Molded part coring

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

Let s consider the humble bottle cap just a little further. It has threads on the inside to attach it to the bottle. Recall that when this part is molded, a core forms the inside of the prodnct After cooling, ejection is actu-... 

Polymer orientation varies through the thickness of the injection-molded part owing to the fountain flow of the melt in the mold cavity. The flow at the center of the cross-section is deformed through extension and the highly stretched flow front rolls up to the cold mold surface, where orientation is frozen in a thin surface layer. The rest of the melt required to fill the cavity flows under this stationary frozen layer in more or less a plug fashion, with minimum orientation. Surface orientation in an injection-molded part can be significantly different from that in the core of the part. 

Entries 1-6 in Table I reveal an approximate constancy of volumetric CTE through the XYDAR 300, 400, and SRT-300 series resins. This result is not surprising, since the molecular structures of these materials are similar. However, the CTE values for a molded part reflect macroscopic structure (skin/core) as well as domain or molecular level morphology. The LCP molded part must therefore be regarded as a composite structure. These considerations as well as differences in basic molecular composition may explain the lower volumetric CTE for Vectra A-950 relative to the neat XYDAR resins, at least over the measured temperature range of 0-150 C. These factors may also be responsible for the lower anisotropy of the neat XYDAR 300 series resin. 

Time, temperature, and pressure are the primary factors influencing the surface flnish of the molded part. Time factors include rate of injection, duration of ram pressure, time of cooling, cross-linking time, and rotation of the screw (RPM). Pressure factors are applied pressure (high or low), back pressure on the extruder screw, and pressure loss before the charge enters the cavity, which can be caused by a variety of restrictions in the mold cavity. Temperature factors are mold (cavity and core), barrel, and nozzle temperatures, as well as the melt temperature because of back pressure, screw speed, and frictional heat. 

Molded samples of LCP s often form a "skin-core" structure. The phenomenon is depicted by a significant dependence of the anisotropic properties of molded parts on part thickness ( 5.). Scanning electron photomicrographs of the cross-section of a molded part reveal a highly oriented "skin" layer surrounding a less ordered inner "core." Apparently, the fraction of disordered core material diminishes as the sample thickness decreases. 

Figure 10.11. Schematics of skin-core effect on phase morphology in injection molded part.

As the mold cavity is filled with polymer melt, the pressure increase within the cavity can produce stresses of up to 10,000 to 180,000 psi in the mold cavity material. The resulting deformation is substantial but can be accommodated provided the elastic limit of the material is not exceeded. However, where the dimensional tolerance of the molded part is critical, it is imperative that the mold material modulus is sufficiently stiff to ensure part dimensional accuracy. Maintenance of proper temperature of the mold cavity and core is also necessary in this respect as well as for the production of a molded part with good physical and mechanical properties. Dimensional accuracy of the part also demands that the dimensions... 

Undercuts should be avoided in molded parts since they require either a split mold or a removable core section. 

As is the case in molds for thermoforming, the design of injection molds must provide for venting to remove trapped air, which otherwise will mar the finish of the molded object, and may, in extreme cases, prevent accurate mold filling. The simplest way to accomplish this is to build vent holes into the mold parting line, where the core and the cavity come together. For large molded objects, this may be insufficient, and additional vents may be needed. 

---