The first thing to do when choosing and matching an injection molding machine is to look at injection volume. However, "maximum injection volume" listed on supplier's specifications may differ from actual weight you can produce by more than 30%.
This isn't because machine is overstated; it's because you haven't understood conversion relationship between material density and screw volume.
Having worked in cost accounting and external procurement pricing for many years, I've noticed a phenomenon: many people, when faced with product information, only look at clamping force when selecting an injection molding machine, completely ignoring injection volume. Result is either over-engineered and wasteful, or under-engineered and unreliable parts. Today, we'll discuss injection volume factor in injection molding machine selection principles in detail, helping you understand core logic of machine selection and matching.

 

Injection molding materials have two densities, but many people only look at one, which is root cause of incorrect injection volume calculations.
Solid density (ρs) is density of product after it has cooled and solidified at room temperature. It's used to calculate actual weight of finished product, which is density listed on material supplier's datasheet.
Melt density (ρm) is density of plastic when it melts at high temperature and is pushed by screw. It's used to match injection volume of injection molding machine.
Core relationship: Melt density = Solid density ÷ Expansion ratio
Image: Non-crystalline: PS/ABS/PC Expansion ratio 1.1～1.2 Crystalline: PP/PA/POM Expansion ratio 1.2～1.35
Why are both densities important?
Because "injection volume" on injection molding machine's parameter sheet refers to volume of melt that can be pushed out by screw's maximum forward stroke, in cm³ or g (PS equivalent). However, weight of product you're molding is solid weight. Product coming out of mold is solid after cooling and shrinkage, and its weight is calculated based on solid density.
Difference between melt volume and solid weight lies in this density conversion.
Conversely, check: Given product's solid weight → calculate required melt volume → check if injection molding machine's injection volume is sufficient.
Common material density comparison table, recommended to save:

�� Practical Reminder: Specific values are based on material supplier's TDS (Technical Data Sheet). Table above shows industry-standard reference values; different grades may vary.

Open parameter table of any injection molding machine, and you'll see data like this: Maximum Injection Volume: 171g (PS); Injection Volume: 188cm³; Screw Diameter: 40mm.
The key point here is: 200g is equivalent weight based on PS material.
Why use PS as benchmark?
Because PS was the first injection molding material to achieve large-scale industrial production, and industry has conventionally used PS as standard reference for injection volume. This is like using "horsepower" to measure engine power—it's an industry practice.
Relationship between Screw Diameter, Stroke, and Injection Volume
Injection Volume = π/4 × Screw Diameter² × Maximum Injection Stroke
Units: Volume cm³, Diameter cm, Stroke cm.
For example: Screw diameter 40mm = 4.0cm. Maximum injection stroke 15cm. Injection volume = 0.785 × 4.0² × 15 = 188.5cm³. Corresponding maximum injection volume for PS = 188.5 × 0.93 (PS melt density) ≈ 175g
�� Key points for confirming parameter table: Some manufacturers' parameter tables directly state "maximum injection volume 200g (PS)". Is this 200g calculated based on solid density or melt density? Different manufacturers have different standards. Most Japanese and European brands use melt density × volume for calculation, while some domestic brands use solid density. Confirm this point first when you get parameter table, otherwise all subsequent calculations will be wrong.

Conversion Principle: Volume of melt that screw can eject each time is fixed (determined by screw diameter and stroke), but different materials have different melt densities. Therefore, actual solid weight of same volume of melt will vary.
Conversion from PS to other materials: Target material maximum injection volume = PS maximum injection volume × (Target material melt density ÷ PS melt density)
That is: W_target = W_ps × (ρ_mtarget ÷ ρ_mPS)
Melt density of PS is taken as 0.93 g/cm^³.
Example calculations using same injection molding machine with a nominal capacity of 200g (PS):

✨ Key Conclusion: Same machine injects 21% less PP than PS, and 27% more POM than PS. If you directly apply nominal PS value without considering density, you'll either choose an oversized machine (wasting resources) or an undersized machine (not filling required volume).

