Injection molding is a popular manufacturing method for many industries because it is fast, efficient, and versatile. One important factor in successful injection molding is shot size.
It is referred to the amount of molten plastic that is injected into the mold and plays a big role in the success of the final product. If the shot size is too small, the molten plastic will not fill the entire mold and the final product will be misshapen or have voids. However, if it is too large, the molten plastic will overflow from the mold and cause defects in the final product.
Have you recently started using the plastic injection molding approach with no idea about the shot size and its importance? Here’s a comprehensive guide covering everything about this significant factor that can make or break the overall performance of injection molding.
A Detailed Introduction to the Shot Size In Injection Molding
In the plastic injection molding process, shot size is the amount of material that is injected into the mold during each cycle and is typically measured in grams or ounces. It is important to have the right shot size for your project in order to create a successful final product.
The shot size of an injection molded part is directly related to the part’s dimensions and weight. The larger the shot size, the larger the part. In general, it is best to use the smallest possible shot size that will still allow the part to meet all of its functional requirements.
The ideal shot size for an injection molded part is determined by a variety of factors, including the volume of the part, the density of the material, the wall thickness of the part, and the desired roughness of the surface finish. Too small of shot size will result in a part that is not filled out completely, while too large of a shot size will result in excessive material waste and potential damage to the mold.
The shot size can be specified in terms of either weight or volume. However, it is more common to specify the shot size in terms of weight, as the weight of the molten plastic material can be more easily controlled and monitored.
The size of the shot also has a direct impact on the cycle time for the injection molding process. Generally, increasing the shot size will increase the cycle time, as more material must be injected into the mold. For this reason, it is important to carefully consider the desired shot size when designing an injection molding process.
What Factors Affect the Shot Size in Injection Molding?
In order to select the right size for your project, you need to understand the factors that can affect it. Here, we will explore those pointers that will help you determine the best shot size for your injection molded project.
The Size of the Part
The size and volume of the part you are injection molding will have a direct effect on the amount of plastic required (i.e. the shot size). A general rule of thumb is that the shot size should be between 10% and 20% larger than the volume of the part. This allows for adequate flow of the molten plastic, while still allowing for enough pressure to fill all the cavities in the mold.
If your shot size is too small, you will not be able to fill all the cavities in your mold and will end up with an incomplete part. In addition, if your shot size is too small, you run the risk of flash, where thin strips of excess plastic come out around the edges of your part.
However, if your shot size is too large, you will end up wasting plastic. In addition, a large shot size can cause warpage and sink marks on your part as the plastic cools and shrinks unevenly. Hence, follow the rule and choose the appropriate shot size.
The Density of the Material
The density of the part being injected also affects shot size. A part with a higher density will require a larger shot size than a lighter part. This is because the molten material must fill the entire volume of the mold in order to create a solid part.
The denser the material, the more pressure is required to inject it into the mold. This means that a higher shot size is needed for denser materials. For example, a material with a density of 1 g/cm3 will require a shot size that is about twice as large as a material with a density of 0.5 g/cm3.
The Thickness of the Part
As the wall thickness of the part increases, the shot size must also increase to maintain a consistent weight. The thicker the walls of the part, the more plastic is required, and therefore the shot size must be larger. If the shot size is too small, the walls of the part will be thinner than desired and may not be structurally sound. Conversely, if the shot size is too large, there will be excessive plastic material and waste.
The Desired Roughness of the Surface Finish
When it comes to injection molding, the roughness of the surface finish can have a significant effect on the shot size. In general, a rougher surface finish will result in larger shot size, while a smoother surface finish will result in a smaller shot size.
There are several reasons why the roughness of the surface finish can impact the shot size. First, a rougher surface can cause the molten plastic to spread out more, resulting in a larger shot. Additionally, a rougher surface can trap more air bubbles, which can also lead to a larger shot.
The Injection Pressure
It is the force that is exerted by the molten plastic as it is injected into the mold cavity. The higher the injection pressure, the more force is exerted on the molten plastic, and the smaller the shot size will be.
The lower the injection pressure, the less force is exerted on the molten plastic, and the larger the shot size will be. The proper injection pressure must be matched to the shot size so that too much plastic is not injected into the cavity.
