The Selection Principle Of Injection Speed For Plastic Parts

One sunny day at Prototool, a concerned manager approached me with a plastic part in hand. The piece, destined for a high-profile client’s latest product, had unexpected warping and surface blemishes. The team was puzzled; our processes were usually impeccable. As whispers of potential production halt and delays began to circulate, Leo, our seasoned production manager, stepped in.

Holding the part up to the light, Leo pointed out the uneven flow marks and slight discoloration on the surface. “These are classic signs of an incorrect injection speed,” he explained. “Too fast, and the plastic doesn’t have time to fill the mold evenly, leading to these flow marks. Too slow, and the plastic can start to cool before it’s fully injected, causing discoloration.” With his years of experience and a trained eye, Leo could spot these subtle signs that many might overlook. Thanks to his keen observation, we were reminded of the critical importance of precise injection speed in achieving molding excellence.

The Pivotal Role of Injection Speed in Molding Excellence

In the dynamic realm of injection molding, controlling the process directly influences the final quality of the parts and the cost-effectiveness of the entire operation. As technological advancements continue to revolutionize household appliances, the complexity of product designs escalates. This evolution demands a heightened emphasis on both the intrinsic and extrinsic quality of injection-molded components. Consequently, molding enterprises face elevated standards in mold manufacturing and process control adjustments.

The Convergence of Advanced Equipment, Precision Molds, and Process Control

With the continuous enhancement in mold processing techniques and precision, intricate product designs have become achievable. Moreover, the relentless improvements in the electrical control designs of injection molding machines ensure the production of high-quality products from complex molds. However, possessing advanced machinery and top-notch molds is just half the battle won. A superior level of process control is imperative to harmoniously integrate the machine, mold, and final product.

Key Parameters in the Injection Molding Process

The most critical conditions in the injection process revolve around factors affecting the plastic flow and cooling. These include temperature, pressure, injection velocity, and their respective durations. These elements interplay in a complex dance; for instance, elevating the melt and mold temperatures can reduce the injection pressure and speed. Conversely, a decrease in these temperatures necessitates an increase in both injection pressure and velocity. At the heart of these influencing conditions lies the viscosity change in plastics. This aspect is crucial for making informed decisions about parameter selection during the injection process.

The Growing Recognition of Injection Rate’s Significance

As research deepens into the flow and deformation of plastic melts during injection, there’s a growing acknowledgment of the importance of injection rate parameter selection in enhancing product quality.

Understanding Injection Speed, Time, and Rate

• Injection Speed: Refers to the movement speed of the screw or plunger during injection. The faster the speed, the shorter the time required to complete an injection cycle, leading to higher production volumes.
• Injection Time: Denotes the shortest time the screw or plunger takes to complete its maximum injection journey or the minimum time needed to fully inject a specific volume of material.
• Injection Rate: Represents the volume flow rate achievable during injection per unit of time. It’s the ratio of injection volume to injection time.

By comprehensively understanding these parameters and their interrelationships, businesses like Prototool can ensure that their injection molding speed, whether it’s the fill rate or the injection velocity, is optimized for the best outcomes.

The Dual Dynamics of Injection Speeds in Molding

In plastic molding, the speed at which the melted plastic is put in greatly affects the quality of the finished item. Both high and low injection speeds, or as some might refer to them, “injection velocities,” have their unique advantages. Let’s delve deeper into the characteristics and benefits of each.

Advantages of High Injection Speeds

1. Efficiency in Time: Using a faster injection rate reduces the overall molding cycle, making the production process more efficient.
2. Enhanced Flow: A rapid injection velocity improves the flow of the plastic, which is especially beneficial for the molding of thin-walled components.
3. Surface Brilliance: Products molded at higher speeds often exhibit a superior surface gloss.
4. Strength at the Seams: High-speed injection can enhance the strength of the weld lines, making them less noticeable.
5. Prevention of Cooling Deformities: A swift injection process can prevent deformities that arise due to premature cooling.

