How (and Why) to Use a Valve Gate for Injection Molding
Learn the Basics Around Valve Gates
The valve gate injection molding technique is an effective and widely accepted approach for large-volume production of plastic components that require top-level quality, such as medical and cosmetic parts. That is because traditional injection molding generally has a disadvantage - the plastic that is injected out of the injection barrel's nozzle tends to run off. Apart from affecting the item’s appearance, this defect might also affect its strength and overall quality. As a result, a method for preventing this drooling effect should be employed, and this is where valve gates come into play.
Valve gates are a type of hot runner gates that are designed with a unique gate pin or valve; their main function is to control the flow of plastic into the mold. This article is about to cover everything you should know about valve gating systems, from their definition to their usage and advantages.
Gate type, placement, and quantity can affect cycle time, part aesthetics, and structural integrity of parts. Choosing the right gate is a highly important part of both the mold design and the gate injection molding process, since selecting the wrong type of gating can result in a variety of issues during processing. There are a few critical aspects to consider when choosing the gate type and location for a molded item. The first is the design of the mold and the placement of the gate. Gating possibilities are limited by the part’s orientation in the mold, the show surface location, and the action locations. Material selection, part volume/size, and manufacturing functionality are other design factors that should be taken into consideration.
Some general gate location guidelines include:
- Gates should be put on the part’s non-functional areas.
- Gates should be placed in the part's thickest cross-section to avoid sinks and voids.
- Gates should be placed in areas where their removal would be simple and straightforward.
Resin flow may be improved by carefully choosing the placement and number of gates, which can furthermore help prevent flow markings and weld lines. Apart from influencing the final product’s appearance, the placement of the gate may also impact its strength.
The difference in hot runner vs cold runner systems is whether plastic is consistently heated on the way to the mold. Most gates are attached to a cold runner and a hot sprue, and are known as cold runner systems. For clarification, sprues and runners are channels the liquid plastic flows through to get to the mold. Hot runner systems (also known as hot manifold systems), on the other hand, keep the plastic consistently heated until it reaches the mold. Valve gates are some of the most commonly utilized examples of hot runner systems.
In a cold runner system, the molds and the runners are both kept at the same temperature. The mold in this method holds two or three plates. The two-plate approach (in which a mold has two halves) is more user-friendly, although it does necessitate the use of an injection mechanism to extract the part from the mold. The runner works on a separate plate in a three-plate configuration. As a result, it may be separated from the runner and ejected independently.
A hot runner mold, on the other hand, consists of only two plates heated through the use of a manifold system. Hot runners can be utilized both externally and internally. Externally heated molds are recommended for materials that are extremely sensitive to heat. Internally heated molds are the superior choice when improved flow control is required. Heating rods, coils, and heating pipes are all options for heating runners.
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Valve gates are a type of gate used only in hot runner systems; their purpose is to control the melt flow by managing the shut-off time and preventing two flow fronts from colliding in the middle. Valve gates are a far more effective alternative to other types of gates, which are difficult to modify. The injection molding process benefits from using hot runner systems: without the valve (in what is known as a hot drip or hot tip system), there’s nothing to block the plastic from flowing out again. This requires additional process optimizations to prevent this. Since the valve gate shuts off the exit and plastic is prevented from flowing out, a valve gate offers an extra layer of reliability in a repeating process.
A valve gate nozzle uses a valve stem to positively seal the gate. The valve stem slides forward and seals the gate orifice with mechanical force. During mold opening and part ejection, the valve stem remains closed, eliminating drool and stringing. Unlike thermal gated systems which require melt decompression, valve gate nozzles eliminate this demand since the seal stays robust even if the hot runner manifold stays pressured. This is important since melt decompression can lead to splay and other aesthetic flaws.
Valve mold gate nozzles may leave a witness mark on the component with the same size as the gate diameter itself. The valve stem protruding through the gate may create a slight depression in the gate's center. Because the molded section part separates from the gate without breaking or shearing the plastic, discoloration or distortion as a result of the gate break is uncommon. Gate quality is consistent over a wide variety of process conditions. However, if the valve stem is kept open for an extended period of time, enabling the resin in the gate to solidify, the solid plastic can prohibit the valve stem from seating in the gate orifice, resulting in a post.
Valve gates improve the part’s aesthetics, reduce cycle time, eliminate the need for a runner removal operation, and reduce waste associated with a discarded runner or sprue. Valve gating is especially important in automated manufacturing because it enables faster mold start-ups, larger processing windows, and prevents melt stringing and drooling at the gate. Let’s take a look at its main benefits in more detail:
Cycle time is reduced as molds no longer require a sufficiently frozen gate. With a valve gate nozzle, the hold time may be shortened, and melt plasticization can begin as soon as the valve gate is closed. Thermally gated molds, on the other hand, require a sufficiently frozen gate before hold pressure is released and screw recovery begins. The reduced shear rate in the gate area also reduces the shear heating of the melt, lowering the part's cooling time.
