Time to read: 7 min
With injection molding, achieving complete part formation is absolutely essential. A short shot defect occurs when molten plastic doesn’t fill the cavity in the mold completely, resulting in parts with missing sections. This not only affects aesthetic quality, but can significantly weaken the structural integrity, increase the chances of product failure, and subsequently lead to expensive rework or scrapped goods.
This article outlines the reasons for short shot errors, their detection during manufacturing, and the corrective measures that can be implemented, including preventive solutions.
What Is a Short Shot in Injection Molding?
A short shot is an injection molding defect that occurs during the molding process when there is insufficient flow of molten plastic material. This causes the mold cavity to be incompletely filled and prevents the component from being fully formed. Simply put, gaps result, often in the form of missing features or incomplete areas of a product.
Any type of thermoset or thermoplastic material can be prone to this defect, which is relatively easy to identify. Short shots are not subtle; they often leave visible scarring or gaps that render the component useless. This defect typically appears as thin edges or missing plastic at corners. They can often occur at the furthest point from the injection point (gate), or near weld lines where plastic material meets in the mold.
Common Causes of Short Shots
Inadequate Injection Pressure
The molten plastic will only reach certain areas of the mold if the injection pressure exceeds a specific limit. Molds with long flow paths and geometric complexity are more likely to have incomplete fills.
Insufficient Heating of Injected Material
Injection molding machine settings have to account for the properties of the chosen feed material and the mold design. A temperature that is set too low will result in poor plastic flow and, therefore, an increased likelihood of short shots.
High Viscosity of Injected Material
High-viscosity materials do not flow as freely. Such feed plastics may not be able to fill the most resistant spaces of large or complex molds. The mold design and injection parameters, including temperature and pressure must account for the material viscosity.
Poor Mold Venting
Poorly designed vents may not have any way to channel air out of certain mold passages. Because air is unable to escape, it becomes pressurized, creating resistance to the molten plastic filling the part and leading to incompletely filled parts.
Poorly Designed Gates and Runners
If gates and runners are too small or poorly positioned, they may restrict the plastic flow or cause uneven material distribution, resulting in short shots.
Dirt, Dust, and Other Contaminants
Moisture contained within the material and any contaminants on the surface can interfere with melt behavior, resulting in unpredictable flows and increased solidification.
Mold Flow Problems
Mold features with thin walls and long, narrow, sharp-cornered sections can significantly slow the flow rate, even under optimal processing conditions.
Preventing Short Shot Defects
Eliminating short shot flaws begins well in advance of the actual production phase, with thoughtful injection molding design and process sequencing.
Following DFM Guidelines
Adhering to DFM (Design for Manufacturing) standards and best practices while optimizing the geometry of your part and mold increases the odds of better flow of the material. This includes the avoidance of overly thin sections and the use of gradual rather than sharp corners, which helps in reducing the flow restriction.
Mold Flow Simulation and Analysis
Early in the design process, engineers can use mold flow simulations to identify and alleviate problems that would otherwise occur during mold filling. Through simulation, identification of flow path deficits, area identification for suspected short shot occurrence, and design alterations to improve fill uniformity can all be addressed.
Part Geometry Optimization
Maintaining a consistent wall thickness throughout a part helps ensure uniform flow resistance, which minimizes the risk of premature solidification. Similarly, designing rounded transitions and edges avoids melt stalling and aids complete filling.
Gate Placement Principles
Gates located at the proper positions ensure a balanced distribution of liquid plastic throughout the mold cavity. These gates and flow channels minimize the loss of flow momentum, thereby assisting proper filling and minimizing pressure loss.
Continuous Mold Upkeep and Examination
Regular inspections to clear vents, runners, and gates eliminate stagnant air and prevent build-ups or blockages. Proper mold maintenance achieves consistency and minimizes defect rates.
Material Selection, Pretreatment, and Storage
In addition to selecting a material with appropriate flow properties, such as viscosity, proper pre-process material handling, storage, and drying are essential to maintaining part quality.
