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This article is written by Rasmus Grusgaard, The Danish Plastics Federation and was published in the Danish media 'Magasinet Plast' on september 11, 2020. Download the article in Danish here

Michael Lundbech A/S had problems with their subjects refusing to let go of the form, in cooperation with Super-Moulds and The Danish Plastics Federation they managed to make the subjects demould automatically with help from advanced coating and ion-implantation of the mould.

Injection moulding is extremely common as a production process and can take place extremely quickly under optimal conditions. A prerequisite is - of course - that you can get the cast item out of the mould.

At Michael Lundbech A/S, this was a problem for a specific subject. The Innovation Fund project Super-Moulds was able to remedy the problem with the help of coating and ion-implantation of the form. It is well known that polymers belonging to the TPE family can be difficult to deform due to their high tack.

Figure 1: Illustration of the cast item (left) and
schematic illustration of the mold used. When the mould opens, the item sticks to the solid part of the mould, as indicated by the black arrow.

The company Michael Lundbech A / S had long struggled to cast a mischievous item in Pebax® 3533 SP 01 resin when they joined the Super-Molds project. The problem was that the moulded workpiece adhered to the mould, which made automatic deformation impossible. It was not even possible to manually pull the item out of the mould between two nails or with tweezers.

Michael Lundbech A/S had tried different approaches to solve the problem by machining the mould (VDI 18, VDI 24 and VDI 30) as well as adding ejector cores but all in vain. Only VDI 30 machining made it possible to demould the subjects, but this was at the expense of the transparency of the items, which thus did not live up to the customer's specification. It was decided to treat the issue as an end-user case in the Super-Moulds project, and a test matrix was established.

The test matrix consisted of three different surface structures made by sparking (EDM - electrical discharge machining), two different surface structures made using laser texturing and three different coatings made using PVD (physical vapour deposition).

Automatic process possible

The test showed that neither the laser-textured nor the spark-textured surfaces caused stable casting. However, better transparency of the blanks could be achieved by using the sparked moulds compared to the laser textured moulds.

Two out of the three tested coatings made it possible to run an automatic process on the casting machine with perfect shaping. The two coatings are both based on chromium nitride made using the special HiPIMS (High Power Impulse Magnetron Sputtering) technology. One of these coatings was ion-implanted with oxygen, and this coating made it possible to run an automatic process on the casting machine with an emission of 500 bar.

For comparison, with the non-implanted coating, one could run an automatic process on the casting machine at 300 bar. Both coatings provided transparent items as desired. The next step will be to test the coated mould over a longer period to ensure that the positive qualities are preserved under production conditions.

- For us, it is a huge benefit to be part of the Super-Moulds project. In addition to solving a specific problem that has caused us several challenges, we have gained new knowledge and good sparring with the other project partners, says René Hansen, project manager at Michael Lundbech A/S.

It is still unknown whether the positive effect of the ion-implanted coating was due to the presence of oxygen or a change in the crystallinity of the coating caused by the powerful ion bombardment. The Super-Moulds consortium is therefore working further with the case and has in this regard planned tests with other coatings to systematically investigate the positive effect of the coating.

Michael Lundbech, CEO at Michael Lundbech A/S. Foto: PR

The partners in Super-Moulds are looking for plastic manufacturing companies, toolmakers, subject builders and tribology technicians.

If your company has a tool where sticking is a problem, Super-Moulds can offer to perform texturing and coating of the tool. Super-Moulds will cover the costs of these processes as long as we can use the results as part of the project results.

If you are interested you can find the relevant contact information here.

This article was brought in the 6th edition of the Finnish magazine Muovi Plast. Download the article here

Summary: A defined laser texturing of a mould insert has demonstrat­ed positive effects on the productivity when applied in combination with the DuPont grade Zytel® HTN51G35HSL NC0J0, which is a PPA polymer containing 35 % glass fibers. A validation test under produc­tion conditions has shown that a surface topography tuned with laser texturing (provided by GF Machining Solutions) has an anti-adhesive effect and allows for a potential gain of 5 seconds cooling time, which corresponds to a cycle time reduction from 33 to 28 seconds or a 15 % productivity increase.

