Polymerization

Chain-reaction, Addition

Monomer (ethylene)

Catalyst (free-radical peroxide)

Initiation

Propagation

Termination (polyethylene)

Step-reaction, Condensation

Oligomer (Nylon 66)

Structure

Linear, branched, cross-linked

Homopolymer

Copolymer

random, alternating, block, graft

Crystalline

solid until melting temperature

Amorphous

brittle, glass temperature, viscous, visco-elastic

Types

Thermoplastics

Covalent in chains, Van der Waals between chains

moldable

Thermosets

covalent cross-linking between chains

not moldable; strong, stable, light

epoxies, urethanes

Additives and reinforcements

carbon, glass fibers

fiber, tape, fabric

carbon, glass beads

strength, tolerances

metal powder

colorants

fillers

foaming agents

plasticizers

lubricating and release agents

Materials

Polystyrene (PS)

cheap, stable

household goods

Polypropylene (PP)

light

packaging

Acrylonitrile Butadiene Styrene (ABS)

stiffer than polypropylene

equipment, toys

Polyvinyl Chloride (PVC)

cheap, inert

pipes, film

High Density Polyethylene (HDPE)

environmental resistance

tanks, bottles

High Impact Polystyrene (HIPS)

polystyrene plus rubber

tougher

Polyester Terephthalate (PET)

stiff, translucent

gears, film, polyester

Polymethly Methacrylate (PMMA, Acrylic)

hard, transparent

signs, windows

Polyamide (PA, Nylon)

abrasion resistance

bearings

Polycarbonate (PC)

clear, high impact resistance

CDs

Styrene-acrylonitrile (SAN)

cheap, clear, strong

models, lenses, mouldings

Green parts

mix material with polymer, mold, then fire/sinter

stainless steel, ceramics

Extrusion

Blow molding

A softened tube is extruded into a mold, then inflated. This is how bottles are made.

Vacuum forming

Softened material is drawn into a mold.

Compression modling

A heated mold is squeezed around a pre-formed blank. It is slower, but the material does not flow as far.

Injection molding

A power or pellet is liquified, injected into a mold, cooled under pressure, and ejected. It is fast, there can be little waste, and it is easy to automate

In the plunger models #45 and #50, the plastic granules in the heating cylinder resist the downward motion of the the injection plunger, causing the heating cylinder assembly to press its nozzle down against the top of the mold. The force exerted against the mold by the machine's nozzle is equal to the pneumatic (or hydraulic) pressure generated by the plunger cylinder. This force pushes the "V" mold into the slot formed by the two mold clamping shoes. The wedging of the mold against the 15 degree included angle of the system is amplified by the inverse of the tangent of the angle which is a factor of 3.73 : 1. This forces the mold shoes apart slightly, which stretches the two tie bars that hold the shoes. This stretching creates a tension, much like a spring, in the tie bars which presses back against the mold , holding it together during injection. The model #55 uses an independent short stroke cylinder to drive the nozzle against the mold top, making its force independent of the injection force. When the carriage cylinder sees a pressure of 1950 PSI, a pressure switch is tripped, starting injection. This insures the mold being fully clamped.

When the hot plastic is injected into the mold, the force in pounds per square inch (PSI) created by the hot plastic is amplified by what is termed the "projected area" or shadow area of the cavities and runners on the same plane as the mold parting line. This force is proportional to the pressure of the molten plastic times the projected area. This force attempts to push the mold halves apart during injection, which would cause the plastic to "flash" out between the mold halves.

The tension force stored in the tie rods resists the injection force, keeping the mold tightly closed until injection is completed. This clamping pressure is measured in tons, which on all three "Wasp" Mini-Jectors is just under 12 tons. That figure is consistent on the model #55, and varies with injection pressure settings on the #45 and #50. Due to varying viscosities of the various plastic resins, the actual melt pressure within the mold cavity is impossible to calculate, but a good rule of the thumb is to have 2 tons of clamping pressure for every square inch of mold projected area. For low viscosity materials such as nylon or high melt index polyethylene, at least 3 tons is recommended. The practical limit of these machines is therefore 6 square inches.

When the injection timer times out, the plunger retracts. The plunger will trip a limit switch about .188" short of retracting fully. The operator then turns the selector switch to "retract-eject " which pulls the ejector bars up fully ejecting the mold from the "V" slot by the ejector bar. The mold can then be removed. The limit switch may be set to permit full retraction and ejection without need to turn the switch. The model #55 has a second timer controlling the overall machine cycle, and ejects the mold when that timer times out.

Typically, a "V" mold with sprues runners and cavities (and including the standard "V" mold bases available from Mini-Jector) may cost less than a complex commercial small mold base alone without any cavities! A "Wasp" machine and its "V" mold can be producing parts before big machine tool- ing is off the drawing board.

Because of the low cost of Mini-Jector machines and tooling, even very short produc- tion runs of 100 pieces or less can be economi- cally possible. A typical case in point would be a model shop, where miniature scale models of industrial equipment and hardware are assem- bled in precision scale models of factories and manufacturing facilities. A large variety of parts in small individual quantities are required in such an undertaking. Prototyping of new or experimental parts, color testing, ASTM tensile test specimens, and educational programs in schools and universities are other typical appli- cations for "Wasp" Mini-Jectors.

Insert and loose-core molding can be per-economically on "Wasp" Mini-Jectors. Insert molding involves molding plastic around a metal or wire inserted object or onto a previ- ously molded plastic article (sometimes called "overmolding") to form a finished assembly. Complete encapsulation of miniature electrical or medical devices can be performed on these machines. Loose cores are metal forms inserted into the mold prior to molding to form hollow portions in the finished part. The core is then removed from the mold and finished piece after the mold is taken apart. An example of loose core molding would be forming the hollow portion of a fishing lure.

Insert molding of electrical parts such as sockets, cord sets, and slip ring and brush assem- blies would be typical insert applications, as would molding handles on tools or knife blades. The beauty of insert molding using "V" molds is that the insert or loose core can be placed in the mold; the two halves assembled and the entire assembly held in one's hand prior to placing it in the machine. This permits molding around very fragile inserts that could not be safely molded in high tonnage machines. The illustrations on the back of this bulletin show very simple insert and loose core molding examples.