Cable grommet moulding comes down to three settings. The mould must grip the cable firmly enough to stop flash without crushing it, the flexible PVC must run in the melt window for its hardness, and the bore must allow for 1.0% to 2.5% shrinkage. Get those right and strain reliefs follow the same rules.
This article covers overmoulded parts, formed directly on the cable, and loose grommets moulded on a core pin and fitted later. The setup steps overlap, and the differences are noted where they matter.
Key Takeaways
- On an overmoulded part the cable is the core: the mould closes on its jacket, so cable diameter tolerance decides flash and crush.
- A flexible PVC compound guide gives melt temperatures from about 163 °C to 199 °C, rising with hardness.
- The same guide says the compound must not sit for more than 10 minutes and asks for vents at the final fill points.
- Linear shrinkage of 0.010 to 0.025 in/in means a core pin is cut at the large end and ground down after trials.
What Makes Cable Grommet Moulding Different?
The cable is part of the mould. On an overmoulded grommet or strain relief, the cable forms the bore and the mould closes on its jacket at each exit. Cable diameter changes alter the seal at those exits, so flash and crushing come from the cable, not only the steel.
A loose grommet is simpler to mould. It forms on a steel core pin and is pushed onto the cable or into a panel hole later. It needs no cable handling at the machine, but it does not bond to the jacket and seals only by interference.
Either part may end up in an enclosure inlet. BIS's summary of IS 14772:2020, for boxes and enclosures for electrical accessories up to 440 V a.c., classifies boxes by inlets and clamping means, among other points. Lids, covers, knock-outs and fixings must protect against ingress of solids and water, so ask which enclosure your grommet must suit.
How Should the Mould Hold the Cable?
With a shut-off at each cable exit sized to the cable's nominal diameter, and support far enough back that the cable cannot move as melt arrives. Measure cable diameter across several reels before cutting steel, because the shut-off has to suit the whole tolerance band, not one sample.
A shut-off that is too large lets melt run along the jacket as a thin sleeve of flash. One that is too small crushes the jacket, marks it and can press on the conductors. A mould can use a slightly compliant insert at the exit to absorb diameter variation.
Venting matters because the cavity is small and fills fast. The flexible PVC guide used here asks for vents at the final fill points. On a strain relief gated at the housing end, that is the far end of the tail, where trapped air would otherwise burn.
Ejection on soft parts needs care. A flexible grommet can stick to a core and stretch rather than release. Larger ejector contact areas, a stripper at the cable exit or an air blast help, and on overmoulded parts the operator can lift the part out by the cable.
What Melt and Mould Settings Suit Flexible PVC?
Settings follow hardness. A Teknor Apex processing guide for flexible PVC compounds puts melt temperature at about 163 °C to 168 °C for Shore A50 to 60, rising to about 193 °C to 199 °C for Shore C80 to 90, with mould cooling at about 10 °C to 43 °C.
Flexible PVC injection settings by hardness, converted from the Teknor Apex guide (°F to °C, rounded)
| Hardness | Typical barrel settings | Melt temperature |
|---|---|---|
| Shore A50 to 60 | 124 to 154 °C | 163 to 168 °C |
| Shore A60 to 70 | 129 to 157 °C | 168 to 174 °C |
| Shore A70 to 80 | 138 to 160 °C | 174 to 179 °C |
| Shore A80 to 90 | 149 to 177 °C | 179 to 185 °C |
| Shore C70 to 80 | 168 to 191 °C | 191 to 196 °C |
| Shore C80 to 90 | 177 to 196 °C | 193 to 199 °C |
The same guide gives back pressure of 50 to 150 psi, about 3.4 to 10.3 bar, a moderate injection rate, a chrome-plated general purpose screw with a 2:1 to 3:1 compression ratio and 20:1 to 24:1 L/D, and stainless 420 or chromed tooling. Drying is typically not required, and up to 20% regrind can be mixed with virgin.
