A buyer puts three samples on your desk: a 300 ml PP mug with a handle, a 150 mm serving bowl and a two-compartment lunch box with a clip lid. Can one 120 t machine run all three? Yes. Moulding mugs, bowls and lunch boxes on 120 t works, one mould at a time, if each mould passes three checks beyond clamp.
Those checks are opening stroke for the part depth, shot volume for the cavities, and cooling time for the wall. Clamp is the fourth, and the applications article in this series covers it in full. Here the focus is on what makes deep houseware different from flat lids and trays.
Key Takeaways
- Allow an opening stroke of at least 2.5 times the part height, so a 100 mm mug needs 250 mm or more.
- Work out shot size in volume at melt density and keep the shot between about 20% and 65% of barrel capacity.
- Cooling time rises with the square of wall thickness, so a 20% thinner wall needs about 36% less cooling.
- Lunch boxes sold to food businesses as packaging fall under the FSSAI Packaging Regulations, 2018.
How Much Opening Stroke Do Deep Mugs and Bowls Need?
Allow an opening stroke of at least 2.5 times the part height, so the part clears the core and can drop or be picked. A 100 mm mug needs 250 mm or more. Then check daylight, because a thick mould with a long core can use it up.
The 2.5 times rule comes from a MoldMaking Technology article by Bruce Catoen on machine sizing. The same article explains daylight. On a toggle clamp, maximum daylight equals maximum shut height plus stroke, so full stroke is available at every mould thickness. On a direct hydraulic clamp, it equals minimum shut height plus maximum stroke.
Mugs add a complication. A handle on the side of the part usually needs a slide or side core to release, and the slide makes the mould wider and often thicker. Check the mould outside size against the tie-bar space as early as you check stroke.
Bowls are wide and open, so ejection is the next concern. A deep bowl can hold on the core by vacuum as it shrinks. An air poppet or a stripper ring at the rim breaks that vacuum and pushes evenly, which a ring of small pins may not do without marking the part.
What tends to limit each part on a 120 t machine
| Part | Likely limit | What to check first |
|---|---|---|
| Mug with handle | Part depth and the side action for the handle | Stroke of at least 2.5 × height; mould width with the slide |
| Serving bowl | Projected area and core cooling | Clamp at the upper PP factor; cooling channels in the core |
| Lunch box with partitions | Uneven walls between base, partitions and rim | Wall ratios, cooling time and warpage of the long sides |
| Clip lid for the lunch box | Latch or living hinge strength | Hinge thickness and flexing the hinge right after ejection |
What Changes When Moulding Mugs, Bowls and Lunch Boxes in PP?
Shot size changes most. Deep parts with 2 to 3 mm walls weigh far more than flat lids of the same footprint, so the injection unit, not the clamp, can become the limit. Size the shot in volume at melt density, at about 20% to 65% of barrel capacity.
John Bozzelli's Plastics Technology column on shot size sets that window. Below 20% of barrel capacity, the melt sits in the barrel too long and can degrade. Above 65%, melt quality suffers. He also warns against buying the biggest screw on offer, because a bigger screw usually means lower plastic pressure.
Units matter here. Machine shot ratings are usually given in grams of PS, and PS melt density is about 0.945 g/cm³ against about 0.74 g/cm³ for PP. So a barrel holds only about 78% of its PS gram rating when filled with PP melt. Add 10% for check-valve leakage and melt uniformity.
Our view: never size a houseware machine on a shot weight in PS grams. Convert the part weight to melt volume first, because the gap between the two figures is the difference between a good mug and a short shot on the fourth cavity.
How Do Wall Thickness and Cooling Set the Cycle?
Cooling can take most of the cycle on thick houseware walls, and its length depends mostly on wall thickness, which is squared in the cooling equation. Thinning a wall from 2.5 mm to 2.0 mm, 20% thinner, cuts the cooling time by about 36%, if stiffness still meets the design.
RJG's guide on cooling time reports a trial on an eight-cavity ABS tensile bar 3.3 mm thick in which about 85% of the cycle was cooling. It notes that only about 40% of the heat has to be removed before ejection, and suggests adding about 20% to the calculated timer as a safety factor.
The thickest section sets the cycle. On a mug, that is often the handle root or the base. On a lunch box, it can be the rim or the junction where a partition meets the floor. Coring out thick spots and keeping walls even shortens the cycle more than a colder chiller.
For lunch boxes, the FSSAI Packaging Regulations, 2018 apply where a food business uses the boxes as packaging. PP must conform to IS 10910, overall migration is limited to 60 mg/kg or 10 mg/dm² tested to IS 9845, and pigments must meet IS 9833. The food business needs an NABL lab certificate for direct-contact packaging.
What Should You Check Before Ordering the Moulds?
For each mould, write down part height, part weight, cavity count, runner type and resin grade. Work out stroke, shot volume and clamp from those figures, and ask the toolmaker for the mould outside size and thickness. See the 120 ton horizontal injection moulding machine page.
Frequently Asked Questions About Moulding Mugs, Bowls and Lunch Boxes
Can a mug handle be moulded without a side core?
Sometimes. If the handle opening lines up with the direction the mould opens, the parting line can run through it and no slide is needed. A closed loop handle at right angles to the opening needs a slide or a collapsible feature. Ask the toolmaker which design the handle allows before finalising it.
Should a lunch box lid use a living hinge or separate clips?
A PP living hinge saves parts and assembly but must be thin, well gated and flexed soon after ejection so it does not crack in use. Separate clips cost an extra mould and assembly step but are easier to replace. The choice depends on price point and how often the lid is opened.
Why do deep bowls stick on the core?
PP shrinks onto the core as it cools, and a deep, smooth bowl can seal against the steel and hold by vacuum. Enough draft, an air poppet to break the vacuum and a stripper ring for even push all help. Polishing direction and core temperature also affect how cleanly the part releases.
Do I need an NABL certificate for household lunch boxes?
Under the FSSAI Packaging Regulations, the food business that packs food in direct contact needs an NABL lab certificate of conformity for that packaging. If your boxes go to such a buyer, expect them to ask you for test support. For retail houseware, confirm the requirement with your buyer and a testing lab.
How do I stop sink marks on a thick mug handle?
Core out the handle so its wall matches the body where the design allows, and gate so the thick section packs before the gate freezes. Longer hold time helps up to the point of gate freeze. If the handle must stay solid, a textured finish can hide sinks that cannot be fully removed.
Related Articles
- What Can You Make on a 120 Ton Household Moulding Machine?
- 120 Ton vs 125 Ton Machine: Which Suits Houseware?
- Starting a Houseware Unit With a 120 Ton Machine
Sources & References
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- RJG Inc.: How to Determine Injection Molding Cooling Time
- 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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