The injection molding technique has taken the thermoplastic manufacturing sector to greater heights due to its ability to facilitate mass production in a fast manner while ensuring product quality. When using this manufacturing technique, the quality of the product is determined by the mold design, especially gate location. Gate location alters the balance and direction of flow of the polymer thus determining the filling times as well as temperature and pressure distribution.
Improper gate location leads to high shear stress, excessive warping, overpacking, poor weld lines, among other issues which affect product quality. This calls for optimal gate location as it is a major variable when it comes to improved parts quality and efficient injection molding.
In this guide, we will cover everything you need to know as far as gate location is concerned in injection molding. Read on to learn the best strategies to optimize your gate location!
Injection molding gate design (image source: pinterest)
Fundamentals of Gate Location
The gate is a vital component of any injection molding machine. Basically, a gate is an opening through which the molten polymer is forced into the cavity which is a representation of your final plastic product. This component works to regulate the pressure at which the molten material flows into the cavity.
In injection molding, there are different types, positions, dimensions and material for gates. These features affect different aspects of the injection molding process which in turn determines the quality of the end product.
For instance, gate shape affects the flow of the molten resin into the cavity, gate location affects the pressure of molten resin while gate type determines the temperature of the molten polymer. All these features when combined together determine the quality of the end product.
With different injection molding projects, gates can be positioned differently either strategically throughout the cavity or along mold separation lines. These gates can also have varying diameters to make them either narrow or wide for molten polymer flow control. When it comes to costs, the cost of gates is dependent on the complexity and number of gates in an injection mold.
Based on the design of the product, some injection molds allow the molten polymer to solidify outside the cavity thus protruding from the surface of the part being manufactured. This calls for a process known as de-gating; which is the process of removing the excess material from the part.
Guidelines for Proper Gate Placement
Molten polymers for injection molding have different properties in terms of the flow direction and cross-flow direction since they are anisotropic. Based on the use case of the final product, these property differences are critical in performance of the injection molded part. You need to consider the anisotropic nature of the molten polymers to determine the best gate location for the products you want to make.
Basically, you can estimate material by eye or flow analysis softwares. However, the latter is the best option of the two as it is more accurate. With high shrinkage grade injection molding materials, the part may shrink near the gate to cause gate packing in case of high molding stress at the gate. This makes it necessary to place gates near the heaviest cross sections to promote part packing and minimize sinks and voids.
In injection molding, some parts with handle-grip-like shape warp towards the gate side of the part. This warping can cause various challenges in the manufacturing process with defects on the end product being one of them. With this defect, gate location can come in handy to solve it. For instance, locating the gate towards the top of the part being molded will curb this issue.
You can also minimize warping in injection molded parts by placing two gates on the opposite side of the part although this technique also pauses a different challenge whereby the part may develop two knit lines. This issue can be resolved by adding flow channels which prevent formation of flow marks and knit lines.
To achieve optimal results with thin walled injection molds, you need to place two gates to properly fill the parts. However, the gates should be placed in a manner that makes the flow path as short as possible. The gate should also be placed strategically away from cores and pins in order to minimize obstruction of the flow path.
Considerations for Material and Part Design
Shrinkage is an unfortunate aspect of polymers, especially for injection molding manufacturing. This natural aspect affects all plastics as they solidify from liquid state. Nonetheless, each plastic material used for injection molding shrinks at a different rate from others.
Plastic mold sprue, runner and gate (image source: pinterest)
The shrinking rate of injection molding materials is dependent on polymer composition and material properties such as temperature, pressure and flow rate. For instance, semi-crystalline polymers like polypropylene have a higher shrinking rate as compared to amorphous polymers like PVC and polycarbonate.
The explanation behind this is the fact that semi crystalline polymers undergo a macromolecular rearrangement to form crystallite that is well arranged in structure. This leads to less space needed for the same atoms when the polymer is cooled down.
Other factors such as part design, wall weakness and mold constraints also affect shrinkage. While it is almost impossible to eliminate shrinkage completely in injection molding, this aspect can be reduced by optimizing the gate location to control the factors that enhance shrinkage.
Shrinkage greatly determines the positioning of the gate in an injection mold. For instance, the gate has to be positioned in a manner that it reduces shrinkage (gate pucker) while not compromising the part aesthetics and function.
