Injection molding resin preparation requires four separate calculations: the mass of each blend component, the water removed during drying, the working inventory in the drying hopper, and the material turnover through the barrel. Each calculation needs its own inputs. A correct blend ratio does not establish acceptable moisture, and sufficient hopper inventory does not establish acceptable barrel residence time.
Start with the approved resin grade, regrind allowance, masterbatch instructions, and processing requirements. Use mass balances and throughput calculations to plan the job, then compare the results with the material supplier’s requirements and the actual equipment configuration.
Define the Blend Percentage Before Weighing Material
The denominator determines how much material to weigh. “20% regrind” can mean 20% of the finished blend or 20 parts of regrind added to 100 parts of virgin resin. Those recipes are not equivalent.
For a two-component blend in which regrind is specified as a percentage of the total:
[
m_r = M\frac{R}{100}
]
[
m_v = M-m_r
]
Where:
- (M) = total blend mass, kg or lb.
- (R) = regrind percentage of the total blend.
- (m_r) = regrind mass, in the same mass unit.
- (m_v) = virgin resin mass, in the same mass unit.
For an illustrative 100 kg batch containing 20% regrind by total blend mass, weigh 20 kg of regrind and 80 kg of virgin resin.
If the instruction instead calls for regrind at 20% of the virgin resin mass, adding 20 kg of regrind to 100 kg of virgin resin produces:
[
\text{Regrind percentage}
=
100\frac{20}{100+20}
=
16.7%
]
Write the recipe as “20 wt% regrind in the final blend,” rather than leaving the percentage basis unstated.
Include Masterbatch in the Total When the Recipe Requires It
If regrind and fresh masterbatch are both specified as percentages of the final blend:
[
m_r=M\frac{R}{100}
]
[
m_b=M\frac{C}{100}
]
[
m_v=M-m_r-m_b
]
Here, (m_b) is the fresh masterbatch mass and (C) is its percentage of the final blend.
For an illustrative 100 kg batch containing 20 wt% regrind and 2 wt% fresh masterbatch:
| Component | Mass |
|---|---|
| Regrind | 20 kg |
| Fresh masterbatch | 2 kg |
| Virgin resin | 78 kg |
| Total | 100 kg |
This differs from adding masterbatch to an already weighed 100 kg virgin/regrind batch. In that case, the final batch exceeds 100 kg, and the original regrind percentage changes.
The calculation only establishes component masses. The allowable regrind content still depends on the approved material and part requirements. BASF’s Ultramid processing guidance, for example, identifies moisture uptake and damage from repeated processing as considerations when reusing ground material; a mathematically correct percentage does not establish its suitability.
Convert Masterbatch Instructions to an Explicit Mass Basis
Before calculating the dose, determine whether the supplier specifies:
- Masterbatch as a percentage of the final blend.
- Masterbatch added as a percentage of the base resin mass.
- A mass ratio with the components explicitly identified.
For masterbatch at (C)% of a final batch of mass (M):
[
m_b=M\frac{C}{100}
]
If a fixed mass of base resin, (m_{\text{base}}), is already available, the masterbatch needed to reach (C)% of the final blend is:
[
m_b=m_{\text{base}}\frac{C}{100-C}
]
For an addition rate of (A)% relative to the base resin:
[
m_b=m_{\text{base}}\frac{A}{100}
]
An illustrative 2 kg addition to 100 kg of base resin is a 2% addition rate, but its concentration in the 102 kg final blend is:
[
100\frac{2}{102}=1.96%
]
By contrast, making masterbatch exactly 2% of the final blend requires approximately 2.04 kg for every 100 kg of base resin.
For a stated base-resin-to-masterbatch mass ratio of (N:1):
[
C=\frac{100}{N+1}
]
Thus, 49 parts base resin to 1 part masterbatch gives 2 wt% masterbatch in the final blend. Do not enter a let-down ratio until its direction and component basis are clear.
Account for Colorant or Additive Already Present in Regrind
Fresh masterbatch percentage is not necessarily the same as total colorant or additive concentration. Regrind may already contain the active ingredient from its previous processing pass.
For a simple three-component mass balance:
[
x_f=\frac{m_vx_v+m_rx_r+m_bx_b}{M}
]
Where (x_f) is the active ingredient mass fraction in the final blend, and (x_v), (x_r), and (x_b) are its mass fractions in virgin resin, regrind, and fresh masterbatch. Enter fractions as decimals: 40 wt% becomes 0.40.
