Alleviating mold adhesion during thermoplastic polyurethane injection molding
Injection molding is one of the most commonly used techniques for the mass production of plastics. Although highly trained technicians can solve many of the issues that arise during the injection molding process for plastics fabrication, the undesirable phenomenon of mold adhesion remains a challenging problem. This issue, which occurs at the interface between a molded part and the cavity surface during ejection, often arises during injection molding of thermoplastic polyurethane (TPU), e.g., in the fabrication of sport shoes, boots, and googles. When serious, the mold adhesion effect can make it difficult to release the molded product from the mold cavity. Moreover, distortions or cracks that occur in the finished product cause lower yield rates and high production costs.
There are four common approaches used in the injection molding process to alleviate mold adhesion. In the first two approaches, a release agent is released into the resin or applied to the cavity surface. As an alternative, it is possible to improve the ejection system used in the mold. The fourth approach—developed over the last decade—involves a special treatment of the cavity surface. For example, it has been found that wolfram carbide, titanium nitride (TiN), diamond-like carbon (DLC), and chromium nitride (CrN) coatings are effective for reducing the demolding force during the injection molding of polypropylene, polyethylene terephthalate, and poly(methyl methacrylate), PMMA.1 In addition, a thin, chemically adsorbed fluorocarbon film has been used to significantly reduce the ejection force during PMMA injection molding.2, 3 Three major elements—DLC, polytetrafluoroethylene, and fluorine—were integrated into this film coating to improve its hydrophobic properties.4–6 To date, however, there have been no studies dedicated to overcoming the adhesion effect during TPU injection molding.
In our work, we have previously developed a mold-adhesion-force tester that can be operated in two different ways, i.e., in an ejection mode7 and a tensile mode.8 To investigate the adhesion effect of TPU in particular, we prefer to use our tester in the tensile mode because TPU is an elastomeric resin. In the past, we have studied five substances that are commonly used for injection molding coatings (i.e., electroplating chromium, Teflon, CrN, TiN, and DLC), to coat the injection molding cavity surface and to thus alleviate the adhesion force. We have found that a chromium-based coating was effective for alleviating the adhesion force during TPU injection molding,7 but we did not have sufficient data to judge whether this coating was effective for all TPU resins. In our most recent work,11 therefore, we have investigated the adhesion phenomenon of three different TPUs and the release mechanism of different coatings. Two of the resins we used (all were obtained from BASF) were esters (Elastollan S70A and Elastollan S74D) and the third was an ether (Elastollan 1174D).
As part of our comprehensive survey, we measured the adhesion force, surface energy, and surface morphology of our different TPU/coating samples. Our adhesion force measurements (see Figure 1) show the variation in adhesion force that results from the surface qualities of the different coatings. For the chromium-based coatings, we find that the single-layer and modifying CrN coatings effectively reduced the adhesion force, whereas the multilayer CrN coating was ineffective. Although DLC exhibits the lowest friction coefficient of our coating materials, we find that this coating also was ineffective in alleviating the adhesion forces for all three TPU resins.
We have also measured (see Table 1) the surface energy of the different coated and polished (half-polished or mirror-polished) cavities (at different contact angles). We obtain an average surface energy of 41.59mJ/m2 for the half-polished surface without any coating, which is substantially higher than for the half-polished cavities with a single-layer CrN or a modifying CrN coating (16.20 and 19.80mJ/m2, respectively). This indicates that the chromium-based treatments are effective for reducing the adhesion force. The half-polished multilayer-CrN-coated surface energy, however, was ineffective (indicated by the higher average surface energy of 53.58mJ/m2). We also find that the surface energy of the DLC-coated cavity (39.85mJ/m2) is similar to that of the untreated cavity and therefore exhibits the same level of adhesion. We observed similar results and trends for the mirror-polished surfaces. The results given in Table 1 also illustrate that the polar components of the surface energy totals are much larger than the dispersive components.Table 1.
Scanning electron microscope and atomic force microscope images of our different coatings are shown in Figures 2 and 3, respectively. These images indicate the multistructure of both the half-polished and mirror-polished surfaces (without coatings). In addition, we observe that the melted resins easily filled the gaps and valleys of the cavity surface and gave rise to a larger adhesion force (caused by mechanical anchoring). With regard to the different surface treatments, we find that the single-layer CrN and the modifying CrN both exhibit a distinctive conical structure, with uniform grain size. In addition, we see that the apex of the cone has a spherical surface. These structural characteristics cause good hydrophobicity. In these cases, the molten resin could not easily fill the grain boundaries, which therefore caused a lower adhesion force. In contrast, we observe significant variations in grain size and height, i.e., the multilayer CrN and the apex of its grains showing sharp peaks. These properties lead to stronger mechanical anchoring and a larger adhesion force. Although the DLC coating exhibits uniform topology, its grain size is much larger and the valleys between the grains are wider. The molten resin can thus easily fill the gaps and cause a large adhesion force.
In summary, we have investigated the effects of a variety of surface treatments on the adhesion force during injection molding of TPU resins. We find that specific chromium-based coating can be used to effectively alleviate adhesion for ester TPUs, and can slightly reduce the adhesion force of an ether TPU. Although certain coatings could be used to reduce adhesion for particular resins, we were unable to find a single treatment that was effective in all cases. In addition, we find that the adhesive phenomenon was more apparent for cavity surfaces with lower surface energies. Although surface roughness can affect the water-repellency characteristics, the inherent properties of the coatings (i.e., surface and surface morphology) are dominated by the adhesion level rather than the surface roughness. In our future work we intend to investigate the effect of the rheological properties of the resins on the adhesion phenomenon.
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