Use of large gates and direct gates also reduce shrinkage. Shrinkage along vertical flow is large while shrinkage in the direction flow is small. In such a setup, it would be more prudent to place the gate across the vertical flow to reduce shrinkage. You can use a multi-point gate rather than a point gate to increase the cavity pressure and extend the flow time, which in turn reduces the shrinkage rate.
Techniques for Effective Degating
Injection molded parts require a finishing touch to ensure that the products are of the desired aesthetic appearance and quality. With injection molding, some products may have protruding and extruding plastic parts which need to be removed in the finishing process. The process of removing these parts caused by gates is what is called gate trimming or degating.
Degating is normally done either manually or automatically using a jig, a robot or an in-mold degating. The manual process of degating usually involves the operator removing or trimming the gate using a clutter. This method is advantageous as it serves as a quality control mechanism because the operators have to inspect the injection molded products one by one.
While this method is great for quality cross-checking, the fact that it is very time consuming, can lead to injuries and causes varying degating results, are major drawbacks. As a manufacturer, the injection molding process is time consuming and taking up further time to perform degating could lead to delays in mass production. The process could also turn out to be more costly as you will need several inspectors to degate the parts.
Automated degating is more efficient in injection molding especially for large scale processes. This method, as mentioned earlier, involves use of a jig and a robot to perform the task. Auto degating has several benefits which include 24/7 running, it is faster, less dangerous (to the operator) and more efficient.
In-mold degating is another form of automatic degating. It involves use of a controller powered degating system which is timer-based to allow for clean trimming. This degating technique does away with secondary processing and you can customize the cutting blade position and size based on the products being manufactured.
You should always place gates in a manner that allows for shorter flow path lengths. Shorter flow path lengths reduce the possibility of having flow marks which occur due to insufficient holding and mold pressure.
Advanced Gate Location Strategies
Gate location is a critical aspect for injection molding. Some gates can be located in tricky sections and this causes a challenge of segregating some areas as compared to others. Additionally, the molded plastic might also develop lines and other malformations based on how gates are closed. This calls for manufacturers to pay close attention to gate locations and the number of gates.
Fan gate in injection molding (image source: pinterest)
There are very many gate designs available for injection molding. These designs include sub gates, pinpoint gates, fan gates, edge gates, among others. All these types of gates are utilized based on the part being molded, resin flow rate, and desired product outcome.
That said, you can also use multiple gates in your injection mold. The number of gates required varies based on certain factors such as temperatures involved and the flow rate of the molten polymer. Basically, using many gates helps with eliminating issues that may arise from incorrect gate size and location as they facilitate proper filling of parts being molded.
With pinpoint gates for instance, you can employ the divide and conquer rule to place gates at several locations. This setup allows for the flowing resin to be divided between the gates. The multiple gate locations lead to shorter resin flow and ensure that the cavity is filled symmetrically.
When it comes to wall thickness, a wall should not be too thin as compared to adjacent walls as this inconsistency in thickness might cause warping. However, some products necessitate thin wall molding in which the gate location plays a crucial role. You should place the gate strategically to minimize the material flow distance and facilitate uniform filling of the cavity.
The gate should also be designed to minimize gate marks on the part. Adding flow channels will also help to minimize sinks and marks on the part surface.
Gate Size and Shape Consideration
Gate size and gate shape are significant in an injection molding project. The two play very critical roles with gate size promoting proper shearing when the mold paseses through the machine. Small gates are associated with high shearing rates and large gates the vice versa.
Normally, gate size is determined by the volume of the cavity and depth is determined by the size of the wall. Nonetheless, the type of plastic is also a factor because different materials have varying flow rates.
Geometry, or rather shape of the gate allows for proper filling of the cavity. Part size and performance determines the choice of gate shape. For instance, cashew-shaped gates are commonly used for parts that require the gate location to be behind or below the show surface. Tunnel gates on the other hand are used when injection molding products that need to be aesthetically pleasing.
Types of injection mold gates (image source: pinterest)
Getting the right balance between gate size and gate shape is crucial in injection molding. This right balance will not only lead to faster production times but also result in defects-free surfaces thus enhancing products appearance.
Conclusion
Injection mold gate location is crucial in ensuring efficient and successful injection molding processes. Optimizing your gate location helps curb defects such as warps, sinks and lines on the product. This further translates to better product quality and higher consumer adoption.
Gate location also has a massive impact on production costs and cycle time thus optimizing it is crucial if you aim to lower costs and minimize cycle times. To optimize your gate location, you will need to consider factors such as desired product output, polymer utilized in the process, temperature and pressure involved, and need for degating.
