How Scrap Assumptions Change an Injection Molding Quote？

There is the same 3D model, resin specification, and production volume given to two injection molders. But when the quotes come, there is a huge gap between the two. Why is this difference? It depends more on the molder’s scrap calculations than their individual profits¹.. This is how scrap assumptions change an injection molding quote.

Simply asking for scrap rate or looking for it on the quote sheet will not protect your budget.You need to know what the scrap assumption covers and its effect on the production cost of the one good part². In this article, we will discuss these scrap assumptions to understand how scrap assumptions change an injection molding quote.

Question	Answer
How scrap assumptions change an injection molding quote?	One quote might look cheaper when the molder has set their prices being more optimistic, i.e., excluding runner waste, less startup defect rates, and by assuming that all the scrap plastic will be reused.

What is “Scrap” in Injection Molding?

Leftover or molded plastic that molder can not sell but can be reused after recycling is called scrap in injection molding. Different types of scrap in injection molding are:

Defective Finished Parts

During manufacturing, some of the plastic items do not meet the set quality standards. These are defective finished parts. Defects can be because of incomplete filling, surface depressions, etc.

Startup and Setup Waste

Time and material that goes into scrap while adjusting the temperature, cycle speed, pressure, etc. to get the best-quality production.

Leftover Melt and Mold Plastic

Molten plastic or excess solid in scrap parts, nozzle, barrel, etc. during injection molding.

Color and Material Changeover Waste

The non-acceptable parts generated in the time when the machine switches from one color or material to another.

Returned and Rejected Parts by Customer

If the components are below quality standards or do not meet customer expectations, there can be rejection or return of it.

How Scrap Assumptions Change an Injection Molding Quote?

What, according to you, is raw material cost? Is it simply this equation:

Part Weight × Resin Price \= Material Cost

Well, this is incomplete. In actual it is the total consumption of the material that is used to produce a single usable component. In simple terms, it goes as per this flow:

Material Required ⟶ Material Consumed ⟶ Good Parts Produced ⟶ Effective Material Cost per Good Part

Let’s say a four-cavity mold produces four 50 g parts. Here the finished parts will be 200 g per cycle. But when the runner and sprue add 40 g, then the machine would process 240 g per cycle. If some of the produced parts had to be scrapped, the amount of plastic used per part would increase to ensure that the demanded quantity is met.

So, this is the reason why buyers should focus on cost per good part, instead of quoted scrap percentage.

The Real Cost of Scrap: From Part Weight to Final Quote

While evaluating the injection molding quote, the common confusion is between part weight and that of actual material consumption. If you see part weight, it is the final plastic that you receive. Whereas if you see material consumption, it has everything that is used to mold it.

Finished Part Weight

The final weight of the fully trimmed non defective single plastic component that is ready to be shipped to you.

Total Shot Weight

It is the weight of the liquid plastic that is injected into the mold in one cycle – finished parts + leftover feed channels (sprues and runners).

Scrap & Process Losses

All the material that is used apart from the primary molding cycle is counted. Setup scrap, warm-up runs, color purges, and defect rejects all are there in this injection molding material scrap.

Real Material per Good Part

For one good part you pay for all the total plastic used in its manufacturing – leftover runners + waste.

Why Two Suppliers Can Use Different Scrap Assumptions

When you see different injection molding scrap rates from two suppliers, it doesn’t mean that only one of them is right. You need to understand their injection molding quotation calculation, only then you will understand how scrap assumptions change an injection molding quote. From technical variables to equipment choices, and quality standards, there are a lot behind those numbers.

Let’s understand these variables.

Different mold and runner designs

If a cold runner is in use, you will pay for all the leftover plastic sticks (runners) every cycle. There is no plastic waste if a hot runner is in use. But in this case mold cost is higher.

Different cavity counts

An 8 cavity mold needs a bigger network of plastic channels in comparison to 2 cavity mold. Moreover, leftover plastic also changes according to mold size. In small networks there is more waste per part. But on the other hand, in larger networks waste spreads across more parts.

