Project Overview
A beauty oxygen injection device — the handheld appliance used in facial oxygen therapy — depends on a small plastic component that most users will never see: the electron tube socket. The socket is a 21 mm diameter ABS ring, 13 mm tall with a 1.2 mm wall, weighing 3.6 grams. Molded into it are five metal contact pins, each 1.6 mm in diameter and 18 mm long, that carry the electrical connection to the oxygen-generation circuit.
The part is small, but the molding problem is concentrated: the pins sit close to the outer edge of the ring, the shut-off distance around each pin is minimal, and five pins of different lengths must be loaded and held in the mold simultaneously. A pin that shifts during injection produces a part that cannot make electrical contact. This case study documents the three engineering challenges and the solutions implemented in a 4-cavity insert mold.
Part Specifications
| Parameter | Specification |
|---|---|
| Product | Electron tube socket for beauty oxygen injection device |
| Plastic part | ABS, 21 mm diameter × 13 mm height |
| Wall thickness | 1.2 mm |
| Part weight | 3.6 g |
| Metal pins | 5 per part, 1.6 mm diameter × 18 mm length |
| Cavities | 4 |
| Tool steel | S136 |
| Process | Insert molding (pins overmolded in the plastic socket) |
Engineering Challenge 1: Precision Insert Locating
The challenge. The clearance between the metal pins and the molded holes and steps is extremely small. The pins sit close to the outer edge of the ring, which leaves a very short shut-off distance around each pin — the sealing land between the pin and the cavity wall. With such a small shut-off, the mold must hold every pin to exact position during injection. Any drift lets melt flash over the pin or leaves the pin proud of its molded seat. The locating difficulty is the core precision requirement of this mold.
The solution.
- Dedicated insert locating system. Each pin position uses a dedicated locating insert machined to the pin diameter, holding the pin at its seat and its tip — a two-point support that fixes the pin against lateral shift during injection.
- Shut-off faces machined to close clearance. The sealing lands around the pins are cut to the minimum clearance that still vents, so melt pressure cannot push flash onto the pin surfaces.
- Locating accuracy verified by dimensional inspection of pin positions on the molded socket, pin by pin, cavity by cavity.
Engineering Challenge 2: Multi-Pin Loading and Assembly
The challenge. The five pins are not all the same length. All five must be loaded into the mold in the same cycle, in the correct position and orientation, every shot — across four cavities, that is 20 pins per cycle. Manual loading is slow and error-prone; a pin loaded backwards or seated high produces scrap, or worse, damages the mold at the next clamp.
The solution.
- Dedicated pin-loading fixture. A purpose-built loading jig holds the five pins in their exact relative positions outside the mold, so the operator transfers a complete, correctly-oriented pin set into the cavity in one motion.
- Quick-locating structure in the mold. The mold-side locating system receives the pin set with guide features that register the pins to their inserts without fine adjustment — the fixture positions the pins; the mold only verifies the position.
- Loading sequence standardized per cavity, reducing the cycle-to-cycle variation that manual loading would otherwise introduce.
Engineering Challenge 3: Runner and Gate Design
The challenge. The melt must flow around five metal pins before filling the ring. Each pin splits the flow front and rejoins it downstream — the classic conditions for weld lines, uneven filling, unbalanced pressure on the two sides of each pin, and localized short-fill of the plastic around the pins. A weld line on the socket wall is a cosmetic and structural weak point; a pressure imbalance bends a pin; a short-fill leaves the pin partially unsupported.
The solution.
- Moldflow analysis of the full 4-cavity layout, modeling the melt path around all five pins in each cavity.
- Flow balancing around the pins. Gate positions and runner sizing were adjusted in simulation until the flow fronts rejoin with balanced pressure on both sides of every pin — the melt wraps each pin symmetrically instead of pushing it sideways.
- Gate and runner geometry sized from the simulation results, with the balanced layout carried into the machined mold.
Mold Design Details
The mold combines the three solutions into a 4-cavity insert-molding design. Each cavity carries the dedicated insert locating system for its five pins; the quick-locating structure receives the pin sets from the loading fixture; and the runner system follows the Moldflow-balanced layout that wraps the melt symmetrically around each pin.
The locating inserts are cut in S136 — the corrosion resistance matters where the pins seat repeatedly across production, and the polishability supports the close-clearance shut-off faces. The full insert-molding process context is documented in the overmolding and insert molding guide.
Quality Control
- Pin position inspection on the molded socket — pin by pin, against the drawing, with the pin-to-edge distance checked as the tightest feature.
- Weld line inspection on the socket wall at the pin-opposite positions where flow fronts rejoin.
- Short-fill and flash inspection around every pin seat, ensuring each pin is fully supported by plastic with no flash on the contact surface.
Results
The mold delivers the three outcomes the design set out to achieve:
- The dedicated insert locating system holds all five pins to position through injection — no pin drift, no flash over the contact surfaces, across all four cavities.
- The loading fixture and quick-locating structure load 20 pins per cycle with consistent position and orientation.
- The Moldflow-balanced runner and gate design keeps pressure balanced on both sides of every pin, eliminating pin deflection and localized short-fill around the pins.
The result is a socket that carries its five pins to exact electrical-contact position — the requirement this component exists to meet. The tooling discipline behind it is the same precision molding capability documented across our insert-molding programs, delivered through our mold design and injection molding services.
