Injection molding heating stage

Intermittent accumulator EBM machines use a normal conventional axially fixed continuously operating extruder to prepare the melt. The accumulator is a heated reservoir where the melt is temporarily stored in the intervals between parison extrusion (similar action of a two-stage injection molding machine (IMM) as reviewed in Chapter 4. Also used is a conventional reciprocating IMM that delivers melt through a die into a blow mold. 

Reinforced thermoset (RTS) plastic B-stage sheet material can be processed with its required heating cycle. However the most popular is to use reinforced thermoplastic (RTP) sheets usually using polypropylene plastics. Compared to injection molding RTPs, these stamped products can provide improved mechanical and physical properties with its longer fibers such as impact strength, heat distortion temperature, and much less anisotropy. 

The two molds needed for this process, an injection mold and a blow mold, may be located in the same molding machine or in different molding machines. In the first case, known as a one-stage process, there are multiple stations in the machine through which the material travels. For example, in a four-station machine the polymer would be injected into the preform, shuttled to a conditioning (profiled heating) station, shuttled to a blow station, and then shuttled to ejection. 

During the plastication stage, the polymer melt is typically plasticized from solid granules or pellets through the combined effect of heat conduction from the heated barrel and the internal shear heating caused by molecular deformation with the rotation of an internal screw. Screws in injection molding have many... 

There are essentially two methods of using the plasticating screw. The first is the screw-plunger system also called the two-stage or screw-pot system. The screw rotates in the heated barrel and consequently plasticizes the polymer material. The plasticized material is then transferred into a second heated cylinder from which it is injected into the mold by a plunger. Figure 11.6 shows the basic features of a screw-pot injection molding machine. 

With knowledge of the material temperature, wall temperature, ambient temperature, plus the specification of the article removal temperature, it becomes possible to analyze the energy balance of the overall mold. Decisions on whether and how to provide the mold with thermal insulation arc made at this stage of the layout. The performance of the temperatiue control system is also established at this stage. Figure 8-5 shows the heat balancing of an injection molding tool. 

Snap-fits are injection-molded into the two components of the assembly. Two general heat staking methods are low profile and high profile. Methods of the two types include spherical, flush, hollow, and knurled. In addition, there are cements and adhesives, and a mechanical fastening assembly. Whatever assembly method is used, the choice is usually made at the design stage. 

Heat capacity, thermal conductivity and pressure-volume—temperature (PVT) are the macroscopic characteristics of polymers that are very important during the processing stage. Typical heat capacity determines the amount of heat required to bring the respective volume of PLA up to the final processing temperatures. Meanwhile, thermal conductivity and PVT can affect the rate of heat transfer and compressibility, which are important in determining the shrinkage of injection-molded products. 

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