Shot capacity, also known as injection capacity, is a crucial concept for anyone involved in injection molding. As we’ll cover, understanding shot capacity helps you avoid common issues like underfilling, poor process control, and material degradation.
In this complete guide, as a professional plastic injection molding manufacturer, we’ll start with the basics:
- What is shot capacity?
- Why shot capacity matters
- Calculating shot size
We’ll also dig into more advanced topics like:
- Shot capacity utilization
- Factors impacting product shot size
- Different injection unit concepts
Let’s dive in.
What is Shot Capacity in Injection Molding?
The shot capacity (or injection capacity) is the maximum amount of material that can be injected into a mold during one cycle.
Specifically, it refers to the total weight or volume of material that the injection screw can deliver per shot.
Shot capacity is typically measured in:
- Cubic centimeters (cc)
- Cubic inches (in3)
- Ounces (oz)
- Grams (g)
For example, an injection molding machine might have a shot capacity of 8 ounces or 266 grams.
This maximum shot weight/volume depends on factors like:
- The size of the injection screw
- Available hydraulic pressure
Now let’s look at why accurately calculating shot capacity matters.
Why Understanding Shot Capacity is Critical
Getting the shot size right is important to avoid several injection molding issues including:
1. Material Degradation
Small shot sizes (less than 20% barrel capacity) often result in long residence times.
Long residence times can degrade the polymer through overheating, oxidation, or even burning. This leads to:
- Discolored parts
- Weak parts
- Parts with odor
2. Underfilling
An inadequate shot size means that there’s not enough material to completely fill the mold.
This underfilling leaves an unfilled section inside the part, causing cosmetic issues.
3. Flashing
On the other hand, large shot sizes (over 65% barrel capacity) provide excess material.
The extra material seeps out of the mold, causing flashing – excess plastic attached to the finished part.
4. Poor Process Control
Both small and large shot sizes make it extremely difficult to control factors like temperature and pressure.
This results in inconsistencies between cycles and rejects.
Now let’s walk through how to calculate shot size for your application.
How to Calculate Shot Size in Injection Molding
Figuring out the optimal shot size involves taking into account:
- Feed system volumes
- Part volume
- Polymer shrinkage
- Impact of any additives
Here’s the basic formula:
Product/Mold Shot Size = Feed System Volume + Part Volume + Shrinkage
The main elements of the feed system are:
- Spruce: Attaches the runner system to the machine nozzle
- Runner: Delivers material from the spruce to the gates
- Gates: Control flow into the cavities
Let’s take a closer look at each factor:
Feed System Volume
The feed system includes the spruce, runners, and gates.
Make sure to calculate the volume of each feed system component based on their dimensions.
For example, a runner with a cross-section of 8 mm x 3 mm and length of 35 mm would have a volume of:
Volume = Cross Section x Length
= 8 mm x 3 mm x 35 mm = 840 mm3 (0.84 cc)
Tally up the volumes of the spruce, runners, and gates to get your overall feed system volume.
Part Volume
As you design or select a part, pay attention to the volume of the cavities.
For example, a cube shaped part that measures 25mm x 25mm x 25mm has a volume of:
Volume = Length x Width x Height
= 25 mm x 25 mm x 25 mm = 15,625 mm3 (15.65 cc)
Accurately determining the volume is critical, so work with your mold designer if needed.
Polymer Shrinkage
As we know, polymers shrink as they cool. Make sure to account for this reduction in volume.
For example, polypropylene (PP) shrinks around 2% as it solidifies.
So if our part has a volume of 15.65 cc, we’d expect it to shrink to:
15.65 cc x 0.02 = 0.313 cc
The amount of shrinkage depends heavily on the material. Work with your resin supplier to get an accurate number.
Impact of Additives
Fillers, reinforcements, colorants and other additives change the density and viscosity of the polymer.
Carefully take into account additives when determining shot size to avoid surprises!
Bringing It All Together
As an example, let’s say we need to figure out the shot size for a PP part with:
- Feed system volume: 25 cc
- Part volume: 15.65 cc
- 2% shrinkage: 0.313 cc
Our total shot size would be:
Shot Size = Feed System Volume + Part Volume + Shrinkage
= 25 cc + 15.65 cc + 0.313 cc = 40.96 cc
This number gives us our total shot size in cubic centimeters.
Next, we’ll look at shot capacity utilization – or how much of your barrel you should be filling.
Shot Capacity Utilization for Injection Molding
General guidelines suggest:
- Commodity resins (PP, PE): Use 20-80% of total shot capacity
- Engineering resins (nylon, ABS): Use 30-50% of total shot capacity
This avoids issues around long residence times or excess material in the barrel.
For example, let’s say our barrel has a total shot capacity of 100 cc.
For polypropylene, we’d want to keep our shot size between 20-80 cc.
And for ABS between 30-50 cc.
If you plan on running different materials, consider getting multiple barrel sizes to give yourself flexibility.
Now let’s look at some of the factors that determine how much material you can inject into the mold.
What Factors Impact Product Shot Size?
The product shot size depends heavily on:
1. Feed System Dimensions
The size and design of components like sprues, runners and gates control how much material can make it to the cavities.
Optimizing these paths means less material gets trapped in the feed system.
2. Number of Cavities
The more cavities in your mold, the more material required. Pay attention to how changes in cavity count impacts shot size.
3. Material Properties
As mentioned earlier, the specific resin chosen and any additives determine volumetric shrinkage and viscosity – both of which impact shot size calculations.
4. Packaging Requirements
If the application demands tight dimensional tolerances, you typically need to use more material to account for post-mold shrinkage.
Carefully factor in any requirements around precision or stability.
Understanding what goes into product shot size is the first step. Next, let’s explore the injection unit that prepares and delivers this shot.
Injection Unit Concepts for Delivering Shot Capacity
The injection unit is responsible for:
- Heating and melting the resin
- Mixing the polymer
- Delivering a consistent shot to the mold
This is difficult with polymers like fluoropolymers that have issues around:
- Thermal conductivity
- Specific heat
- Melt viscosity
There are a few common injection system configurations:
1. Single-Stage Plunger or Ram
Uses a plunger to inject material without pre-processing. Simple but inefficient for heating and mixing.
2. Dual-Stage Ram
Attempts to improve single-stage performance but still ineffective.
3. Screw with One Stage
Screw provides mixing/heating. Ram provides pressure for injection.
4. Screw/Ram Combo
Most advanced option. Optimized for both heating/mixing (screw) and injection pressure (ram).
Despite concept differences, injection units are rated on:
- Maximum injection pressure
- Available injection volume
Take these specs into account when selecting a system.
Now that you understand shot capacity basics, let’s recap some key takeaways.
Conclusion and Summary
Determining shot size and having the barrel capacity to deliver is critical for injection molding success.
To recap:
- Shot capacity is the max material per shot in cubic centimeters or other units
- Avoid issues with material degradation, flash and process control
- Carefully calculate feed system, part, and shrinkage volumes
- Typical shot capacity utilization is 20-80% for basic resins
I hope this guide gave you a solid overview of shot capacity in injection molding. Please let me know if you have any other questions!