Co-injection molding is the term used to describe the process of feeding several melts, one after the other, into one mold during the injection molding process. The melts should make contact with each other but should not flow into one another. The resultant composites may be permanent or it may be possible to move the individual components separately. In-mold decoration and back-molding processes are sometimes also included in this category. Here, a substrate, e.g. in the form of a fabric or film, is inserted into the mold before the melt is injected in.

 

Typical multi-color injection molded part

 

Strengths of the process

 

Reduction of assembly costs through integration of parts (multi-colored, rigid/flexible combination)

 

Good tactile properties, improved design (e.g. sandwich)

 

Integration of printed circuits in three-dimensional components (MID)

 

Material savings through the use of recycled material as the inside component (sandwich)

 

Reduction of storage and transport costs

 

Multi-color technique possible

 

Co-injection molding (sandwich molding, multi-material molding):

Co-injection molding is the term used to describe the process of feeding several melts, one after the

other, into one mold during the injection molding process. The melts should make contact with each other but should not flow into one another. The resultant composites may be permanent or it may be possible to move the individual components separately. In-mold decoration and back-molding processes are sometimes also included in this category. Here, a substrate, e.g. in the form of a fabric or film, is inserted into the mold before the melt is injected in. According to this definition, co-injection molding includes the following processes:

Two-material co-injection molding / two-color co-injection molding, overmolding

 

three-color co-injection molding / three-material co-injection molding / multi-color co-injection molding

 

Moving composites in injection molding

 

Multi-material molding, sandwich molding

 

3D MID (Molded Interconnect Devices)

 

Back-molding of film and fabrics

 

However, in this article, co-injection molding covers only with the first three items on the list. For the

others, please refer to the special articles.

Two-material co-injection molding:

By two-material co-injection molding we mean the injection of two different melts into the mold, one afte

the other. They may comprise the same material in different colors or two different materials. Other term

to describe this process are two-color co-injection molding or two-component sandwich molding. The

principle is basically the same, although there are essentially five different methods of carrying out the

co-injection molding process. We can divide them first of all into rotating and non-rotating mold systems.

 

Rotary plate

 

Rotating mold

 

Turnstiles and rotating cores

 

Transfer technique

 

Core-back technique

 

Stack-mold technique

 

Reference is made here to the respective mold chapter.

The arrangement of the two units is more or less independent of the mold technology but is governed

essentially by the geometry of the molded part. The units can be arranged either parallel or perpendicular to each other.

The use of the two-material co-injection molding process makes it possible to produce parts that satisfy

stringent specifications. These may be multi-part products for everyday use such as food packaging, toys, components for electrical equipment or other engineering components such as rubber bearings, rollers,

and sealing elements or various kinds of damping elements and housings with integrally molded seals.

The parts can be produced economically by co-injection molding in one step. Only one mold and thus only one machine is needed to manufacture the moldings. There is no need to pre-treat the metal as is usual today, for example, by degreasing with solvents. There are many reasons in favor of using co-injected structures. For example, the two-material co-injection molding method allows several functions to be integrated in one part. It also improves quality and minimizes costs, e.g. by reducing assembly work.

Furthermore, it is possible to obtain or improve certain performance properties such as functional, tactile

or design features. Composites of rigid and flexible materials open up a wide range of new possibilities.

These composites have a variety of functions, with the flexible component exhibiting the typical springy   

and elastic properties to provide resilience and a cushioning effect, or non-slip characteristics for good

grip, while the rigid component contributes its strength and stiffness to prevent distortion where loads are

applied.

Three-color and three-material co-injection molding:

To manufacture, for example, car rear lights, three or more different components are needed. For this

and similar cases of application, the three-color or three-material co-injection molding method is used.

This process is comparable to the previously described two-material co-injection molding process as far

as the mold technology, processing and machinery are concerned. All that is needed in addition is a third

injection unit and a suitable mold with a gating system for the three components, which of course is

correspondingly more complex.

Continuing this principle, several plastics can be combined together to create one part (multi-color

co-injection molding).

