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Short Shots in SolidWorks Plastics - 8 Common Problems and Their Solutions

Ricky Huynh

By HawkRidge Systems Engineering Team

 

What is a short shot in SolidWorks Plastics? In Injection Molding, a short shot occurs when a mold cavity does not completely fill with material.  In some cases, short shots are intentionally used to visualize how the flow front is progressing.  In this way, it can be viewed as an analysis tool.  However, short shot can occur unintentionally; usually, in locations where there is very thin geometry or when injection pressure of the mold machine is too low.  This will cause incomplete parts.  There are a variety of ways short shots can occur, as well as solutions. What does a short shot look like? Below is an example of a short shot in a real part.

SolidWorks Plastics - Short Shots - Lego Example

SolidWorks Plastics can help you visualize these issues during the part design phase to prevent costly rework and production stalls later on during manufacturing.

SolidWorks_Plastics_-_Short_Shots_-_2

There are many possible causes for a short shot; here we will look at some of the most common issues. These problems and solutions are from a variety of sources around the internet and in plastics design literature.

What conditions create a short shot and how do I avoid them?
Problem 1:  Lack of material due to empty hopper or blocked gates.
Solution:  Make sure to properly load the hopper with material and check gates, runners, and sprues to make sure that the previous shot’s material is properly evacuated.
Problem 2:  The material used is not reaching its proper injection temperature.
Solution: Make sure to check the manufacturer’s specifications for the given material to make sure it is being heated to the proper melt temperature.  Verify that the injection molding machine being used is rated for the material being used.
Problem 3:  The speed of the injection is too slow.
Solution:  A slow injection allows the flow front to cool prematurely.  By increasing the injection speed, the material will fill a larger percentage of the mold before cooling, but will also maintain a higher temperature longer due to viscous friction between the material and mold that creates additional heat.
Problem 4:  The injection pressure is too low.
Solution:  Increase the injection pressure so that the flow front is pushed through the entirety of the mold.  However, make sure not to exceed the machine’s maximum injection pressure as this can lead to damage in the machine’s injection system.
Problem 5:  Not enough gates for material injection or poor gate location.
Solution:  Ideally, gates should be positioned away from thin-walled areas so that the thick areas can be filled first.  If the gate location is too close to a thin-walled region, a condition called hesitation can occur.  This issue is caused when a flow front encounters both thick and thin geometry.  The material will have a tendency to fill the thick area and stagnate, or hesitate in the thin region.  This allows the material to cool prematurely.
Problem 6:  Thin geometry or areas of transition from thin to thick geometry
Solution:  In situations where the gates cannot be better positioned, increasing the thickness of the walls can alleviate the issue.  In cases where geometry and gates location cannot be modified, a less viscous material may be used.
Problem 7:  Mold temperature is too cold.
Solution:  Increasing the temperature of the mold walls will allow the material to flow more freely.  However, make sure avoid prolonged exposure to elevated temperatures as this can degrade the quality of some materials.
Problem 8:  Lack of mold vents to allow air to escape the mold.
Solution:  Add vents to the mold in areas where air traps occur.  These locations are typically where the material fills last.  Adding proper vents will allow the material to freely flow into these pockets without added resistance.
Short shots in injection molding are a fairly common issue.  With SolidWorks Plastics, you can analyze your mold for these issues during the part design phase to prevent costly rework and production stalls later on during manufacturing. In particular, SolidWorks Plastics can allow you to vary parameters to address Problems 2, 3, 4, 5, 6 and 7, and air traps can be reviewed in the tool to help review problem 8.   This will save you time, money, and decrease your time to market. If you’d like to find out more information about SolidWorks Plastics or request a demonstration, please give us a call at 877.266.4469 in the US or 866-587-6803 in Canada, or email us at info@hawkridgesys.com.
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