Engineering plastics have contributed to weight reduction in automotive parts, electronics and electrical products, and many other industrial products, by being used in place of metal materials.Today rather than simply replacing metal materials, many metal-resin composite materials are manufactured which still have some metal materials, and replace the rest with engineering plastics. metal-resin composite materials deliver the advantage of simultaneously using both the properties of metal materials (such as high rigidity, electrical conductivity, etc.) and the properties of engineering plastics (such as low density, electrical insulation, etc.). However, since there is a bond interface between the two materials, it presents a higher level of technical difficulty when bonding force and airtightness are required.
Past methods used to produce metal-resin composite materials include simple insert molding, as well as physical joining through caulking or screw tightening, bonding with adhesives, and sealing with potting agent. However, simple insert molding, caulking and screw tightening often did not provide enough functionality because they provide poor airtightness. Bonding with adhesives and sealing with potting agent involve large numbers of parts and extensive procedures, making high costs inevitable.
In order to resolve this problem, technologies appeared from the year 2000 onward which give the bond interface of the metal and resin properties of bonding force and airtightness with injection molding alone by giving the metal surface physical texture and chemical affinity in advance and making it into insert molding.
However, since many factors (metal parts, resin materials, mold structure, injection molding conditions, etc.) in this method have an effect on bonding, it does not provide stable bonding. As a result, there are still not many examples of this going into mass production.Thus here we will introduce some points to keep in mind to getmetal-resin bonding capability with injection molding alone.

■Benefits gained from metal-resin bonding technology

	Pattern①	Pattern②	Pattern③
Conventional technology	Outsert molding,adhesion, screw tightening	adhesion, radiation sheets	O-rings
Benefits of metal adhesion	Better appearance （better strength,design）	Better functionality （better heat release）	Added functionality （added water resistance）
Application examples	Resin boss bonding to metals	Release of heat to that sink of heating elements	Water resistance for mobile device connectors

１. Points for successful metal adhesion

Figure 1-1 shows the process of direct metal-resin bonding with metal insert molding.
Direct metal-resin bonding is a process which produces parts in which metal parts and resins are firmly bonded by introducing molten resin through injection molding to metal parts which have already undergone sufficient surface treatment, and the resin then cools and solidifies. Since there are many influential factors in this process, all of the factors shown in figure 1-1 (metal parts, resin materials, mold structure, injection molding conditions, etc.) have to be optimized in order to produce a stable bond. Below are the points to keep in mind for each of these different factors.

（1）Metal surface treatment

Reports are already out on our Quick-10^® technology, which uses quick heating and cooling of insert metals to produce direct metal-resin bonding with metal insert molding alone, without needing any special surface treatments for the metal side. In any case, from among the various technologies offered for metal surface treatment up to now, here we will introduce metal surface treatment technology that does not require quick heating or cooling systems.
NMT2）、TRI, and PAL-fit^® 3) are some of the treatments that use chemical products, but technologies such as Laseridge^® 4) and DLAMP^® that use laser irradiation treatment to perform surface treatment are now being put into use, and these do not use chemical products. Each different surface treatment has its own various characteristics. For example, surface treatments using chemicals treat the entire insert metal uniformly, but laser irradiation surface treatment has characteristics such as only treating part of the insert metal. For details about surface treatment technologies please refer to information in our publication “pla-topia”^® , or to the manufacturer websites.

（2）Resin materials

To ensure bonding, normal metal insert molding has been shaped either to cover all of the metal with resin, or to have resin parts physically secured by some metal parts. However, we know that when there are various changes to the environment after molding (heat aging, heat cycles), bonding capability can decline due to differences between the linear expansion of the metal and the resin, and other such factors. Resin materials need to have properties such as the following5) in order to produce good metal-resin bonding that can withstand these environmental changes post-molding and at any point afterwards

①Mechanism (affinity): Injection – dwelling – cooling – mold release

　Increase adhesiveness through mechanisms such as intermolecular force and hydrogen bonding.

②Good surface transferability (flowability): Injection – filling

　Increase bonding strength and adhesiveness through gapless introduction bonding on fine uneven metal surfaces without any gaps.

③Low molding shrinkage and linear expansion rate close to that of metal : dwelling – cooling – post-mold release

　Get molding shrinkage and linear expansion rate to be close to that of metal, to prevent separation at the interface due to temperature changes.

Figure 1-3 shows analysis results based on experiments performed by Polyplastics.
Relative values of surface transferability on the horizontal axis (smaller values indicate better surface transferability) and shrinkage rate on the vertical axis (smaller values indicate lower shrinkage rate) are shown without dimensions.
Affinity improvement agent is an additive that gives the metal-resin interface better affinity. It is composed of elements chosen to make it distribute uniformly within the resin, and be highly adhesive to metal surfaces.
The left part of figure 1-3 shows bonding when affinity improvement agent has not been added, and this is where standard materials are categorized. Bonding can be improved by optimizing surface transferability and shrinkage rate even in standard materials, but there is a limit. The right side of figure 1-3 shows bonding when affinity improvement agent has been added. Metal adhesion grades are categorized for this, and materials are designed to meet the aforementioned three requirements for resin materials.
Table 1-1 shows the metal adhesion grades that Polyplastics sells, which follow the aforementioned guidelines for resin materials design. 1135MF1 and 940MA are the set standard metal adhesion grades for PPS and PBT respectively, and these also have metal adhesion grades 1150MF1 and 930MA which have added functionality. We are also currently developing even more metal adhesion grades with new functionalities in order to cater to new demands.