Advantages of Low Injection Speeds

1. Precision Edges: Slower injection rates can prevent the formation of flash or excess material along the edges of the molded product.
2. Surface Clarity: It helps in avoiding jetting or flow lines that can mar the appearance of the product.
3. Burn Mark Prevention: Reduced speeds can prevent burn marks that occur due to trapped air.
4. Avoidance of Air Traps: Ensures that the molten plastic doesn’t trap air as it fills the mold.
5. Molecular Orientation: A slower speed can prevent molecular orientation deformities, ensuring the plastic’s properties are consistent throughout the product.

However, it’s essential to understand that the benefits of high-speed injection are often the drawbacks of its slower counterpart and vice versa. This realization has led to the adoption of a combined approach, utilizing both high and low speeds at different stages of the injection process. This technique, commonly referred to as multi-stage injection molding, ensures that manufacturers can harness the advantages of both speeds while minimizing their respective downsides. It’s a testament to the evolution of modern injection molding machines that many now come equipped with capabilities to adjust between five to six levels of injection pressures and speeds, along with three to four stages of holding pressures. This adaptability ensures that during the holding phase, when the mold is already filled with molten plastic, the influence of holding speed becomes negligible, ensuring optimal product quality and process efficiency.

Mastering the Art of Injection Speed Selection

The injection molding process is a delicate balance of various factors, with the speed of injection being one of the most crucial. Given the intricate shapes of plastic products and the complexities of molten plastic flow, the selection of the right injection speed is paramount.

Principles of Injection Speed Selection

Plastic products have complex shapes. This makes the flow of melted plastic during the molding process, especially in certain parts of the mold, very complicated. Recent studies in rheology, combined with CAE technology, have concluded that to achieve high-quality products with minimal internal stress, it’s essential to maintain a uniform melt flow front. In simpler terms, the velocity of the molten plastic, or its linear speed, should remain constant throughout the process.

Products manufactured with this principle in mind tend to be of superior quality, with fewer defects like shrinkage or inadequate filling. Moreover, the uniform flow of the melt inside the mold ensures a rational orientation of polymer molecules, resulting in a better surface quality of the product.

Given the geometric complexities of injection-molded products and the varying resistances they offer to the flowing melt, the flow rate (product of speed and cross-sectional area) varies over time. To ensure quality while also considering the economic aspect of shortening the injection time and molding cycle, multi-stage injection techniques are employed.

Rational Selection of Injection Speeds

Based on the flow of the melt during the injection process, injection speeds can be broadly divided into five parts:

Main and Sub-Runners: The principle here is to use high-speed filling, which shortens the molding cycle. However, it’s crucial to ensure that this rapid injection doesn’t compromise the surface quality.

Gate and Nearby Product Areas: Depending on the type of raw material and gate design, this stage might require a slower injection speed. Especially for high-viscosity resins like PC, PMMA, and ABS, and for certain gate designs, a slower speed helps prevent defects like jetting or gate blushing. However, for low-viscosity resins like PP, PA, and PBT, and where surface quality near the gate isn’t a primary concern, high-speed injection might be suitable.

Main Body of the Molded Product: Once the area near the gate is filled and until the product reaches about 70%-80% of its weight, high-speed injection is the norm. This not only shortens the molding cycle but also minimizes viscosity changes of the melt inside the metal mold cavity. It can also enhance the surface gloss of the product, reduce deformities, and improve the appearance of weld lines.

Near Full Cavity (around 85%-90%): This stage employs medium-speed injection, serving as a transition to the next low-speed phase. It helps prevent defects in thicker parts of the product and uneven filling that might lead to flash.

Final Filling of the Cavity: The last stage requires slow injection to prevent defects like flash or burn marks. It ensures the dimensional and weight stability of the product and allows for a controlled reduction in mold-locking pressure.

Fine-Tuning the Injection Speed Curve for Optimal Results

The art of injection molding is not just about understanding the principles of speed selection but also about accurately setting and adjusting the injection speed curve during the actual operation. This makes sure the finished product is of good quality.

