A plant moves a 16-cavity cap mould with a 32 g shot onto a press with a much larger barrel, because that press was free. Within days, caps show black specks and faint colour streaks. The clamp is fine and the mould is clean. The melt is staying in the barrel for too many cycles.
That is why the closure injection moulding screw should be sized to the shot, not to the biggest job you might one day run. For light closure shots, a small screw such as 35 mm keeps the shot in a healthy share of the barrel and gives more melt pressure for thin skirts and stiff closure grades.
Key Takeaways
- John Bozzelli's guidance is to use 25% to 65% of barrel capacity per shot; below 25%, melt uniformity can be poor.
- On the same injection unit, pressure scales with 1 ÷ D². A 35 mm screw gives about 1.3 times the pressure of a 40 mm screw and 1.65 times that of a 45 mm one.
- Residence-time formulas understate reality, because screws trap resin in dead spots; measure it with a colour test.
- Indian closure grades can be stiff-flowing. Reliance lists an MFR of 1.6 g/10 min for its bimodal HDPE cap grade Relene 54GC016.
Why Does a Light Shot Need a Small Closure Injection Moulding Screw?
A shot should use roughly 25% to 65% of the barrel. A light cap shot on a big barrel falls below that band, so the melt gets few screw turns, mixes poorly and sits hot for many cycles. A smaller screw puts the same shot back inside the band.
This is the guidance John Bozzelli gives in Plastics Technology. Below 25%, he writes, melt uniformity is often poor because the resin sees few screw revolutions; above 65%, unmelt and long recovery appear. He adds that about 80% of the melting energy should come from shear.
In a second column he warns against buying the biggest screw by default, because big screws usually mean lower plastic pressure. He also explains that machine shot ratings are given in grams of PS, so you should work in volume: PS has a melt density of about 0.945 g/cm³, against about 0.74 for PP.
Three screw sizes on the same injection unit (calculation, same stroke and hydraulics)
| Screw diameter | Screw area | Swept volume per 100 mm of stroke | Pressure relative to 35 mm |
|---|---|---|---|
| 35 mm | 9.62 cm² | 96 cm³ | 1.00 |
| 40 mm | 12.57 cm² | 126 cm³ | 0.77 |
| 45 mm | 15.90 cm² | 159 cm³ | 0.61 |
How Does Screw Diameter Change Pressure for Closures?
The hydraulic force behind the screw is shared over the screw's area. A smaller screw has less area, so the same force gives higher melt pressure. On one injection unit, pressure scales with 1 ÷ D², which is why 35 mm out-pushes 40 mm by about 30%.
Closures can use that pressure. Skirts are thin, threads are fine and tamper bands are thinner still, so the melt meets high resistance near the end of fill. A screw with pressure in reserve fills those features at a controlled speed instead of running at its limit.
Resin grade adds to the need. Reliance describes Relene 54GC016 as a bimodal HDPE for injection and compression moulded caps, with an MFR of 1.6 g/10 min, density of 0.954 g/cm³ and processing temperatures of 180 to 220 °C. Its sister grade 54GC017, with slip additive, is listed for carbonated soft drink closures at an MFR of 1.25.
The same data sheet lists compliance with IS 10146 and IS 16738:2018, the positive list for PE and PP in food contact, and the BIS designation IS 7328-3B-CB-FDB. Low-MFR grades like these resist flow more than general moulding grades, which favours a smaller, higher-pressure screw.
How Long Does the Melt Stay in the Barrel?
Longer than most formulas suggest. Residence time is often estimated from barrel capacity, the share used per shot and the cycle time. Bozzelli argues those formulas miss resin trapped in dead spots at the base of the flights, which can stay in the screw for a long time.
He reports finding such trapped resin nearly 90% of the time when screws are pulled for service. In one case, red resin from a short run clung to the flights for over a month while the machine ran only black.
His fix is an experiment. With the machine on cycle, drop a few granules of a different colour into the feed throat, count shots until that colour is gone, and multiply by the cycle time. Swirls or streaks in the mixed colour also tell you the melt is not uniform.
For closures this matters because the shot is small and the hot runner adds its own melt volume. A smaller screw keeps the barrel's share of that total under control.
When Is One Screw Size Enough?
One screw size is enough when every job you plan sits in the same shot band, such as a closure line running caps of similar weight. A choice of screws matters when the same machine must run light caps one month and heavy containers the next.
Our opinion is plain: for a dedicated closure line, buy the screw for the shot you run every day, not the largest shot you might run once. A screw sized for occasional heavy jobs puts every daily cap shot at the bottom of the barrel's range.
If your product mix is truly wide, Bozzelli notes that a second barrel ordered with a new machine can cost little, and a skilled operator can change it in under 30 minutes. Cycle time and cavity count for closures are covered in the cap making article in this series.
What Should You Send to Choose a Screw for Closures?
Send the cap weight, cavities, runner type and any hot runner volume, the resin grade and its MFR, the target cycle, and any heavier jobs you plan for the same machine. With those, the shot's barrel share, pressure margin and residence can be checked. See the 140 ton container injection moulding machine page.
Frequently Asked Questions About Closure Injection Moulding Screws
What does single-screw mean on a machine datasheet?
Here it means the machine is offered with one screw diameter rather than a choice of two or three. It does not mean a different type of injection unit. A single option suits buyers whose jobs sit in one shot band, such as caps and closures of similar weight.
Can a small screw handle a heavier container shot later?
Only if that shot stays below about 65% of the barrel. Above that, recovery time grows and unmelt can appear, as Bozzelli warns. If heavier jobs are planned, check them now and consider a second barrel, which can be cheaper to order with the machine than later.
Why do black specks appear in light-coloured caps?
Degraded resin can cause them, from melt staying too long in the barrel or hanging up in dead spots on the screw, check ring or hot runner. Check the shot's share of the barrel, purge thoroughly, and inspect the screw and check ring if specks persist after purging.
Does screw speed matter for closure resins?
Yes. Most melting energy should come from shear, so screw speed and back pressure decide melt temperature and uniformity. Too fast can overheat a low-MFR grade; too slow can leave unmelt. Recovery must still finish inside the cooling time, or it adds directly to the cycle.
How do I convert a PS shot rating to HDPE?
Work in volume. Divide the PS rating by PS melt density, about 0.945 g/cm³, to get the swept volume, then multiply by your resin's melt density from the supplier. Bozzelli gives about 0.74 g/cm³ for PP; ask your HDPE supplier for that grade's value rather than guessing.
Related Articles
- What Can You Make on a 140 Ton Container Moulding Machine?
- Cap and Closure Production on a 140 Ton Machine: Cavities and Cycle Time
- 140 Ton vs 180 Ton for Thin-Wall Cups and Small Tubs
Sources & References
- Plastics Technology, J. Bozzelli: Revisiting Shot Size vs. Barrel Capacity (Jan 2020)
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity (2017, updated 2022)
- Plastics Technology, J. Bozzelli: Injection Molding: A Practical Approach to Calculating Residence Time (Apr 2018)
- Reliance Industries: Relene 54GC016 HDPE for caps and closures, technical data sheet (Feb 2026)
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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