There are several other factors that affect the shot size in injection moldings, such as the type of resin being used, the size and shape of the mold, and the temperature of the molten plastic. By understanding how these factors influence the shot size, you can fine-tune the process to produce parts that meet your specifications.
What is the Short Shot Size?
If you are new to injection molding, you may have come across the term “short shot” and wondered what it meant. Here, we’ll explain what a short shot is in relation to injection molding and how it can affect your product.
A short shot occurs when the molten material does not fill the entire mold cavity. This can happen for a number of reasons, such as incorrect machine settings or poor material quality. Short shots can cause defects in the final product, so it is important to avoid them.
The terms “short shot” and “long shot” are used to describe how much material is being injected into the mold during each cycle. A short shot occurs when not enough material is injected to fill the entire cavity, resulting in a defective part. A “long shot” occurs when too much material is injected, resulting in excessive flashing or trimming. The ideal injection cycle would result in a “full shot”, where just enough material is injected to fill the cavity without any excess.
The 4 Major Causes of Short Shot Size in Injection Molding
The main causes of short shots in injection molding are typically attributed to either the injection process or the mold design. While Injection-related causes include incorrect set points for melt temperature, pressures, or screw speed, Mold-related reasons cover flash, inadequate venting, or a small runner system.
Here’s a quick rundown of the major causes that result in the short shot size in injection molding. Check them out here and keep them in mind to avoid the problem in the future.
Incorrect Mold Design
Inappropriate mold design is one of the most common causes of short shot size in injection molding. It can be caused by a number of factors, including incorrect tooling dimensions, poor material selection, and poor gate location.
Make sure that your tooling dimensions are correct to avoid having trouble with your injection molded parts.
Oversupply of Plastic
In the world of plastic injection molding, “short shot” refers to a condition in which the mold cavity is not completely filled with molten plastic. This can happen for a variety of reasons, but the most common cause is an oversupply of plastic.
When too much plastic is injected into the mold, it causes the pressure inside the cavity to rise too high. This can cause the plastic to flow back out of the nozzle, resulting in a short shot. Short shots are one of the most common problems in injection molding, and they can be very costly.
Inadequate venting can also cause short shots by trapping air in the molded part and preventing it from filling out. If the vents are not properly placed or sized, the air will not be able to escape from the mold cavity as the plastic is injected. This can cause the plastic to cool too quickly and prevent it from filling the cavity, thus resulting in short shots.
Unstable Production Style
It can be one of the major causes that trigger short shots in injection molding. They can cause defects in the final product, and lead to downtime and expensive repairs.
There are several factors that can cause an unstable production cycle. One is improper machine setup. If the machine is not set up correctly, it can cause problems with the injection process, which can lead to instability.
Another factor is poor raw material quality. If the raw materials are not of good quality, they can cause problems during the injection process, leading to the instability of the production cycle.
How to Avoid Short Shots in Injection Molding?
Since now you are covered with the basics of short shots, it’s time to understand the methods to fix the issue. Remember that short shots lead to several problems which further increase the cost of the overall project, hence, it should be avoided as much as possible. Here, we have mentioned some effective solutions to resolve the issue.
Make sure all vents are accurately placed
Proper venting is always the first place to start when troubleshooting this issue. By ensuring that your vents are properly placed and sized, you can help prevent this problem from happening in the first place.
Control the plastic supply
The best way to avoid short shots is to carefully control the amount of plastic that is injected into the mold. This can be done by adjusting the screw speed or setting a precise fill weight. If you are having trouble with short shots, your best bet is to consult with an experienced injection molder and get to know the amount you should add when carrying out the process.
Check the design of the mold
Make sure that your tooling dimensions are correct. The last thing you want is for your parts to be too small because your tooling was not properly sized. Also, examine your gate location from where the molten plastic enters the mold cavity. If it is not placed correctly, it can cause short shots.
The size of the shot has a direct impact on the quality of the final product. If the shot is too small, it will not fill the mold completely and there will be voids in the finished product. If the shot is too large, it will cause the material to flow too quickly and result in a poorly defined final product. The ideal shot size is one that fills the mold completely and slowly so that the material has time to cool and set properly. So, all in all, choose the right size shot for your project in order to avoid these problems.