FMEA for Injection Molding Processes: A Practical Guide 2026

Building an FMEA for injection molding processes means walking every stage of the molding cycle, naming the ways each stage can go wrong, scoring the harm, the frequency and the chance your current controls catch it, then fixing the rows that score worst. It is a workshop, not a form, and for a molding line it usually takes one day to build and a day a year to keep honest.

Most molding teams already have the raw material. What they lack is the structure that turns a pile of defect complaints into a ranked list of engineering changes. That is the gap this guide closes.

Table of Contents

• What You Need
• What FMEA Is and the Five Steps Behind It
• Step-by-Step: How to Create an FMEA for Injection Molding Processes
  • 1. Define the Injection Molding Process and Boundaries
  • 2. Build the Process Flow and Function List
  • 3. Identify Failure Modes and Effects
  • 4. Identify Causes and Prevention Controls
  • 5. Score Severity, Occurrence, and Detection for Molding Defects
  • 6. Calculate the RPN and Set Action Priorities
  • 7. Define Detection Measures and Corrective Actions
  • 8. Review and Revise the FMEA After Validation
• Common Mistakes
• Frequently Asked Questions
  • What are the 5 steps of the FMEA process?
  • What are the four types of FMEA?
  • What is the difference between a PFMEA and an FMEA?
  • Is FMEA lean or Six Sigma?
  • How often should an injection molding PFMEA be reviewed?
  • What is the difference between an FMEA and a control plan?
• Conclusion: Where to Start

What You Need

Gather these before you book the meeting. Missing one of them is the usual reason an FMEA workshop turns into an opinion session.

• A process flow diagram for the specific part. Not a generic molding cycle diagram. Number the steps the way your cell actually runs them, because that numbering has to match the Control Plan later.
• The current process parameter sheet. Barrel zone temperatures, melt temperature, screw speed, fill time, hold pressure, hold time, cooling time, clamp force, back pressure, cushion size.
• Twelve months of defect data. Scrap records, rework logs, customer complaints, the 8D reports you have closed, and the last-shot inspection notes.
• The Control Plan and the PFMEA or DFMEA it descends from. You are updating, not starting from a blank page, unless this is a brand new part.
• Material and equipment specifications. Resin data sheet, drying recommendation from the supplier, machine tonnage and shot capacity, mold construction notes, and the list of special characteristics on the drawing.
• A cross-functional team. A process engineer, a quality engineer, a production lead who runs the machine, a tooling or maintenance representative, and someone from design if the part has cosmetic or sealing faces.

Add capability data if you have it. Cpk or at least a scrap rate and defects per million for the last three months is what separates a defensible Occurrence rating from a guess.

What FMEA Is and the Five Steps Behind It

An FMEA is a preventive risk analysis. For each step of a process you document how it can fail, what that failure does, what causes it, what controls exist today, and how urgent the problem is. FMEA stands for Failure Mode and Effects Analysis, and the name now covers a whole family of documents rather than one specific sheet.

Type	What it analyses	When you use it on a molding part
Design FMEA (DFMEA)	The product design, before the tool is cut	Confirms the part can physically be molded at all, and flags geometry that guarantees sink or warpage
Process FMEA (PFMEA)	The manufacturing and assembly process	The core document for a molding cell, and the subject of this guide
Interface FMEA	Interfaces between parts or suppliers	Your mold versus the incoming insert, or the molded part versus the assembly fixture
Concept or Functional FMEA	Early design intent and function	Rarely used in a molding shop, mostly in product development

So the difference between a PFMEA and an FMEA is a matter of scope rather than method. An FMEA is the analysis. A PFMEA is the one that asks how your process can produce a part that does not meet specification.

The five steps of FMEA are the same whatever the industry:

1. Prepare. Assemble the team, the process flow and the historical data, and write down scope, assumptions and special characteristics.
2. Identify failure modes and effects. For every process step, ask what can go wrong and what the result is for the part, the tool, the equipment, the operator and the customer.
3. Identify causes and existing controls. For each failure mode, list the process parameters, material conditions, tool features and human actions that produce it, and the controls already in place.
4. Score and prioritize. Rate each failure mode for Severity, Occurrence and Detection from 1 to 10, then derive an Action Priority.
5. Act and re-score. Assign an action, an owner and a date to every high-priority row, implement it, and re-rate Occurrence and Detection to show the residual risk dropped.

