English
Plastic Injection Molding Process
Created on 01.04
Injection Molding Process Flow
The injection molding process primarily consists of six stages: mold closing, filling, holding pressure, cooling, mold opening, and demolding. These six stages directly determine the molding quality of the product and form a complete, continuous process. This chapter focuses on the filling, holding pressure, cooling, and demolding stages.
Filling stage
Filling constitutes the initial stage of the entire injection moulding cycle, commencing from the moment the mould closes and continuing until the mould cavity is approximately 95% filled. Theoretically, a shorter filling time translates to higher moulding efficiency; however, in actual production, the moulding time (or injection speed) is subject to numerous constraints.
High-speed filling. During high-speed filling, the shear rate is elevated, causing the plastic to exhibit reduced viscosity due to shear thinning, thereby lowering overall flow resistance. Localised viscous heating effects also contribute to a thinner solidification layer. Consequently, during the flow control stage, filling behaviour is often determined by the volume to be filled. That is, in this phase, the significant shear thinning effect of the melt during high-speed filling typically outweighs the cooling influence on thin-walled sections, allowing the rate effect to prevail.
Low-speed filling. During low-speed filling with thermal conduction control, shear rates are low, local viscosity is high, and flow resistance is significant. As the replenishment rate of molten plastic is slow, flow progresses gradually, making thermal conduction effects pronounced. Heat is rapidly dissipated by the cold mould walls. Combined with minimal viscous heating, the thicker solidified layer further increases flow resistance in thinner wall sections.
Due to the flow dynamics of the fountain flow, the polymer chains in the plastic melt align nearly parallel to the flow front. Consequently, when two streams of molten plastic converge, the polymer chains at their interface become mutually parallel. Compounded by the differing properties of the two melts (varying residence time in the mold cavity, temperature, and pressure), this results in a microstructural weakness at the convergence zone. When the part is positioned at an appropriate angle under light and observed with the naked eye, a distinct weld line becomes visible. This represents the formation mechanism of weld lines. Weld lines not only compromise the appearance of plastic parts but also exhibit a loose microstructure prone to stress concentration. Consequently, the strength of this area is reduced, leading to potential fracture.
Generally speaking, weld lines formed in high-temperature zones exhibit superior strength. This is because polymer chains exhibit greater mobility at elevated temperatures, enabling them to interpenetrate and intertwine. Furthermore, in high-temperature regions, the temperatures of the two molten streams are closer, resulting in nearly identical thermal properties of the melts, which enhances the strength of the welded area. Conversely, weld strength is poorer in low-temperature zones.
Pressure holding stage
The purpose of the holding pressure stage is to continuously apply pressure, compacting the melt and increasing the plastic density (densification) to compensate for the material's shrinkage behaviour. During holding pressure, as the mould cavity is already filled with plastic, the back pressure is relatively high. Throughout the holding pressure compaction process, the injection moulding machine screw can only advance slowly in minute increments, and the plastic flow rate is also relatively sluggish; this flow is termed holding pressure flow. During the holding pressure phase, the plastic cools and solidifies rapidly against the mould walls, causing the melt viscosity to increase sharply. Consequently, significant resistance builds within the mould cavity. Towards the latter part of holding pressure, material density continues to increase and the part gradually takes shape. The holding pressure phase must persist until the gate solidifies and seals. At this point, the cavity pressure during the holding pressure phase reaches its maximum value.
During the holding pressure phase, the plastic exhibits partial compressibility due to the relatively high pressure. In areas of higher pressure, the plastic becomes denser with increased density; conversely, in regions of lower pressure, the plastic becomes more porous with reduced density. Consequently, density distribution varies with both position and time. Throughout this phase, plastic flow velocity is extremely low, rendering flow dynamics no longer the dominant factor; pressure becomes the primary determinant influencing the holding process. By this stage, the plastic has filled the mould cavity. The gradually solidifying melt now acts as the medium for pressure transmission. Pressure within the cavity is transmitted through the plastic to the mould wall surfaces, creating a tendency to expand the mould. Consequently, adequate clamping force is required to secure the mould. Under normal conditions, mould expansion force slightly widens the mould, aiding venting. However, excessive expansion force may cause flash, spillage, or even mould separation. Therefore, when selecting an injection moulding machine, one should choose a machine with sufficient clamping force to prevent mould expansion and ensure effective holding pressure.