The latest development involves 5-color rear lights in which the metallizing of the reflector surfaces is

integrated inline into the production process. With hygroscopic plastics (e.g. PMMA, PC), this prevents

moisture absorption and produces a high-grade coating.

Bond strength in sandwich molding:

(based on a paper from Dr.-Ing. Karl Kuhmann, KT University of Erlangen, "Decorative surfaces of

injection molded parts", Seminar Carl Hanser Verlag, Wiesbaden, April 1999).

When several plastic parts are joined by sandwich or co-injection molding, the aim is to achieve a

positive, permanent bond of the plastics components. The attainable bond strength of adhesion-compatible plastic combinations under mechanical load and on exposure to certain media is of particular interest. As far as the adhesion and manufacturing precision are concerned, advantages are

generally obtained through the simultaneous production of pre-moldings and encapsulated finished

components, as well as sequential two-step production in one mold without intermediate demolding. The

first step involves injecting a pre-molding from one of the components, then encapsulating certain area

of the partly cooled pre-molding with one or several plastic components.

Factors influencing the bond strength:

The bond strength can be influenced among other things by the process parameters during injection

molding. The extent depends, however, on the material combination and thus on the specific prevailing

composite mechanism. In addition, attention must be paid to the local rheological and thermodynamic

conditions (also with regard to time) at the interface of real two-dimensional parts [94]. These influences

were reproduced on a modular specimen mold. It is only possible to apply the results to practical parts on

a qualitative basis.

Observations on the formation of a permanent material bond during co-injection or sandwich molding

make use of adhesion theories, including the diffusion theory. A reliable theoretical prediction of the bond

strength is, however, not possible with this theory, nor is it possible to predict the qualitative influence of

the injection molding parameters, if, for example, interactions exist between the injection molding

parameters and if the boundary conditions for the model do not fully apply to the physical processes

actually occurring in the sandwich molding for a material/part combination. A positive bond in the sense

of "injection welding" necessitates adhesive bonding of the components through sufficient wetting of the

partially cooled pre-molding through the melt heat of the newly injected component and a

"rapprochement" of the molecules from both components. Fig. 21 shows different adhesion mechanisms

that (depending on the material combination) overlap and can contribute towards the "technical" bonding

strength. The formation of adhesive forces is based among other things, on intermolecular interactions

(dipolar, dispersion and induction forces).

Apart from the material combination, the injection conditions in the real molded part also influence the

bond formation. As the distance from the gate varies, so the flow and cooling conditions vary, and this

has differing effects on the bond. In addition, the conditions for the second component may change

through different gating geometries, flow cross-sections and flow directions compared with the

pre-molding. Even with constant flow cross-sections, the gating conditions change in different areas of

the molding, for example through the later cooling of the melt, the time between the wetting and pressure

build-up or the relaxation of orientations.

Finally, for good adhesion compatibility, there are other criteria to take into account, especially the

so-called property compatibility. For example, depending on the component geometry, excessive

differences in shrinkage or thermal expansion can lead not only to distortion but also, if combined with

different stiffness of the components in relation to temperature, to failure of the material composite

through the overlapping of internal stresses. The various influences described here involve the

processes taking place during co-injection molding and they must be taken into account when developing

and evaluating a molded part.

Basically, any material that is suitable for injection molding can also be used for co-injection or sandwich

molding. The only proviso is that the adhesion of the materials to one another has to be good.

Part design

The distribution of the individual plastics will depend on the molding geometry and the gating point.

Where geometric structures such as long thin bars are to be encapsulated, care must be taken to provide

openings through the bars to support them.

Processing

When working with the co-injection molding process, attention must be paid to the shrinkage of the

individual components. Due to differences in the shrinkage characteristics of the plastics during cooling,

shear forces are generated that influence their adhesion to one another and affect the tendency of the

part to become distorted. Stress cracking can occur, for example, due to shrinkage. The picture shows

the different stresses during encapsulation/insert molding of the second component. Because the first

component has already cooled, the component injected second will shrink more strongly.

Basically, the molds are perfectly normal injection molds. Any type of gating system can be used to inject

the first component, while the subsequent component can be fed into the cavity by direct gating

recesses, breakthroughs or flow channels.