（3）Mold design

This was mentioned in the previous item for resin materials, but in the steps from injection to filling in (2), it is desirable to have good surface transferability (flowability) for gapless introduction bonding on fine uneven metal surfaces. Surface transferability is a property that closely corresponds to resin flowability. Gas generated at the time of molding can sometimes cause problems by obstructing flow of resin and resin welded parts, and at the end of the flow area.
Proper resin thickness settings and expulsion of gas through gas vents are necessary when resin flow obstruction causes bonding problems.

（4）Injection molding

Injection molding conditions, particularly mold temperature, have a strong effect on bonding when performing direct metal-resin bonding with metal insert molding. For this we can devise injection molding conditions to achieve the good surface transferability and low molding shrinkage with linear expansion rate close to that of metal, which we examined in the section on resin materials. Figure 1-4 shows experiment results from the test specimen. These results indicate that no effects on bonding were observed for molding conditions other than mold temperature. However, in actual products we believe that there are cases where it is necessary to also optimize molding conditions other than mold temperature in order to achieve the two aforementioned objectives.

（5）Quality control

In addition to optimizing each of the conditions mentioned up to now, it is also important to check whether the necessary bonding is being achieved during mass production. There are times when control is administered indirectly through manufacturing parameters (injection molding conditions, etc.) and times when product inspections are performed directly. The direct method is best at providing certainty in quality control, but it involves more new added costs than the indirect method. Costs could be reduced if inspections for the direct method could be done automatically over the internet, so there are high expectations for internet-based automated inspection systems that would utilize machinery such as X-ray CT scanners and ultrasonic flaw detectors.

■ Figure 1-4. Relation between injection molding conditions and bonding

Here we will share the bonding strength data we have produced for metal-resin composites. The bonding strength is evaluated by finding the shear fracture toughness between the metal and the resin, using test specimen of metal-resin composites based on ISO19095 as shown in figure 2-1. We used aluminum alloy A5052 for metal, and standard grade PBT and PPS for resin. Figure 2-3 shows the results. When the proper surface treatment was not applied, neither PBT nor PPS resins produced any bonding force at all, as the interface of the metal and resin separated when releasing the metal-resin composite from the mold with insert molding. However, when proper surface treatment was applied, there was enough bonding force to cause cohesive failure on the surface layer of the resin in the initial bonding strength test. This showed that applying the proper treatment to metal surfaces produces sufficient bonding force in PBT and PPS even when using standard grades.

Here we will share the metal-resin composite airtightness data that we produced. We used two different types of test specimen, as shown in figure 3-1 and figure 3-2. For metal we used aluminum alloy A5052 (with and without surface treatment) and for resin we used standard grade and metal adhesion grade PPS. In the airtightness test we used helium leak tester and evaluated the relative quality of the airtightness in terms of leakage speed (Pa m3/s). Table 3-1 shows the results. Standard grade PPS did not show good airtightness, with leakage speed of 5 × 10-5 (Pa m3/s) and higher regardless of whether or not the metalsurface was treated. Using metal adhesion grade PPS also did not produce good airtightness when there was no proper surface treatment. Using metal adhesion grade PPS and also giving proper surface treatment to the metal side did produce good airtightness, with leakage speed of 5×10-7 (Pa m3/s).
In addition to initial airtightness, we also observed airtightness after durability treatment for the combination of surface treated metal and metal adhesion grade. We did not detect any increases in leakage speed for any of the durability treatment conditions (1,000 heat cycles, or 1,000 hours of high temperature and humidity). Leakage speeds can only practically be considered ballpark measurements, since these can change depending on the shape and size of the test specimen, but we gained a general idea that we can attain a practical level of airtightness by combining metal that has undergone surface treatment and metal adhesion grade

■Table 3-1. Airtightness evaluation results

○：Leakage of 5 × 10-7 Pa・m3/s or less    ×：Leakage of 5 × 10-5 Pa・m3/s or more

There are now many more insert molding metal-resin composite parts than there were in the past, but it is still difficult to determine just by looking at them whether they are simply insert molded articles or if they are metal adhesion molded articles. Reported examples of items that have been mass produced up to now using metal adhesion technology include various types of mobile device parts (mobile phones, tablets, digital cameras, etc.) and some auto-related parts. In order to expand its practical applications in the future, it will be necessary to define (1) bonding mechanisms, and (2) quality control indexes for bonding interfaces. Polyplastics will also continue striving to make progress in this area as the leading company for engineering plastics.