What is Short Shot in Injection Molding?
Among the common injection molding defects is short shot. Usually, short shot phenomenon where injection molded components miss certain components. That is, there is an incomplete formation of parts during injection molding process.
It usually occurs as a result of incomplete filling of injected plastic material into the mold cavity.
Causes of Short Shot during Injection Molding
Less or Insufficient Venting – Less venting results into incomplete filling of the mold since the pressurized air inside the mold cavity restricts the flow of molten material into the mold cavity leading to short shot defect.
Once you have sufficient venting in the injection mold tooling, air will have room to escape. As a result, as material enters the mold, there will be “zero” resistance.
Mold Design – Problem with mold design such as improper wall thickness, complex geometries or inadequate draft angles can result in short shots. They can restrict the flow of plastic to all areas of the mold or create areas where the material is more likely to solidify prematurely.
Choice of the Material – The choice of material can impact the occurrence of short shots. Usually, the material to be used should be selected based on; its viscosity, degree of shrinkage, compatibility to the requirement of the process and the design complexity of the part.
Materials with high viscosity tend to flow more slowly hence they may end up not filling all the areas of the mold cavity therefore, resulting into short shot defect.
Inappropriate Injection Speed and Pressure – An increase in the injection pressure facilitates faster flow of the molten plastic material into the mold cavity and this can result into complete filling of the mold cavity as well.
Low Temperature of Mold Cavity – Low temperature of the mold cavity may interfere will the flow speed of molten material leading to incomplete filling of mold cavity which then result into short shots.
On the other hand, high temperature (recommended threshold) ensures free flow of material into the mold. As the molten material enters the mold, temperature will have no impact thereby hindering short shot formation.
Insufficient Cooling of the Mold – Usually this can lead to “early” solidification of injection molded components which ultimately causes short shot. Therefore, you must design the cooling channel properly. Again, there should be a reasonable allowance for cooling to take place.
Improper Monitoring and Control of the Injection Molding Process
Improper management of the injection molding process can result into: –
- inadequate process parameter settings such as injection speed, holding pressure etc.
- lack of mold cavity pressure control.
- Lack of process optimization
- Inadequate sensor placement
Signs of Injection Molding Short Shot Defects
Short shot can lead to various defects in the molded parts and they include;
· Flow Marks – These are visible marks that occurs due to the interruption of the plastic material flow. They occur on the surface of the molded part when the molten material does not completely fill the mold and can impact the surface quality and aesthetics of the part.
· Incomplete Filling – It leads to portions of the part being small or undersized which can further result into improper functionality of the product, dimensional inaccuracies and structural weaknesses.
· Brittle or Weak Areas – Short shots can create thin sections due to incomplete filling which can further result in brittle or weakened parts within the molded part.
These weakened parts compromise the durability, resistance to external force and strength to the molded part.
· Sink Marks – They occur in the form of dents or depressions especially on the thicker areas of the molded parts.
These areas with sink marks tend to cool more slowly due to their thickness as compared to other areas therefore, it can result into different rates of shrinking and contracting of inner parts and the outer part of the molded component.
· Warpage – It also occurs as a result of uneven cooling and shrinkage of the material and can affect the assembly, functionality and the overall performance of the molded part.
They often appear in the form of bends or unintended twists.
· Cosmetic Defects – Various cosmetic effects such as void, bubbles or air traps can occur in the molded parts.
These can negatively impact the surface smoothness, part’s appearance and even the overall quality.
Mold Design Consideration to avoid Short Shot Defects
Some of the considerations include;
· Selection of Appropriate Wall Thickness
When the mold walls has many uneven wall thickness, there will be hindrance in material flow. Due to uneven flow of material, there will be a high possibility of short shot injection molding.
Parts should be designed with uniform wall thickness to promote a balanced and consistent filling thereby preventing short shot defect.
· Gate Placement
Strategic positioning of the gate allows for balanced and complete filling which prevents short shot defects. The shape and the size of the gate should be considered to ensure adequate flow of the material.
· Proper Venting of the Mold Cavity
Proper venting allows air inside the mold cavity to escape, allowing for smooth flow of the molten plastic material into the mold cavity thereby leading to complete filling of the mold cavity. The vents should also be located in the rightful position to avoid creating flow restrictions.
· Part Design
In designing the mold, avoid sharp corners or complex geometries that could compromise the flow of material.
· Mold Surface Finish
The surface finish should be smooth and well-polished to help in reducing friction during injection and ensure proper flow of material.
· Monitor and Control Temperature
Always ensure the temperature of the material is right and the temperature of the mold cavity is high enough to facilitate smooth and faster flow of molten material into the cavity.
In case there is need, you can also incorporate special channels for cooling. With these channels, you can regulate temperature during injection molding process.
Detecting Short Shot in Injection Molding
It involves micrometers, coordinate measuring machines and calipers to measure the dimensions of the molded parts. They are then compared with the expected dimension specified in the design. It is indicated by a consistently smaller dimension to the expected measurement.
Scale balance is used to weigh the molded part. The measured weight is compared with the expected weight for the specified molded part. A consistently smaller weight indicates a short shot.
Involves checking for any visible defects such as underfilled areas, voids or incomplete fillings. Presence of some of these visible defects may indicate a short shot in injection molding.
You will use X-ray to examine the internal structure of injection molded components. This method reveals any incomplete filling or voids within the molded part which indicate the presence of short shot.
Uses a process monitoring system to track the injection process parameters such as injection time, injection pressure, velocity profiles, screw positions among others.
This helps identify the deviations and inconsistencies from the desired fill behavior hence indicate a short shot.
How to Minimize Short Shot Injection Molding Defects
Conduct Mold Flow Analysis
The injection molding process can be monitored and analyzed using mold flow analysis software. This will help in identifying potential defects such as short shots early enough and make necessary adjustments to the parameters r mold.
Regular Maintenance
Regular inspection and maintenance of the mold is required to ensure it remains in optimal condition. Always clean the mold thoroughly after every production runs to prevent any blockage or contamination that can affect the material flow and the overall result.
Appropriate Wall Thickness
Proper wall thickness throughout the part designs facilitates uniform flow of the material and prevent the occurrence of short shots.
Adjust Time and Holding Pressure
Holding time and pressure refers to the duration and the pressure applied after filling the mold cavity to compensate for shrinkage and ensure part quality. Therefore, adjustment of these parameters helps in improving the overall part filling and preventing the occurrence of short shots.
Proper and Adequate Venting
Adequate venting provisions in the mold can help in minimizing short shot defects. The vents will allow for easier release of trapped gases thereby allowing for complete filling of the mold.
Increase Temperature of the Mold
Optimal increase in the melt temperature can improve the flow of the material and minimize the chances of short shots occurring.
However, it is essential to balance the material degradation with the temperature increase to prevent the occurrence of any other negative effect on the part quality and short shot as well.
Optimize Gate Design
It ensures smooth and evenly material flow throughout the cavity thus allowing for complete filling of the mold cavity and minimizing short shots. An optimized gate design considers factors such as gate shape, thickness, size and even the gate location.
Use Flow Enhancers or Flow Aids
It prevents the occurrence of short shots by improving the material flow characteristics. Usually, it reduces the friction between the mold and molten material thus ensuring that the material reaches all parts of the mold cavity, reducing short shot defects caused by incomplete filling.
Conduct Mold Flow Analysis
Involves the use of mold flow analysis software in identifying potential areas of concerns and optimize process parameters to minimize short shot before manufacturing process.