Principles of Injection Molding
A comprehensive guide to the fundamental processes and mechanisms behind the injection molding procedure
The Injection Molding Procedure Fundamentals
The basic equipment for injection molding consists of an injection molding machine and an injection mold. The injection molding procedure utilizing a screw-type injection molding machine follows a precise sequence of operations that transforms raw plastic materials into finished products.
As illustrated in the diagram, the injection molding procedure begins by adding granular or powdered plastic into the injection machine's barrel. The plastic is heated and melted, then the machine's screw pushes the molten plastic at high pressure and speed through the nozzle at the front of the barrel.
This molten material is rapidly injected into a closed mold cavity [Figure 1-2(a)]. The melt that fills the cavity, under pressure, cools and solidifies while retaining the shape of the cavity [Figure 1-2(b)]. The mold then opens, and the product is removed [Figure 1-2(c)] as the final step in this phase of the injection molding procedure.
During the injection molding procedure, plastics undergo a series of transformations: softening, melting, flowing, shaping, and solidification, as shown in Figure 1-3. Each stage of the injection molding procedure is critical to the quality of the final product, with precise control required at every step.
Stages of the Injection Molding Procedure
(1) Softening and Melting
A critical phase in the injection molding procedure is the transformation of solid plastic into a molten state. Figure 1-4 shows the barrel and screw structure of an injection molding machine, which plays a vital role in this part of the injection molding procedure.
L1 - Feed section; L2 - Compression section; L3 - Metering section; h3/h1 - Compression ratio; D - Screw diameter
The barrel is equipped with circular heaters that provide the necessary thermal energy for the plastic transformation. As the screw rotates, the plastic moves forward while simultaneously melting, eventually being injected into the mold through the nozzle. This part of the injection molding procedure requires precise temperature control to ensure proper material preparation.
Several key changes occur during the plasticization phase of the injection molding procedure: First, as plastic enters the compression section (L2) from the feed section (L1), it is compressed due to the decreasing volume of the screw channel and undergoes degassing.
Before entering the metering section (L3), the plastic reaches its melting temperature and becomes a molten mass. A crucial aspect of the injection molding procedure is ensuring that the plastic is sufficiently degassed before melting. If the plastic melts too early in the compression section, its degassing efficiency would be significantly compromised, affecting the quality of the final product.
The metering section (L3), also known as the mixing section, features an even smaller screw channel depth (h3). In this part of the injection molding procedure, the plastic undergoes intense shear force during screw rotation, resulting in thorough mixing and complete melting. This ensures the molten plastic has uniform consistency, which is essential for the subsequent stages of the injection molding procedure.
Key Transformations in the Injection Molding Procedure:
- Plastic is introduced into the system
- Heating begins the transformation process
- Degassing removes trapped air and volatiles
- Softening prepares the material for melting
- Adiabatic compression increases pressure and temperature
- Complete melting occurs in the metering section
- Uniform mixing ensures consistent material properties
(2) Flow
When molten material is injected into the mold under high pressure and speed during the injection molding procedure, two significant phenomena typically occur. First, the plastic in a compressed molten state within the barrel undergoes sudden decompression, causing expansion. This rapid expansion, known as adiabatic expansion, results in a temperature drop in the molten plastic, similar to the principle of adiabatic expansion in refrigerators.
Examples from industrial injection molding procedure applications demonstrate that this temperature drop can reach 50°C for polycarbonate and 30°C for polyoxymethylene plastics. Additionally, when the molten plastic enters the mold and comes into contact with cold wall surfaces, another rapid temperature decrease occurs, affecting the material's flow characteristics.