Step 5 is the one that gets skipped, and it is the one an auditor opens first.

Step-by-Step: How to Create an FMEA for Injection Molding Processes

1. Define the Injection Molding Process and Boundaries

Write the scope block before anything else: part number and name, resin and grade, mold identification and cavity count, annual and peak volume, number of shifts, and the customer. State what is out of scope, such as incoming resin inspection or downstream assembly, so nobody argues about those rows later.

Mark the special characteristics on this part explicitly. A sink mark on a cosmetic housing is a visual defect. The same sink mark on a sealing face is a leak path and a customer line stop. Same geometry, different severity, and only the scope block tells you which one you wrote.

2. Build the Process Flow and Function List

Most injection molding PFMEAs key to eight or nine steps. Adapt the numbers to your cell, but keep this shape:

1. Resin receiving, incoming check and drying
2. Material preparation, blending, regrind ratio and feed to the machine
3. Mold warm-up, purge and first-shot setup
4. Fill and injection
5. Pack and hold
6. Cooling and mold temperature control
7. Ejection and part removal
8. Trim, cut and secondary handling
9. Final inspection, measurement, packing and labeling

For each step write one line of required function. The function is what the step must achieve, stated positively and measurably: dry resin below 0.02 percent moisture, hold cavity pressure at a minimum of 35 MPa, remove the part without ejector marking on the visible face.

A function written as a task name rather than a requirement gives you nothing to test later. “Dry material” is a task. “Deliver pellets to the throat below 0.02 percent moisture” is a function with a detection method attached.

3. Identify Failure Modes and Effects

At each step, ask what the step can produce that is wrong, and write the effect at two levels. The local effect is what the operator or the machine sees. The end effect is what the customer or the downstream process sees. A short shot locally looks like an incomplete fill; downstream it is an open circuit on an assembly that was never populated.

Use the vocabulary your operators already use, because that is the vocabulary the line will understand. Short shot, sink mark, weld line, jetting, flash, burn mark, silver streak, void, flow mark, witness line, ejector pin mark, insert shift. Generic wording like “part out of specification” is what makes a PFMEA unusable on the floor.

Here is the mapping from what you see to the row it belongs in and the parameter that causes it. This is the one-hop lookup that most troubleshooting guides skip.

Defect	Typical cause	Process parameter at fault	PFMEA step
Short shot	Trapped air, low melt or mold temperature, long flow path, weak hold	Melt temperature, fill pressure, venting, clamp force	Fill and injection
Sink marks	Thick section, insufficient packing, hot melt	Hold pressure, hold time, melt temperature, cooling time	Pack and hold
Warpage	Uneven cooling, unbalanced packing, ejection stress, material orientation	Mold temperature balance, cooling time, ejection speed	Cooling and ejection
Flash	Clamp force too low for cavity pressure, worn or damaged tooling, parting line mismatch	Clamp force, hold pressure, parting line condition	Mold tooling and fill
Burn mark	Poor venting, air compressed at the end of fill, high back pressure	Vent depth and position, back pressure, fill speed	Fill and injection
Weld line	Multiple flow fronts meeting around a hole or boss	Melt temperature, fill speed, gate location	Fill and injection
Silver streak or splay	Moisture in the resin, trapped gas, degraded material	Drying temperature and time, melt temperature	Resin drying
Void	Air trapped at a thick section, regrind contamination	Hold pressure profile, back pressure, regrind ratio	Pack and hold
Flow or weld-mark streaks	Melt temperature too low, cold slug, poor plastication	Melt temperature, screw speed, back pressure	Material preparation
Ejector pin mark	Ejection too early or too fast, pin imbalance, small draft	Ejection speed, cooling time, mold draft	Ejection and part removal
Insert shift or damage	Insert not retained, high injection force against the insert, no insert cooling	Fill pressure, packing force, insert temperature control	Fill and injection
Color streak	Poor pigment dispersion, low screw speed, excessive regrind	Screw speed, melt temperature, back pressure, regrind ratio	Material preparation

4. Identify Causes and Prevention Controls

For each failure mode, write every plausible cause you can act on, then attach the control that stops the cause from ever occurring. A prevention control changes the process so the failure never appears. A detection control finds it after it appears, before the part leaves the cell.

