3-Component Plastic Scissors

Going through some old plastic samples which I collected in the past, I bumped into an interesting product: a 3-component plastic scissors. Where the 3 different materials are co-injected at the same time, with the hinge installed by in-mold-assembly techniques. (At least that’s what the packaging tells me.)

This triggered me enough to put the model into our CT-scanner and examine it in a bit more detail. Below you can see some results of the scan.

What is in-mold assembly?

Als known as integrated injection molding, it is a manufacturing process where pre-fabricated parts are inserted into a mold cavity and then overmolded or encapsulated during the molding process.

This eliminates post-molding assembly and reduces part count, leading to improved product quality and efficiency.

Principle:

Tooling acts as an assembly fixture, holding previously molded parts during subsequent molding stages.
Special technology for in-mold assembly (IMA) is found in the tooling, rather then in the molding press.
In some cases rotating or sliding plates are used to change cavity configurations between shots.

Benefits

Reduced part count: In-mold assembly eliminates the need for separate assembly operations and reduces the numbers of parts required, leading to a more streamlined and cost-effective process.

Improved product quality: The assembly is performed within the mold, resulting in a more consistent and robust component.

Increased productivity: Eliminating assembly operations and reducing the need for separate presses and molds leads to higher productivity and reduced labor and floor space requirements.

Faster production: In-mold assembly reduces the total time to produce and assemble.

Enhanced functionality: This approach allows for the creation of parts with features that would be difficult or impossible to produce through conventional assembly processes.

Examples

Articulated joints: In-mold assembly can be used to create plastic products with articulated joints by first molding the hole and then molding the pin inside the hole.

Multi-shot molding: In-mold assembly is often used in multi-shot molding where multiple layers of material are applied during the molding process.

Electronics encapsulation: In-mold assembly can be used to encapsulate electronic components within a plastic housing.

In-mold assembly is distinct from general multi-shot molding or hard/soft overmolding, as the core concept is assembling separate components inside the mold that would otherwise be assembled outside the mold through methods like snap-fits, welding or adhesive bounding.

Benefits of CT-scanning

Interal assembly verification

Correct placement/alignment of subcomponents that are assembled within the mold.
Ensures mechanical interlocks or snapt-fits formed during molding are engaged properly.
Identifies shifting or rotation of parts during overmolding.

Bonding and weld quality

Checks the interfaces between the different material layers.
Can reveal lack of adhesion, cold weld or voids between overmolded sections.
useful for TPE-over-hard plastic combinations where adhesion quality is critical.

Void detection

Internal air pockets or bubbles trapped during molding.
Especially relevant in high-stress zones or tight tolerances areas.
Can also identify voids at the interface between molded parts and inserts.

Cracks and microstructural defects

Detect micro-cracks, delamination or resin-rich/resin-starved areas.
Useful for brittle materials or parts with glass/mineral fillers.

Material flow analysis

Reveals flow lines, knit lines and swirl patterns that may indicate improper gating, poor temperature control or material incompatibility.

Insert placement analysis

Verifies position and orientation of embedded metal or plastic inserts.
Ensures no shift due to injection pressure during subsequent molding stages.

Wall thickness & dimensional consistency