Short Shots – Why They Happen and How to Fix Them

Injection molding defects can present significant challenges. Among the most definitive of these is the "short shot," a term for a part that has not been fully formed. Unlike cosmetic blemishes, a short shot results in a functionally incomplete and unusable product, representing a direct loss of material, cycle time, and production capacity. This defect occurs when molten plastic fails to fill the entire mold cavity, leaving voids or missing sections in the final piece. Understanding the causes of short shots, which can range from simple process adjustments to complex tooling issues, is the first step toward implementing effective, lasting solutions and ensuring every part is made to specification.

Read More About The Power of Hollow and Internal Channels in Medical Devices

What Is a Short Shot in Injection Molding?

A short shot is an incomplete molding. It manifests when the molten polymer solidifies before it has had a chance to fill all the spaces within the mold cavity. The appearance of a short shot can vary, from a part with slightly rounded or missing edges to one that is substantially incomplete, with large sections of its geometry absent. Regardless of its severity, a short shot is always a critical defect because the resulting component does not meet the required dimensional or functional specifications. This failure to completely form the part signals a fundamental problem with the material flow within the mold, which must be diagnosed and corrected.

For industries like medical injection molding and ISO 13485 injection molding, avoiding short shots is crucial for safety and regulatory compliance.

What Are the Primary Causes of Short Shots?

The root cause of a short shot is always that the plastic material stops flowing prematurely. This flow hesitation can be attributed to two main categories of problems: issues with processing parameters that create unfavorable flow conditions, or physical obstructions and design flaws within the mold tool itself.

Process Parameters

Often, the cause of a short shot can be found in the settings of the injection molding machine. If the plastic is too cold or is injected too slowly, its viscosity (resistance to flow) increases, causing it to solidify before it can reach the furthest extremities of the mold. A low mold temperature can also contribute significantly by causing the plastic to freeze upon contact with the cold cavity walls, halting its advance. Another common process error is simply an insufficient shot size, where the machine is not programmed to inject enough material to fill the entire volume of the part and its associated runner system.

For industries requiring injection molding quality control, optimizing process parameters is part of standard best practices.

Mold Design

If processing parameters are within the material’s recommended range, the issue may lie with the physical design of the mold. One of the most frequent culprits is inadequate venting. Molds are designed with tiny vents to allow the air inside the cavity to escape as the plastic enters. Without proper venting, trapped air gets compressed by the incoming plastic, creating back-pressure that can be strong enough to stop the material flow front, resulting in an incomplete part. Proper venting is essential because it allows the molten plastic to fill the mold completely and evenly, which leads to better-quality parts.

Similarly, the design of the runner system, the channels that guide the plastic from the machine nozzle to the part cavity, is vital. If the runners or the gates (the entry points into the cavity) are too narrow or restrictive, they can cause the plastic to cool and solidify prematurely. A poorly designed runner system hinders the mold from filling correctly and efficiently, which can interrupt the process and lead to inconsistent part quality.

This is where partnering with a design and manufacturing company that offers plastic injection mold design services and tooling solutions becomes critical.

Read More About Design for Minimal Post Processing | Save Time & Costs

How Can Short Shots Be Prevented and Corrected?

Correcting a short shot defect involves a systematic troubleshooting process, starting with the easiest and most common fixes before moving to more complex solutions.

1. Adjust Process Parameters: The first step is to review and modify the machine settings. This includes increasing the injection pressure, injection speed, or both, to help push the material into the far corners of the cavity. Raising the melt temperature or the mold temperature can lower the plastic's viscosity, improving its ability to flow. Finally, verifying and, if necessary, increasing the shot size ensures that enough material is being injected to begin with.
2. Inspect the Mold and Material: Check for any physical obstructions that could be blocking material flow. This includes ensuring the nozzle and all gates are clear of any solidified plastic from previous cycles. It is also wise to confirm that the plastic material is properly dried, as excess moisture can affect flow properties.
3. Evaluate Tooling for Flow Restrictions: If process adjustments do not solve the problem, the mold itself must be evaluated. Inspect the vents to ensure they are open and not clogged with residue. If the part consistently shows a short shot in the same location, it is a strong indicator that a vent may need to be added or enlarged in that area to let trapped air escape. If venting is sufficient, the runner system and gates may need to be analyzed. In some cases, they may need to be enlarged to allow for a less restrictive flow of material into the part cavity.

A design for injection molding (DfIM) approach during the initial design phase can help avoid many of these issues upfront. Additionally, working with a tooling company that offers precise tooling solutions can greatly reduce the risk of short shots during production.

Why Is a Systematic Approach Important for Solving Short Shots?

Short shots are a foundational defect in injection molding that can halt production and impact quality. Because the causes can be numerous, a systematic approach to troubleshooting is the most efficient path to a solution. By first examining the process and then the tool, manufacturers can resolve the issue without undertaking expensive tooling modifications unless necessary. The design and quality of the mold are fundamental to the success of the molding process. A well-designed and properly maintained mold allows a part to be molded consistently and accurately, without defects like short shots.

For low-volume injection molding and custom injection molding solutions, where production runs are small and part consistency is critical, addressing short shots quickly is essential to maintaining cost and timeline goals.

If short shots are plaguing your parts and causing frustration, don't hesitate to get in touch with us at Aprios. Our team has the experience and tools to ensure your parts come out complete and ready for use, every time.