A more common scenario in actual work is: I have a product, I know material and weight, what size machine should I choose?
STEP 01 Determine Total Product Weight
Total Weight = Product Net Weight + Runner/Gate Weight (Cold Runner)
Hot runner system runner weight ≈ 0, only product net weight needs to be considered.
STEP 02 Determine Safety Factor
Actual injection volume should be 30%-80% of machine's maximum injection volume.
Below 30% (too small) – Material Degradation
- Insufficient pre-plasticizing amount in barrel, resulting in excessive residence time of molten material in barrel.
- Repeated heating of material leads to molecular chain degradation, glass fiber breakage, and additive volatilization.
- Heat-sensitive materials (PVC, POM, flame retardant) are particularly prone to black spots, yellowing, and screw corrosion.
Above 80% (too large) – Unstable Process
- Pre-plasticizing amount is close to limit, screw stroke is too short, melting section length is insufficient, resulting in uneven plasticization.
- Insufficient cushioning, causing screw to "bottom out" during holding pressure stage.
- Injection begins before check ring has had time to reset, causing material backflow and large fluctuations in product weight.
- Difficulty in controlling position switching, leading to flash or short shots.
In short: Too small an injection volume → material is damaged in barrel; too large an injection volume → no operating window, process becomes extremely hot.
STEP 03 Calculate Equivalent Injection Volume of PS
Actual Selection: Three-Step Calculation
Step 1: Calculate Total Injection Volume
Total Injection Volume = Product Net Weight + Runner Weight + Cold Slug Well
If it's a multi-cavity mold, multiply by number of cavities.
Step 2: Convert to Machine Standard Value
Machine specifications are generally based on PS (polystyrene, density 1.05).
Equivalent PS Weight = Total Injection Volume × (Material Density / 1.05)
For example, a 100g PE (density 0.95) product has an equivalent PS weight of only 100 × 0.95 / 1.05 ≈ 90g.
Step 3: 30%-80% Back-calculation of Machine Range
Maximum injection volume of a suitable machine = Equivalent PS Weight / 0.3 ~ Equivalent PS Weight / 0.8
Example: Machine Selection for ABS Products
An ABS product with a net weight of 85g and a cold runner system runner weight of approximately 15g, how do you select a machine? Total weight = 85 + 15 = 100g
ABS melt density = 0.90 g/cm³
PS melt density = 0.93 g/cm³
Required PS equivalent injection volume = 100 × (0.93 ÷ 0.90) ÷ 0.6 = 100 × 1.033 ÷ 0.6 = 172g
Refer to machine parameter table: Select a machine with a nominal injection volume ≥ 200g (PS), corresponding to approximately 250T (depending on machine model).

 

If screw can be changed on same machine, changing to a screw of different diameter will change injection volume.
Injection Volume ∝ Screw Diameter²
If standard screw is 40mm:
Changing to a 35mm small screw: Injection volume change = (35/40)² = 0.77 (decrease of 23%)
Changing to a 45mm large screw: Injection volume change = (45/40)² = 1.27 (increase of 27%)

Screw diameter and injection pressure are inversely proportional: the smaller screw, the higher injection pressure.
�� Note when changing screws: After changing screw, injection volume and clamping force must be recalculated; do not rely solely on standard values in parameter table.

STEP 01 Check Density: Confirm solid density and melt density of product, based on material's TDS.
STEP 02 Calculate Weight: Net weight of product + runner waste = Total weight
STEP 03 Conversion and Machine Selection: Use PS equivalent formula to calculate backwards, multiply by safety margin, and select corresponding tonnage.
A quick formula to remember:
Machine PS injection volume ≥ Total product weight × 0.93 ÷ Material melt density × 1.67 (1.67 is reciprocal of 0.6 utilization rate)
Injection volume matching, in essence, boils down to understanding one relationship: how much melt volume machine can eject, and how heavy that melt will be after cooling.