Different production volumes

Every time when a machine starts up, it needs time tto warm up the press and clear the barrel. During this time wastage is there. For example, in a big order of 100,000 parts, if there is a loss of 15 kg of plastic, it’s like a drop in a bucket. What if you break that into fifty 2,000-part batches? That same startup waste will eat your margins .

Different process assumptions

Setting a low price with strict process assumptions is not always a savings. Suppose a custom injection molding supplier assumes there will be a “perfect run” with zero machine downtime and also near-zero scrap. But when the cycle runs in real, disruptions occur and it increases your production costs.

Different regrind policies

Reusing leftover plastic can sound like a cost saver. But the cost and quality of injection molding regrind depends more on molders, like how they handle the regrind process.

• There is a lower material price if there is full reuse of leftover plastic. But when there is re-melting of plastic too many times, it can weaken parts or can also cause visual defects.
• If there is zero regrind, it is because of 100%virgin plastic that offers maximum strength at a higher cost.
• For partial reuse, 80-20 ratio for virgin-to-regrind blend saves raw material cost without compromising on the performance.

Myth vs. Reality: What Scrap Assumptions Really Tell You

Here are the common myths regarding quotes and what is in reality.

Myth	Reality
Less quote means less scrap	Not necessary. Other assumptions such as shot weight, runner design, startup losses, or regrind treatment, can change the final cost.
Scrap only means rejected finished parts.	There can be production loss during startup, changeover, purging and other stages. (Source)
All runner material is waste.	It depends on the runner design, recovery method and material requirements. Cold-runner systems, for example, generate runner material that can potentially be reprocessed. (Source)
Regrind makes recovered material free.	There is value in recovered material, but it still has processing and application considerations. (Source)
A 0% scrap assumption means a highly efficient process.	It may simply mean certain losses are excluded or accounted for elsewhere.
Matching scrap percentages means two quotes are comparable.	Only if both suppliers define and calculate scrap in the same way.
The lowest unit price is always the cheapest option.	The lowest price may reflect different assumptions or exclusions.

How to Compare Scrap Assumptions Across Injection Molding Quotes

Are you comparing injection molding quotes just based on the final unit price?.

A lower price can be just because of optimistic scrap assumptions. This can result in extra costs or price changes after you place the purchase order. So to spot hidden costs and compare quotes fairly, you need to check the manufacturing assumptions behind each quote before placing the purchase order.

Ask the Supplier	Why it Matters
Finished part weight	Establishes baseline material content
Shot weight	Shows actual material used per cycle
Cavity count	Affects output per cycle
Runner type	Affects material recovery/waste
Assumed rejection rate	Shows expected process yield
Startup scrap	Identifies launch/run-up losses
Changeover losses	Important for multi-colour/material production
Regrind treatment	Shows how recovered material is accounted for
Resin price assumption	Prevents material-price differences from being confused with scrap differences
Included/excluded operations	Prevents hidden costs

Buyer’s Rule: If a supplier cannot or will not break down their scrap assumptions in writing. Then assume that the missing costs will resurface later as setup fees, scrap surcharges, or unit price hikes.

A Worked Example: Why the “Cheaper” Quote May Not Be Cheaper

Imagine two suppliers are quoting the same four-cavity part. The part weighs 45 g, and both suppliers are asked to deliver 100,000 good parts.

You will notice Supplier A looks cheaper because its quoted price is lower. But to understand the difference between two quotes, you need to look at the scrap assumptions.

Quotation Assumption	Supplier A	Supplier B
Quoted price per part	$0.58	$0.64
Part weight	45 g	45 g
Cavities	4	4
Shot weight	230 g	205 g
Runner system	Cold runner	Hot runner
Assumed rejection	1.5%	4%
Startup scrap	Not included	Included
Changeover losses	Not included	Included
Regrind	80% runner recovery assumed	No regrind assumed
Production volume	100,000 good parts	100,000 good parts

This is an Illustrative example only.

