Introduction to Injection Compression Molding Technology

Time:2021-06-26 11:24:43 / Popularity: / Source:

Injection compression moulding (icm) is an advanced form of traditional injection moulding.
It can increase ratio of injection length/wall thickness of injection molded parts; use smaller clamping force and injection pressure; 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, miniaturized parts, optical lenses, and parts with good anti-attack characteristics.
Compared with traditional injection molding process, main feature of injection compression molding is that obvious feature of injection compression molding is that mold cavity space can be automatically adjusted according to different requirements. For example, it can close mold guide part before material is injected into cavity, cavity space can be expanded to twice finished wall thickness of part. In addition, according to different operation methods, size of cavity space can be controlled during injection of material or after injection is completed to match injection process to keep polymer in a proper pressure state and achieve effect of compensating for material shrinkage.
According to geometry of injection molded parts, surface quality requirements, and different injection molding equipment conditions, there are four types of injection shrinkage protection to choose from. 
They are: sequential; co-acting; breathing and local compression
Injection Compression Molding Technology 

Sequential icm (seq-icm)

In sequential injection compression molding process, injection operation and push-fit of mold cavity are carried out sequentially. At beginning, mold guide part is slightly closed, and there is a cavity space about twice part wall thickness. When resin is injected into mold cavity, movable part of mold is pushed until it is completely closed, polymer is compressed in cavity. During this process, since there will be a moment when polymer flow pauses and stops from completion of injection to beginning of compression, it may form a streamline trace on the surface of part. Visibility depends on the color of polymer material, texture structure and material type of part during molding. .
Operation process in this way. Crank rod type equipment can be used to perform this kind of icm.  

Co-moving icm (sim-icm)

Same as sequential icm, mold guiding part is slightly closed at the beginning of co-moving icm. Difference is that when material starts to be injected into cavity, mold starts to push and press. During joint movement of extrusion screw and mold cavity, there may be a delay of s2. Since polymer has always maintained a stable flow state in front of it, it will not appear as seq-icm process pause and surface streamline traces.
Because above two methods leave a large cavity space at beginning of operation, when molten polymer is injected into cavity and has not yet encountered directional pressure, it may first flow into lower side of cavity due to gravity, undesirable foam may appear due to being temporarily under pressure. Moreover, the larger part wall thickness, the larger cavity space, extension of injection length will also increase time period for mold to be completely closed, which may aggravate above phenomenon.

Breathing icm (breath-icm)   

With breathing icm, mold is completely closed at beginning of injection. Therefore, once polymer is injected, it will remain under pressure. This overcomes potential problems that may occur in aforementioned two methods. When polymer is injected into cavity, mold is gradually opened to form a larger cavity space, and polymer in cavity is always kept under a certain pressure. When material is close to full cavity, mold has begun to push back and close until it is completely closed, so that polymer is further compressed and reaches required finished thickness of part. Movement between expansion cavities of above-mentioned mold can be realized by means of injection pressure transmitted by polymer injected into cavity or a preset movement program of injection molding machine.   

Local compression type icm (select-/com-icm)

When using local pressure type or line pressure type icm, mold will be completely closed. There is a built-in line pressure head that presses from a certain part of cavity to cavity when polymer is injected or after injection is completed, so that larger solid part of the part is locally compressed and thinned.  
This kind of local pressure can be controlled by injection molding equipment or a separate hydraulic device preset built-in line program. 

Design of injection parts and molds  

Injection compression molding is suitable for injection molding parts with curved appearances, such as laptop computer shells, car tailgates, car dashboards, and flat car fenders. It is necessary to select inlet of injected part and position of flow channel to achieve a good effect of filling cavity. Some commercial injection filling procedures can be used to detect pushing force and injection pressure. Some standard rules formulated for plastics can also be used, such as thickness ratio of stiffener/wall thickness, and some combination techniques.  
Attention should be paid to guiding blade rail, guiding core and cavity that make mold extend. There are tight tolerances to prevent polymer leakage from overflowing cavity. There must be a nozzle with a check switch to prevent polymer from flowing back into injection molding machine. It is also possible to install a thermal injection nozzle with a check valve on mold instead of above nozzle.  
For parts with through holes, lock nails fixed on one side of mold should be inserted into the other side of mold and have a good sliding fit to prevent mold cavity from forcing pins to loosen or get stuck. In addition, since cavity pressure in icm injection molding process is lower than that of traditional injection molding, mold structure does not have to be as solid and bulky as traditional injection molding. 

Injection molding equipment   

Because icm's pushing force clamping and movement of feeding screw are different from corresponding operations of traditional injection molding, some software functions must be added to injection molding machine. In order to obtain simultaneous movement of molds and screws such as sim-icm and breath-icm, fluid flow rate of hydraulic injection molding machine must be increased. In addition, when hydraulic main injection molding equipment is used for seq-icm, hydraulic valve used for mold clamping in traditional injection molding can be used to realize pushing movement of mold.
Most hydraulic injection molding equipment can be used for injection compression molding of large parts. But closing movement of cavity should be controlled by a pre-programmed pressure program, otherwise it will encounter some troubles.
Pay attention to keeping movement of mold cavity linear, because non-linear mold movement will have a temporary effect of polymer flow, which will lead to abnormal appearance of surface gloss of part.  
Because icm can have a longer injection length, lower locking force and injection pressure than traditional injection molding, it can use smaller equipment than traditional injection molding to produce large parts. 
Tests on noryl gtx964 show that under conditions of same wall thickness and part geometry, icm reduces clamping force by 75% and injection pressure by 30% compared with traditional injection molding. Test of using sim-icm to make car body panels shows that when mold adopts central pouring channel and part wall thickness is 1.5mm, injection length can be increased by 200% compared with traditional injection molding.
In addition, reduction of clamping force largely depends on when mold is closed. Too fast or too slow clamping will increase injection pressure and clamping force.

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