Is Your Injection Molding Screw Tip Assembly Leaking?
Much like other manufacturing sectors, the injection molding industry is heavily focused on improving productivity and efficiency. This noble endeavor brings with it a list of new process training programs, philosophies, sensors and the like. It is important for the industry to aim for these high-end goals, but it is crucial to not ignore or overlook the bedrock components of the system. The screw and valve is the heart of any injection molding operation and more than any other component can determine the overall part quality, and have a major influence on profitability. Many, if not most, injection molding firms do not fully understand non-return valve design. Most often the screw tip is treated as a wear component that is replaced as needed, when in reality it is one of the most influential components of the operation. There has been an interest in exploring improved valve designs in the industry, most notably when production limitations can no longer be processed around. To best understand the importance of adequate valve design, it is best to explore the fundamental mechanism for closure and how that may impact production. Is your injection molding screw tip assembly leaking?
How do Injection Molding Check Ring Assemblies Work?
Simply put, a check ring is a one way valve that prevents backflow during injection. Traditional valve designs may vary to some degree, but generally consist of a valve body, a ring with an angled sealing surface, and an angled rear seat. Closure is accomplished primarily by a minimal clearance between the ring OD and barrel ID. The ring is moved forward to an open position when screw rotation is initiated. During recovery, the screw is moved rearward along with the ring until the desired shot size is achieved. Finally, injection rapidly moves the screw forward; ideally bringing the valve to a closed position to fill the mold cavity. This is accomplished by the friction generated by the tight clearance between the ring OD and barrel ID. This necessary ring movement and tight clearance is an obvious source of barrel wear, ring wear, and front retainer wear on the valve body.
To eliminate the frictional wear on the retainer and extend wear life, valve designers opt for the interlocking variants, as well as 4 piece designs that incorporate replaceable front retainers. Although front retainer wear can be reduced or even eliminated, the ring to barrel wear is still apparent and often exacerbated with interlocking variants, as the minimal ring to barrel clearance is still required for proper closure and the ring now rotates with the screw. As the barrel wears, rear seat closing valves lose the friction necessary for sealing and performance becomes erratic.
Molders are often forced to utilize more and more decompression to help alleviate this issue, oftentimes resulting in the introduction of splay or other part defects. Velocity is also a critical component to positive closure in these valve types. The greater the velocity the greater the initial sealing force. At low velocities the separating force created by the resin leaking upstream and the increasing pressure on the upstream side of the ring can be great enough to resist complete closure and cause cushion variation or in worst cases, non-closure. This quick, surface level analysis is important yet insufficient for better understanding the downfalls of your check rings and potential solutions. To best understand these designs and make better decisions one must understand pressure drop and leakage flow calculations, as these are the primary ingredients to proper valve design.
Pressure Drop and Leakage Flow in Injection Molding Valves
As previously stated, the purpose of a check ring is to prevent backflow during injection. An ideal valve would close near instantly, work properly on every shot, have minimal pressure drop, have no dead spots for material to hang up and degrade, and have minimal wear on equipment. The limitations of traditional rear seat closing valves makes balancing these criteria essentially impossible. Fast consistent closure that provides accurate part weight and low cushion variation requires a short distance to close. This logical conclusion becomes muddled when one is faced with the requirement of a large sealing surface area. As previously stated, low injection velocities are prone to cushion penetration. To help prevent this, a larger sealing surface is required. Other obvious factors include contamination and wear. Because material flows over the sealing surface (rear seat and upstream ring surface), the sealing surface becomes a wear surface. Small sealing surfaces or point-to-point contact designs would quickly wear or misalign and become erratic. To better demonstrate this design limitation, pressure drop and shear rate calculations are utilized. For this example, we will “unwrap” a valve to display a slit orifice configuration using dimensions from an original OEM rear seat closing valve to estimate both pressure drop and shear rate. In this example, the land length is the sealing surface, the width is the circumference of the valve, and the clearance is the distance to close.
