Shot Weight Analysis：A Key Factor in Injection Molding Machine Selection

Shot weight refers to the total weight of plastic material actually injected into the mold during each injection molding cycle. Simply put, it is the weight of molten plastic actually injected by the machine during one molding cycle. This value is not only the weight of the product itself, but the total material required to fill the entire mold.

In general, cold runner molds, shot weight usually includes both the product and the entire runner system. Therefore, engineers must not only consider product weight when estimating, otherwise it may lead to miscalculation of capacity. Shot weight usually includes the following：

• Product weight
• Sprue weight
• Runner weight
• Gate weight

If a hot runner system is used, because the runner remains molten and does not eject with the product, only the product weight needs to be considered.

 

 

Further reading：Sprue, Runner, and Gate in Injection Molding: A Practical Guide

 

 

Maximum shot weight is the maximum theoretical melt weight an injection molding machine can deliver in a single injection cycle. It essentially represents the maximum plastic capacity that can be injected when the screw is at its maximum forward stroke. General injection molding machine specifications are typically marked using polystyrene as the standard material because of its stable flowability and long-term use as a capacity conversion reference. However, in practice, it must be noted that different plastic materials have different densities. Even if the injection volume is the same, the actual weight will vary significantly.

Material	Density (g/cm³)
Polypropylene	approx. 0.90
Polyethylene	approx. 0.95
ABS resin	approx. 1.05
Polycarbonate	approx. 1.20
Polyvinyl chloride	approx. 1.40

Under the same screw displacement volume, if polypropylene and polyvinyl chloride are processed on the same machine, the actual shot weight will differ significantly. Therefore, maximum shot weight is not a fixed value but changes with material density. This is also a key factor often overlooked by engineers when selecting injection molding machines.

 

 

In the field of injection molding, “ shot weight ”and“ shot capacity ”are often confused, but they are actually completely different concepts. Shot weight refers to material weight, while shot capacity refers to the volume displaced by the screw. The differences are as follows：

Item	Shot Weight	Shot Capacity
Definition	Actual weight of injected material	Volume or stroke of screw displacement
Unit	Grams, Ounces	Cubic centimeters, Millimeters
Influencing factors	Material density	Screw size and stroke
Nature	Weight concept	Volume concept
Purpose	Quality control	Machine capacity design

The same shot capacity corresponds to different actual weights depending on different plastic materials.

 

 

Many people consider shot weight to be only a material parameter, but in fact, it is closely related to overall molding quality. If not properly controlled, even if the mold and machine are normal, various molding defects may still occur, including short shot due to insufficient filling, flash caused by excessive pressure, dimensional instability leading to tolerance deviation, and inconsistent melt state affecting appearance and structural quality.

When the shot weight is insufficient, the melt cannot fill the cavity, resulting in short shots. Common phenomena include：

• Edge underfill
• Rib underfilling
• Incomplete hole formation
• Incomplete thin sections

This is especially obvious in thin-wall products and high flow-length ratio products.

If the shot volume is too large, cavity pressure may become abnormally high, causing melt to overflow from the parting line and form flash. In addition to affecting appearance, it may also cause：

• Increased post-processing cost
• Mold wear acceleration
• Dimensional defects
• Yield reduction

Therefore, a larger shot volume is not always safer.

A stable shot weight helps maintain：

• Consistent holding pressure
• Stable shrinkage rate
• Consistent product weight
• Stable dimensional tolerance

This is especially important for precision electronic parts, medical products, and automotive components.

If shot weight is not properly controlled, it may lead to：

• Overfilling
• Runner waste
• Unnecessary holding pressure
• Increased material consumption

This is especially significant when using expensive engineering plastics.

 

 

The most common calculation method for shot weight in injection molding is：

Shot weight = ( Product volume + Runner volume ) × Material density × ( 1 + Shrinkage rate ) × Packing compensation factor

Parameters	Description
Product volume	Volume of the finished part
Runner volume	Volume of runner and sprue
Material density	Plastic density
Shrinkage rate	Mold cooling shrinkage ratio
Packing compensation factor	Compensation during the holding pressure stage

Among them, the shrinkage rate and packing compensation factor are often ignored, but they have a very significant impact on actual molding, because plastics shrink in volume after cooling. If shrinkage is not considered, it may lead to insufficient filling.

