A four-cavity plug mould shooting 28 g of PVC from a barrel rated 100 g of polystyrene uses only about 21% of that barrel's PVC capacity. The melt then sits through nearly five cycles before it is injected: on a 45 s cycle, about 3.5 minutes at temperature, before counting any resin trapped in dead spots. That is the core of PVC moulding for plugs and grommets. Control how hot the melt gets and how long it stays hot, and run it through a screw, barrel and check ring built for a heat-sensitive, corrosive material.
The figures in this article come from compound suppliers' processing guides. Your own compound's data sheet always comes first.
Key Takeaways
- Flexible PVC melt temperature rises with hardness; one guide runs from about 163 °C for soft Shore A grades to about 199 °C for hard Shore C grades.
- Keep the shot at 50% or more of barrel capacity, and never let PVC sit hot for long: one flexible PVC guide sets a 10 minute limit.
- Use a chrome-plated or stainless screw, a sliding check ring rather than a ball check, and a barrel free of high-iron or cobalt metals.
- Never shut down with PVC in the barrel, and never let PVC meet acetal at processing temperature.
What Temperatures Suit PVC Moulding for Plugs and Grommets?
It depends on hardness. One flexible PVC guide gives melt temperatures from about 163 to 168 °C for Shore A50 to 60 compounds up to about 193 to 199 °C for Shore C80 to 90. A rigid PVC guide gives 199 to 207 °C. Measure the melt at the nozzle.
Flexible PVC barrel settings and melt temperature by hardness (converted from one compounder's guide)
| 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 melt runs hotter than the barrel settings because screw rotation adds shear heat. That is why the guide asks for a pyrometer reading on material leaving the nozzle rather than trust in the controller display. Soft grommet compounds and harder plug body compounds can sit 20 °C or more apart, so a barrel set for one is wrong for the other.
Injection rate matters for the same reason. The flexible guide suggests a moderate rate, since a fast fill through a small gate adds shear heat exactly where the melt is already hottest.
Mould water for flexible PVC runs cool: the same guide gives about 10 to 43 °C. Keep it even between halves, because uneven cooling shows up as warped tails and sink near thick sections.
The cord jacket is PVC too, with its own class. IS 5831 groups PVC cable sheaths as Type ST1 (general purpose, 70 °C conductor), ST2 (heat resisting, 90 °C) and ST3 (general purpose for flexible cables, 70 °C). Ask the cord supplier which type you will receive, and trial your overmould compound against that sheath, not a sample from another source.
Which Screw, Barrel and Check Ring Does PVC Need?
A screw with a protective surface, a moderate compression ratio and no dead spots, a sliding check ring, and a barrel lining that does not react with PVC. Flexible and rigid PVC guides differ in detail, so the table compares one of each.
Screw and barrel guidance from a flexible and a rigid PVC processing guide
| Item | Flexible PVC guide | Rigid PVC guide |
|---|---|---|
| Screw | General purpose, chrome plated | Stainless highly recommended; 4140 steel needs triple chrome |
| Compression ratio | 2:1 to 3:1 | 2.2:1 recommended (2.0 to 3.0 used) |
| Flight split | Not stated | 35% feed, 50% transition, 15% metering |
| Check valve | Not stated | Sliding check ring; ball checks not recommended |
| Barrel | Not stated | Bimetallic ideal; no high-iron or cobalt metals; no vented barrels |
| Shot size | At least 50% of capacity | 50% to 80% (30% to 90% possible) |
| Back pressure | 3.4 to 10.3 bar | Start at 5.2 to 10.3 bar |
| Purge | HDPE if necessary | GP ABS, PS or acrylic regrind; never PE or PP |
When PVC overheats it yellows and gives off hydrochloric acid, which is why both guides call for protected screw surfaces. Any corner where melt can hang, such as a worn ball check or a gap at the nozzle, becomes a source of brown streaks. Note the purge conflict: follow your own compound supplier's choice.
The mould needs the same care as the screw. The flexible guide calls for stainless steel grade 420 or chromed tooling, liberal gating because PVC is shear and heat sensitive, and vents at the final fill points. On a grommet, those are often the sealing lips; on a plug, the corners farthest from the gate.
How Do You Stop PVC Degrading in the Barrel?
Keep residence short and never leave PVC hot and still. Size the barrel so the shot uses at least half of it, keep the cycle steady, retract and purge during stops longer than about 15 minutes, and never shut down with PVC in the barrel.
Formula estimates of residence miss resin trapped at the base of screw flights, and one long-time troubleshooter reports finding such trapped resin nearly 90% of the time when screws are pulled. Measure instead: drop a few granules of the same compound in another colour into the feed throat and count shots until the colour is gone.
After a power cut, the rigid PVC guide gives a slow restart: hold 121 °C for one hour, raise 14 °C every 30 minutes to 177 °C, jog the screw, then purge. Keep PVC away from acetal at processing temperature; the two together release formaldehyde with violent gas release.
Our view: never leave PVC hot in the barrel over a break. Purge it or bring temperatures down every time, even if it costs ten minutes at restart.
What Should You Agree With Your Compound Supplier?
Get the data sheet with melt and barrel ranges, the density, the purge material, regrind limits and the maximum hold time at temperature. Then match screw, barrel and shot window to them before buying. See the horizontal clamp vertical injection moulding machine page.
Frequently Asked Questions About PVC Plug and Grommet Moulding
Does PVC need drying before moulding?
Usually not. A flexible PVC processing guide says drying is typically not required, and a rigid PVC guide says it is not usually needed. If a compound has picked up moisture in storage, the rigid guide suggests about 2 hours at 49 to 66 °C. Follow your compound's data sheet.
Why measure melt temperature at the nozzle instead of reading the controller?
Barrel zones show heater settings, not the melt. Screw rotation, back pressure and residence all add heat, so the melt can run hotter than any zone. A pyrometer reading on an air shot leaving the nozzle shows what the plug actually receives, and it should be repeated after any change.
What should you do after a power cut with PVC in the barrel?
Do not restart at full temperature. One rigid PVC guide holds the barrel at 121 °C for an hour, then raises it 14 °C every 30 minutes to 177 °C, jogs the screw and purges before moulding. Ventilate the area, since degraded PVC releases hydrochloric acid.
Can PVC and acetal run on the same machine?
Only with great care, and separate machines are safer. PVC and acetal must never meet at processing temperature, because the reaction releases formaldehyde with violent gas release. If one machine must run both, purge completely with a neutral material approved by both compound suppliers before changing over.
Why do PVC plugs turn yellow or show brown streaks?
Overheated PVC yellows and gives off hydrochloric acid. Common causes are high melt temperature, long residence from an oversized barrel, dead spots at a worn check valve or nozzle, and barrel metals with high iron or cobalt. Lower temperatures, shorten residence and inspect the check ring.
Related Articles
- Cable and Wire Overmoulding on Horizontal Clamp Vertical Machines
- Moulding Plugs on Power Cords: Machine and Mould Setup
- Horizontal Clamp or Vertical Clamp? Choosing the Right Vertical Machine
Sources & References
- Teknor Apex: Processing Guide for Injection Molding of Apex Flexible PVC Compounds
- GEON Performance Solutions: Injection Molding Guide: GEON PVC Molding Formulations (PG0019BC, 2021)
- Bureau of Indian Standards, Electrotechnical Department: Compendium of Indian Standards on Power Cables and Accessories (IS 5831)
- Plastics Technology, J. Bozzelli: Injection Molding: A Practical Approach to Calculating Residence Time (Apr 2018)
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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