Difference between Plastics

Hello,

I will try to make this as simple as possible.

Plastic injection molding: is a process to create a product using a closed mold in which there is a shape 'your part' inside the closed faces of the mold. This shape is called a cavity and you can have one or many in the mold.

The mold is placed into a machine called an Injection molding machine and fastened tightly to 2 large faces called platens. Plastic pellets are heated in a barrel on the outerside of one of the platens and forced by way of a plasticising screw into the mold, through a small orifice (sprue) in through the rear plate or side of the mold. The mold temperature is regulated depending on the type of material being molded by a cooling system incorporated in the mold. When the plastic has cooled sufficiently the machine platens open and the part is ejected from the cavity. This is done by way of an ejection system built into the mold and activated by the injection mold machine. Normally you can see round witness marks on an injection molded part where the ejector pins sit against the molding surface. A typical mold for a computer keyboard will fill in .6 of a second and will take 45-60 seconds for a complete molding cycle. A cycle is injection time, hold time, cooling time, opening, ejection and closing time. The design intent is to have a finished part ejected from the mold with little extra work to be done to complete it.

Vacuum form molding: is very different in that you typically have a single sided, open mold over which you drape a pre-heated sheet of plastic material and then suck the plastic down onto your form. A good example would be the clear clam-shell packaging used to hold an easter egg or computer mouse etc, suspended within its package. Vacum forming by its nature uses a large sheet of plastic from which you must later die-cut your finished part from. It is a limited process although cheaper all around than injection molding but can not compete where accuracy and range of useable materials is necessary. It also creates more scrap as the web has to be discarded, mostly recycled.

Plastic Extrusion: is very different again. In this process you typically plasticize pellets of plastic in much the same fashion as the injection molding machine but that is where the similarity stops. You attach a die to the end of the plasticising unit or barrel and you force the molten plastic through the die in a continuous stream. Cooling is done after it leaves the die, typically by passing through a cooling trough. This is similar to producing pasta, licqorice, PVC water pipes. Plastic film is also produced in sophisticated operations of extrusion. Profiles such as that used for windows, both aluminium and PVC are produced in this way. Cost wise it is a relatively cheap process after the initial die has been made and debugged. Automatic cutting equipment is employed to cut lengths on the fly and so long as you fill the hopper with material it is pretty much a continuous process. I have seen diameters of upto 16 inch thick of UHMW extruded. At this diameter the melt in the center is hard to control but is being done. UHMW can not be injection molded but you can used yet another molding process called compression molding.

Compression Molding: is a process utilizing a closed mold to form a plastic preform into the final cavity shape that you need. It fills the need for molding thermosetting plastics and the sintered type of processing that produces Teflon and UHMW etc. Urethane polymers are processed in this process. But injection molding encroaches on the viability of a lot of compression molded products as there are thermoplastic urethanes and that are suitable for some of the products formerly made of urethane. Likewise for other products fomerly produced by this process solely.

Blow Molding: Injection blow molding, Rotomolding and RIM Molding are further processes to shape plastics. There are many hybrid processes of all of the above. There is gas assisted injection molding to reduce the plastic wall section to a viable thickness while producing injection molded parts with their speed and accuracy. A blob (charge) of molten plastic is injected into the closed injection mold and then nitrogen is forced into the center of the melt stream pushing the plastic to the walls of the cavity and allowing a hollow part to be produced.

I hope this helps.