What does this tell the buyer?

Supplier A looks significantly cheaper ($0.58 vs. $0.64), but it also uses more optimistic assumptions. Supplier B’s higher price includes additional allowances for startup, changeover, and material losses.

One important point: don’t claim that Supplier A is actually more expensive overall unless you calculate the complete material and production cost. The purpose of this injection molding quote example is to show that the headline price and scrap percentage alone are insufficient for comparing quotes.

Buyer’s Checklist Before Approving an Injection Molding Quote

Here is your checklist to understand how scrap assumptions change an injection molding quote of the different suppliers.

Questions about material

• What resin grade is included in the quote?
• Which resin price did you use for this?
• Does the calculation include runner and sprue material?

Questions about scrap

• What is the assumed rejection rate?
• What is there in the stated scrap percentage?
• Do you include startup scrap?
• Does it cover purge and colour/material changeover losses?

Questions about tooling

• Cavity count?
• Hot runner or cold runner?
• What’s the runner/sprue weight per shot?
• Can runners be reground and reused?

Questions about production

• What production volume is the quote based on?
• Assumed cycle time and production yield?
• Are startup, purge, and setup scrap included in the cost?
• What happens if scrap rates exceed the estimate?

Higher Scrap Assumption May Actually Be the Better Quote – When?

A higher scrap allowance may be more realistic when:

• The part has strict appearance or size requirements.
• The material is difficult to process.
• The job requires frequent colour or material changes.
• Mould is new and to become stable for production it requires more adjustments.

Strict quality requirements can also increase scrap because parts that do not meet the required standards may need to be rejected.

The important question is not:

“Which supplier has the lowest scrap assumption?”

It is:

“Which supplier has the most realistic assumption for producing the required number of good parts?”

A realistic scrap allowance can make a quote look higher initially. But getting a more accurate estimate of the production cost is important to save costs later.

How to Level Injection Molding Quotes (Apples-to-Apples)

ere is the 5-point checklist we use to compare suppliers before picking one:

Step	What to Verify	Why Molders Hide Here
Batch size	Order quantities and production run sizes	Molders quote annual volume to look cheaper, but charge extra for smaller batch runs. .
Material Base	Same resin grade and current market price per kg	Molder using outdated prices or bad plastic, their low price nothing but an illusion.
Runner & Shot Mass	Part weight vs. total shot weight (runner system)	Cold runners waste plastic. Make sure you are not paying for heavy runner scrap on every cycle.
Scrap Baseline	Setup waste, purges, and expected yield percentages	“Zero-scrap” assumption will result in high billing later when actual defects come.
Landed Unit Cost	Total cost divided by accepted units	Tells you what a single usable part actually costs you to make, including all the hidden expenses.

The lowest quote is not usually the cheapest shop. it’s just the one hiding the most assumptions.

Final Takeaway

The scrap percentage number does not explain anything, unless you know what type of scrap waste is counted in it. So, never only go for the cheapest quote and end up getting low standard products or paying more later. Look for the details of the scrap assumptions to get the best and real price for good products.

For help with your injection molding project, CustomMoldHub can help you review your requirements and fulfill them at the best quotes.

FAQ

1. How can you calculate injection molding?

Material cost + Machine cost + Labor cost + Tooling cost + Scrap/waste cost + Other production costs

All these together gives you the injection molding cost calculation

2. What is shot size in injection molding? ?

In injection molding, shot size is the total plastic material that an injection molding machine prepares and injects into the mold during one molding cycle.

3. Is injection molding actually profitable?

Yes, but it depends more on volume and tooling recovery. It is more profitable for high-volume production because you can produce high-quality parts in large quantities at low per-unit cost.

4. Does using regrind actually lower your scrap costs?

Yes. It does so by converting rejected parts, sprues, and runners into reusable materials. The amount of regrind depends on certain factors like material, contamination, processing history, part requirements, and more.

---