The EUREKA project Super-Moulds is co-financed by Innosuisse (Switzerland) and the Innovation Fund Denmark and unites 11 Swiss and Danish partners. The aim of this almost five-year project (initiated in 2017) is to quantify the influence of polymer material, mould geometry and mould surface treatment on the overall productivity of plastic injection moulding.

Sytematic demoulding tests have been performed and the ejection force has been measured as a function of time using a specially designed test tool that is equipped with a force detector. The test tool was mounted in an Arburg 270C Allrounder 500-250 press. The maximum force and the integral value of the force curve are used as criteria to quantify the influence of a given test parameter. The maximum ejection force corresponds to the peak value of the curve, and the integral value of the force curve multiplied by the ejection speed corresponds to the energy necessary for demoulding the part.

In the initial phase of this project, industrial end-user companies have been approached and concrete test geometries have been defined. Figure 1 shows the original set of mould inserts and the conceived study geometry for a specific case. For this star-shaped geometry the DuPont grade Zytel® HTN51G35HSL NC0lO, a PPA polymer containing 35% glass fibers, has been defined as the reference polymer.

Figure 1 Left: Original star-shaped inserts of the production mould. Right: Test insert for studying the effect of surface treatments on the demoulding properties. All parts are made in stainless steel (1.2379).

A series of tests was made using star shaped cores made of KllO Bohler Steel (1.2379) and using seven different polymers. A series of cores was modified by laser texturing strategies to obtain different surfaces to be tested in real injection processes. Here, the laser textures are described by their corresponding surface roughness (S,). The roughness scale-was chosen to cover surfaces with low values of Sa (0.5-0.9 µm). Although the surface topography cannot be defined only by its roughness value, a trend was observed.

Figure 2: Ejection force depending on surface roughness.

For the majority of the polymers, the results reveal a minimum in ejection force around Sa = 0.6 µm. The laser textured cores were further investigated by surface contact angle analysis to determine the surface energy. It was found that with increasing roughness, the surface energy decreases, which presumably leads to lower adhesion between the polymer and the core surface. On the other hand, increasing roughness gives rise to interlocking between the core surface and the polymer and hence an increase in the static friction.

From these two general trends, the effect of surface roughness can be summarized as follows:

According to these results and depending on the polymer, the impact of the laser textures varies from a 10 % increased ejection force to a 60 % reduction as compared to the untextured reference. In particular, a random laser blasting pattern of s. = 0.6 µm was the best performing surface treatment in combination with the DuPont grade Zytel® HTN51G35HSL NC010 PPA grade (Figure 3). ln this case, laser texturing gave rise to a reduction of the initial peak force by 60%.

After the injection tests, an end-user validation test was carried out under production conditions. Inserts fitting to the production mould were fabricated and then textured by laser using the identical parameters as for the most performant test insert. For the validation tests, reference plastic parts were produced using the standard injection parameters. In a second phase, the cycle time was systematically reduced by decreasing the cooling time. Standard quality control tests (one example shown in Figure 4) were conducted to evaluate the minimum allowable cycle time for which the geometric tolerances of the produced plastic parts were met.

Figure 3: Ejection force versus time curve for a reference insert, which has been polished in the direction of demoulding, and a laser textured insert
(S, = 0.6 µm). The initial peak indicates the combined effect of static friction and adhesion and the integral under the curve (multiplied by the ejection speed} corresponds to the energy necessary to demould the part. For both parameters, the laser textured insert was the best performing. The test was performed with DuPont grade Zytel"' HTN51G35HSL NCOlO, a PPA polymer containing 35 % glass fibers.

The validation test showed that the cooling time can be reduced bt 5 seconds when the laser textured insert is used instead of a standard surface-finished insert. Thus, the total injection cycle time could be reduced from 33 seconds to 28 seconds, corresponding to a productivity gain of 15 %.

Figure 4: Example of a key quality control test of the star shaped axis of the plastic part. The plastic parts produced using the laser textured inserts fulfilled this quality criterion, even after reducing the cooling time by five seconds.

Contact information:

Ejection force measurements and validation test: Stefan Hengsberger, Haute ecole d'ingenierie de Fribourg, email: stefan.hengsberger@hefr.ch

Project Management (Super-Moulds): Sascha Louring, Danish Technological Institute, email: salo@teknologisk.dk