Introduction
Injection molding holds the title for the most versatile manufacturing method, including for high volume and custom-made plastic parts. Considering its high precision and intricate abilities it’s no wonder injection molding is a favorite for many manufacturers.
Despite all of these accolades, common defects like short shots in injection still contend with its innovative contribution to production, sometimes giving manufacturers sleepless nights. Essentially, short shot is a situation where molten plastic fails to reach certain areas in the mold cavity especially corners and far ends of the molds leaving them void.
Nonetheless, following careful instructions like the ones laid out in this article will lessen the fears of short shorts in a production run leading to a flawless product. Ultimately, this guide addresses causes of short shots as well as effective strategies to prevent this defect. So, without further ado let’s dive in.
Definition of a Short Shot:
To begin with, let’s define short shots. As we’ve previously hinted, short shots occur with molten plastic flowing through the mold cavity but failing to completely fill the voids in the mold. Areas where complex geometries occur as well as the far end of the mold and difficult-to-reach areas like corners are often the most affected parts of plastic products. Also, thin-walled plastic often experiences short shots especially when the wall areas are distant from the gate. Additionally, non-uniformed wall thickness is another cause of this unpleasant injection mold defect.
However, several more reasons exist why there are short shots in injection molding. Limited plasticizing abilities of the molten plastic as well as inadequate shot calibration are some of these other reasons. Also, very viscous plastic materials which often solidify prematurely can lead to short shots in injection molding. Still, inadequate venting and degassing causing the trapping of air bubbles which eventually leads to voids can also be the cause of short shots.
Nevertheless, addressing this issue only involves the consideration of certain strategies such as opting for a less viscous plastic material. This can increase the fluidity of the molten plastic in the mold thereby avoiding defects like short shots.
Again, increasing the melting temperature of the molten plastic can increase the flow of the fluid within the mold. Additionally, design considerations that prioritize the smooth flow of molten plastic in the mold is another important strategy used to avoid short shots but more on these strategies later.
Nevertheless, the place of the injection molding machine in all of these cannot be taken lightly. For instance, the feeding rate of materials in an injection molding machine contributes significantly to the quality of the product. Short shots can arise where it is found to be inadequate and increasing this feed rate becomes crucial.
Now, let’s explore the primary causes of short shots in more detail.
Primary Causes of Short Shots:
Causes of short shots in injection molding are numerous and can either be primary or secondary causes or a combination of both. However, the primary causes are often the common ones which is why we feel the need to highlight them below.
1. Injection Pressure
The injection pressure refers to the force which pushes the molten plastic into the mold ensuring that it fills every void in the mold cavity including every corner and far-to-reach area in the mold. Now here’s the thing: both excessive and inadequate injection pressure contributes equally to short shots. In the case of inadequate injection pressure, the molten plastic usually lacks the force to propel it to reach the far ends of the mold cavity resulting in premature solidification before the mold is filled. On the other hand, excessive injection pressure results in extreme flow speed causing stresses and burrs in the mold which ultimately results in short shots. Hence, optimizing the injection pressure and taking cognizance of the flow rate of the plastic when in the molten state is crucial to avoiding short shots. Steps which include mold flow analysis with simulation provided by design software often come in handy in determining the optimal injection pressure in this case.
2. Injection Speed
Similar to the effects of injection pressure, the failure to optimize injection speed can also result in short shots. You can observe this situation when the injection speed is too low for the fluid to achieve the momentum required to reach every part of the mold cavity resulting in premature solidification and short shots. Also, excessive injection speed can cause short shots from the effects of the turbulent flow arising from the extreme speed. Hence, the need to optimize the injection speed to avoid irregular fluid flow within the mold. Again, it is advisable to utilize flow simulation software in analyzing the flow characteristics of the molten material to ascertain the right injection speed. Still, considering critical factors including the complexity of the part, materials type, and mold design can enable designers to specify a suitable injection speed that guarantees a quality product. Ultimately, carefully adjusting the injection speed to match vital production parameters can enhance the production process and ensure that defects like short shots are avoided.
3. Molten Plastic Temperature
The impact of temperature and the cooling time of molten plastic is significant and pivotal to the smooth operation of the injection molding process. Especially with filling the mold and solidification of plastic parts, the temperature can serve as the boundary between success and failure in any injection molding operation.