This equation assumes the active ingredient is retained without loss or reaction. It calculates nominal composition, not dispersion, shade, or functional performance. Use the approved formulation when the regrind composition is unknown rather than treating it as uncolored virgin resin.
Establish Moisture Requirements Before Calculating Water Removal
Obtain the preparation requirements for the exact resin grade and any masterbatch or additive being processed. Record:
- Maximum permitted moisture at processing and the applicable measurement method.
- Recommended drying temperature and time.
- Required drying-air conditions and dryer type.
- Any restrictions on prolonged drying, storage after drying, or re-drying.
Do not substitute a resin-family name for a grade-specific instruction. BASF’s Ultramid guidance states that excessive processing moisture can affect surface quality and mechanical properties and directs processors to the processing data sheets for detailed recommendations. Conair likewise notes that drying temperature and time vary with material type, grade, and manufacturer.
A dryer setpoint is an operating condition, not a measurement of pellet moisture. Check the material using the approved sampling and measurement procedure, with sampling representative of the material entering the process.
Keep Moisture Percent, PPM, and Mass Basis Consistent
For moisture expressed on the same mass basis:
[
\text{Moisture in ppm}
=
\text{Moisture in percent}\times10{,}000
]
Therefore:
- 0.30% = 3,000 ppm.
- 0.020% = 200 ppm.
These conversions do not change the denominator. Confirm whether moisture is reported on a wet basis or a dry basis:
[
w=\frac{m_{\text{water}}}
{m_{\text{dry solids}}+m_{\text{water}}}
]
[
u=\frac{m_{\text{water}}}{m_{\text{dry solids}}}
]
Here, (w) is wet-basis moisture fraction and (u) is dry-basis moisture fraction. Their relationship is:
[
u=\frac{w}{1-w}
\qquad
w=\frac{u}{1+u}
]
Use “dry solids” to mean all nonwater material, including polymer, filler, pigment, and additives. Do not combine measurements with different moisture bases without converting them.
Calculate Batch Mass Change by Conserving Dry Solids
For a batch with initial mass (M_i), initial wet-basis moisture fraction (w_i), and final wet-basis moisture fraction (w_f):
[
M_s=M_i(1-w_i)
]
[
M_f=\frac{M_s}{1-w_f}
]
[
m_{\text{water removed}}
=
M_i-M_f
=
M_i\frac{w_i-w_f}{1-w_f}
]
Where:
- (M_s) = dry-solids mass.
- (M_f) = final batch mass.
- All masses use the same unit.
- Moisture percentages are divided by 100 before substitution.
The calculation assumes dry-solids mass remains constant and water is the only material removed. It does not account for spilled pellets, lost fines, other volatiles, or decomposition.
Example: Reducing Moisture in a 100 kg Batch
Assume an illustrative batch starts at 100 kg and 0.30% wet-basis moisture. The calculation target is 0.020% wet-basis moisture; this is an example input, not a universal processing limit.
[
M_s=100(1-0.003)=99.700\text{ kg}
]
[
M_f=\frac{99.700}{1-0.0002}
\approx99.720\text{ kg}
]
The estimated water removed is approximately 0.280 kg, or 280 g. Approximately 20 g of water remains in the final batch.
Simply multiplying the initial batch mass by the difference in moisture fractions gives a close low-moisture approximation. The dry-solids balance is more exact because the final moisture percentage refers to the smaller final batch mass.
If the target moisture exceeds the initial moisture, the same equation produces a negative removal value, indicating a calculated water addition. That result is a mass balance—not an instruction to wet pellets before molding. BASF distinguishes drying resin before processing from conditioning molded polyamide parts afterward to develop their intended moisture-dependent properties.
The water-removal result cannot determine drying time, temperature, airflow, or dryer capacity. Those require material-specific drying requirements and equipment performance information.
Size Dryer Working Inventory From Sustained Consumption
Estimate how much material the machine consumes while cycling:
[
Q=m_{\text{shot}}\frac{3600}{t_c}
]
Where:
- (Q) = throughput, kg/h.
- (m_{\text{shot}}) = total material discharged per cycle, kg.
- (t_c) = complete cycle time, seconds.
Include all molded parts plus any cold runner and sprue discharged each cycle. Do not use single-part weight unless the mold produces one part and no additional material is discharged. In a hot-runner mold, retained manifold material is inventory, not material newly discharged every cycle.
For a required hopper residence time:
[
M_{\text{working}}=Qt_d
]
Where (M_{\text{working}}) is usable material inventory in kg and (t_d) is required drying residence time in hours. Conair uses throughput multiplied by drying time as a hopper-sizing step, while separately requiring selection of a suitable dryer and hopper.
