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New test method quantifies injection molding release effect

Epluser Co.,Limited | Updated: Apr 26, 2016

New coatings with improved release properties for the injection molding process have been developed, tested and validated by the Tribology Centre at the Danish Technological Institute (DTI; Aarhus, Denmark) operating in a consortium with two Danish companies and partners in Switzerland and Spain (see fact box below).

The release mechanism in an injection molding process is complicated, and the ejection force depends on several factors, including:

  • the applied plastic type;

  • the design and geometry of the injection mold;

  • applied injection molding parameters;

  • shrinkage around the applied core;

  • surface characteristics; and

  • the application of coatings combined with ion implantation.

Surface characteristics can be tailored by changing the surface roughness using CNC milling, spark erosion or other methods or through different secondary treatments, such as microblasting, polishing and so forth.

DTI’s current research project, named Super-Slip under the Eurostars program, was initiated under E-SURF EUREKA, a publicly funded, intergovernmental European Union network with the goal of enhancing European competitiveness. Good progress has been made in designing, developing, implementing and validating a new test method for quantifying the release properties in an industry-relevant injection molding geometry, says Lars Pleth Nielsen, PhD, HD(O), Director of DTI's Tribology Centre. “It has been a quite complicated task just to be able to measure a reproducible ejection force,” says Nielsen. “This has been achieved with such a high reproducibility that we have realized that all test parts must serve as their own references. This implies that the measuring technique is so accurate and the reproducibility so high that we can measure a difference between individual cores—in nominal terms—when they are manufactured in the same way to the same specifications.”

Winther Mould Technology A/S developed the injection molds and measuring principle with a built-in force sensor, while DTI's Tribology Centre developed the coatings and ion implantations used in the test matrix. The practical implementation and refinement of the test molding took place at the Département de physique Ecole d’ingénieurs et d’architectes in Switzerland, whereas the model and pilot tests of industrial-scale injection molding were performed at research institute IK4-Tekniker and the company PROINYEC, both of which are located in Spain. DTI's Tribology Centre selected the applied molding parameters, which mimic industry conditions, in cooperation with Novo Nordisk A/S.

The comprehensive test protocol involves four different types of plastic—PP, POM, ABS and TPU—and different types of surface finish—laser-textured, microblasted, machined and so forth—and a large number of different coatings and ion implantation parameters.

It was a long process, says Nielsen. “For instance, it was necessary to modify the core by adding four parallel ribs on the outside (Figure 1C). “We did this in order to make the molded part more rigid and to prevent it from deforming and separating from the core surface during the demolding process,” explains Nielsen. Experiments revealed that, without the four longitudinal ribs, it is impossible to see the effect of the different coatings, according to Nielsen, as the applied ejection force caused the parts to warp away from the core, and the actual interaction between the plastics and underlying surface became insignificant.

Figure 1A

DTI: injection molding release effect 

Figure 1B

 DTI: injection molding release effect

Figure 1C

DTI: injection molding release effect 

Figure 1. Renderings A and B show the injection mold with a built-in force sensor (yellow), which records the ejection force after molding of the plastic part (Figure 1C). The dimension of the mold is a = 206 mm, b = 196 mm and h = 387 mm. The molded part has been optimised with four parallel fins on the inside to increase rigidity and prevent it from bending away from the substrate at the center and, thus, separate from the core surface when the part is pushed from the bottom during demolding. The molded plastic part is 69 mm long, and the contact area between the internal core and molded part is 31.7 cm 2.

DTI: injection molding release effect 

Figure 2.  Click on image to view full-size version. Shown here is the response from the force sensor when pulling and pushing actions are performed, respectively. As the force meter is able to record positive and negative forces, it will be possible not only to measure the ejection force but also the force during mold filling.

The researchers ultimately observed that the optimised geometry made it possible to measure a huge and reproducible difference in the ejection force between, for example, a TPU part and core with and without the CrN SuperSlip (CrN-SS) coating from the Tribology Centre, as illustrated in Figure 3.

DTI: injection molding release effect

Figure 3.  Click on image to view full-size version. Shown here is the ejection force required to demold a TPU part with and without the CrN-SS (SuperSlip) coating. The ejection force drops by approximately a factor of seven when the core is CrN-SS coated. The actual core in the experiment has a grinded surface structure.

A considerable amount of experimental and analytical work remains to be done. However, the consortium already has developed a new, innovative in-situ method for assessing the effect of surface treatments and coatings. “Indeed, we may be the first in the world to observe consistency between what we know works in practice and what we can now actually measure and quantify directly in an injection mold,” says Nielsen. “The first results are very promising (see Figure 4) and verify our experience from dialogues with our many customers around the world. They confirm that it is easier to demold using a CrN SuperDense (CrN-SD) coating or, taking it a step further, the CrN-SS coating than from an untreated steel surface,” says Nielsen.

DTI: injection molding release effect 

Figure 4.  Click on image to view full-size version. Normalized ejection forces for four different plastic types (PP, POM, ABS and TPU) are shown. The same reference core is used in three different states: Untreated, coated with CrN-SD and coated with CrN-SS.

Empirical knowledge is good, says Nielsen, but having reproducible data for the release effect is better. “The next years will prove the results of our close cooperation with our partners in Switzerland and Spain. It is absolutely necessary to quantify the ejection force to be able to improve mold performance by developing new surface coatings combined with ion implantation,” says Nielsen. The DTI Tribology Center is preparing to launch a

High Power Impulse Magnetron Sputtering (HiPIMS) chrome-nitride coating (CrN-HiPIMS), which reportedly is about 20% harder than traditional CrN. Moreover, it is denser, has fewer surface defects and is even smoother, adds Nielsen.

It is worth noting that not all chrome-nitride coatings are created equal (see Figure 5). Physical vapor deposition (PVD) coatings vary from supplier to supplier, notes Nielsen. “Coatings such as CrN, DLC and TiAlN will be different depending on the supplier. The coating properties will depend on the applied process technology and process parameters. This is where the experience of DTI's Tribology Centre makes a difference. For several years, we have invested a great deal of time and effort in optimising our various coating families,” says Nielsen.

The Tribology Centre has chosen to focus exclusively on the so-called magnetron sputter technology, as it provides some of the best conditions for making coatings with few surface defects and good overall performance, according to Nielsen. Furthermore, it is versatile and flexible, although not quite as fast as the so-called arc technology, for example. “For instance, we can see that the crystal structure and column growth in a CrN coating can be altered significantly by varying, for example, the amount of nitrogen (N2) used during the coating process. It is evident that such changes have a huge impact on the mechanical, chemical and physical properties of the coating, not least on its performance during demolding in an injection molding process,” says Nielsen.

DTI: injection molding release effect 

Figure 5. Click on image to view full-size version. Shown here are electron microscopy photos of a traditional chrome-nitride coating (CrN) and the Tribology Centre’s optimized CrN-SD (SuperDense) coating, which is also the underlying basis for the CrN-SS coating (SuperSlip).

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