What is the difference between sequential, breathing and co-acting injection compression molding?

injection compression moulding/icm is an advanced form of conventional injection moulding. It increases the flow length/wall thickness ratio of injection molded parts; uses less clamping force and injection pressure; reduces internal material stress; and increases processing productivity.

Injection compression molding is suitable for a variety of products made of thermoplastic engineering plastics, such as large curved parts, thin-walled, miniaturized parts, optical lenses, and parts that require good resistance to attack characteristics.

The main feature of injection compression molding is that the mold cavity space can be automatically adjusted to different requirements compared to the traditional injection molding process.

    For example, it is possible to close the mold guides 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, the size of the cavity space can be controlled during or after the material is injected, depending on the operation, to match the injection process, to keep the polymer under proper pressure, and to compensate for material shrinkage.

Depending on the geometry of the injected part, surface quality requirements, and different injection equipment conditions, there are four injection shrinkage protection divisions to choose from. They are: sequential; coactive, breathing and partial pressurization.

Sequential icm (seq-icm)

Sequential injection compression molding process in which the injection operation and the pushing together of the mold cavities are performed sequentially. At the beginning, the mold guide is slightly closed and there is a cavity space of about twice the wall thickness of the part. Once the resin is injected into the mold cavity, the movable part of the mold is pushed until it is completely closed and the polymer is compressed inside the cavity.

During this process, there is a pause in polymer flow and a moment of rest between the completion of injection and the start of compression, which may create a flow line on the surface of the part, depending on the color of the polymer material, as well as the texture structure and material type of the part being molded. The process is operated in this manner. This icm can be performed using a crank bar type device.

Co-active icm (sim-icm)

The same as sequential icm, the common type icm starts with a slightly closed die guide, except that the die starts to push and apply pressure at the same time as the material is injected into the cavity. The extrusion screw and the die cavity may have a delay of s2 or s2 during the co-moving period.

Since the polymer flow front is always in a steady state of flow, it does not show the pauses and surface flow traces as in the seq-icm process.

Since both of these methods leave a large cavity space at the beginning of the operation, and before the molten polymer is injected into the cavity and encounters directional pressure, it may flow first into the lower side of the cavity due to gravity and may develop undesired bubbles due to the temporary unpressurized state.

Moreover, the larger the wall thickness of the part, the larger the cavity space will be, and the longer the flow length will increase the time period for the mold to close completely, all of which may exacerbate the above phenomenon.

Breath-icm (breath-icm)

With breath-icm, the mold is fully closed at the beginning of the injection. As a result, the polymer remains under pressure as soon as it is injected. This overcomes the potential problems that can occur with the two aforementioned methods. As the polymer is injected into the cavity, the mold is gradually pulled apart and a larger cavity space is created, while the polymer in the cavity is always under pressure.

As the material approaches a full cavity, the mold begins to push back until it is completely closed, allowing the polymer to compress further and reach the desired finished thickness of the part. The above movement between the extended cavities of the mold can be achieved with the help of the injection pressure coming from the polymer injected into the cavity or a pre-programmed motion of the injection molding machine.