1 - Injection machine; 2 - Resin injection into mold (actually consisting of sprue and gate); 3 - Mold (inside cavity); 4 - Central portion with faster flow rate; 5 - Portion with extremely slow flow rate along mold cavity walls; 6 - Resin molecules stretched and oriented by flow; 7 - Entangled resin molecules
The second phenomenon in this stage of the injection molding procedure is that the macromolecules of the molten plastic become oriented along the direction of flow. As evident in Figure 1-5, the melt flows very slowly near the mold cavity walls, while the central portion flows much faster. Plastic molecules in the faster-flowing regions become stretched and oriented, a characteristic feature of this phase of the injection molding procedure.
When plastic solidifies in this oriented state during the injection molding procedure, differences in shrinkage rates between directions parallel and perpendicular to the flow often lead to product deformation and warpage. This is a critical consideration in both the injection molding procedure design and mold design phases, as it directly impacts the dimensional stability of the final product.
Flow Dynamics in the Injection Molding Procedure
The flow behavior during the injection molding procedure is influenced by several factors:
Material Properties
Viscosity, melt flow index, and temperature sensitivity affect how the plastic flows during the injection molding procedure.
Process Parameters
Injection speed, pressure, and temperature directly impact flow characteristics in the injection molding procedure.
Mold Design
Cavity geometry, gate design, and cooling channel placement influence flow patterns during the injection molding procedure.
Molecular Orientation
Flow-induced orientation affects mechanical properties and can lead to anisotropic behavior in the final product.
(3) Shaping and Solidification
In the final stages of the injection molding procedure, molten plastic is injected through the nozzle into the mold, where it acquires the desired shape and then cools and solidifies to form the final product. The filling of molten plastic into the mold typically occurs within a few seconds, making direct observation of this phase of the injection molding procedure challenging.
To better understand and optimize this critical stage of the injection molding procedure, researchers like Stevenson in the United States have employed computer simulation methods. These simulations depict the filling process for complex parts, such as a polypropylene automobile door panel molded using a hot runner mold with two gates. The simulations calculate important injection molding procedure parameters including filling time, weld lines, and required clamping force.
The flow patterns observed in the simulation closely match expectations, providing an accurate representation of the actual filling process for automobile door panels during the injection molding procedure. The dotted lines represent flow front positions at different times, while the circular lines indicate weld lines formed where different flow fronts meet.
Various methods have been developed to simulate the flow processes in the injection molding procedure, including the FAN method, CAIM simulation system, and Moldflow simulation system. These sophisticated tools allow engineers to predict how molten plastic fills the mold cavity during the injection molding procedure, enabling more rational mold design, optimal gate placement, and improved process efficiency.
After the molten plastic has been shaped in the mold during the injection molding procedure, it enters the solidification phase. A key phenomenon during solidification is shrinkage, which results from both cooling and crystallization processes occurring simultaneously. Understanding and controlling shrinkage is crucial in the injection molding procedure to ensure dimensional accuracy and minimize part distortion.
Figure 1-7 illustrates the shrinkage behavior of three types of polyethylene with different crystallinity levels as temperature decreases. This data is invaluable for optimizing the cooling phase of the injection molding procedure, as it helps determine appropriate cooling times and temperatures for different materials.
The injection molding procedure's solidification phase requires careful control of cooling rates to balance production efficiency with part quality. Rapid cooling can reduce cycle times but may introduce internal stresses, while slower cooling can improve crystallinity and reduce stresses but increases production time.
Figure 1-7: Density changes of polyethylene (PE) at different temperatures
a - PE with relative density 0.9645; b - PE with relative density 0.95; c - PE with relative density 0.918; d - Cooling rate curve
The relationship between temperature and density during the injection molding procedure directly affects the final dimensions and properties of the molded part. By understanding how different materials behave during cooling, manufacturers can optimize the injection molding procedure to produce parts with consistent quality and performance.
The entire injection molding procedure represents a delicate balance of thermal and mechanical processes, where each stage influences the final product quality. From material preparation through melting, flow, shaping, and solidification, every aspect of the injection molding procedure requires precise control and understanding to achieve optimal results in modern manufacturing.