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Molding Defect and Reject Log Worksheet

A defect log lets you see which defects, molds and presses cost the most. Enter each reject event and the sheet calculates the reject rate and scrap cost, so you can fix the expensive problems first and keep a record for customers.

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Date	Press	Mold	Defect	Parts inspected	Rejects	Reject rate	Cost per part	Scrap cost
						3%		$15
						1.2%		$9
						3%		$21
						1.2%		$12
						4.1%		$22

Total inspected

6,900

Total rejects

148

Total scrap cost

$79

Overall reject rate

2.1%

Weighted by quantity, not an average of row rates.

Average scrap cost per entry

$16

Use the cost per part that your finance team recognizes, usually material plus conversion cost up to the point of rejection. Sort by scrap cost, not by count, to pick the next problem to attack.

Connect rejects to ERP scrap postings

Optional: look at how defect reasons can flow into your ERP scrap and cost reports.

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How to use the log

Log each defect event with the date, press, mold, defect type and the counts of parts inspected and rejected. Enter the cost of a part at the point it was rejected. The sheet then calculates the reject rate for the row and the scrap cost. Review totals weekly and sort by scrap cost to see where money is lost, rather than where noise is greatest.

Use consistent defect names across shifts
Count what you inspected, not only what you rejected
Use a realistic cost per part
Review weekly with the setters and quality

Common molding defects and first checks

Short shots often point to insufficient shot size, low melt or mold temperature, restricted venting or a blocked gate. Flash suggests excessive pack pressure, worn parting lines or low clamp force. Sink marks come from thick sections with insufficient pack or cooling. Splay is commonly moisture or degraded resin. Black specks suggest contamination or carbon from over heated material. Treat these as first checks, then confirm with your own process data.

Short shot: shot size, melt temperature, venting, gate
Flash: pack pressure, clamp force, parting line wear
Sink: pack time, gate size, wall thickness
Splay: drying and residence time

Reading the numbers

The overall reject rate is calculated as total rejects divided by total inspected, so large inspection samples carry more weight. Be careful comparing rates across jobs with different inspection plans. A press with a low rate may simply be inspected less. Where possible, track the same defect category on the same mold over several weeks to see if a corrective action holds.

Common mistakes

Too many defect categories make reporting messy, and too few hide cause. Using Other for everything is the usual failure. Another mistake is logging only the worst shifts, which skews the picture. Finally, do not treat a high reject rate as a process failure until you confirm the inspection criteria are correct and stable.

Frequently Asked Questions

What is an acceptable reject rate in injection molding?

It varies widely by part and industry. Commodity parts may target well under one percent, while cosmetic or medical parts may have different expectations. Set your target against your own cost and customer requirements.

Should regrind and purge be counted?

Track them separately if they are significant. This log focuses on rejected parts. Purge and startup scrap can be logged with a defect type of Other and a note.

How is scrap cost calculated?

It multiplies rejects by the cost per part you enter. Choose material plus conversion cost, and add secondary operations if the part was already processed.

Can I add a root cause column?

Yes. Export to Excel and add columns for cause and corrective action, or use a separate 8D or corrective action report for serious events.

If your scrap and reject reasons are posted in an ERP, we can help connect them to this kind of log so one entry feeds both quality and cost reports.

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