clamping will increase the injection pressure and clamping force.