For instance, in the case where the temperature is excessively high, cooling the molten plastic becomes hard and takes longer to solidify. This situation can cause excessive shrinkage leaving voids in the plastic part.
On the other hand, a very low temperature can cause the molten material to solidify too early leaving a part of the mold cavity unfilled leading to short shots. Hence, the need to regulate mold temperature as well as the cooling period of the molten is crucial to facilitate a flawless product.
Again, inadequate fluidity of the material due to high viscosity and nature of the material can tell on the quality of the final product. This can also result in premature solidification of the material before getting to critical areas like grooves and the far end of the mold leaving the part with incomplete formation and short shots. In addition to selecting a plastic material with suitable fluidity – which is often the first point of action, adjusting the gate position of the mold can equally solve this challenge. This ensures that plastic material can flow to difficult-to-reach areas without hiccups.
Still, the use of temperature sensors, controllers, and thermocouples are more innovative way of solving the challenge of inappropriate temperature. It allows operators to observe the inconsistencies and adjust them to a suitable temperature in real-time.
4. Ventilation in the Mold
Equally important in the efforts to avoid short shots is the design and implementation of suitable mold ventilation. Implementing a suitable ventilation design helps to eliminate the possibility of trapped air bubbles in the molten material as it solidifies. These trapped air bubbles are one of the causes of short shots in injection molding as they disturb the free flow of the fluid in the mold leading to incomplete filling of the mold.
Pre-design analysis of the mold and simulation of the mold flow helps in designing and implementing suitable mold ventilation. Often, these vents are deep enough, allowing trapped air to escape while also disallowing the formation of other defects like burrs.
Another thing that can be done is employing devices like vent pins, slots, or inserts to enhance the ventilation system within the mold.
Furthermore, innovative ventilation techniques including corridor ventilation, standard ejector, and peripheral ventilation are sometimes the solution to inadequate ventilation that causes short shots.
However, a nuanced understanding of the mold peculiarities will inform mold designers on specific areas to put additional ventilation channels for adequate ventilation of the mold. This is advisable for areas experiencing short shots as it ensures the escape of air bubbles leading to complete mold filling and also preventing short shots.
5. Wall Thickness
Irregularities in wall thickness are yet another reason short shots exist in injection molding. This non-uniformity often leads to cooling disparities in different areas of the mold and consequently initiates short shots.
While uniform wall thickness enhances the quality of the plastic part, material wastage is also minimized resulting in a more cost-effective production. Additionally, the existence of thin-walled parts can reduce the flow of molten plastics in the mold with some of the hot runner systems being blocked.
Consequently, ensuring that the walls are not just uniform but also sufficiently thick enhances the flow of resin within the mold for complete mold filling.
For cold runner systems, it is best to ensure the runners are large enough to guarantee a smooth flow of resin and avoid constraints in the runners that lead to short shots.
In all, these considerations are critical to facilitate uninterrupted material flow while also mitigating short shots leading to high product quality and manufacturing efficiency.
6. Mold Design
While most people often think of design as just diagrams and dimensions, designing a suitable mold that helps to achieve manufacturing objectives of quality and precision is much more than drawing lines. Determining critical mold parameters including the gate size, shape, and location can significantly impact the formation of short shots. These parameters are vital for the easy and smooth flow of molten plastics in the mold.
For example, the gate’s function is to grant the resin entrance into the mold, hence, choosing the right size for the gate ensures that the right amount of fluid is able to pass quickly to completely fill the void. Also, ensuring a suitable gate location based on quality requirements and precision is critical. This will minimize trapped air and the risk of short shots. Collaboration between mold designers and molders can yield positive results especially as it relates to aesthetic requirements, flow rate, and part geometry. These parameters largely determine the suitability of a gate’s location. Avoiding complex features, thin walls, and sharp corners is important to prevent any hindrances with molten plastic flow.
Ultimately, the use of Design for Manufacturability (DFM) – a design concept, can add immense value to the injection molding process while also helping to avoid short shots in injection molding. The major advantages of this manufacturing concept are that it reduces the cost of production, increases production efficiency, and improves the quality of products.
