Calculate virgin resin requirements by multiplying the gross feed eligible for regrind by the virgin fraction of the approved blend. Add any material that must run as virgin only, then subtract available virgin inventory to determine the net purchase requirement.
For a two-component blend:
[
V = G(1-r)
]
where:
- (V) = virgin resin required, in kg or lb
- (G) = gross feed to be supplied as the approved blend, in the same mass unit
- (r) = regrind mass divided by total blend mass, expressed as a decimal
A 20% regrind blend therefore contains 20% regrind and 80% virgin resin by mass. The calculation estimates material requirements; it does not establish whether that recipe is approved or whether enough usable regrind will be available.
Establish the Approved Percentage and Its Mass Basis
Use the regrind percentage authorized for the specific material, part, and application. Distinguish between a maximum permitted percentage and a fixed production recipe:
- A maximum of 20% establishes a ceiling. A lower percentage is acceptable only if the applicable recipe and process approvals permit it.
- A fixed 20% recipe requires that blend. Regrind shortages cannot automatically be covered by increasing the virgin fraction without approval.
Supplier guidance is not a universal authorization. For example, BASF’s Elastollan Processing Recommendations, page 6, makes regrind use conditional on product quality, matching material type and Shore hardness, and freedom from contamination. It excludes contaminated or degraded material and notes that specified quality requirements may prohibit regrind. Those conditions concern Elastollan TPU and do not establish a limit for other polymers or approve a particular part. _-_Processing_Recommendations)
Define the percentage on a total-blend mass basis:
[
r=\frac{R}{V+R}
]
Here, (R) is regrind mass and (V) is virgin mass.
Adding 20 kg of regrind to 100 kg of virgin resin produces a 120 kg blend containing approximately 16.7% regrind—not 20%. To make a 100 kg blend at 20% regrind, weigh 20 kg of regrind and 80 kg of virgin resin.
These equations assume virgin resin and regrind are the only separately metered components. If the recipe includes color concentrate or other additives, establish whether the approved regrind percentage is based on the resin portion or the complete formulation. Account for each component on that stated basis rather than treating all non-regrind material as virgin resin.
Calculate Gross Feed Before Applying the Regrind Share
Gross feed is the cumulative material fed through the process for the planned job. It is not just the mass of acceptable finished parts, and it is not necessarily the mass of newly purchased resin.
For a mold producing identical parts:
[
N_s=\left\lceil\frac{Q}{C(1-s)}\right\rceil
]
[
G_p=\frac{N_s(Cw_p+w_r)}{1000}
]
where:
- (N_s) = estimated production shots, rounded up to a whole shot
- (Q) = required acceptable parts
- (C) = active cavities producing one part each per shot
- (s) = expected rejected fraction of molded parts, entered as a decimal
- (w_p) = resin weight of one molded part, in g
- (w_r) = total ejected sprue and runner weight per shot, in g
- (G_p) = gross production feed, in kg
The rejection adjustment assumes the stated average yield across the active cavities. It provides a planning quantity, not a guarantee that the job will produce exactly the required acceptable count.
Use actual molded-material weights when available. For insert-molded parts, exclude insert weight. Include the cold runner and sprue once per shot, not once per cavity. Material remaining inside a hot runner is not an ejected runner allowance for every shot; account for relevant filling and purging requirements separately.
Add startup, sampling, restart, and handling allowances only where they are not already included in the production estimate. Separate allowances that may use the approved blend from those that must use virgin resin:
[
G=G_p+G_b
]
[
V=G(1-r)+G_v
]
[
R=Gr
]
where (G_b) is additional feed permitted to use the blend and (G_v) is additional feed required to be virgin only. All masses must use the same unit.
Do not apply the regrind percentage to startup material that must be virgin only. Likewise, do not add a general scrap allowance for rejects already included through (s).