Setting the Injection Speed Curve

When setting the injection speed curve, technicians should focus on two main aspects:

1. Determining the Speed: The magnitude of the injection speed.
2. Positioning the Injection: The exact point or phase during the molding process where the speed is applied.

A common practice is the “0 injection method” to set the screw injection position. This involves setting the second-stage injection pressure and speed to zero and adjusting the injection position to observe the actual size and shape of the molded product. This observed position is then compared with the desired position to finalize the settings. The process is repeated for subsequent stages, adjusting the speed from low to high, ensuring that the product’s surface quality is not compromised while aiming for a shorter molding cycle.

Adjusting the Injection Speed Curve

Adjustments to the injection speed curve should be made in reverse order. This means first determining the adjustment in injection speed and then reviewing the transition positions of speed changes. This is because, during the initial setting of the curve, the injection positions generally align with our expectations. However, changes in injection speed can cause deviations in the filling results at the same transition positions.

In multi-stage injection, injection pressure and speed are interrelated and influence each other. When the filling time is short and the flow path is long, the flow rate (or injection speed) is higher, necessitating a higher injection pressure.

Furthermore, increasing the filling time (i.e., reducing the injection speed) means the polymer has a longer cooling time. This results in a thicker frozen layer when the polymer encounters the cold mold, increasing the melt’s viscosity and resistance. Therefore, a higher injection pressure is needed to fill the mold cavity.

To address this, the injection pressure should be set slightly higher. This allows for precise control over product defects, such as flash and short shots, by adjusting the various stages of injection speed. This embodies the design philosophy of injection molding machines: one segment for injection pressure and six for injection speed. Adjusting the injection speed helps overcome product defects.

Causes and Solutions for Common Defects

1. Flow Marks at the Gate: These are caused by the melt jetting, creating flow lines. To resolve this, the mold design should ensure that the gate jetting angle is aligned with the core to prevent direct jetting. Additionally, the injection speed should be reduced as the melt flows through the gate. Given the reduced speed in the first stage, the melt encounters the cold mold and solidifies with a thicker frozen layer, requiring a higher injection pressure. Therefore, the second-stage injection speed should be increased to reduce the filling time and lower the injection pressure, preventing product shrinkage. As the mold cavity is almost filled, the injection speed should be reduced to prevent flash.

2. Weld Lines: These are formed due to the rapid cooling of the melt after it encounters the cold mold walls, causing the melt to break and form weld lines. To mitigate this, the mold design should change the direct gate to a fan-shaped gate, increasing the fusion angle. This ensures that the two streams of melt begin merging closer to the gate, minimizing temperature differences. Additionally, the first-stage speed should be increased, and as the melt flows through the gate, the injection speed should be reduced to decrease the shear rate. Once 50% of the product is filled, the injection speed should be increased to ensure complete filling while maintaining a lower injection pressure, reducing the appearance of weld lines. As the mold cavity is about to be filled, the speed should be reduced to minimize uneven gloss on the weld lines.

Drawing Conclusions from Injection Speed Analysis

The speed of plastic molding greatly affects the quality and success of the finished item. Delving deep into the theoretical discussions and practical examples, we can derive the following insights:

1. Overcoming Defects with Speed Adjustments: Adjusting the injection speed can help fix molding issues. This improves the product’s quality and makes the process more cost-effective.

2. Tailored Speed Adjustments: Different product designs, material types, and gating systems demand distinct choices and adjustments in the injection speed curve. It’s not a one-size-fits-all scenario; customization is key.

3. The “0 Injection Method” in Practice: Implementing the “0 injection method” during actual production operations proves to be an effective strategy for setting the injection speed curve accurately.

4. Understanding the Multifaceted Nature of Injection Speed: There are numerous factors influencing the injection molding speed. It’s crucial to grasp these intricacies correctly to make informed adjustments. By making these changes, we improve the molding process. This also makes sure the final product is high-quality and accurate.