Gating is performed via a coaxial needle in which the nozzles are arranged concentrically. After injection

of the first component, the outer nozzle is closed and the needle of the inner nozzle drawn back so that

the second component can be injected. Depending on the desired distribution of the individual plastics,

the cavity of the mold can be altered slightly by retracting one half of the mold. In such cases, it is also

possible to work with mold cores which, depending on the injected material, dip into the mold cavity or

are retracted.

Rotary plate:

With this rotating mold technique, one raw material is first injected into a cavity designed in such a way

that it only releases space for the first plastic, but remains closed for the second (different color or

different material). After opening the mold and turning the moving mold half (or the part of this mold half)

in which the pre-molding is located, the latter moves into the second cavity. The rotating process is

carried out with the aid of a rotary plate mounted between the platen and the moving mold half. The

second cavity now releases the space for the second plastic, which is then injected. After the part has

cooled, the mold is opened again and the finished part ejected. In parallel with this second step, the next

pre-molding is already in production in the first cavity.

Rotating mold:

With this technique, the rotary movement is performed with a plate belonging to the mold instead of with

the rotary table described above.

Turnstiles and rotary cores:

Instead of the complete moving mold half turning, only an intermediate plate in the mold is turned by

means of turnstiles or rotary cores.

Transfer technique:

A part is produced in the first cavity and then transferred by hand or by means of a handling device into

the second cavity, where the second material is molded on. This technique can also involve the

production of a part on a machine, removing the pre-molding and inserting it in a second machine, where

the second material is molded on.

Core-back technique:

With the non-rotating core-back technique, the first component is first injected in and, by means of

movable inserts or cores, a space is kept closed off for the second plastic. Without opening the mold, the

cavity is released for the second component as these moving elements in the mold change position. The

second plastic can now be injected after a delay of just a few seconds. The components can be arranged

next to, above or in one another.

The advantage of the rotating mold technique over the core-back technique is that melt can be injected

into both cavities in parallel, whereas it can only be done in sequence in the case of the core-back

technique. On the other hand, because of the moving elements, there is no need to open the mold with

the core-back technique in the middle of the process, and the interface temperature when the melt of the

second material arrives can be varied more easily as a result. Which process is used for which part must

be decided individually for each application, taking into account both technical and economic aspects.

Stack-Mold technique:

With these rotary molds, it is not one half of the mold that is turned as in a normal rotary mold, but a

middle third plate of the mold. It is basically a stack technique with a rotating center section having its

own hydraulic cylinder drive system. The axis of rotation can be either horizontal or vertical. It is of

particular advantage here that the mold opening forces at both parting surfaces more or less correspond

and that no eccentric loading of the clamping unit occurs. This means that the machines used in this

process can be equipped with much lower clamping forces than those for the rotary plate technique -

probably around 60 % lower, significantly reducing investment and production costs. The production cost

for a car rear light, for example, was just under 7 % down. The patent situation has not yet been clarified.

Regardless of whether it is a one-material or a multi-material co-injection machine, the basic structure of the injection molding machine remains the same:

The machine frame accommodates the plasticating unit and the clamping unit. The latter performs the

opening and closing movements of the mold during the production cycle. The injection molding cycle i

coordinated via an instrumentation and control unit that is generally accommodated in a control cabinet

separate from the machine.

The screw in the plasticating unit is normally driven hydraulically. The pumps needed to provide the

hydraulic oil flow are accommodated in the machine frame and are driven electrically. The hydraulic

valves needed for controlling the individual motions are, wherever possible, positioned in the immediate

proximity of the consumer to which they are assigned.

To activate additional functions, e.g. ejectors, shut-off nozzles or to move core pullers, additional

electromechanical or pneumatic drives are used. A mold temperature control device may be integrated in

the machine, but is normally accommodated in an external housing.

The most significant difference between a one-material and a multi-material co-injection molding

machine is the different number of plasticating units. In most cases, they are screw plasticating units with

universal screws. In the case of multi-material or sandwich molding machines, it is normal to have a

plasticating unit for each component, arranged either parallel to each other or perpendicular to each

other.


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