Worked Example: Production Blend Plus Virgin-Only Startup
Assume the following hypothetical job. The values illustrate the calculation, not a recommended recipe or reject rate.
| Planning input | Assumed value |
|---|---|
| Required acceptable parts | 12,000 |
| Active cavities | 4 |
| Resin weight per part | 24 g |
| Total cold runner and sprue per shot | 16 g |
| Expected production reject rate | 4% |
| Approved production regrind share | 20% of total blend mass |
| Additional virgin-only startup allowance | 20 kg |
| Additional blended-feed allowance | 0 kg |
| Separately metered additives | None |
Estimated production shots:
[
N_s=\frac{12{,}000}{4(1-0.04)}=3{,}125
]
Total ejected material per shot:
[
4(24)+16=112\text{ g}
]
Gross production feed:
[
G_p=\frac{3{,}125(112)}{1000}=350\text{ kg}
]
Production regrind requirement:
[
R=350(0.20)=70\text{ kg}
]
Total virgin requirement, including startup:
[
V=350(0.80)+20=300\text{ kg}
]
The job therefore requires an estimated 300 kg of virgin resin and 70 kg of usable regrind. Together, they supply 370 kg of cumulative feed. Only the 350 kg production portion uses the 20% regrind recipe.
Limit the Regrind Credit to Material That Can Actually Be Used
Before using the calculated virgin requirement for purchasing, confirm that the planned regrind quantity is eligible and available when needed. Count material only after accounting for applicable identification, release, drying, processing-history, and handling requirements.
If the recipe permits any regrind share up to an approved maximum:
[
R_{\text{use}}=\min(Gr_{\max},R_{\text{available}})
]
[
V=G-R_{\text{use}}+G_v
]
Here, (R_{\text{available}}) is the usable regrind allocated to this job, not the total unverified scrap inventory. This calculation assumes a lower regrind share is authorized.
In the example, if only 45 kg of eligible regrind is available, the production blend would contain approximately 12.9% regrind. Virgin demand would increase to:
[
V=350-45+20=325\text{ kg}
]
If the recipe instead requires a fixed 20%, the job has a 25 kg regrind shortage. Obtain additional approved material or authorize a recipe change before releasing that production plan.
Account for Same-Job Recovery Without Counting It Twice
Runners and rejected parts generated during production may contribute to the regrind supply, but only if their reuse is permitted and they can be prepared and returned in time.
For a same-job recovery plan:
- Provide enough opening regrind for production before recovered material becomes available.
- Credit only the usable recovered quantity after collection, grinding, screening, drying, and release, as applicable.
- Track any restrictions on repeated processing.
- Exclude material recovered too late to be used in this job.
Keep runner and reject mass in gross feed even when it will be recovered. Recovery changes the source of subsequent feed; it does not eliminate the material already fed through the process. Deducting recoverable scrap from gross feed and then applying a regrind share that already includes it would count the same benefit twice.
Convert Virgin Demand into a Purchase Quantity
The calculated virgin requirement is the job’s material demand, not necessarily the order quantity.
[
P_{\text{net}}=\max(0,V+H-I_v)
]
where:
- (P_{\text{net}}) = net virgin resin to purchase
- (V) = calculated virgin demand
- (H) = additional virgin purchasing reserve not already included in the job estimate
- (I_v) = usable virgin inventory allocated to this job, excluding material committed elsewhere
Use matching material grades and approved inventory. Do not subtract unrelated stock or material awaiting release.
For the 300 kg virgin requirement above, suppose 80 kg is on hand but 10 kg is committed to another job. Available inventory is 70 kg. With no additional reserve:
[
P_{\text{net}}=300-70=230\text{ kg}
]
If the purchasing unit is a 25 kg bag, order 10 bags, or 250 kg. The additional 20 kg results from package rounding, not from a change in calculated process consumption.
Record the gross-feed basis, approved blend, usable regrind allocation, virgin-only allowances, inventory deduction, and package rounding separately. These quantities make the estimate traceable and allow purchasing to revise the order when the production plan or available regrind changes.