Four cavities at about 175 t, or two at about 140 t. That is the clamp for a 250 ml round thin-wall tub with an 85 mm rim, at published thin-wall factors of 5 and 8 US t/in² with a 10% reserve. Thin-wall food container moulding on a 180 t machine works, but the flow ratio of the part decides how many cavities it can carry.
The machine build, the mould and the resin grade then decide whether those cavities fill at a useful cycle. This article takes each in turn, then covers the food-contact rules that apply in India.
Key Takeaways
- Thin-wall parts have flow length to wall thickness ratios of about 100:1 to 200:1 and a published clamp range of 5 to 8 US t/in², against 3 to 5 for conventional moulding.
- They can need injection pressures above 30,000 psi (about 2,109 kg/cm²) and fill times under one second.
- Reliance lists Repol H200MA, a PP homopolymer with an MFR of 25 g/10 min, for thin-walled containers and long flow paths.
- FSSAI sets an overall migration limit of 60 mg/kg for plastics in direct food contact and requires an NABL lab certificate of conformity.
What Makes a Food Container Thin-Wall?
The ratio of flow length to wall thickness, not the wall alone. A PlasticsToday article by Gary Tremblay of Sabic Innovative Plastics puts thin-wall designs at about 100:1 to 200:1. At those ratios, the melt must travel far through a narrow gap before it freezes.
A tub gated at the centre of its base with a 0.5 mm wall and 100 mm of flow to the rim sits at 200:1, the top of that range. The same tub at 0.7 mm sits near 143:1, which is a different job.
Flow ratio for a tub with 100 mm of flow from gate to rim (calculated)
| Wall thickness | Flow ratio | Where it sits |
|---|---|---|
| 1.2 mm | 83:1 | Below the thin-wall range; conventional moulding |
| 0.8 mm | 125:1 | Inside the range, lower half |
| 0.6 mm | 167:1 | Inside the range, upper half |
| 0.5 mm | 200:1 | Top of the published range |
| 0.4 mm | 250:1 | Beyond the range; needs a fill study before quoting |
The article says thin-wall resins need high flow and must withstand shear heating of 50,000 s⁻¹ or more. It also gives the clamp range of 5 to 8 US t/in², which is about 0.703 to 1.125 t/cm². The general clamp method is in the applications article in this series.
Can a 180 t Machine Handle Thin-Wall Food Container Moulding?
It can if the machine is built for speed. Tremblay's article lists injection pressures above 30,000 psi, injection velocities above 3 in/s (76 mm/s) and often 6 in/s (152 mm/s), and fill times under one second. A general-purpose press may not deliver all three together.
The article describes thin-wall machines with accumulator-assisted clamps for fast movement and extra-thick platens. On better machines, the tie-bar distance to platen thickness ratio is 2:1 or less, so the platen does not flex under high cavity pressure. It advises against adapting a conventional machine for this work.
Control matters too. The article lists resolutions of 0.04 in/s on speed, 1 bar on pressure, 0.004 in on position and 0.01 s on time. At fill times under a second, coarser control shows up as short shots in one cycle and flash in the next.
Our view: for flow ratios near 200:1, buy the speed and the platen stiffness first and the cavity count second. A four-cavity mould that fills on a fast machine earns more than an eight-cavity mould that only fills on a good day.
How Should a Thin-Wall Mould Be Built?
Stiff, well vented and fast to cool. Tremblay recommends steels harder than P-20, thicker plates, as many heavy support pillars as fit, and interlocks. Demanding parts vent core pins and ejector pins as well as the parting line, because trapped air burns at these fill speeds.
For cooling, the article favours non-looped lines and larger lines for more flow rather than colder coolant, plus high-conductivity inserts. For ejection, a rule of thumb is double the pins at twice the size, with blade or sleeve ejectors on ribs. Thin walls shrink little, so they grip the core.
Gates and runners are a trade-off. Large ones cut pressure drop but raise residence time; small ones may not fill. A mould-filling analysis before steel is cut settles the balance.
Which Resin and Food Rules Apply in India?
Use a high-flow food-contact PP and check FSSAI rules. Reliance lists Repol H200MA for thin-walled containers, multi-cavity moulding and long flow paths, with an MFR of 25 g/10 min, melt at 210 to 250 °C, mould at 20 to 50 °C and IS 10910 compliance.
Under the FSSAI Packaging Regulations 2018, plastics for food containers must conform to listed Indian Standards, including IS 10910 for PP, IS 10146 for PE and IS 10142 for PS. Overall migration is limited to 60 mg/kg or 10 mg/dm², tested to IS 9845, and pigments must meet IS 9833.
Food businesses need a certificate of conformity from an NABL-accredited lab for packaging in direct food contact. Send that requirement to your resin and masterbatch suppliers with the order.
What Should You Send Before Ordering a Thin-Wall Mould?
Send the container drawing with rim size, wall thickness, gate position and flow length, the resin grade and the target cavity count. With those, the flow ratio, clamp and fill demands can be checked before steel is cut. See the 180 ton horizontal injection moulding machine page.
Frequently Asked Questions About Thin-Wall Food Container Moulding
Why do thin-wall containers crack at the rim?
The rim is the last place to fill, so it gets the least packing and the most orientation. High-flow homopolymers also have modest impact strength; Reliance lists 27 J/m notched Izod for Repol H200MA. If rims crack in drop tests, review gate size and pack time, then consider a copolymer grade.
Can a cold runner be used on a thin-wall mould?
It can, but it adds projected area to the clamp and pressure drop to the fill, both of which thin-wall moulds can least afford. It also adds regrind on every shot. For multi-cavity thin-wall food containers, a hot runner tends to give the better balance between cavities and the shorter cycle.
How thin can a PP food container wall go?
There is no fixed figure. The limit depends on flow length, grade flow, injection pressure and speed, and mould venting. Published thin-wall work runs to flow ratios of about 200:1. Run a mould-filling analysis with your grade and gate position before you commit to a wall below what has run before.
Does a thin-wall mould need special steel?
It needs harder steel and more support than a conventional mould. Tremblay's guidance calls for steels harder than P-20, thicker plates, support pillars with a small preload and interlocks to keep the halves aligned. These add cost to the mould but protect it from the high cavity pressures thin walls create.
What tests must a food container pass in India?
The resin must conform to the listed Indian Standard, such as IS 10910 for PP. The packaging must meet the overall migration limit tested to IS 9845 and the specific migration limits for listed metals, pigments must meet IS 9833, and the food business needs an NABL lab certificate of conformity.
Related Articles
- What Can You Make on a 180 Ton Container Moulding Machine?
- 40 mm or 45 mm Screw? Choosing for Thin-Wall Packaging
- Dairy Cups and Small Tubs: Output Planning on a 180 Ton Machine
Sources & References
- PlasticsToday, G. Tremblay (Sabic Innovative Plastics): Never too thin
- Reliance Industries: Repol H200MA Technical Data Sheet
- Food Safety and Standards Authority of India (FSSAI): Food Safety and Standards (Packaging) Regulations, 2018, compendium
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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