Click here to download a free PDF of this article.
The Trouble with Traditional Ring Check Valves
When speaking to injection molding companies about their production efficiency and their choice of non-return valves, the most common response is that their valves are performing just fine. Almost as if a script is passed around from company to company, big and small; performance is great, quality is good, all is well, why are you calling me? The most obvious follow up question to this is centered on what other options have been explored. Naturally the majority of companies only use traditional ring check valves. One must consider that if only one option has been explored, the standards of what is good or acceptable are limited solely on the experience of that option. There is no standard of comparison. The issue plaguing the injection molding industry stems from the notion that screws and valves are simply replacement parts for the injection unit. Many of the individuals surveyed have the mindset that the valve and screw that came with the injection unit is the exact assembly needed in order to function. This culture of thinking is detrimental to the effectiveness of a company and unknowingly is costing thousands. Being complacent with mediocre performance leads to excessive wear on equipment, lower quality parts, reduced productivity, and wasted material. A large dollar amount can be assigned to these issues but alternately, there are potential environmental implications. To confront this culture of thinking in the injection molding industry one must discuss the downfalls of the traditional seat closure ring check valve and how they can be addressed. The ring check valve is widely used in the injection molding industry. The traditional design simply relies on a ring sealing against a rear seat. The fundamental premise of how these valves function is not the issue but rather the execution, or lack there of. Due to the nature of the design one is faced with inevitable shortfalls: excessive wear, decompression (pull back) – that may cause splay, inconsistent closure, and quality.
When investigating the traditional ring check valve the limitations are clear. Due to the dependence of the ring sealing against the rear seat, positive closure can only be achieved with firm consistent pressure. The key issue with this design is that the sealing surface area is limited to the ring thickness which is limited by the ID of the barrel. Not only is the sealing area limited to the dimensions of the barrel, in order to achieve positive closure the sealing capability is dependent on the friction between the OD of the ring and the ID of the barrel. The valve design already relies on the relationship between the maximum material conditions of the barrel and ring, increasing the sealing surface area is simply not feasible. This limitation of sealing capabilities leads to a far greater issue: excessive wear.
As described above, the very design of the valve relies on friction to properly seal, this leads to excessive wear on the upstream side of the retainer, the downstream side of the ring, the OD of the ring, and more costly: the barrel itself. Undeniably the components of the injection molding machine will wear but a very clear design flaw is the reliance of friction between a relatively cheap valve and a very expensive barrel. To combat the lifespan of the valve, one may opt for an interlocking design. This variation utilizes interlocking between the upstream side of the retainer and the downstream side of the ring. Interlocking valves may very well reduce the wear potential between the ring and retainer but still due to the reliance of friction to properly seal, barrel wear is unaddressed. Since the ring now rotates in conjunction with the screw, barrel wear can actually be increased. The most crucial aspect of the valve, the sealing capability, hinges on its greatest downfall, friction. In order to achieve positive closure, firm consistent pressure must be obtained. As the barrel and valve components wear the necessary friction to seal consistently is lost which leads to unpredictable performance, as seen in cushion variation. Cushion variation directly correlates to part quality and material waste, through over packing or short shots. To reduce costly barrel wear, molders are forced to utilize greater decompression.
Typically, decompression is intended to eliminate mold drool. When dealing with traditional ring check valves and the excessive wear, molders are required to use greater and greater pullback distances to maintain optimal ring-to-barrel clearance. The clear consequence is sporadic performance and quality due to the irregular wear patterns down the length of the barrel ID. As the molders combat the issues of wear and erratic performance, they are ultimately faced with more quality issues. On top of cushion variation encountered before, molders may be faced with splay due to excessive decompression. Splay defects ultimately lead to even more wasted material.
Lastly, aside from friction, resin velocity is crucial to positive closure in all seat type closing valves. It is apparent that greater velocities will lend to greater initial sealing forces. The trouble is that 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 nonclosure. As discussed earlier, cushion variation leads to part weight variation: wasted material. To ensure positive closure, molders tend to focus on using the injection unit to properly close the valve rather than to fill the mold. This inefficiency and waste is blatant.