Molding has unusually strong prevention controls available, and most teams under-use them because they think in detection terms. Parameter lockouts on the machine, cavity pressure sensors, monitored purge and drying cycles, resin moisture verification per lot, and gated parameter recipes per cavity are all prevention. First-article inspection, in-process dimension sampling, and last-shot checks are detection.

Control	Type	What it moves
Machine parameter lockout with changeover password	Prevention	Occurrence
Resin moisture check per drying lot	Prevention	Occurrence
Monitored drying cycle with logged dew point	Prevention	Occurrence
Cavity pressure sensor monitoring	Prevention	Occurrence
Periodic parting line and vent inspection in the TPM plan	Prevention	Occurrence
First-article inspection at changeover	Detection	Detection
In-process dimension sampling, say every two hours	Detection	Detection
Last-shot check before the runner is cleaned	Detection	Detection
Automated vision or cavity monitoring	Detection	Detection

Cycle time is the real constraint in most shops, and it deserves an honest mention. Controls that add zero seconds come first: parameter lockouts, cavity pressure sensing, moisture verification, and TPM inspection intervals. Only after those are exhausted should you propose longer cooling time, extra cooling circuits, or 100 percent inspection, and when you do, say what the added seconds cost you in annual capacity.

5. Score Severity, Occurrence, and Detection for Molding Defects

Use the same three definitions every time, because inconsistent wording is the single biggest reason two engineers score the same row differently.

Severity is the worst harm the failure can cause if it reaches the customer, judged from the customer end, not from the convenience of your inspection. Occurrence is how often the cause is likely to appear under your current controls. Detection is how likely your current controls are to catch it before the part leaves the line, judged by the control that exists today, not the one you plan to add.

Score	Severity anchor for a molded part	Occurrence anchor (evidence required)	Detection anchor
1 to 2	No effect on function; a cosmetic mark on a hidden surface	No occurrences in process history; capable process with Cpk at or above 1.33 for the driving characteristic	Certain detection: 100 percent automated or gauged, proven capable
3 to 4	Visible cosmetic defect on a customer-facing surface; minor dimensional deviation inside tolerance	Very low rate; under 1 defect per million, or scrap well under 0.1 percent with stable capability data	Reliable detection: capable gauge, low sampling, 100 percent attribute check by a trained operator
5 to 6	Dimensional deviation near the tolerance limit, or a cosmetic defect on a surface the customer inspects	Moderate: scrap between 0.1 and 0.5 percent, or ppm in the hundreds, without stable capability evidence	Moderate: sampling inspection with an attribute gauge of limited capability
7 to 8	Loss of a function: sealing leak, insert loose, part out of tolerance at the customer	High: scrap 0.5 to 2 percent, or repeated customer complaints on the same defect	Low: visual only, infrequent sampling, or a control with known poor detection
9 to 10	Safety or regulatory exposure, or a defect that stops the customer line with no containment available	Very high: scrap above 2 percent, or the failure recurs routinely	No effective control, or detection only after the part reaches the customer

Two hard rules keep the ratings honest. Sampling inspection never earns a Detection score of 3 or better, because a sample cannot reliably catch every part. An Occurrence score of 2 or 3 requires capability data on the record, not a verbal assurance. Those two rules alone stop most over-scoring.

6. Calculate the RPN and Set Action Priorities

The Risk Priority Number is Severity multiplied by Occurrence multiplied by Detection, giving a value from 1 to 1000. Older AIAG guidance treated an RPN above 100 as an automatic action trigger. That is a blunt instrument, and the more useful lesson is that a low RPN can still hide a serious problem: a severity of 9 paired with two low ratings produces a modest number.

AIAG-VDA replaced the RPN rule with the Action Priority table, which reads Severity, Occurrence and Detection together and returns High, Medium or Low. A High Action Priority obliges you to act and to identify a responsible owner, whether the RPN is large or not. Keep RPN in the sheet as a sorting aid if your customers expect it, but let Action Priority drive the work list.

