An experimental study has been carried out to determine the effect of viscoelasticity in comparison to viscosity on micro-injection molded parts. In this study, two different polymeric materials — Polystyrene (PS) as a viscous material and High Density Poly-Ethylene (HDPE) as a viscoelastic material — have been selected to observe the effect of melt elasticity on the filling phase of micro molding based on cavity pressure of molded part. All process parameters except temperature are the same for both polymers. Process temperatures have been selected in order to match the viscosity for both polymers used. Polymer viscosity was characterized at different shear rate and temperature. Viscoelasticity of both polymers were investigated using rotational rheometry in the oscillation mode. The mold geometry with high aspect ratio has been used and the effect of viscoelasticity on cavity pressure has been discussed. It was observed that there is retardation on the response of pressure because of elastic response of material during filling. Despite the differences in slope, peak value, area, and cycle time between two curves, they share similar trends. The only difference is their response during solidifying because of material property.

Due to the micro-scale dimensions in the microinjection molding proces, it is difficult to inspect the part quality without using costly microscopic observation methods. To address this issue, a suitable process monitoring method such as cavity pressure monitoring can be employed to detect any process deviation that causes defects in part quality. The objective of this study is to investigate how cavity pressure responds to different molding conditions that lead to varying part quality of a molded hollow cap.

The study focuses on the ability of a polymer to replicate micro-features when processed at an elevated mold temperature without externally applied pressure. Replication is performed using four different polymers—High Density Polyethylene (HDPE), Polypropylene (PP), Polystyrene (PS), and Poly (Methyl Methacrylate) (PMMA) on a silicon mold containing surface features as small as 500nm. Feature replication is assessed using scanning electron microscopy (SEM) and atomic force microscopy (AFM) to compare feature dimensions of the mold to those of the replicated parts. Shrinkage in dimensions is observed to be anisotropic in the molded parts and its extent of varies among the different polymers. Crystalline HDPE shows a higher degree of shrinkage relative to amorphous polymers such as PS and PMMA. These results verify the theoretical value of shrinkage calculated from the coefficient of volumetric shrinkage values and density. By increasing the mold temperature well above the melting point of the polymer, a depth ratio of 70–80% can be achieved in parts having aspect ratios of around 0.5. The result is comparable to the values achieved by similar studies. Varying aspect ratios are fully replicated by all four polymers at elevated mold temperature. This clearly shows that increasing mold temperature results in significant improvement in depth ratios for micro-featured parts. The amorphous materials provide better feature replication and lower surface roughness than the semi-crystalline polymer.

This investigation focuses on determining why polystyrene ASTM specimens exhibit an increase in tensile strength when processed by vibration assisted injection molding (VAIM) while polycarbonate parts do not. VAIM is one of several polymer processing methods that attempt to improve product properties via manipulation of the polymer melt. Observation of birefringence patterns in VAIM processed polystyrene samples show a significant impact on molecular orientation. The same studies were conducted on opaque polycarbonate and were unable to determine the degree of molecular orientation via birefringence measurement. It was theorized that VAIM did not produce significant orientation due to its higher thermal conductivity and stiffer backbone. It has been determined by this investigation that VAIM processing does impart significant molecular orientation in polycarbonate specimens but still does not increase its UTS. It is proposed that increased molecular orientation induced by VAIM processing inhibits crazes from growing into cracks. VAIM therefore favors polymers that fail by crazing (e.g., polystyrene) rather than those that fail by shear yielding (e.g., polycarbonate).

This study investigates the effects of processing of styrene acrylonitrile (SAN) by injection molding using a delayed packing stage. The concept of Delay Pack Injection Molding (DPIM) evolved from an in-situ study of vibration-assisted injection molding (VAIM) which indicated that the beneficial effects of VAIM came not from the vibration itself, but rather from the delay in the onset of packing resulting from the application of the vibration. Conceptually, DPIM involves normal filling of the mold immediately followed by a slight retraction of the injection screw for a specified time period before the final packing pressure is applied. Application of DPIM results in increased birefringence in the molded parts and increases in the ultimate tensile strength of molded parts very similar to the effects seen using VAIM. A parametric study using a design of experiments framework was carried out to determine the delay pack parameters affecting SAN and resulted in a maximum increase in UTS of 11.6%. Observation of birefringence patterns in Delay Pack processed samples shows a significant impact on molecular orientation while observation of failed specimens and their fracture surfaces shows distinctly different modes of crack growth and failure. Growth of craze cracks resulting from tensile loading appeared to be arrested by oriented areas surrounding the part core allowing the specimen to sustain higher loads relative to conventionally molded parts. All of the above observations are consistent with the observed effects of vibration-assisted injection molding.

Solid freeform fabrication technology has shown a great deal of promise for the plastic injection molding industry due to its ability to produce complex geometry tooling relatively quickly. However, one shortcoming of metal-based SFF processes is that they have difficulty producing parts with acceptable surface quality. As such, secondary operations, such as machining, are frequently required thereby increasing fabrication time and cost. In addition, there is variation in the surface quality that is dependent upon the surface orientation during the build process. For example, parts produced using the metal-based 3-D printing process have vertical faces with a typical roughness 52% greater than the horizontal faces. This work investigates the effects on part surface quality resulting from the application of a contact surface "blank" to the part free surfaces during the infiltration stage of a powder metalbased rapid manufacturing process. Specifically, the effects of "blank" surface roughness and contact pressure are studied with respect to resultant surface roughness and uniformity of the infiltrated part. Application of a smooth contact surface on vertical faces resulted in R a values at least 25% lower than that of vertical free surfaces. It was also revealed that there is a correlation between surface roughness of the blank and the surface roughness of the infiltrated part. Such blanks could be used to impart desired surface finish and texture to critical surfaces of a mold tool.