Why does it all matter? The common thread that ties the traditional ring check valve design together is waste. Aside from the obvious financial and physical waste involved in replacing worn valve components and very expensive barrels, companies are faced with downtime. Every opportunity for excessive wear leads to excessive maintenance costs. Time is money. A company’s resourcefulness in avoiding downtime and defects by tweaking machining parameters is yet another instance of waste. The time and money invested in training individuals or troubleshooting needless scenarios could be more aptly spent on marketing, expansion, R&D, etc. Material costs are a key factor in determining the profitability of a company. When dealing with sporadic performance, part quality is concerning. Whether dealing with splay, obvious short shots, or excessive flash, material is discarded. Sometimes less obvious is the issue of over packing. The erratic cushion variation lends to over packed parts in order to avoid other defects. This over packing is yet another opportunity to waste material. The parameters associated with part quality are directly correlated to cost and customer satisfaction but more broadly, wasted plastic is simply bad for the environment. It is no surprise to companies that are deeply rooted in the plastics industry that there is an inherent responsibility when dealing with environmental causes. Unlike some industries, plastic recycling is lagging. Apart from the decrease in profitability, the rate of material waste should be a key component in production to help solidify the growth of the plastics industry.
How do you address this dilemma? The injection molding industry must change the standards of efficiency. In the name of continual improvement, millions of dollars are spent on new state of the art machinery, automation, software, facilities, training of new production philosophies, etc. all while ignoring the downfalls of one of the most commonly replaced parts, the non-return valve. Many companies surveyed rely on the traditional valves because they simply do not know that they are being complacent. The standard of what is good is not overall good but rather as good as it can be within the limitations of the valve design. Whether third party variations or OEM, the traditional ring check valve is a needless bottleneck to the industry. These general purpose designs are not meant for high performance. In order to redefine the standards of efficiency, one must embrace innovation.
The R. Dray APV family of non-return valves may very well redefine the standards of efficiency. Rather than relying on the traditional rear seat closure and excessive friction, the APV utilizes sliding closure. A positive seal is obtained when the ring slides past a circumferential groove on the valve body. As the ring contacts the upstream retainer, an even more positive seal is achieved. This method of closure allows for consistent sealing without the reliance of decompression. Sliding closure enables the valve to close positively while withstanding pack and hold pressures of 20,000 PSI without creeping into the cushion. Unlike traditional ring check valves that rely on the relationship of the maximum material conditions of the ring and barrel, the APV does not require excessive ring thickness and friction to obtain positive closure. This allows for a minimal land length which translates to minimum pressure drop during recovery. Consequently, the APV uses three times less distance to close as compared to traditional valves.
As a result, the APV closes much faster, eliminating leakage and reducing cushion variation. This less restrictive design enables faster recovery times, less erratic temperature changes, and an increased ring to barrel clearance - eliminating excessive wear on the barrel and valve components. The short distance to close incorporated in the APV design provides an opportunity for dispersive mixing. Dispersive mixing occurs when high shear forces are acted upon particles or agglomerates; the particles are elongated, exposing more surface area to the adjacent resin, then transferred in a laminar flow. When resin is forced through the orifice into the linear groove, mixing the material, the APV design incorporates distributive mixing. The circumferential groove in communication with the longitudinal grooves of the valve body allow for particles to be spread throughout the medium to achieve good spatial distribution and better uniformity. Perhaps more important than the features and benefits that remedy the downfalls of the traditional valve is the basis of modification. The APV family of non-return valves is designed to be easily modified to maximize the performance potential of a given injection unit and material relationship. Rather than being a single general purpose option, the APV is engineered to optimize production.
In conclusion, the traditional ring check valve is an unnecessary bottleneck to the injection molding industry. Molders should not simply adjust to the downfalls of equipment but rather seek the most cost effective ways to eliminate waste and increase productivity. Innovation is the driving force to any industry. The APV non-return valve can help set new standards of efficiency and expectations in the injection molding industry. The rate of innovation in the parts being molded and the materials used is a clear indication. The injection molding industry can not afford to depend on unreliable general purpose equipment when customers expect cheaper, higher quality, sophisticated products. In any case, competition dictates that lowering the cost of production is far more beneficial than raising the price of the product.
Improve your production efficiency today!