 

 

Different runner systems directly affect shot weight calculation.

In a hot runner system, the runner remains heated and does not eject with the product. Therefore, shot weight calculation usually only considers the product itself. Its advantages include reducing material waste, reducing runner weight, improving overall material utilization, and further reducing actual shot weight.

In a cold runner system, all runner structures must be included in the shot weight calculation, including sprue, runner, and gate. Therefore, the overall shot volume is higher, material loss is greater, and manufacturing cost increases. However, its mold structure is relatively simpler, and the maintenance cost is lower.

 

 

Further reading：Hot Runner v.s. Cold Runner in Injection Molding: Engineering Comparison & Mold Design Guide

 

 

In the injection molding industry, there is an important rule stating that shot weight should be controlled within 20% to 80% of the machine's maximum capacity, known as the 20/80 rule. Its purpose is to avoid molding issues caused by too small or too large a capacity, such as excessive residence time or incomplete plasticization. Therefore, an appropriate shot capacity ratio should be maintained according to actual production conditions to ensure process stability and product quality consistency. Recommended shot capacity ratios are as follows：

Shot ratio	Status
Less than 20%	Capacity too large
20%–80%	Recommended range
More than 80%	Capacity insufficient

When the actual shot volume is less than 20% of the barrel capacity, plastic remains in the barrel for too long, which may lead to：

• Thermal degradation
• Plastic carbonization
• Black spots
• Yellowing
• Viscosity instability
• Process drift

This is especially significant for engineering plastics such as PVC, POM, PA, and PC.

If the shot volume exceeds 65% – 80% of the barrel capacity, it may cause：

• Melt inconsistency
• Incomplete plasticization
• Unmelted particles
• Slow screw recovery
• Unstable holding pressure
• Increased flash

Therefore, maximum shot weight does not represent the optimal working capacity. In actual production, a sufficient safety margin must still be reserved to ensure plasticization stability and molding quality.

 

 

Many people only consider material density at room temperature when calculating shot weight. However, injection molding processes molten plastic. After melting, materials expand in volume and decrease in density due to increased molecular spacing. Therefore, melt density is usually lower than room temperature density.

Material	Room temperature density	Melt density
Polystyrene	1.04	approx. 0.94
Polypropylene	0.90	approx. 0.74

This is why the same weight of polypropylene requires a larger injection volume in the molten state. Therefore, in precision machine selection, engineers usually prioritize melt density, actual melt volume, and screw plasticizing capacity rather than relying solely on theoretical weight.

 

 

When selecting an injection molding machine, in addition to clamping force, shot capacity should also be evaluated to ensure it meets product requirements. Comprehensive planning should be carried out based on material properties, product dimensions, and actual molding conditions to ensure process stability and product quality.

It must be confirmed whether shot capacity matches product requirements. If capacity is insufficient, it may cause short shot, incomplete filling, and insufficient holding pressure. Conversely, if capacity is too large, material may remain in the barrel for too long, leading to melt degradation, carbonization, or unstable plasticization. Therefore, proper shot capacity configuration is very important for molding quality and process stability.

Different plastic materials have differences in melting behavior, viscosity, shrinkage, and plasticization difficulty. Therefore, machine selection must also consider material properties. For example, semi-crystalline materials such as polypropylene, polyamide, and POM usually require larger safety capacity and more stable plasticizing ability to ensure uniform melt and stable molding.

In actual production, it is generally recommended to reserve about 10% additional safety margin to reduce the impact of non-return ring leakage, unstable plasticization, and process fluctuations. A proper safety factor not only improves production stability but also helps prevent quality defects or machine issues caused by excessive load.

If product size ranges are wide or future product specifications may vary, it is recommended to retain flexibility during machine selection, such as adopting dual-barrel configurations or different screw specifications, to improve adaptability to different products and materials, thereby increasing future production planning and equipment utilization flexibility.

 

 

Shot weight is a key parameter for injection molding machine selection and process stability. If shot volume does not match machine capacity, problems such as short shot, flash, melt degradation, and dimensional instability may occur. Therefore, when selecting an injection molding machine, it is not enough to only consider clamping force. It is also necessary to confirm whether the actual shot weight falls within a reasonable operating range, and to evaluate it together with material properties and mold requirements to ensure stable product quality and production efficiency.

 

 

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