Many plastic resins absorb significant amounts of moisture from the air. This not only affects the quality of the finished part but also the flow characteristics when being injected. If moisture contamination is suspected, warming procedures and drying times should be implemented to avoid unpredictable flow properties.
Identifying a Short Shot Defect
To reduce waste and prevent further complications, early detection of short shots is crucial. The good news is that such defects are usually quite obvious and predictable.
Visual Signs
A short shot will often reveal itself through missing contours, incomplete edges, and underfilled features. These might be described visually as rough or frayed sections, or blunt ends where the plastic fill did not complete the intended shape. Unlike cosmetic flaws, these defects compromise the function or structural integrity of the component.
Inspection Tools
In addition to visual and microscopic observation, several ergonomic aids can confirm and assess short shot defects. Calipers or CMMs, alongside other dimensional measurement devices can detect and quantify precise underfills and size discrepancies.
Production Patterns
Repeated short shots within a single mold cavity or a particular part feature often indicate a systemic problem. For example, inadequate venting, blocked flow channels, or variegated material characteristics. Documenting and analyzing process data and defect trends will help identify patterns. Doing so can lead to the efficient implementation of corrective measures far more effectively than making random adjustments or examinations.
How to Fix Short Shots in Injection Molding
With proper design for injection molding, short shots can often be mitigated on the manufacturing side through the following solutions:
Modify the Injection Rate or Mold Temperature
Check the injection flow rate, pressure, and temperature first. Adjusting these may help propel the melt deeper into the mold prior to cooling and solidification. Watch out for excessive improvement, as this may lead to other defects such as burn marks.
Modify Gate Design
Enhancing the plastic flow reduces the risk of premature hardening. Sometimes, modifying the gate is all that is needed to balance the fill by distributing the melt more evenly.
Refine Mold Venting
Blocked vents also lead to short shots due to stalling the fill. Enhancing mold venting or using vacuum venting improves filler flow as air is removed while the cavity fills.
Alter the Material Viscosity
If material flow is a problem, switching to a more flowable resin or adding flow enhancers can resolve filling issues without the need to change the mold.
Proper Material Drying
During injection, moisture causes resin vaporization, which leads to gas pockets. Use proper protocols and equipment to eliminate gas pockets during injection. This can be as wide ranging as using porous inserts …to adopting multi-stage injection pressure levels.
Related Injection Molding Defects
Although short shots remain a concern in injection molding, they are just one of several defects that can undermine part quality. Understanding where short shots sit among a hierarchy of other frequent issues can help teams in problem diagnosis and bolster their confidence in design work.
Weld Lines
Weld lines are flaws formed by the incomplete joining of two or more flow front divisions. They generally tend to encircle holes, ribs, or obstructive features. Unlike short shots, weld lines provide weak points instead of absent portions. Those poor flow conditions that are responsible for weld lines, such as a low melt temperature or insufficient injection pressure, may also lead to short shots.
Sink Marks
Sink marks are small depressions that occur on the surface of a part. These features often occur as a result of thicker sections cooling non-uniformly and shrinking more than the portions surrounding them. While not involving missing material like short shots, they both arise from poor thermal and flow control management in the molding process.
Voids
Voids refer to internal air pockets trapped within a part. Voids often remain unnoticed until some post-processing or testing is done on the part. They commonly arise due to trapped gas, inadequate packing pressure, and insufficient venting, factors that also lead to conditions of short shots. Although voids do not always affect the external appearance of the part, they can weaken its structural integrity and are comparable to short shots in load-bearing features.
Prevent Injection Molding Defects With Fictiv
The quality of the molded part, the efficiency of production, and the total costs will be affected drastically due to injection molding defects like short shots. Proactive design choices, precise mold flow analysis, and stringent process control are critical to curbing short shot defects in injection molding.
With Fictiv, you gain access to manufacturing experts and advanced tools designed for precision and efficiency in resolving mold-filling challenges. We help optimize your part design, material choice, and other crucial process parameters through DFM insights and moldability analysis. Ensure every shot taken counts by using our services to prevent incomplete parts.
Start a quote today, and see how we help you achieve consistent, high-quality production of plastic components at scale.