With the given design data we find that there is a reasonable 3.52 psi pressure drop and a shear rate of 26.14 in 1/sec. This low pressure drop is achieved by sacrificing repeatability with the very large distance to close. With almost 3/8” of travel there is more than enough opportunity for material to leak rearward during closure. Because reducing the sealing surface area (land length) negatively impacts reliability and the limitation of hoop strength (ring thickness), the only viable option is to reduce the distance to close (clearance). As the clearance decreases, both pressure drop and shear rate increase dramatically. This still doesn’t address the concerns of excessive wear, as friction is a function of closure in these designs.
Shorter Distance to Close in Non-Return Valve Designs
If we were to redesign this valve assembly to incorporate a much shorter distance to close to improve backflow, we would quickly face extreme limitations in both pressure drop and shear rate through the valve assembly. To highlight this, below are the same previous calculations, this time with a more reasonable distance to close of 1/16”.
Depending on the resin being processed, the increased shear rate in some cases could aid the process by acting as a dispersive mixer, similar to a blister ring style mixing device. However, much like blister ring style mixers, this approach becomes a pressure consuming device, negatively impacting throughput. Furthermore, the sealing surfaces are still wear surfaces that are prone to contamination and erratic performance. As previously discussed, the limitations of traditional rear seat closure force valve designers to rely heavily on tight ring to barrel clearances. Not for preventing leakage over the ring, but simply to utilize friction for adequate closure. Again, as these surfaces wear, rear seat closing valves lose the friction necessary for sealing and performance becomes erratic. Unfortunately, because of the need for tight clearances for closure, one cannot leverage larger clearances for reduced component wear – despite the lack of leakage over the ring. It is very common for molders to expect that resin simply leaks over the ring, rather than through the valve body itself. To highlight this, we can again turn to simplified annular orifice flow rate calculations using the same original OEM valve geometry.
In this example we see that the ring has only 0.001” radial clearance. This provides a flow rate over the ring of approximately 0.00028 in3/sec, or effectively zero. This is largely due to the viscosity of the resin being processed, the clearance, and more importantly, the length of the ring. With higher viscosity materials, flow rate over the ring is further reduced, as with longer ring lengths. To exemplify this theory, we can triple the ring to barrel clearance. This can be further advanced by estimating cushion creep or penetration at this flow rate.
Even in this example, the theoretical leakage flow and cushion penetration is very low. However, as previously stated, friction is a function of closure in traditional valve designs. Therefore, equipment wear is inevitable, despite the lack of leakage over the ring. Also, extending the length of the ring significantly is rarely possible because of the predetermined valve envelope designated by the OEM. To change this, one would have to modify both the screw and the valve to accommodate the new length.
The SRV Valve Family: Improved Repeatability and Wear Life
The SpiraMelt SRV (shot replicator valve) family of valve designs was developed to remedy the common downfalls of traditional screw tip assemblies. Rather than a throwaway replacement part, the SRV family of valves are specifically engineered to meet the demands of your application. Each valve assembly is custom-designed, tailored to provide best-in-class repeatability. The unique shut-off of the SRV valve family allows for shorter distance to close without excessive shear rates, no dead spots for material hang up and degradation, increased ring-to-barrel clearance for better wear life, and no excessive pressure drop under the ring that limits throughput. The SRV family of valves includes the standard SRV, the SRVi interlocking variation, and the SRV Vortek mixing valve to improve color mixing and melt quality on poor screw designs. Each valve is tailored to meet your production requirements and are made from premium, high-wear resistant tool steels.
Injection Molding Non-Return Valves are More Than Just Replacement Parts
Simply put, check ring assemblies should not be treated as simple wear components that are replaced as needed. The non-return valve is a vital component to the entire injection molding process. After all, valve closure controls the accuracy of the final molded part. Although it is important for injection molding firms to invest in their employees by enrolling them into advanced processing training, equal efforts should be spent on better understanding melt stream components that most control the injection molding process. Rather than using the machine to close the valve assembly, we should best utilize the machines to effectively fill the mold quickly and efficiently to provide the highest quality components to the end user at an affordable price.
