A sweets maker wants 500 g tubs and lids with a printed label that will not peel off in the fridge. The only free press is a 220 t machine already running plain tubs. IML tub moulding can put the label inside the mould, but it brings a robot, a label magazine, a way to hold each label in the cavity and a longer cycle.
This article covers how labels are held, what changes in the cycle and the mould, and how to choose labels and resin for tubs and lids.
Key Takeaways
- IML places a printed label in the open mould so it becomes part of the tub, removing a separate printing or labelling step.
- Labels are held by vacuum ports in the mould or by a static charge; static avoids vacuum passages in the tool.
- A charged label needs an insulating face toward the steel, ideally 10¹² ohms/sq or higher.
- Each label placement adds robot time, so plan output on the labelled cycle, not the plain one.
What Does IML Tub Moulding Add to a Standard Tub Job?
A robot with an end-of-arm tool, a label magazine, a way to hold each label in the cavity and labels built for the process. The machine still needs the clamp, shot and stroke of a plain tub job, plus room and interlocks for the robot to work safely.
Scott Shelton's Plastics Technology article lists what IML replaces: a separate pad or screen printing or labelling step, along with corona or flame treating. The decoration is permanent, handling drops, and recyclability improves when the label is made from the same material as the tub, such as a PP label on a PP tub.
The clamp check is the same as for an unlabelled tub, as the applications article in this series explains. The label covers part of the cavity wall, not the rim, so it does not change the projected area.
Should the Label Be Held by Vacuum or by Static Charge?
Both work. Vacuum suits complex preformed labels and textured surfaces but needs ports machined into the mould. Static charge holds the label against the grounded steel with no ports, which keeps the tool simpler and avoids melt being drawn into passages if a label is missed.
Vacuum and electrostatic label holding compared, as described by Plastics Technology
| Point | Vacuum ports | Static charge |
|---|---|---|
| Mould cost | Ports and passages add significant tooling cost | No vacuum needed in the mould |
| Missed label | Melt can enter the ports, with costly cleaning downtime, unless vacuum is sensed before injection | No ports to block |
| Mould temperature | Passages may cause uneven die temperatures | No passages to disturb cooling |
| Label | Must be stiff enough not to be pulled into ports, which can leave pimples | Needs an insulating face that holds charge |
| Best fit | Complex preformed labels, textured surfaces | Film labels on smooth cavity surfaces |
In the static method, the robot picks the label by suction, a charging applicator puts a high static charge on it, and the robot releases it against the cavity. Shelton describes a high-voltage DC supply with up to 30 kV adjustable output, charging times of about 0.5 to 2 seconds and emitters about 1 in (25 mm) behind the label.
Flat lid labels can be charged by a fixed bar that the robot passes on its way to the mould. Round tubs with a wrap-around label need the mandrel holding the label to rotate past the charger for full coverage. Shelton also warns that charging and placing labels by hand is labour-intensive, unreliable and slows the cycle.
How Does IML Change the Cycle and the Mould?
It adds robot time while the mould is open, and it changes how the labelled face cools. The mould must open far enough for the end-of-arm tool to enter, place labels and take out finished tubs, and the gate must sit where melt flow will not push the label.
Each cycle now includes the robot entering, placing labels and removing tubs, often in one move. How much time that adds depends on the robot, the stroke and the label count, so measure it on a trial rather than assuming the plain-tub cycle.
The label also sits between the melt and the steel. A film label conducts heat less well than the steel it covers, so that face can cool more slowly, and the tub can warp if the cooling layout does not allow for it. Shelton lists moulding temperature, polymer compatibility, gate location and melt flow among the factors that affect adhesion.
Thin-wall IML tubs also bring the speed and pressure demands covered in the screw options article in this series.
Which Label and Resin Choices Matter for Tubs and Lids?
Use labels made for IML, with an insulating face toward the steel if you hold them by static, and a resin grade that flows well enough for the wall and the cavity count. For food tubs, the inks and pigments on the label must also meet FSSAI rules.
Shelton recommends a contact-face resistivity of 10¹² ohms/sq or higher, so the label keeps its charge on the steel. Conductive inks, coatings or foils must sit on the back of the label. Curl, thickness and texture matter too: a thin, smooth label on a smooth cavity holds best.
For resin, Reliance lists Repol H200MA, a PP homopolymer with an MFR of 25 g/10 min, for thin-walled containers, multi-cavity moulding and long flow paths. Its data sheet gives a melt temperature of 210 to 250 °C, a mould temperature of 20 to 50 °C and compliance with IS 10910 for food contact.
Under the FSSAI Packaging Regulations, printing inks must meet IS 15495 and pigments IS 9833. The applications article in this series covers the wider food-contact rules.
Our view: run the label trial before the mould is finished. Label stock, charging and gate position interact, and fixing a drifting label on a trial insert costs far less than fixing it on a finished multi-cavity tool.
What Should You Prepare Before an IML Tub Trial?
Prepare the tub and lid drawings, the label artwork with its material and thickness, your preferred holding method, the cavity count and the target output, and ask your label supplier to attend the trial. See the 220 ton horizontal injection moulding machine page.
Frequently Asked Questions About IML Tubs and Lids
Can IML tubs and lids run on the same machine?
Yes, as separate moulds, provided each fits the clamp, tie bars and stroke. Lids are flat, so a fixed charging bar can often charge their labels, while round tubs with wrap-around labels need a rotating mandrel. Plan the robot tooling for both before you order either mould.
Why does a label slip or move during injection?
Common causes are a label that loses its charge on the steel, curl from storage or an uneven laminate, a textured label or cavity surface, or melt from the gate pushing the label. Check the label's surface resistivity, store labels flat and review gate position with the mould maker.
Do IML tubs need a special resin grade?
Not special, but suitable. The grade must fill the wall at the planned cavity count, bond with the label and meet food-contact rules where needed. High-flow PP homopolymers made for thin-walled containers are one option; confirm label adhesion on a trial, because it also depends on melt and mould temperature.
Is IML better than printing tubs after moulding?
It depends on volume and design. IML removes a separate printing or labelling step and gives a permanent decoration, but it adds robot, label and tooling cost and a longer cycle. Long runs of one design can pay that back; many short runs of different designs may suit after-printing better.
What safety points apply to static charging equipment?
Shelton recommends current-limited applicators, which will not hard arc if they come too close to the mould. Arcs can pit the cavity surface and cause radio interference with robot or machine controls. Support the high-voltage cable along the robot arm with enough slack, and inspect it weekly.
Related Articles
- What Can You Make on a 220 Ton Container Moulding Machine?
- Three Screw Options on a 220 Ton Container Machine Explained
- Ice Cream Containers: Mould Height and Cycle Time on a 220 Ton Machine
Sources & References
- Plastics Technology, S. E. Shelton (Simco Industrial Static Control): In-Mold Labeling: Electrostatics Are the Way to Go
- Reliance Industries: Repol H200MA Technical Data Sheet (July 2024)
- 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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