For preliminary volume planning:
[
V_{\text{working}}
=
\frac{M_{\text{working}}}{\rho_{\text{bulk}}}
]
Use kg/L for bulk density to obtain liters. Bulk density is the mass of loose pellets or regrind per occupied volume, not the solid polymer density. Conair notes that pellet size and shape affect bulk density and that regrind bulk density can vary widely.
Example: Inventory for a 120 g Shot
Assume:
- Total shot mass: 0.120 kg.
- Complete cycle time: 30 seconds.
- Required drying residence time: 4 hours, supplied as an illustrative input.
- Bulk density of the material being dried: 0.60 kg/L.
Then:
[
Q=0.120\frac{3600}{30}=14.4\text{ kg/h}
]
[
M_{\text{working}}=14.4\times4=57.6\text{ kg}
]
[
V_{\text{working}}=\frac{57.6}{0.60}=96\text{ L}
]
The result is approximately 58 kg, or 96 L, of working material inventory. It is not automatically a recommendation for a nominal 96 L hopper. Confirm usable fill volume, operating level, material flow, and the dryer’s performance at the required conditions.
Apply throughput to the correct stream. If only virgin resin passes through this hopper, use virgin-resin consumption—not total blend consumption. If the complete blend is dried together, use complete-blend consumption and its measured bulk density.
Use sustained running consumption rather than a shift average diluted by downtime. At startup, provide the required drying exposure before consuming newly loaded material; adequate steady-state inventory does not eliminate the initial drying period.
Estimate Barrel Turnover Separately From Hopper Residence
Drying-hopper residence concerns material exposure to drying conditions. Barrel residence concerns material moving through the heated plasticating unit. The two calculations use different inventories and answer different questions.
For a steady-running barrel turnover estimate:
[
t_b=\frac{M_b}{Q}
]
Where:
- (t_b) = estimated turnover time, hours.
- (M_b) = estimated material inventory within the defined barrel boundary, kg.
- (Q) = material throughput through that boundary, kg/h.
Using shot mass and cycle time:
[
t_b=
\frac{M_b}{m_{\text{shot}}}
\frac{t_c}{60}
]
This form gives minutes when (t_c) is in seconds and both masses use the same unit.
For an illustrative barrel inventory of 0.72 kg, a 0.120 kg shot, and a 30-second cycle:
[
t_b=
\frac{0.72}{0.120}\frac{30}{60}
=
3.0\text{ min}
]
If cycle time rises to 45 seconds with inventory and shot mass unchanged, the estimate rises to 4.5 minutes.
Define What the Barrel Inventory Includes
Use an inventory estimate appropriate to the actual screw and barrel geometry, and identify whether it includes the nozzle. Do not substitute the machine’s maximum rated injection weight for total barrel inventory. Maximum injection capacity does not include all material along the screw channels, and a rating expressed as polystyrene-equivalent shot weight is not an inventory mass for another resin. Plastics Technology’s residence-time analysis specifically distinguishes injection-stroke calculations from the additional material conveyed along the screw.
The inventory-throughput calculation is an average turnover estimate. It does not identify the longest thermal exposure or establish how long every portion remains fully molten. Screw-channel material, mixing, and stagnant regions prevent all material from following one identical path.
Hot-runner inventory also needs separate consideration. Adding it to the barrel inventory can estimate combined average turnover only if the boundary and throughput are consistent; it does not reveal long residence in individual manifold branches.
Compare the estimate with the supplier’s guidance for the actual material, melt temperature, and plasticating unit. Covestro’s processing guidance relates residence time to material thermal sensitivity, screw selection, and metering stroke, and describes empirical assessment using an indicator such as color concentrate. Its published ranges apply to the materials discussed there, not to every injection molding resin.
Recalculate When the Recipe or Production Rate Changes
A preparation plan remains useful only while its inputs match production:
- A regrind change alters the recipe and may alter the colorant contribution, moisture condition, and bulk density.
- A shot-weight or cycle-time change alters both dryer consumption and barrel turnover.
- A hopper operating-level change alters available drying inventory, even if the hopper itself is unchanged.
- A production interruption adds thermal exposure to material already in the barrel and hot runner. The steady-state turnover equation does not describe that interruption.
Record component weights and percentage bases with the batch recipe. Keep moisture results, drying conditions, hopper inventory, and barrel-turnover assumptions as separate entries. For interruptions, use the material supplier’s and equipment manufacturer’s approved standby, shutdown, and restart procedures rather than treating a calculated average residence time as a permissible hold time.