Detection of Short Shots
After examining the major causes of short shots, the next point of call is knowing the various methods you can use to detect this defect during manufacturing. You basically want to ascertain if there are any cases of incomplete mold filling.
One way to identify short shots is through visual inspection. Assess the molded part to see if you’ll notice things like voids or sink marks which often indicate the presence of short shots. The signs often show that there has been an incomplete filling of the plastic in the mold.
Another way to detect this defect is by measuring key areas of the molded part to ascertain consistency. If the measurements fail to meet the specified dimension for that part especially where you have intricate designs or complex geometry, you may have a short shot right there.
Moreover, the presence of other defects like warping and sink marks can be an indication of short shots as these are often attributed to incomplete filling of the molds during injection molding.
More importantly, using innovative methods and technologies, including sensors and controllers helps to monitor pressure, temperature, and injection speed. It also helps to catch short shots in real-time with the opportunity to proffer the necessary solutions promptly.
Solutions and Preventative Measures
Identifying is just one step in ensuring the quality and accuracy of the injection molding process. One other important step in this process is proffering solutions to this anomaly or even better, preventing this defect from happening. Here are some important preventive measures and solutions you can apply to solve or avoid short shots entirely.
1. Adjusting Injection Parameters:
Injection parameters such as feed rate, temperature, and pressure are critical to the quality of plastic injection molding especially when it involves defects such as short shots. Thus, escaping from the horrors of short shots may only require adjusting these parameters to achieve the required precision and quality.
For example, the need to adjust the feed rate of the injection machine can come in handy once you realize the material is not getting fed into the machine fast enough to fill the mold completely. Also, too fast a speed can lead to the turbulence of the molten plastic in the mold and adjusting the speed appropriately can help minimize the possibility of short shots.
For the temperature and pressure, adjusting these two elements helps in the smooth flow of the fluid within the mold as too low or too high can be disastrous for the production process. The result of adequately adjusting them is a flawless final product devoid of any defects including short shots.
2. Improving Mold Design:
The success of the plastic production process or the lack of it often begins at the mold design stage which is why great care is often taken by mold designers to ensure that mold is suitable for the project. However, making every effort to improve this mold design is even more critical, especially with considerations that include quality, cost-effectiveness, and the efficiency of the whole process.
One such improvement that usually improves the mold design often deals with the size, dimensions, and locations of the mold gates. These parameters ensure that the resin quantity and rate of feed at any instance are adequate to avoid defects like short shots. Another important consideration in optimizing mold design and avoiding this flaw is proper ventilation of the mold. This can increase the chances of getting a flawless defect-free product at the end of the day.
Furthermore, ensuring that the runner systems are suitable and support the plastic injection projects is another vital factor in design improvement. All of these design elements contribute in no small ways to an optimal mold design that allows the easy and smooth flow of molten plastic within the mold while avoiding short shots.
3. Material Considerations
Right from selecting the material for the plastic injection project, you can set the process in motion to avoid anomalies in the plastic product, including short shots. Essentially, a suitable material will have parameters like viscosity and flow rates that are compatible with the product.
Moreover, the manufacturer has the responsibility of ensuring that these parameters are considered against factors such as mold wall thickness, geometrical complexity, and other intricacies in the mold design.
Consequently, partnering with a suitable resin manufacturer who can design the best resin mix for your project can be very helpful. In the end, using either stiff or light resin will largely depend on the mold design and the product specification.
Conclusion
In a nutshell, product quality is often the target of various defects including short shots in injection molding. Thankfully, there are several preventive methods that can help eradicate or at least significantly reduce the possibility of short shots.
For instance, critically examining the interplay between pressure, temperature, and resin feed rate can be of huge help in preventing this injection molding flaw. Also, material choices with their associated parameters including their viscosity and flow rate are critical to the quality of plastic injection molding and should be taken seriously during the design stage.
Nevertheless, identifying short shots’ root causes can come in handy, especially with the intention of adjusting the affected parameters in real-time. This will prove immensely helpful in dealing decisively with short shots. All in all, a proactive approach as against a reactive mindset is the best manufacturing practice to avoid defects like short shots. This will help arrest the root cause of short shots before it becomes unmanageable.