Under the new injection moulding environmental conditions, we must consider novel injection moulding processes such as gas-assisted moulding, water-assisted moulding, and foam injection moulding.
Cooling phase
In injection moulding, the design of the cooling system is of paramount importance. This is because the moulded plastic part must be cooled and solidified to a certain rigidity before ejection to prevent deformation under external forces. As cooling time accounts for approximately 70% to 80% of the entire moulding cycle, a well-designed cooling system can significantly reduce moulding time, enhance injection moulding productivity, and lower costs. An inadequately designed cooling system prolongs moulding time and increases costs; uneven cooling further exacerbates warping and deformation of the plastic product.
Experiments indicate that the heat entering the mould from the melt is dissipated in two primary ways: approximately 5% is transferred to the atmosphere via radiation and convection, while the remaining 95% is conducted from the melt to the mould. Within the mould, the plastic product's heat is transferred via thermal conduction from the cavity plastic through the mould base to the cooling water pipes, where it is then carried away by the cooling fluid through convection. A small portion of heat not removed by the cooling water continues to conduct within the mould, eventually dissipating into the ambient air upon contact with the external environment.
The moulding cycle of injection moulding comprises clamping time, filling time, holding pressure time, cooling time, and ejection time. Among these, cooling time accounts for the largest proportion, approximately 70% to 80%. Consequently, cooling time directly influences both the duration of the plastic product moulding cycle and the production output. During the demoulding stage, the temperature of the plastic product must be cooled below its heat deflection temperature to prevent relaxation caused by residual stresses or warping and deformation resulting from external demoulding forces.
Factors influencing the cooling rate of the product include:
In the design of plastic products, wall thickness is a primary consideration. Greater product thickness necessitates longer cooling times. Generally, cooling duration is proportional to the square of the product thickness or to the 1.6th power of the maximum runner diameter. That is, doubling the product thickness quadruples the cooling time.
Mould materials and cooling methods. The materials used for the mould core, cavity, and mould base significantly influence cooling rates. Higher thermal conductivity coefficients in mould materials enhance the transfer of heat away from the plastic per unit time, thereby reducing cooling time.
Cooling water pipe configuration. The closer the cooling water pipes are to the mould cavity, the larger their diameter, and the greater their number, the more effective the cooling and the shorter the cooling time.
Coolant flow rate. The greater the cooling water flow rate (generally achieving turbulent flow is optimal), the more effectively the cooling water removes heat via thermal convection.
Coolant properties. The viscosity and thermal conductivity coefficient of the coolant also influence the mould's heat transfer efficiency. Lower coolant viscosity correlates with higher thermal conductivity, lower temperatures, and superior cooling performance.
Plastic selection. This refers to the plastic's ability to conduct heat from hot to cold areas. A higher thermal conductivity coefficient indicates better heat transfer efficiency. Alternatively, a lower specific heat capacity means the plastic's temperature fluctuates more readily, facilitating heat dissipation and thus achieving better thermal performance with reduced cooling time.
Process parameter settings. Higher material temperatures, higher mould temperatures, and lower ejection temperatures all necessitate longer cooling times.
Design principles for cooling systems:
The cooling channels must be designed to ensure uniform and rapid cooling.
The purpose of designing a cooling system is to maintain appropriate and efficient cooling of the mould. Cooling holes should employ standard dimensions to facilitate machining and assembly.
When designing a cooling system, the mould designer must determine the following design parameters based on the wall thickness and volume of the plastic part: the position and dimensions of the cooling holes, the length of the holes, the type of holes, the arrangement and connection of the holes, and the flow rate and heat transfer properties of the coolant.
Demoulding stage
Demoulding constitutes the final stage of an injection moulding cycle. Although the product has already cooled and solidified, the demoulding process significantly impacts its quality. Improper demoulding methods may result in uneven force distribution during removal, potentially causing deformation or other defects. Two primary demoulding approaches exist: ejector pin demoulding and ejector plate demoulding. When designing moulds, the appropriate demoulding method must be selected based on the product's structural characteristics to ensure quality.
For molds employing ejector pins, these should be positioned as uniformly as possible, located where ejection resistance is greatest and where the part exhibits maximum strength and rigidity. This prevents deformation or damage to the plastic component.
Ejector plates are typically used for deep-cavity, thin-walled containers and transparent products where ejector pin marks are unacceptable. This mechanism delivers substantial yet uniform ejection force, operates smoothly, and leaves no discernible traces.
Contact
Leave your information and we will contact you.