Plastic injection molding for outstanding molded parts
The process of plastic injection molding
The injection molding process is an essential production process for the production of different plastic parts. Injection molding has one of the most advantageous applications in today's mass processing industry with large quantities up to 10 million, also suitable for the production of complicated plastic parts. The injection molding has the following advantages:
✔ Large series in a short production cycle
✔ Cost-effective production
✔ Automatable production feasible
✔ Easy and fast from raw material to injection molded part
✔ High reproduction of the injection molding
✔ Little or no post-processing of the injection molded part
First the plastic granulate is filled in the filling funnel, the plastic is plasticized by the rotation of the screw. The plasticized plastic is then injected under pressure in an injection mold. Then, by cooling the tool, the liquefied material turns back to the solid state. The molded injection-molded part is removed as a plastic part after the mold has been removed from the mold. The cycle time of the process can range from a few seconds to minutes.
The injection molding machine basically consists of two units, injection unit and clamping unit.
Injection unit consists mainly of a cylinder with a reciprocating screw, a spray nozzle and a hopper. The task of the unit is, on the one hand, to plasticize the plastic granules through the heated, rotated screw and, on the other hand, to inject the plasticized granules into the injection molding tool. The key component of the injection unit is the horizontally inserted screw, which is rotated and heated after the drive.
In the clamping unit there are three plates arranged vertically. The stationary plate connects directly to the injection nozzle. It is immobile and carries half of the injection mold on the nozzle side. The second plate is moving platen. Then the other half of the injection tool is arranged as ejector side. This serves to close the tool and then reopen it, to demold the injection molded part. The third is base plate which supports the second plate and provides the closing force, which is electric or hydraulic, to close and open the injection mold.
Usual plastics for plastic injection molding
Basically, there are different forms of materials for injection molding, most commonly used like thermoplastic, thermoset and elastomer, which differ from each property as shown below,
Thermoplastic: called plastomer. On the one side, it is a plastic and can be formed by heating arbitrarily. On the other side, this raw material can be reformed by cooling and reheating in the liquid state, which is characteristic feature of the plastomer. Thermoplastics include plastics such as PE, PC, PET, PP, PVC and ABS, etc.
Thermoset: This material is usually brittle and hard (e.g. socket housing). After heating, the polymers are hard-crosslinked and deformed, this process is irreversible. This includes the plastics of PF, UP, MF, EP and SI etc.
Elastomer: It is elastic and bendable by hand because of its wide-meshed polymers, such as BR, SBR, NBR, EPDM, FRM, etc.
In order to produce the special injection molded part, it makes sense to add the additive of the plastic, colorant, stabilizer and plasticizer depending on the field of application of the molded part.
Higher profit through faster cooling in the injection molding tool
What is most important to increase the efficiency of the injection molding process? The speed of the cooling time in the tool, every engineer knows that. Faster cycle time means more profit. Cooling is the most crucial factor for the production of the molded part, especially in mass production. A small change in the cooling system can have a significant impact on increasing or decreasing the cycle time.
The picture shows that the cooling time accounts for more than two thirds of the total production cycle time in injection molding process.
Theoretically, the injection molded part in the tool is simultaneously and uniformly cooled after injection. In practice, however, this is not easy to achieve. Because the complexity of the contours of a molded part can prevent uniformly transferring the heat in the injection tool and evenly cooling the molded part.
Uniform temperature in the injection mold: The challenge lies in the uniform temperature of the cavity and the core, which has the most influence on the cycle time. In any case, the temperature difference between them muss not exceed 5 degrees. This condition comes from many factors, but injection tool steel plays a crucial role. The more expensive tool steel has a small distortion of the injection molded part in a shorter time, but increases the investment. In addition, the deformation risk increases under shorter cycle times with cheaper plastic such as H13 and stainless steel.
Uniform temperature between molded part and the mold: The material of the molded part is an important influence point on the temperature, it can also cause the temperature difference between the molded part and injection mold. In practice, this difference should not be more than 25%, otherwise the surface and quality will be affected.
Uniform pressure in the coolant: The water as coolant in the cooling channels ensures that the heat is dissipated to the outside. The water pressure generates enough turbulent flow in the cooling system. In an ideal state, the consumption of water pressure in the injection tool does not exceed 0.35 bar, so that the heat transfer in the injection molding tool can be best controlled.
Frequently asked questions about the injection molding
Which forms are there for plastic injection molding processing?
In the process of molding there are different industrial technologies to form the plastic parts like injection molding, pressing and transfer molding.
How to fix the tool on the injection molding machine?
The two-platen tool must be fixed to the respective platen. While the mold half is firmly held by the nozzle side with centering ring, the other is movable to the clamping side. Through the guide pins and bushing, both mold halves will be positioned.
What is shrinkage during injection molding?
Practically, the shrinkage appears after cooling because of the difference of mass between mold cavity and molding, which must be noted during production.
How is the deformation of the injection molded part created?
There may be two possible causes for this, one is the mass shrinkage of the molded part due to heat shrinkage in mold cavity, and moreover, the internal stress of the part can also create the deformation.
How does the temperature in the injecting nozzle affect the molding part?
If the temperature of the nozzle is too high or too low, the material dosage is unstable, which damages the mass and appearance of the molding.
Why avoid the undercut?
The undercut prevents demolding in the normal separation direction because it is transverse to the demolding direction. To solve this difficulty you need additional component in the tool, the slide, which leads to higher tool costs and the longer cycle time. The engineer must take this into account in the designing.
What is the slider in the tool?
In the special case with undercuts in the molded part, it is impossible to demold the injection part directly after opening. You need a slider unit to release the undercuts. Normal slide unit consists of jaws, oblique bolts, curve locking. It makes sure to demold the molded part with undercuts in the slide direction. This component increases the cost of mold making.
Which shape type is available for sprue gating system?
By sprue gating is meant a flow system between the mold cavity and the nozzle. It delivers the molten plastic to the cavity. For the Arte of pinpoint gate, disk gate, rod gate, tunnel gate, ring gat is available.
What is the purpose of venting system in the mold?
The vent plays a necessary role for the quality of the molded part, also for promoting the production cycle time in injection molding. The air exists both in the cavity, cooling line, as well as volatile gas of molten material occurs. All these airs must be removed in time to avoid the harmful consequence in the molding.