Thin wall molding applications are characterized by high fill rates, high pressures, and rapid cooling. Filling the cavity quickly - in the range of 0.5 seconds or less - is required before the frozen layer solidifies and hinders further cavity filling. Valve gates are an excellent choice for these applications. Fast filling is possible thanks to the wide gate widths and lack of flow constraints, which reduce pressure drop and shear heating. Rapid component cooling allows the valve stem to seal promptly after the cavity fills in many thin wall molding applications.
Nozzle tip configuration is mostly determined by the properties of the resin used. For amorphous resins, the gate region should be thermally isolated from the nozzle and nozzle tip. Heat transmission from the nozzle to the gate region can cause deformation in the gate area by delaying solidification. To keep the resin molten, a valve gate nozzle intended for semi-crystalline and high-temperature amorphous resins must transfer heat to the gate seal-off area. This is accomplished by using a nozzle tip with an integrated sealing surface that extends to the molding surface. Moreover, valve gates have been utilized effectively with abrasive polymers comprising glass and carbon fiber fillers. It's critical to utilize wear-resistant materials for the valve stem and nozzle tip. For abrasive resins, a replacement nozzle tip with an incorporated gate seal is the best option.
Injection molding gates are used by matching the correct gate size and runner system, then determining their closures according to the final part size and plastic flow. To use a valve gate correctly, the first step is to choose the correct valve gate size, together with the hot runner system. The primary factors in sizing a valve gate are the material to be molded and the overall part design. In an injection press, the mold is to be connected to a hot runner system and a valve gate. The valve gate's electrical controls must be linked to the injection cycle's push timer. Valve gates are also often activated by pneumatic or hydraulic means. Before taking any action, it should be verified that the valve's actuation is controlled properly.
Next, the rate of material flow and the part's size is going to determine the time when the gate should be closed during the injection cycle. There is no need to inject extra plastic once the mold has been adequately filled with molten plastic. Once the gate is closed, the following cycle's melt operation may start. The valve gate actuator should be configured to either close or open the gate, depending on the current state of the process.
During a valve-gated process, knowing if there’s enough plastic in the mold is of the utmost importance. The least effective way to check this is by measuring the amount of time plastic flows into the mold. A better option is to inject the material from different positions in order to ensure everything is covered. The most reliable and newest way, however, is to have external pressure sensors inside the mold showing the interior pressure and thus base the assessment of whether there’s enough plastic in the mold cavity. This method is gaining more ground nowadays, even though most companies still use the second technique.
Sequential valve gate controllers open groups of gates independently of each other. It is common practice to put multiple distinct gates in the valve gate system. In such a scenario, these gates can be handled in two different ways. The first is where all the gates are controlled together, i.e. all gates are open and closed at the same time and hence, only one valve gate controller is required for all valve gates. The second way is by utilizing sequential valve gates which open independently of one another. In this case, a separate controller for each gate or group of gates is needed. For example, a tool could have eight valve gates, grouped into four independent groups of two gates. This would require four independent controllers to open two gates at a time. Each group of two gates can be also referred to as a zone. The controllers that are being used for both valve gate systems would be the same; the only difference is the larger number of controllers used for sequential gating.
The sequential valve gate is a standard technique for filling a mold quickly and efficiently. One of the key advantages of this technique is that it improves dimensional stability by preventing gas bubbles. Moreover, the SVG process provides higher flexibility and has a shorter cycle time, lower costs, and increased production.
When utilizing a sequential valve gate system, the fill valves of the mold are gradually filled from the center. The material is uniformly distributed in the cavity thanks to a sequential fill valve-controlled system. Apart from reducing cycle time and costs, this approach can also help eliminate splay and blemishes in molded parts. While the sequential filling procedure does not affect the quality of a molded product, it does improve the injection process' efficiency.
This article has provided an in-depth overview of a specific type of hot manifold system – the valve gate system. This type of gate works by using mechanical shut-off pins to regulate the flow of plastic into the mold cavity and thus allows the hot runner nozzle to open and close at the tip. In summary, valve gate nozzles can enhance component quality, increase productivity, and provide superior gate quality. Though they can cost more to install and purchase, it’s better to use a valve gate in situations where part geometry or applications require it, as they give more quality control and offer greater versatility.
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