Injection Molding Process
Injection Molding Process
Injection molding is a typical process for the production of parts molded in large
volumes, frequently used in high mass production. The associated mold costs are
in comparison to other processes relatively high [1 to 7]. Experts know that the
profitability of injection molding starts at 10,000 pieces, in some cases it starts at
3,000 pieces depending on the complexity of the molded part and expenses for the
mold. The number of cavities in a mold is mostly under 100. Molds for extreme
mass production though have even more “drops” .
The percentage of production costs for example, the demand of energy or space
decreases, the more cavities are brought into a production unit. However, the
fall-out risk increases so, that a lower cavity number has been implemented than
technically possible. It is a special advantage that very complex elements of the
molded part geometry for example, external and internal undercuts, snap-in noses,
hinges, spring elements or internal threads can be produced with high degree of automation. These can be in the weight range of micro-injection molding of a few
milligrams up to large parts of more than 50 kg .
Injection molding machines are characterized according to their clamping force:
From 50 up to 100,000 kN, for example the production of boat bodies or wet cells
of pre-fabricated houses. It is typical that machine sizes in the range of 100 to
30,000 kN are manufactured in small series.
On the other hand the injection molds are generally individual items and therefore
very valuable production resources. Their availability can be of crucial significance.
It is possible that a higher investment is required into the mold than into the
machinery itself.
As heart of the machine the heated barrel of the injection unit can be taken containing a screw which can rotate and move axially backwards during plasticizing and works
as a non-rotating piston when injecting and during holding pressure time.
The function of the clamping unit is to move the one mold-half called “clamp”,
“movable” or “ejector” half towards the stationary, “fixed” half and the application
of clamping force to seal against the separating force of the cavity pressure.
Relatively high pressures are needed, not under 200 bar, mostly in the range of 300
to 1,200 bar.
Due to the involved areas, forces are created which must be taken into
consideration when designing the side walls of the mold or the needed clamping
force of the machine. Calculations within mold design will take oen a mean value
of 1,000 bar. Such a high pressure level is especially needed because sufficient
shrinkage compensation must occur during the compression of the melt.
