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 after plastic mold making. The associated mold costs are in comparison to other processes relatively high . 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 by some plastic injection molding companies 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. Pellet melting occurs during the screw rotation, generating energy by the barrel heaters and by shear heat which part mostly predominates. Simultaneously there is a forward melt feed towards the closed nozzle so that the screw presses itself back due to its own feed effect.

During the reverse motion the screw has to overcome the frictional resistances of the system. To improve the mixing effect, to increase the amount of shear heat or to reach an enlarged degassing effect an additional reverse resistance the so called “back pressure” will be applied. The rotation is turned off when the given rear position corresponding to the “metering stroke” is reached. The melt volume in front of the screw tip is now ready for the injection process. Thanks to the check valve on the screw tip , representing a standard equipment the screw can work as an injection piston and introduce the melt into the relatively cold mold “cavity” until the cavity is completely wetted called “volucmetric filling”. The type of melt flow is “fountain flow” which means that stationary layers are formed outside in contact with the cold mold wall and the melt has to pass through.  Too intense growth of the stationary layer can bring the filling process to a complete standstill, the melt “freezes off”, for example if the injection speed is too low. In addition, a thin skin layer forms at the flow front which offers a certain self-sealing at tight gaps of the mold. This is the basis of the “ventilation” of cavities in the mold design because the entering melt has to push air out of the cavity. Therefore venting gaps of 10 to 30 μm are possible without flashing, because of the flow front skin thickness of about 10 μm.

In this range, it is possible to allow the mold to “breath” so improving the ventilation. After the volumetric filling, the dynamical phase which is dominated by the “injection pressure” of the filling process moves on to the quasi-static phase. This “holding pressure phase” can on one hand get the melt to compress and on the other hand deliver additional melt during the initial cooling, where the pressure in the cavity remains oen almost constant. Such a post-supply of melt is possible as long as the gate, the junction of the runner into the cavity is still permeable (not yet “sealed”). Aer the “sealing point” follows a larger reduction in pressure according to the actual cooling rate. The shrinkage of the molded part starts aer the complete pressure release, which is mostly connected with a separation of the plastic from the inner wall of the cavity .