7. Define Detection Measures and Corrective Actions

Every High and Medium row needs an action, an owner, a due date and a verification method. An action that cannot be verified is a wish. Prefer actions that change the process, and set residual Occurrence and Detection values that assume the action is complete, not values that repeat the original numbers.

Here is a worked injection molding PFMEA matrix for an automotive interior clip in glass-filled nylon, four cavities, single shift, with the critical characteristic being the retention clip dimension.

Step	Failure mode	Effect	Potential cause	Current controls	S	O	D	AP	Recommended action	Residual AP
2. Material preparation	Resin not dry, moisture above 0.02 percent	Silver streaks, splay, reduced strength on the clip	Drying time below specification, hopper lid not closed, dew point not controlled	Prevention: monitored drying cycle with logged dew point. Detection: moisture check per lot	5	4	5	M	Interlock the dryer with the machine so it cannot run outside the cycle; add a weekly hopper seal check to the TPM plan	L
2. Material preparation	Regrind ratio above 30 percent	Variable viscosity, inconsistent color, lower retention strength	Regrind not weighed at the crusher, batch mixing without records	Prevention: locked ratio set at the crusher with a keyed counter	6	5	7	H	Install a batch recipe per shift and record the actual regrind weight on the run sheet	M
3. Mold warm-up and first shot	First-article shot taken before the mold reaches temperature	Sink, flow marks and short shots recorded as the new standard	Cycle start timer set by the operator, no temperature interlock	Detection: visual first-article inspection	4	6	6	H	Block cycle start until zone temperature is inside the process window, enforced in the machine recipe	L
4. Fill and injection	Short shot in one of four cavities	Part not formed, cavity blocked, downstream assembly incomplete	Trapped air, cavity imbalance, melt temperature drift, low fill pressure	Prevention: venting inspection in the TPM plan. Detection: first-article and hourly last-shot check	8	5	7	H	Add cavity pressure monitoring with a per-cavity low-pressure alarm, and rebalance the runner	M
4. Fill and injection	Insert shift on the overmoulded feature	Part fails functional test, customer line stop	Insert retention insufficient, high fill force, insert thermal growth	Prevention: none today. Detection: functional test at final inspection	9	4	6	H	Add insert retention geometry and cycle the first tool through a thermal check; escalate to the tooling engineer	M
5. Pack and hold	Sink mark on the visible face	Cosmetic reject, customer complaint	Hold pressure drift, hold time shortened for cycle time, melt temperature high	Prevention: parameter lockout on the machine. Detection: hourly visual check against the boundary sample	4	6	4	H	Lock hold pressure and hold time in the recipe, and set a cosmetic boundary sample for hourly comparison	L
5. Pack and hold	Void inside a thick boss	Reduced strength, crack under assembly load	Hold pressure profile not staged, back pressure high, regrind contamination	Detection: no routine internal check	7	5	9	H	Stage the hold profile, then qualify with cut sections on the first ten production lots	M
6. Cooling and mold temperature	Warpage beyond the flatness limit	Part fails assembly, misaligned feature	Cooling circuit imbalance across cavities, cooling time cut to raise output	Prevention: none today. Detection: go/no-go flatness gauge every two hours	7	6	6	H	Balance the coolant circuits and log inlet and outlet temperature per cavity for one week	M
7. Ejection and removal	Ejector pin mark on the visible face	Cosmetic reject	Ejection speed set high, cooling time shortened, draft below 1 degree on the pin boss	Prevention: parameter lockout. Detection: hourly visual check	4	5	4	M	Lower ejection speed in the recipe and raise draft at the next tooling modification; no cycle time cost	L
9. Final inspection and packing	Label wrong or missing on the carton	Wrong part delivered to the customer line	Label verification is visual, part number changed at short notice	Detection: single visual check at packing, no scan	7	3	8	H	Scan-to-verify at packing and add a part number change gate requiring quality sign-off before the label is reprinted	L

Here is the reasoning behind the four rows an auditor will press hardest. This is the part most published examples leave out.

