The plastic injection molding process comprises four main stages: filling, packing, cooling, and demolding. These four stages are crucial in determining the quality of the molded product, and they constitute a comprehensive and continuous process.
Filling is the first step in the entire injection molding cycle, starting from the closure of the mold and continuing until the mold cavity is filled to about 95%. In theory, the shorter the filling time, the higher the molding efficiency. However, in practice, molding time or injection speed is constrained by many conditions.
– During high-speed filling, the shear rate is high, and the plastic experiences a decrease in viscosity due to shear thinning, reducing the overall flow resistance. The local viscous heating effect can also thin the solidified layer thickness. Therefore, during the flow control stage, the filling behavior often depends on the volume to be filled. In this stage, high-speed filling often has a significant shear thinning effect, while the cooling effect on thin walls is not apparent, giving the upper hand to the rate utility.
– During low-speed filling, heat conduction controls the process. With low shear rates, local viscosity is high, resulting in greater flow resistance. The slow replenishment rate of thermoplastics leads to a slower flow, making heat conduction effects more pronounced. The heat is quickly carried away by the cold mold wall. Additionally, the lesser amount of viscous heating phenomena increases the thickness of the solidified layer, further increasing flow resistance in thinner areas of the wall.
Due to the fountain flow, the plastic polymer chains in front of the flow wave tend to align almost parallel to the flow wave. Thus, when the two streams of molten plastic meet, the polymer chains in contact are parallel to each other. Moreover, the two streams have different properties (due to different residence times, temperatures, and pressures in the mold cavity), resulting in a region of poor structural strength where the molten plastics intersect. Observing the part at an appropriate angle under light reveals the clear formation of weld lines, which is the mechanism behind weld line formation. Weld lines not only affect the appearance of the plastic part but also, due to the loose microscopic structure, can cause stress concentration, leading to a reduction in strength and potential fractures.
Generally, weld lines formed in the high-temperature zone exhibit excellent strength. In such high-temperature conditions, the activity of polymer chains is higher, allowing them to penetrate and entwine with each other. Additionally, in the high-temperature region, the temperatures of the two molten streams are closer, and the thermal properties of the molten material are nearly identical, enhancing the strength of the weld zone. Conversely, in low-temperature areas, the strength of the weld is weaker.
The purpose of the packing stage is to apply continuous pressure to compact the molten plastic, increase plastic density, and compensate for plastic shrinkage behavior. During the packing process, the back pressure is higher due to the mold cavity being already filled with plastic.
In the molds used for injection molding, the design of the cooling system is crucial. This is because molded plastic products need to cool and solidify to a certain rigidity before demolding to prevent deformation caused by external forces.
As cooling time constitutes approximately 70-80% of the entire molding cycle, a well-designed cooling system can significantly shorten the molding time, enhance injection molding productivity, and reduce costs. Improperly designed cooling systems can lengthen the molding time, leading to increased costs. Uneven cooling can further result in warping and deformation of plastic products.
According to experiments, the heat from the molten plastic entering the mold is roughly divided into two parts of dissipation. About 5% is transferred to the atmosphere through radiation and convection, with the remaining 95% being conducted from the molten plastic to the mold. In the mold, due to the action of cooling water pipes, heat is conducted from the plastic in the mold cavity through thermal conduction to the mold frame, and then carried away by the cooling liquid through thermal convection. A small amount of heat not taken away by the cooling water continues to conduct in the mold and dissipates into the air upon contact with the outside.
The initial stage of injection molding consists of mold closing time, filling time, holding pressure time, cooling time, and demolding time. Among these, the cooling time has the largest proportion, approximately 70-80%. Therefore, the cooling time will directly impact the length of the plastic product molding cycle and the production yield.
During the demolding stage, the temperature of the plastic product should be cooled below the thermal deformation temperature of the plastic to prevent relaxation due to residual stress or warping and deformation caused by demolding forces.
Demolding is the final step in an injection molding cycle. Although the product has already cooled and solidified, demolding still has a significant impact on the quality of the product. Improper demolding methods may result in uneven forces during demolding, causing defects such as deformation when ejected.
There are mainly two demolding methods: ejector pin demolding and stripper plate demolding. When designing the mold, it is crucial to choose the appropriate demolding method based on the structural characteristics of the product to ensure product quality.
For molds that use ejector pin demolding, the arrangement of ejector pins should be as uniform as possible. The positions should be selected where demolding resistance is highest, and the strength and rigidity of the plastic part are maximum to prevent deformation or damage to the plastic part.
On the other hand, the stripper plate is generally used for demolding deep-cavity thin-walled containers and transparent products where ejector pin marks are not allowed. This demolding method features large and uniform demolding, smooth movement, and no obvious residual marks.
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