I last discussed the process of plastic extrusion. As you will recall, the end product of the plastic extrusion process is principally a profile (tubes, rods, pipe, molding, etc.), sheet or pellet. These pellets and other plastic granules are used in various process heating equipment to manufacture every type of plastic component you can imagine.
Injection MoldingIn this technology, plastic pellets are introduced -- with or without other additives such as color, mold release agents and lubricants -- from a hopper into the molding machine. This feeding system is common to most plastic-forming processes.
Simplistically, a molding machine consists of a reciprocating single-screw extruder or ram, a holding/heating section, and the mold. The mold is split and consists of two halves with cavities that are the negative of the final part. The mold has a runner system for the plastic to flow to the part and cooling channels running throughout to cool the part. The two halves of the mold are held together with a hydraulic or mechanical device.
The plastic is processed through the extruder, which mixes and melts the pellets, and is fed into the heated holding section. On demand, the extruder screw moves forward and injects the melt through a nozzle into the runner and into the cavities in the mold. The volume of plastic injected is programmed and consistent with the cavity volume. The molten plastic fills the mold and is cooled to a temperature that allows the plastic to crystallize (solidify) and form a solid part. The mold opens to release the part and then closes. This cycle is repeated.
Injection molding can produce parts that are of high quality and with excellent dimensional accuracy. Various plastics can be used including polyethylene, polypropylene, ABS and other thermoplastics. Occasionally thermosets are injection molded but may cause processing issues.
Blow MoldingBlow molding is used to manufacture hollow objects. As the name suggests, molten plastic is blown up to the final shape by air, much like a balloon. There are three primary blow-molding techniques.
An extrusion blow-molding machine consists of an extruder that melts the plastic and forms it into a molten tube (called a parison or preform) through a conventional-type die and a split-body mold. The die closes around the parison, sealing both ends, and a blow pin is inserted into the parison to inflate it, causing it to expand and conform the shape of the mold cavity. Again, the mold is cooled and once the part has solidified, the mold opens and the part is removed. Extrusion blow-molding is a continuous process that is used to mostly to manufacture small, thin-walled parts but can produce parts as large as 44-gal drums.
An injection blow-molding machine consists of a number of stations with various devices at each station. In one such machine in the first station, the mold is closed and, with the aid of a mandrel, a hollow injection-molded preform is created. (A mandrel is a piece of steel that allows a hollow to be formed in extrusion or injection molding by filling the part of the cavity that would otherwise be filled by the melt. It is sometimes called a tongue.)
The mold then opens and the hot and soft preform is indexed to the blow station on the machine, where the final shape mold closes. Air is introduced through the mandrel to inflate the part to conform to the internal cavity of the mold. Once cooled, the mold opens, and the part is indexed to the ejection part of the machine where the finished part is removed from the mandrel.
Stretch blow-molding is similar to injection blow-molding in that it uses preforms that are injection molded as blanks for the blow-molding process. They commonly are placed into a separate blow-molding machine (called a reheat-blow (RHB) machine) that heats up the preform to the required temperature before blow molding. Stretch blow-molding physically stretches the plastic, aligning the molecules to provide a biaxial molecular orientation. What this means is that the final part has improved physical properties such as strength, clarity and gas barrier properties.
Drums, bottles, containers, toys and furniture are common examples of blow-molded parts.
Next time, I’ll pick up with thermoforming.