Plastic Injection Molding: Checking Part Completeness with Darkfield Lights

Plastic injection moulding produces billions of components every year. Even small process variations cause defects that compromise part function, assembly fit, or structural integrity. Short shots — where the mould cavity is not completely filled — are among the most common and economically damaging defect categories. Detecting them reliably at production speed requires the right illumination strategy.

Darkfield illumination is the technique best suited to revealing the subtle surface and geometric differences that distinguish a complete part from a defective one. By positioning low-angle LED illuminators around the part, machine vision systems can detect incomplete features, missing geometry, and surface voids that are invisible under conventional front-lighting or diffuse dome illumination.

1. Defect Types in Injection Moulded Parts

Understanding which defects need to be detected determines the illumination strategy for the vision system. Injection moulding defects fall into several categories.

Short Shots

A short shot occurs when molten plastic does not completely fill the mould cavity before it solidifies. The result is a part with missing geometry: incomplete ribs, absent bosses, missing snap-fit features, or unfilled thin sections. Short shots are caused by insufficient injection pressure, low melt temperature, inadequate venting, or contaminated material. They are among the most critical defects because a short-shot part is dimensionally non-compliant by definition.

Sink Marks and Surface Voids

Sink marks are depressions on the part surface caused by localised shrinkage during cooling. They typically appear opposite thick sections or ribs where cooling is uneven. Surface voids are similar but represent a complete absence of material at a specific point. Both defects alter the surface geometry of the part and affect the fit and finish of the assembly.

Flash, Burrs, and Mould Line Irregularities

Flash is excess material that has escaped from the mould cavity along the parting line, ejector pin locations, or side core faces. It represents a geometric excess rather than a deficit and is equally detrimental to assembly and function. Mould line irregularities include raised parting lines and ejector pin marks that exceed tolerance.

2. Why Darkfield Illumination Works for Plastic Part Inspection

Plastic injection moulded parts present a specific optical challenge. Most production plastics have a semi-gloss or gloss finish that is highly specular under conventional illumination. Diffuse dome lights or front-lit ring lights tend to wash out the subtle surface features that distinguish a defect-free part from a short-shot or sink-marked one.

Darkfield illumination uses grazing-incidence light. The LED illuminator is positioned at a low angle relative to the part surface — typically between 10° and 25° from the horizontal plane. At this angle, smooth areas of the part surface reflect the light away from the camera. The camera sees a dark background. Any surface irregularity — a missing rib, a sink depression, a void, a raised flash — scatters light towards the camera and appears bright against the dark background.

Contrast Enhancement for Subtle Geometry

The contrast advantage of darkfield for plastic surfaces is significant. A sink mark that is only 0.2 mm deep may produce negligible grey-level difference under diffuse illumination. Under darkfield at a 15° angle, the same feature generates a bright highlight at its leading edge and a dark shadow at its trailing edge. The signal-to-background ratio increases by an order of magnitude or more. This translates directly into more reliable detection and fewer false rejects.

Detection of Missing Features

Short shots that leave a completely absent feature — a missing boss, an incomplete rib — create a geometry discontinuity at the boundary of the missing area. Under darkfield illumination, this edge is revealed as a sharp bright line against the surrounding dark surface. The absence of expected geometry is detected by comparing the acquired image with a reference golden-part image. Any area where bright edges are expected but absent indicates a short shot.

3. Illumination Setup for Injection Moulding Inspection

Effective darkfield inspection of injection-moulded parts requires careful design of the illumination geometry, wavelength selection, and synchronisation with the vision system trigger.

Illuminator Geometry and Angle

For parts with rotationally symmetric geometry — round caps, cylindrical housings, symmetrical connectors — a ring illuminator centred on the camera axis provides omnidirectional darkfield coverage. All surface features and edges are equally illuminated regardless of their orientation. This eliminates the directionality problem of linear darkfield illuminators, where features parallel to the light direction may not scatter enough light to be detected.

For flat or rectangular parts — panels, covers, flat brackets — a combination of directional low-angle illuminators from two or four sides provides uniform grazing-incidence coverage. The illumination angle should be chosen to match the surface finish: glossy surfaces require lower angles (10° to 15°) for best contrast, while semi-matt surfaces may perform better at 20° to 25°.

Wavelength Selection for Plastic Surfaces

White light is the standard choice for general plastic inspection when the part must be inspected for both geometric defects and colour or surface quality simultaneously. Red or near-infrared wavelengths (630–850 nm) provide good contrast on many plastic surfaces and reduce sensitivity to ambient light. Blue wavelengths (450–470 nm) enhance contrast on light-coloured or transparent plastics.

Strobe Operation and Synchronisation

Injection moulding inspection is typically performed inline at the output of the moulding machine, where parts are conveyed at rates of 10 to 60 parts per minute. Strobe illumination — synchronised with the camera trigger — freezes part movement during exposure, rejects ambient light, and extends LED illuminator lifetime by operating in pulsed mode for most of the production cycle.

4. Multi-View Inspection Strategies

Complex injection-moulded parts — connectors, housings, medical components — cannot be fully inspected from a single camera viewpoint. Short shots and missing features may be located on any face. Multi-view inspection stations use two, four, or more camera-illuminator pairs positioned around the part to provide complete 360° coverage.

Sequential Multi-Illuminator Firing

When multiple illuminators and cameras are positioned around the part, strobe synchronisation becomes critical. Each illuminator must fire independently and synchronise with its corresponding camera exposure. Sequential firing ensures that each image is acquired with only one illuminator active, preventing cross-illumination. RODER Vision LED illuminators support external trigger input for precise synchronisation with vision system controllers.

Combining Darkfield with Backlight

For transparent or semi-transparent plastic parts — medical vials, optical lenses, clear packaging components — combining darkfield front illumination with backlight provides complementary inspection capability. The darkfield image reveals surface defects and geometric features. The backlight image reveals internal voids and contamination through transmitted light inspection. Both images can be acquired sequentially at the same station.

5. Integration Considerations

Ambient Light Control

The effectiveness of darkfield illumination depends on excluding stray ambient light. Factory overhead lighting falling on the part surface brightens the dark background and reduces defect contrast. Inspection enclosures with blackout panels are the standard solution. Where a full enclosure is not practical, high-intensity strobe with short camera exposure times (50–200 µs) provides effective ambient light rejection.

Part Positioning and Fixture Design

Darkfield illumination is sensitive to part height and tilt. A height variation of 2–3 mm can change the effective illumination angle enough to alter the contrast of surface features. Parts should be presented to the inspection station in a repeatable position using mechanical stops, nests, or vacuum fixtures.

Thermal Stability of the Illuminator

Injection moulding machines operate continuously for extended production runs. RODER Vision illuminators incorporate HTTM (High Temperature Thermal Management) technology, which maintains LED junction temperature within tight limits regardless of ambient temperature variations. This ensures that the darkfield illumination level remains constant from the start to the end of the shift, without requiring periodic recalibration of the vision system threshold.

Products and Technologies

RODER Vision Illuminator Families for Plastic Injection Moulding Inspection

The following RODER Vision product families are well suited for darkfield and direct inspection of injection-moulded plastic parts.

Frequently Asked Questions