Short Shot in Injection Molding: Causes and Prevention Measures

Injection molding is known for being one of the most precise and accurate manufacturing solutions available for the creation of plastic components. However, during large scale production, there are bound to be a few mishaps—particularly during high-volume manufacturing. One such defect is short shot. Short shot in injection molding, also known as short filling, happens when the molten material injected into the mold does not completely fill the cavity. This results in an incomplete part, which obviously impacts the quality and functionality of the component. Fortunately, there are several things manufacturers can do to prevent or address short shot. But first, let’s take a look at some of the most common causes and the impact of short shot in injection molding.

What Causes Short Shot in Injection Molding?

As with most defects that can occur during the injection molding process, the causes can sometimes be multi-faceted. In general, these causes include:

Insufficient Material Supply—One of the most straightforward causes of short shot in injection molding is an insufficient material supply. If the shot size, which is the amount of material injected into the mold, is set too low, there simply won't be enough material to fill the mold completely. 

Inadequate Injection Pressure—Injection pressure plays a crucial role in ensuring that molten plastic reaches all areas of the mold cavity. Insufficient injection pressure can prevent the material from fully filling the mold, leading to short shots. These pressure losses can occur due to long flow paths or restrictive mold designs.

Material Viscosity—The viscosity of the material being injected can also contribute to short shot in injection molding. Highly viscous materials are more challenging when it comes to flow and can lead to incomplete filling, especially in molds with complex geometries. Incorrect material temperature can exacerbate this issue by increasing viscosity, making it harder for the plastic to flow and fill the mold. 

Mold Design and Condition—The design and condition of the mold itself are significant factors in preventing short shots. Poor venting in the mold can lead to air traps, which block the material from filling the cavity completely. Complex mold geometries, thin walls, or deep cavities can also hinder the filling process. Another factor is the placement and size of gates, which are the entry points for molten plastic; these must be carefully designed to facilitate smooth flow. 

Injection Speed—The speed at which the material is injected into the mold can influence the occurrence of short shots. If the injection speed is too slow, the material may begin to solidify before the mold is completely filled, leading to incomplete parts. Optimizing the injection speed and balancing the cycle time, including the cooling phase, are critical to preventing this defect.

Machine Parameters—Various machine parameters, such as back pressure and screw speed, can affect the injection process and lead to short shots if not properly set. Incorrect back pressure settings during the plasticizing phase can impact the material's density and flow, while the speed at which the screw rotates and plasticizes the material must be precisely controlled to ensure consistent injection.

What are the Impacts of Short Shot?

Like most defects, short shot in injection molding can have a number of negative effects on the parts and the efficiency of the manufacturing process itself. For instance, short shots increase scrap rates and lead to material waste, which in turn raises production costs and reduces manufacturing efficiency. 

On top of that, addressing short shots often requires reworking parts and adjusting the production process, which can result in delays and higher labor costs. This places a significant burden on quality control processes, requiring more resources to inspect and identify defective parts.

Financially, short shots also escalate production costs due to increased material waste, labor for rework, and extended production times. Delivering defective parts to customers can certainly cause dissatisfaction and, naturally, potential loss of business.

How Manufacturers Prevent Short Shot in Injection Molding

Material Control

One of the primary ways manufacturers prevent short shot in injection molding is by ensuring that there is an adequate supply of molten plastic in the injection unit. Setting the correct shot size, which is the amount of material injected into the mold, is crucial for this reason. Regularly checking and maintaining material levels also helps avoid insufficient material supply.

Optimize Injection Parameters

Adjusting injection pressure to match the mold’s requirements is essential for achieving full cavity filling. Properly setting injection speed also ensures that the material flows into every part of the mold before it starts to solidify. Regular adjustments to these parameters help maintain optimal conditions for mold filling.

Mold Design Optimization

Good mold design and regular maintenance are critical for preventing short shots. Proper venting prevents air traps that can block material flow. Additionally, strategically placing and sizing gates ensures smooth material flow into the mold. Regular mold upkeep helps avoid issues that could lead to incomplete filling.

Temperature Management

Keeping the plastic material at the right temperature is important for maintaining its flowability. Ensuring that the mold and material are heated to appropriate temperatures prevents short shots caused by materials that are too thick or too viscous. Proper temperature control is essential for consistent material behavior.

Process Monitoring

Continuous monitoring of the injection molding process helps identify and address issues that could lead to short shots. Regularly checking and adjusting machine parameters like back pressure and screw speed ensures that the material is processed consistently and effectively. Implementing real-time process monitoring helps maintain quality and prevent defects.

Thogus Delivers Precision-Crafted Plastic Components

At Thogus, we specialize in mid- to high-volume injection molding for a range of applications. Our Ohio-based facilities are equipped with the latest in advanced manufacturing technology and automation. Between that, our decades of experience, and our team of seasoned experts, we deliver tailored manufacturing solutions to fit your specific needs. 

Get in touch with us today to get started on your next project.