A 1 litre ice cream tub 100 mm deep needs at least 250 mm of opening stroke, and its two-cavity mould could be about 450 mm thick (an assumed figure). Those two numbers, more than clamp, decide whether an ice cream container making machine at 220 t can run the tub. Cycle time then comes mostly from cooling the wall.
This article covers mould height and stroke checks for deep tubs, what sets cycle time, and resin choice for containers that live in a freezer.
Key Takeaways
- Opening stroke should be at least about 2.5 times the part height, so deep tubs need long strokes even at modest clamp.
- On a toggle machine, maximum daylight equals maximum mould height plus stroke; check the mould thickness against both limits.
- Cooling time scales with the square of wall thickness, so a wall 20% thinner needs about 36% less cooling.
- PP homopolymers turn brittle in the cold; impact copolymers trade some stiffness for toughness.
What Mould Height and Stroke Does an Ice Cream Container Making Machine Need?
Enough mould-height range to accept the closed mould, and enough stroke to open at least about 2.5 times the tub height. A deep tub mould is thick because the core and the cavity are both long, so it can reach the upper end of a machine's mould-height range quickly.
These checks come from Bruce Catoen's MoldMaking Technology guide to machine sizing. The mould must be thick enough for the machine's minimum shut height and thin enough for its maximum. On a toggle clamp, maximum daylight equals maximum shut height plus stroke, so full stroke is available at every mould height. On a direct hydraulic clamp, daylight equals minimum shut height plus maximum stroke.
The 2.5 times guide leaves room for the core to clear the tub and for the tub to drop or be picked. A robot, or a tub stripped off a deep core, can need more. Lids are the opposite case: they are shallow, so stroke seldom limits them.
Two more checks follow from depth. A deep tub needs enough ejector stroke to push it clear of the core, or a stripper plate or air ejection instead. A thick mould is also heavy, and Catoen's guide advises checking the maximum mould weight the machine can carry on both platens.
Clamp is often not the limit on these tubs. A 1 litre round tub with a 120 mm rim has about 113 cm² of projected area, so two cavities need only about 105 t at the upper PP factor, as the method in the applications article in this series shows.
Opening stroke and daylight check for deep tubs (stroke at 2.5 × height; mould thickness at 2 × height + 250 mm, an assumption for illustration)
| Tub height | Minimum opening stroke | Assumed mould thickness | Daylight needed (thickness + stroke) |
|---|---|---|---|
| 60 mm | 150 mm | 370 mm | 520 mm |
| 80 mm | 200 mm | 410 mm | 610 mm |
| 100 mm | 250 mm | 450 mm | 700 mm |
| 120 mm | 300 mm | 490 mm | 790 mm |
| 150 mm | 375 mm | 550 mm | 925 mm |
What Sets Cycle Time on Ice Cream Tubs and Lids?
Mostly cooling. Wall thickness is squared in the cooling equation, so small changes in wall move the cycle a lot. Mould opening and closing, ejection or robot time, and fill and hold make up the rest, and a deep tub adds a longer stroke every cycle.
RJG's guide to cooling time reports a trial in which about 85% of the cycle was spent cooling, and notes that only about 40% of the heat must be removed before the part can be ejected. It recommends adding about 20% to the calculated cooling timer as a safety factor.
Because cooling scales with the square of the wall, a 0.8 mm wall needs about 64% of the cooling of a 1.0 mm wall. Catoen's guide gives dry cycle times of 1 to 2.5 seconds for thin-wall machines and 3 to 5 seconds otherwise, and a long stroke for a deep tub adds to that.
Deep cores are harder to cool than shallow ones, because heat from the inside of the tub has to leave through a long, narrow core. Core cooling, such as baffles or bubblers, can decide the cycle more than the machine does.
Which PP Grades Suit Containers That Go Into a Freezer?
Grades that stay tough when cold. PP homopolymers are stiff but turn brittle at low temperature, while impact copolymers add a rubbery phase that keeps them tougher. An ice cream tub has to balance the two: stiff enough to stack when full, tough enough to survive a cold drop.
Reliance's data sheets show the difference at room temperature. Repol H110MAS, a nucleated homopolymer for rigid packaging, lists a notched Izod impact of 30 J/m and a flexural modulus of 1,650 MPa. Repol B120MA, a medium-impact copolymer, lists 85 J/m and 1,000 MPa. Both comply with IS 10910 for food contact.
Neither sheet gives a sub-zero impact figure, so ask the supplier for low-temperature data before you choose. Food-contact rules are covered in the applications article in this series.
Our view: for a tub that will be handled straight from a deep freezer, start trials with an impact copolymer and move to a homopolymer only if stiffness or clarity demands it. A tub that cracks in a retailer's freezer costs more than a slightly heavier wall.
What Should You Send for an Ice Cream Container Machine Check?
Send the tub and lid drawings with height, rim, wall thickness and resin, the mould's closed thickness if the mould exists, the cavity count and the planned take-out method. Mould height, stroke, daylight and cycle can then be checked together. See the 220 ton horizontal injection moulding machine page.
Frequently Asked Questions About Ice Cream Container Moulding
Can the same machine mould ice cream cups and family packs?
Yes, if each mould passes its own checks. Small cups are shallow and can run more cavities, so clamp and shot become the limits. Family packs are deep, so stroke and mould height decide. Check the deepest pack against stroke and daylight first, because it is the mould most likely to fail.
Why do deep tubs stick on the core?
PP shrinks onto the core as it cools, and a deep core gives a large gripping area. Draft angle, core surface finish, core temperature and air poppets that break the vacuum under the tub all help release. Ejecting too early can deform the rim, so balance cooling time against sticking.
Does a stack mould suit ice cream lids?
It can, because lids are shallow and a stack mould adds a second parting face without doubling the clamp. Catoen's guide sizes a stack mould's clamp as one face plus about 10% for each extra level. A stack mould is thicker, though, so check mould height and daylight as well.
How can I cut cycle time on an existing ice cream tub mould?
Start with cooling: check water flow in every circuit, core cooling and the gap between chiller set point and actual mould temperature. Then trim mould movement with faster, safe open and close profiles, and confirm ejection is not waiting on a robot. Any change to wall thickness needs customer approval.
Should ice cream tubs be clear or opaque?
Opaque tubs hide colour changes in the product and need no clarifier. Clear tubs show the product but need a clarified grade, and they show flow lines and gate marks more readily. A clarified grade may also be less tough in the cold than an impact copolymer, so test drop strength.
Related Articles
- What Can You Make on a 220 Ton Container Moulding Machine?
- IML Tubs and Lids on a 220 Ton Machine
- Three Screw Options on a 220 Ton Container Machine Explained
Sources & References
- MoldMaking Technology, B. Catoen: Machine Sizing and Specification for Optimal Mold Performance, Part 1
- RJG Inc.: How to Determine Injection Molding Cooling Time
- Reliance Industries: Repol H110MAS Technical Data Sheet (July 2024)
- Reliance Industries: Repol B120MA Technical Data Sheet (July 2024)
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