A carton of plug tops comes back from a customer's test lab. The pins are in gauge, but two bases cracked at the earth pin under the pendulum hammer, and one cord grip let the cable slip. None of that shows at the plug top making machine, yet every fault traces back to the mould, the resin or the way the pins were loaded.
This article covers the base that carries the pins: whether to mould the pins in or fit them later, what the cord grip and screw bosses must survive, how the IS 1293 tests land on the moulding, and how many cavities 60 t can carry.
Key Takeaways
- Pins can be moulded into the base or fitted afterwards; heat staking and ultrasonic fitting work only on thermoplastic bases.
- IS 1293 tests include abnormal heat and fire, pendulum hammer strength and 10,000 insertion and withdrawal cycles.
- IEC 60884-1 includes a cord anchorage test of 100 pulls at 60 N, so the cord grip and its bosses are load-bearing features.
- At 60 t with a 10% reserve, clamp allows about five to eight cavities of a 14 cm² base; pin loading can cap the count first.
What Does a Plug Top Making Machine Need to Do?
Hold three brass pins at the right spacing and height in each cavity, fill the base around them without voids, and form the cord grip and screw bosses cleanly. A vertical clamp with an upward-facing lower mould keeps the pins standing in their pockets while the operator loads every cavity.
A rewireable plug top can be two mouldings: a base that carries the pins and terminal screws, and a cover that closes over it. The base is the insert job. The cover is a plain moulding and can run on any machine of suitable size.
The earth pin is longer and thicker than the line and neutral pins on a three-pin plug, so its pocket and the plastic around it need their own attention. A crack or void here shows up in the strength test first.
Pocket fit and pin pitch follow the same rules as the plug pins and terminals article for 40 t machines: measure pins from several batches before finishing the pockets, and keep the gate off the pins.
Should the Pins Be Moulded In or Fitted Afterwards?
Either can work. Moulding in gives full encapsulation and no assembly step, but adds loading time to every cycle. Fitting afterwards keeps the mould simple. A Plastics Technology article on threaded inserts notes that heat staking and ultrasonic fitting only work on thermoplastics, because they remelt the plastic.
For a thermoset base, the options narrow to moulding the pins in or cold-pressing them later. A BIS article on plugs and switches notes that these accessories are made from both thermoset and thermoplastic materials, so check which your customer's approval covers.
A PlasticsToday summary of DuPont insert moulding guidance calls moulding metal in a last resort, and lists pressing, snap-fitting and ultrasonic insertion as alternatives. It also lists the costs of moulding in: floating inserts, flash, slow loading, preheating and expensive rejects.
Our view: for plug tops with an earth pin under strength testing, mould the pins in and design the base for it. A fitted pin that passes the first article can still loosen after 10,000 insertions; a well-moulded pin is locked in place by its knurl.
Which Tests Does the Moulded Base Have to Pass?
IS 1293 covers plugs and socket-outlets. A BIS article lists tests for earthing, shock protection, dimensions, resistance to abnormal heat and fire, pendulum hammer strength, and normal operation of 10,000 insertions and withdrawals. The table links each test to the feature of the base it loads.
IS 1293 test areas mapped to plug top base features
| Test area | Base feature it loads | Moulding point to watch |
|---|---|---|
| Dimensions | Pin spacing, pin height, body outline | Pocket wear; shrinkage held steady by a fixed process |
| Earthing | Earth pin seating and terminal | Earth pin fully seated before the mould closes |
| Abnormal heat and fire | Whole base, especially around terminals | Approved grade only; regrind within the approval |
| Pendulum hammer | Corners, earth pin boss, thin walls | No voids or weak weld lines at the pin bosses |
| 10,000 insertions | Grip of the base on each pin | Knurl or undercut on the pin; full packing |
| Cord anchorage (IEC 60884-1 reference) | Cord grip and its screw bosses | Boss walls filled and free of voids |
The cord grip deserves attention because it is easy to under-design. Interpower, a cord-set supplier, describes the cord anchorage test in IEC 60884-1, the international plug and socket standard: 100 pulls of 60 N on the cord, one per second, followed by a torque test. How this maps to IS 1293 should be confirmed against the standard itself.
How Many Plug Top Cavities Fit 60 t?
For a base of about 14 cm² plus 1.5 cm² of runner per cavity, 60 t with a 10% reserve carries about five cavities at 5 US t/in² and eight at 3. Each cavity adds three pins to load, so eight cavities means 24 pins every shot.
The range comes from the clamp method RJG's Jeremy Williams gives in Plastics Technology, with factors from a PC datasheet quoting 3.0 to 5.0 US t/in². Flame-retardant PC and filled grades with long flow to thin walls sit toward the top of that range, so plan at 5 until trials prove otherwise.
Pin loading then decides the real count. The more cavities you add, the longer the mould stays open while pins go in, and the cooling time gained by more cavities per shot is lost to loading.
What Should You Prepare Before Quoting a Plug Top Mould?
Prepare the base and cover drawings, pin drawings with tolerances and knurl detail, pin samples from more than one batch, the resin grade and its approval, and the test report format your customer uses. Then time pin loading for one cavity. See the 60 ton vertical injection moulding machine page.
Frequently Asked Questions About Plug Top Moulding
Why does the earth pin boss crack before the others?
The earth pin is the largest insert, so the plastic around it carries the highest shrinkage stress and can sit near a weld line where flow meets behind the pin. Rounded knurls, a thicker boss, a gate placed so the weld forms away from the pin, and a tougher grade all help.
Can the plug top cover run on the same machine?
It can, but the cover has no inserts, so a vertical insert machine adds nothing for it. If covers would take up hours the insert machine needs for bases, run them on a horizontal machine with automatic part drop, and keep the vertical machine loading pins.
Do brass pins need cleaning before moulding?
Oil or residue from stamping and turning can stop the plastic gripping the knurl and can leave marks around the pin. Agree a cleanliness standard with the pin supplier, store pins covered, and check a sample from each batch before it goes to the moulding cell.
What causes pins to stand proud after moulding?
A pin lifting in its pocket as melt flows under it, a pocket worn deeper on one side, or a pin shorter than drawing. Check pin length from the batch, inspect the pocket floors, and look at fill direction around the pins. A gauge on the first shots catches it early.
Is a 60 t machine needed for a single-cavity plug top mould?
Not for clamp. One base of about 15 cm² needs well under 15 t even at a high tonnage factor. A larger machine makes sense only when you plan more cavities, a larger lower mould or other parts that need the extra clamp.
Related Articles
- What Can You Make on a 60 Ton Vertical Insert Moulding Machine?
- Threaded Bush Inserts: Getting Pull-Out Strength Right
- Connector Insert Moulding: Terminal Location and Flash Control
Sources & References
- BIS Blog, N. Agarwal: Standards for Plugs-Socket-Outlets and Switches
- Interpower: Cord Anchorage Pull Test
- Plastics Technology (PennEngineering, sponsored): Four Ways to Tackle Threaded Inserts for Plastics (Jan 2018)
- PlasticsToday, summarising A. Miller (DuPont): Design and material advice from the pros, Part 3: Insert molding essentials
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
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