Short shot in one of four cavities. Severity 8 because an unformed clip stops an assembly operation downstream, with no field containment available. Occurrence 5 because the current scrap record shows roughly 0.3 percent on this family of defects, and there is no stable cavity-to-cavity capability data, so a lower score would need evidence that does not exist. Detection 7 because an hourly last-shot check samples roughly one part in several hundred, and cavity balance problems appear intermittently.

Sink mark on the visible face. Severity 4, not 8, because the clip is an interior component and the affected face is not a sealing surface. Rate the same sink mark at 8 or 9 if it lands on a weather seal and the severity changes completely. Occurrence 6 from a cosmetic reject rate between 0.5 and 1 percent on the shift log. Detection 4 because the boundary sample comparison at check frequency is a 100 percent visual on the inspected parts with a defined limit sample, which is stronger than a sampled dimensional check.

Void inside the thick boss. Severity 7, since the part may pass visual inspection and fail under assembly load. Occurrence 5 based on two recorded complaints in twelve months at this volume. Detection 9 because there is no routine internal check at all today, and finding this defect requires a cut section.

Wrong label at packing. Detection 8 because a single visual verification with no scan is exactly the kind of control that cannot earn a good score. Sampling rules apply here too, and the fix is a scan-to-verify system rather than a second visual check.

8. Review and Revise the FMEA After Validation

The first PFMEA is a hypothesis. Revise it after the validation runs, and then on any of these triggers: a new tool, a new cavity count, a resin or supplier change, a machine or software change that alters the parameter window, a customer complaint, a process deviation, an internal audit finding, or a new special characteristic on the drawing.

Set a calendar review as well. Annual is the usual minimum for a stable, high-volume part; quarterly for a new program still in launch, or for a part with a defect history that has not settled.

At each review, close out finished actions, update residual ratings with real post-implementation data, add rows for any new failure mode found, and record what changed and why. Version the document, keep the previous one, and note the change in the header. An FMEA with no visible history looks like a template someone filled in last week.

Common Mistakes

Most weak injection molding FMEAs fail for the same handful of reasons, and all of them are visible in an audit.

Mistake	Why it fails an audit	Fix
Generic failure modes like “part out of specification”	No technician can act on it, and no control can be designed against it	Use the defect name the operator uses, on the surface where it appears
Arbitrary ratings with no evidence	Every rating is challenged, and the whole document loses credibility	Anchor Occurrence to scrap rate, ppm or Cpk; anchor Detection to the actual control
Too many functions, or functions written as tasks	Rows multiply, scores dilute, nothing gets priority	One requirement per step, phrased so it can be tested
Only detection controls	Rates the process as more likely to fail than it is, and hides design and prevention opportunities	Add at least one prevention control per high-priority row
No owner and no due date	Actions never close, residual ratings never improve	Name a person and a date on every High and Medium action
Never updated after launch	Scores stop matching reality, and the audit asks why the file predates the process change	Trigger a review on every change, and version the document
Recommendations that add cycle time	Actions get rejected on the floor, so the residual rating is fiction	Propose zero-cycle-time controls first, then price the time you are asking for

One more habit pays for itself. Keep the step numbering identical across the process flow diagram, the PFMEA and the Control Plan. When those three documents disagree on step numbers, an auditor stops listening to your content and starts looking for a bigger problem.

For context on why this matters day to day, a thread on r/MechanicalEngineering asked how experienced engineers actually troubleshoot molding defects, and the answers described an iterative parameter-change loop built on personal judgment. A PFMEA replaces that tribal loop with something a new engineer can read, challenge and improve. On safety-side analysis such as guarding and lockout, that is a separate document, but the two can share step numbering so the machine-level steps line up.

Frequently Asked Questions

Conclusion: Where to Start

Start with your worst repeat defect and build a single-page PFMEA for it: the step, the failure mode, the effect on the customer, the cause, the parameter that controls it, the S/O/D scores with the evidence behind each, and one action that does not cost you cycle time. That page, done properly, is worth more than a forty-row worksheet nobody updates.

Then run the full build in a single workshop with the people who run the machine in the room, keep the step numbering aligned with your Control Plan, and set the review triggers before the meeting ends. Revisit this guide when the process changes, and the document keeps pace with the process instead of the audit cycle.