CNC Machining & Injection Molding — DFM/Moldflow Support, CMM Inspection, Prototype to Production Solutions.
Ensure zero-defect manufacturing with our comprehensive engineering checklist. Covering AQL sampling, Class A/B/C visual standards, and FAI documentation requirements for US-market procurement.
Acceptance criteria for injection molding are the measurable pass/fail rules used to accept or reject molded parts. A complete list typically includes CTQ dimensions and GD&T compliance, cosmetic defect limits by surface class (A/B/C), AQL sampling for visual checks, material verification (COA), functional fit tests, and packaging/traceability requirements.
Standardizing these criteria ensures alignment between engineering, quality, and procurement teams, minimizing disputes and guaranteeing that every batch meets the functional and aesthetic requirements of the final assembly.
Use FAI criteria to approve the first parts from a tool or revision (full layout + material + visual standard). Use lot acceptance for ongoing production with defined sampling plans (CTQ stricter, cosmetics by AQL). Incoming verifies shipments meet specs; final ensures cosmetic protection, labeling, and traceability before release.
A CTQ feature is any dimension or requirement that can cause functional failure, safety risk, sealing leakage, assembly interference, or regulatory nonconformance. CTQ criteria are usually zero-tolerance for out-of-spec and often require 100% checks or dedicated gauges. For multi-cavity tools, sample at least one part per cavity to control cavity-to-cavity variation.
AQL sampling is best for visual/cosmetic inspection where minor variation is acceptable. Start with Major defects AQL 0.65 and Minor defects AQL 1.0–2.5, then tighten based on risk and customer expectations. Do not use AQL for CTQ dimensions, safety items, cracks, short shots, or functional failures—those should be pass/fail (often 100%).
To avoid cosmetic disputes, define Class A/B/C surfaces and inspect under consistent conditions (e.g., 500–1000 lux, 18–24 in distance). Use a defect matrix listing each defect type (flash, sink, weld lines, splay, burn marks) with allowed limits per surface class. When possible, approve master/limit samples and reference them in inspections.
*Standardizing these environment variables is critical to eliminate subjectivity during Quality Audits.
| Defect Type | Class A Limits | Class B Limits | Class C Limits |
|---|---|---|---|
| Flash / Burr | None Allowed | Max 0.10 mm (Non-sharp) | Max 0.25 mm (Function-safe) |
| Sink Marks | None Visible | Slight (No depth felt) | Allowed (Non-structural) |
| Weld / Knit Lines | None on Logos/Text | Visible but smooth | Allowed (Non-cracking) |
| Splay / Silver | Not Allowed | Slightly Visible | Allowed |
| Burn Marks | Not Allowed | Not Allowed | Small & Light Only |
Material verification ensures resin grades match BOM/PO requirements via Certificates of Analysis (COA). Robust traceability links raw material lots to specific molding dates and cavity IDs. A strict regrind policy defines the maximum allowable percentage to maintain structural integrity and prevent degradation.
Every incoming batch must be accompanied by a Certificate of Analysis (COA). Quality teams verify brand, grade, and lot-specific properties (melt flow index, moisture content) against the technical data sheet (TDS).
Unless specified otherwise, a standard maximum of 20% regrind is allowed. For high-performance or Class A components, regrind may be strictly forbidden (0%) to prevent mechanical failure or cosmetic splay.
We maintain a digital thread linking: Resin Lot # → Molding Date/Time → Machine ID → Cavity/Tool ID. This allows for surgical precision in the event of a containment or quality hold.
Fields to fill in (blanks, thresholds, references): Replace all ______ with specific engineering tolerances or AQL levels. Reference your CAD drawings and Quality Control Plan (QCP) to define CTQ features.
Example defaults: Use AQL 0.65 for Major Defects and 1.5 for Minor Defects as an industry starting point.
Purpose: Define clear, measurable acceptance criteria for molded parts across dimensional, cosmetic, material, functional, and packaging requirements. This template is designed for US B2B engineering, quality, and procurement workflows (FAI / Lot Acceptance / Incoming / Final).
Surface Class: Class A = primary cosmetic surface visible to end-user; Class B = secondary visible surface; Class C = non-cosmetic or hidden surface / internal features.
Defect Severity: Critical = safety / regulatory / functional failure (typically zero tolerance); Major = affects fit, function, key appearance, or customer requirements; Minor = does not affect fit/function; limited cosmetic imperfections allowed per matrix.
| Item | Category | Requirement / Acceptance Criteria | Method / Gage | Sampling Plan | Severity | Stage | Reference |
|---|---|---|---|---|---|---|---|
| D-01 | Dimensional / GD&T | All drawing dimensions shall meet specified tolerances. | CMM / Caliper | Per drawing | Major | FAI / Lot | Drawing |
| D-02 | Dimensional / CTQ | CTQ dimensions must be 100% compliant. | Dedicated fixture | 100% | Critical | FAI / Lot | Drawing CTQ |
| V-01 | Visual / Cosmetic | Cosmetic defects shall comply with Visual Defect Matrix. | Visual @ 500 Lux | AQL 0.65 | Major | Final | Visual Spec |
| Defect Type | Class A | Class B | Class C | Severity |
|---|---|---|---|---|
| Flash / Burr | Not allowed | ≤ 0.1 mm | ≤ 0.25 mm | Major |
| Sink Marks | Not visible | Slightly allowed | Allowed | Minor |
Data-driven rejection analysis shows that 85% of molded part non-conformances stem from four specific engineering failure points. Identifying these during DFM and inspection is critical to avoiding lot-wide rejects.
Excess resin at parting lines or sealing surfaces. This prevents airtight/watertight assembly and is the #1 cause of functional leakage rejection.
Uncontrolled deformation leading to flatness or circularity failure. Even a 0.2mm bow can make automated assembly or snap-fits impossible.
Visible knit lines in high-stress zones. Beyond aesthetics, these create structural weak points susceptible to cracking under load.
Inconsistent masterbatch mixing or resin degradation. Causes ΔE mismatches when multi-batch parts are assembled together.
Critical defects are zero-tolerance. These include cracks, short shots, severe burn marks, and any non-conformance affecting safety, regulatory compliance, or primary functionality (e.g., failed snap-fits or leaking seals).
Acceptance must be performed on a per-cavity basis. During FAI, a full layout is required for every cavity. During ongoing production, at least one sample per cavity should be included in the AQL lot to detect cavity-to-cavity variation.
In the absence of customer specs, we default to SPI (Society of Plastics Industry) standard finishes and Class B surface criteria. However, we recommend approving a "Golden Sample" or "Limit Sample" before mass production to avoid subjectivity.
An Acceptance Checklist is a static pass/fail gate for finished parts. A Control Plan is a dynamic document that defines the specific process parameters (temp, pressure, cycle time) and inspection points required to produce parts that meet that checklist.
Yes. AQL levels are often tightened (e.g., from 1.0 to 0.4) if a quality trend is negative, or relaxed once a process achieves high stability (Cpk > 1.33) and a history of zero defects over multiple lots.
Typically, no. For high-cosmetic (Class A) or transparent parts, 100% virgin resin is preferred to prevent splay, black specks, or gloss variation. Any regrind usage (usually capped at 10-20%) requires written customer approval.
