The development of acrylonitrile-butadiene-styrene (ABS) resins for use in rotational molding would provide a medium performance material, thus opening doors to new markets for the rotational molding industry. Unfortunately, ABS resins have shown serious problems during the rotational molding process, namely discoloration, bridging, and poor impact strength. It is believed that these effects are due to degradation of the carbon-carbon double bond in the butadiene, through attack by either oxygen or heat. Previous efforts have shown some success in addressing these issues. However, additional improvements are necessary to make ABS resins commercially viable to rotational molders. This study, fourth in a series of similar projects conducted though Brigham Young University, was focused on remediation of the ABS difficulties via two different approaches. First, a survey of several additives was performed with the intent of investigating four different strategies: increased protection from oxygen, decreased butadiene concentration, increased butadiene concentration, and promotion of flow. The best formulation was achieved when 15 wt % of a benzoate ester (XP-2280 available though ChemPoint) was blended into MAGNUM 342 EZ, an ABS resin (The Dow Chemical Company). This formulation showed the best balance between increased impact strength and improvement of cosmetic properties. Second, optimization of several rotational molding processing parameters was executed. These included particle size distribution of the resin, drying of the resin, internal mold atmosphere, and oven temperature. It was found that using coarse particle sizes (ground at 20-mesh rather than the industry standard of 35-mesh) increased the impact strength by about 19%. None of the other parameters proved to have a significant effect upon the system, except for the use of a nitrogen atmosphere, which lowered the impact strength. Final properties testing of this best formulation at the optimal processing conditions showed increased impact strength from 2 ft-lbs (the previous best value) to 8 ft-lbs. There was also a marginal decrease in surface hardness (95 to 78 on the Rockwell R scale) and yield tensile strength (3,900 psi to 3,300 psi). Larger differences were observed in flexural modulus (200,000 psi to 110,000 psi) and heat distortion temperature (95°C to 61°C). Therefore, these formulation and processing changes show a trade-off where stiffness and thermal stability (i.e. flexural modulus and heat distortion temperature) can be sacrificed for an increase in toughness and aesthetics, made manifest by increased impact strength, elimination of bridging, and eradication of discoloration.

This study focuses on the cycle-averaged mold temperature of additively manufactured injection molding tooling and how it is affected by conformal cooling channels. This was done by producing a benchmark mold out of Digital ABS produced by Stratasys, an acrylic based photopolymer, which was then used to produce injection molded parts until tool failure. Another, more cost-effective material, High Temp Resin produced by Formlabs, another acrylic based photopolymer, was also tested but yielded very little success. Then the mold design was altered by adding conformal cooling channels and again tested by producing injection molded parts while tracking the mold temperature. This experimentation was then compared to an injection molding cooling channel model in order to validate the model for use with additively manufactured tooling with conformal cooling channels for use in injection molding. The benchmark Digital ABS mold was able to produce 66 shots in the injection molding machine before complete mold failure. The Digital ABS mold had a cycle-averaged mold temperature of about 155°F. The High Temp Resin mold was able to produce 3 shots before complete mold failure. The High Temp Resin material is much more brittle, and the mold design did not take into account how brittle the material was. The Digital ABS mold with conformal cooling channels had a cycle-averaged mold temperature of 111°F. This is significantly lower than without cooling channels and has a high potential for improving tooling life. The cooling channel model predicted the cycle-averaged mold temperature to be 116°F. This proved to be a very good model and can be used as a design tool when choosing cooling channel geometry and position in additively manufactured tooling. This research shows the potential that conformal cooling channels have to help improve additively manufactured tooling life for injection molding. As shown in other research done, the ability to maintain the mold below 120°F significantly improves the life of additively manufactured tooling. The results of this study demonstrate the effectiveness of conformal cooling channels in controlling mold temperature. It should be researched further, but the use of conformal cooling channels has the potential to produce more production or prototype parts with additively manufactured tooling for injection molding.

The effect of coatings on the interfacial strength of a hollow glass microsphere/polyester composite and their capacity to endure thermal stresses were studied by mechanical testing and an active Acoustic Emission (AE) method. AE was postulated to provide more local information at or near the glass/polyester interface due to the sensitivity of elastic waves to the rigidity of polymer chains at the glass sphere/polyester interface compared to mechanical testing. Three frequency ranges identified by multivariate statistics yet consolidated for the initial analysis into a band of 140-240 kHz, were found to be changing with the different coated glass filler for different glass content and heating state. Considering the acoustic behavior of the composites containing different levels of glass sphere content (1-10 vol%), a lower concentration (aminoethylamino)-propyl-trimethoxy silane coated glass (AS6), demonstrated the lowest attenuation after heating (associated with higher interfacial strength). As anticipated, the highest attenuation after heating was observed for uncoated glass (16K) due to expectedly weaker associations. Mechanical testing results after heating were consistent with the AE response for AS6 and 16K for this frequency range. Trends in amplitude for the three narrower, frequency ranges of 130-160 kHz, 180-220 kHz and 230-260 kHz were compared against that of 140-240 kHz and very small differences were observed. It was found that the frequency range of 130-60 kHz was more descriptive of the changes of interfacial strength in composites (at 10 vol%), being consistent with the mechanical test results. Considering the AE response at 130-160 kHz and mechanical data, higher concentration (aminoethylamino)-propyl-trimethoxy silane (AS12), better endured thermal stresses compared to other coatings. A smaller trial looked at the effect of moisture aging and subsequent thermal cycling on the glass/polymer interface strength as another method to perturb the interface. Attenuation for the band of 180-260 kHz was studied for aged versus non-aged composites. The commercial coating, L21 demonstrated a better moisture resistance before and after thermal cycling compared to uncoated glass spheres. An improved evaluation of interfacial strength in glass/polyester was expected using AE technique versus mechanical testing due to its higher sensitivity to changes in internal structure, however; no significant improvement compared to mechanical testing was observed, at least based on the analysis technique currently being used. / Thesis / Master of Applied Science (MASc) / Sheet molded compound (SMC) is a polymer material reinforced by fibers providing a combination of light weight and high mechanical properties and is used in automotive industry. Light weight fillers (hollow glass microspheres) are used to obtain further weight reduction; however, addition of these fillers leads to reduced mechanical properties and further problems during painting process known as ‘paint popping’. The former is due to uncertain interfacial state between polymer and fillers and the latter results from different thermal expansion behavior of the polymer and filler materials while the material is exposed to high temperatures for painting process. This research aims to devise a highly sensitive technique and evaluate its suitability compared to mechanical testing for investigation of the origin of aforementioned problems. Acoustic Emission (AE) is a method with high sensitivity to changes in internal structure of the material which is postulated to provide a better insight on material microstructure compared to more commonly used method i.e. mechanical testing. Use of interfacial controlling agents was examined to reduce the problems as a result of introduction of fillers. The effect of using surface modified fillers and the effect of thermal stresses on material was investigated using AE technique. Application of AE method in this study provided a good insight about the changes in material internal structure; however, it did not demonstrate a significant improvement in detecting the origins of studied problems compared to mechanical testing at least based on the analysis technique used in this study.

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