Two warnings matter most. The compound must not sit for more than 10 minutes, so retract and purge before a long break. The guide suggests HD polyethylene for purging flexible grades, while the GEON rigid PVC guide warns against PE or PP purges for rigid PVC, so follow your own compound supplier.
One rule holds for every PVC: keep it away from acetal. The GEON guide warns that the two must never meet at processing temperature. Our view: give the PVC machines in a cable shop their own purge stock and never run acetal on them.
How Do You Size the Bore for Shrinkage?
Start from the finished bore you need, divide by one minus the shrinkage, and cut the core pin at the larger result. The flexible PVC guide gives linear shrinkage of 0.010 to 0.025 in/in, or 1.0% to 2.5%, so on a small bore the spread is under a millimetre.
That spread matters on a grommet that seals by interference. A bore shrinking onto a core pin is also restrained, so it may not shrink as freely as a test bar. Treat the published range as a guide and settle the final size from trial shots.
Overmoulded parts avoid this question at the bore, since the cable is the core and the part shrinks onto it, which helps the grip. The outer diameter still shrinks, so cut cavities at the small end of the range and open them up after trials.
What Should You Check Before the First Trial?
Collect cable samples from several reels, the compound grade and hardness, the enclosure or panel the grommet must fit, and the pull or seal test your customer will apply. Set melt by hardness, vents at the last fill points and steel-safe core pins. See the 40 ton vertical injection moulding machine page.
Frequently Asked Questions About Cable Grommet Moulding
How hard should a strain relief be?
Hard enough to stop the cord bending sharply at the exit, soft enough to flex with it. Set the hardness with your customer's bend or flex test, not by habit. Remember that harder flexible PVC grades need higher melt temperatures, so a change in hardness means a change in machine settings too.
Why does my grommet show burn marks at one end?
Burn marks at the last point to fill usually mean trapped air heated by compression. Add or deepen vents at that point, check that existing vents are clean, and try a slower final fill speed. If the marks persist, check melt temperature against the range for the compound's hardness.
Can a grommet be moulded in TPE instead of PVC?
Yes, but check the bond. A TPE grommet moulded onto a PVC jacket may not fuse to it the way PVC does, so it may grip only mechanically. Ask the TPE supplier for a grade rated for adhesion to your jacket material, and confirm it with a pull test on moulded samples.
How do I stop the cable moving during injection?
Support the cable close behind each shut-off so it cannot slide or bow, and position the gate so the melt does not push the cable against one wall. Balanced flow around the cable keeps the wall even. Check wall thickness on cut sections from the first trial shots.
What happens if the machine stops with PVC in the barrel?
Overheated PVC turns yellow and releases hydrogen chloride, which attacks steel. Rigid PVC guidance says never to shut down with PVC in the barrel, and flexible grades should not sit hot for more than 10 minutes. Retract the screw and purge before any long stop or shutdown.
Related Articles
- What Can You Make on a 40 Ton Vertical Moulding Machine?
- Moulding Power Cord Plugs on a 40 Ton Vertical Machine
- 40 Ton vs 55 Ton Vertical Machine: Which Suits Your Cable Part?
Sources & References
- Teknor Apex: Processing Guide for Injection Molding of Apex Flexible PVC Compounds
- Bureau of Indian Standards: IS 14772: 2020 Boxes and Enclosures for Electrical Accessories (summary)
- GEON Performance Solutions: Injection Molding Guide: GEON PVC Molding Formulations (PG0019BC, 2021)
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.
Limitation of Liability: Pratishna Engineers makes no representation or warranty, express or implied, regarding the completeness, accuracy or suitability of the information for any particular application. To the extent permitted by applicable law, Pratishna Engineers shall not be liable for any loss, damage, cost or consequence arising from reliance on or use of the information contained in this document. Final machine selection, configuration and application suitability should be confirmed with Pratishna Engineers based on complete and accurate project requirements.
