This article is from a senior process engineer, who shares some understanding of the injection molding process.
Mold temperature of the injection molding process
The mold temperature affects the injection molding process cycle and injection molding process quality. In practice, the mold temperature is set from the lowest appropriate mold temperature for the material used and then adjusted upward according to the quality condition.
The correct term for mold temperature is the temperature of the cavity surface while molding is in progress. In mold design and molding process conditions, it is important to maintain not only the proper temperature but also a uniform distribution.
Uneven mold temperature distribution can lead to uneven shrinkage and internal stresses, which can cause deformation and warpage of the molded part.
Increasing the mold temperature will result in the following.
Increase the crystallinity and more uniform structure of the molded product.
Make the molding shrink more fully and reduce the post-shrinkage.
Increase the strength and heat resistance of the molded products.
Reduction of residual internal stress, molecular alignment, and deformation.
Reduce the flow resistance during filling and reduce pressure loss
Better appearance of molded products
Increase the chance of burr in molded products.
Increase the chance of denting near the gate and reduce the chance of denting far from the gate.
Reduces the degree of visible bonding lines
Increase cooling time
The pressure of the injection molding process
The pressure of the injection molding process
Pressure Regulation in Injection Molding Process Whether it is a hydraulic or electric injection molding machine, all movements in the injection molding process generate pressure. Proper control of the required pressure is necessary to produce a finished product of reasonable quality.
Pressure regulation and metering system On oil-pressure injection molding machines, all movements are performed by oil circuits responsible for the following operations.
Screw rotation during the plasticizing phase.
Sliding seat channel (injection nozzle near the injection bushing)
Axial movement of the injection screw during injection and holding pressure
Closure of the substrate on the injection rod until the elbow rod is fully extended or the piston closing stroke has been completed.
Initiate assembly of the ejector bar to eject the part
On full voltage machines, all movements are performed by brushless synchronous motors with permanent magnets. The rotary motion is converted into a linear motion by means of a ball bearing screw, which has always been used in the machine tool industry.
The efficiency of the entire process depends in part on the plasticization process, in which the screw plays a key role.
The screw must ensure that the material is melted and homogenized. This process can be adjusted with the help of counterpressure to avoid overheating. The mixing element must not generate too high a flow rate, which would otherwise lead to polymer degradation.
Each polymer has a different maximum flow rate and if this limit is exceeded, the molecules will stretch and polymer backbone breakage will occur.
However, the focus remains on controlling the forward axial motion of the screw during injection and holding pressure. The subsequent cooling process, including aspects such as inherent stresses, tolerances, and warpage, is important to ensure product quality.
This is all determined by the quality of the mold, especially when optimizing the cooling channels and ensuring effective closed-loop temperature regulation. The system is completely independent and does not interfere with mechanical regulation.
Mold movements such as die closing and ejection must be precise and efficient. Velocity distribution profiles are often used to ensure the accurate proximity of moving parts. The contact maintenance force can be adjusted.
It can therefore be concluded that, without taking into account energy consumption and mechanical reliability, additional conditions being the same (e.g. mold quality), product quality is mainly determined by the system controlling the forward movement phase of the screw.
In the case of hydraulic injection molding machines, this regulation is achieved by detecting the oil pressure.
Specifically, the oil pressure activates a set of valves through a control panel, and the fluid acts through a manipulator, which is regulated and released.
Injection speed of injection molding process
Injection speed control includes open-loop, semi-closed-loop and closed-loop control options. Open-loop systems rely on a common proportional valve of the injection molding process.
Proportional tension is applied to the desired proportion of fluid, which creates pressure in the injection barrel and allows the injection screw to move at a certain forward speed.
Semi-closed-loop systems use a closed-loop proportional valve. The loop closes at the location where the closure port is located, and the closure port controls the proportional flow of oil by moving within the valve. The closed-loop system closes at the translational speed of the screw.
A speed sensor (usually potentiometer type) is used in the closed-loop system to detect tension drops at regular intervals. The oil flow from the proportional valve is adjusted to compensate for speed deviations that occur.
Closed-loop control relies on specialized electronic components integrated with the machine. The closed-loop pressure control ensures uniform and consistent pressure during the injection and holding phases, as well as uniform backpressure throughout the cycles.
The proportional valve is adjusted by the detected pressure value and compensated for deviations according to the set pressure value.
In general, the hydraulic pressure can be monitored, but detecting the melt pressure in the injection nozzle or the mold cavity is another effective method.
A more reliable solution is to manage the proportional valve by reading the nozzle or cavity pressure readings. Adding temperature detection to pressure detection can be particularly beneficial for process management.
Knowing the actual pressure the material can withstand also helps to predict the actual weight and size of the molded part based on the set pressure and temperature conditions.
In fact, by varying the holding pressure value, more material can be introduced into the mold cavity to reduce part shrinkage and meet design tolerances (which include preset injection shrinkage).
Near melting conditions, semi-crystalline polymers show a large change in specific volume. In response, overfilling the mold does not prevent part ejection.
These are the 3 understanding of the injection molding process, have you got them?
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