IML Thin-Walled Yogurt Cups/Milk Tea Cups: Key Production Points and Quality Control

In the food packaging field, IML (In-Mold Labeling) thin-wall yogurt and milk tea cups have become the mainstream packaging choice due to their high appearance, good sealing, and environmental recyclability. Their production process integrates two core technologies: injection molding and label bonding, which have extremely high requirements for precision, speed, and stability. However, many manufacturers frequently encounter quality defects such as label deviation, cup deformation, poor sealing, and uneven cup wall thickness during IML thin-wall yogurt/milk tea cup production, which directly affect the product appearance, sealing performance, and use safety. This article systematically analyzes the core points of IML thin-wall cup production and provides a complete set of solutions from process, equipment to raw materials, helping enterprises achieve efficient and high-quality production.

I. Production Points of IML Thin-Wall Yogurt/Milk Tea Cups

The core of IML thin-wall yogurt/milk tea cup production is the simultaneous completion of “injection molding + in-mold labeling”, which needs to balance the high speed and efficiency of thin-wall molding and the precision and stability of label bonding. The core points run through the three links of process, equipment and mold:

Precise matching of process parameters: The wall thickness of thin-wall cups is usually 0.2-0.5mm. Multi-stage injection process is required to control the melt flow speed and pressure to avoid defects such as material shortage, flash and shrinkage marks; at the same time, coordinate the labeling sequence to ensure that the label and melt are bonded synchronously without bubbles or wrinkles.
Equipment performance adaptation: It is necessary to select a high-speed thin-wall injection molding machine, equipped with a servo drive system, to achieve fast mold clamping, high-speed injection and precise holding pressure, and shorten the molding cycle (usually 5-10 seconds per mold); at the same time, configure a special IML labeling mechanism to ensure the precision of label picking, placing and positioning.
Mold structure optimization: The mold needs to adopt high-precision cavity and core to ensure the dimensional accuracy of the cup body; design a reasonable gate and exhaust system to avoid melt turbulence and gas trapping; configure a label positioning device to prevent label deviation and ensure perfect bonding between the label and the cup body.

II. Common IML Thin-Wall Production Quality Defects and Solutions

During IML thin-wall yogurt/milk tea cup production, quality defects are mainly concentrated in three aspects: label bonding, cup molding and sealing. Their roots are closely related to process, equipment and raw materials. The specific solutions are as follows:

Label deviation, wrinkles and bubbles: The reasons are insufficient label positioning accuracy, too fast injection speed, poor mold exhaust or poor flatness of the label itself; the solutions are to adjust the positioning accuracy of the IML labeling mechanism, optimize the injection speed (slow and stable in the thin-wall filling stage to avoid turbulence), add exhaust grooves on the mold parting surface, and select IML special labels with qualified flatness.
Cup deformation and uneven thickness: The reasons are insufficient clamping force, unstable holding pressure, uneven mold temperature or too high melt temperature; the solutions are to increase the clamping force, adopt a stepped holding pressure curve, optimize the mold cooling water channel (ensure uniform cooling of all parts of the cup body), and control the melt temperature within a reasonable range (adjust according to the type of raw material).
Poor sealing and liquid leakage: The reasons are out-of-tolerance flatness of the cup mouth, flash or shrinkage marks on the cup mouth, and poor fluidity of raw materials; the solutions are to optimize the precision of the mold cavity and core, adjust the injection and holding pressure parameters, eliminate the cup mouth defects, select suitable high-fluidity raw materials, and ensure that the cup mouth and cup lid fit closely.
Label falling off and blurring: The reasons are poor compatibility between the label and the raw material, insufficient melt temperature or too short holding time; the solutions are to select IML labels with good compatibility with injection raw materials (such as PP, PET), increase the melt temperature, extend the holding time, and ensure that the label is firmly combined with the cup body.

III. Equipment Requirements for IML Thin-Wall Production

In view of the particularity of IML thin-wall yogurt/milk tea cup production, the equipment needs to balance high speed, precision and stability. The following core configurations are recommended, which are suitable for mass production of thin-wall cups (yogurt cups, milk tea cups) below 500ml:

Injection molding machine selection: Select a high-speed special injection molding machine for thin walls, with a clamping force range of 80-200T, an injection speed ≥300mm/s, and equipped with a servo energy-saving system to balance production efficiency and energy consumption control; for different raw materials such as PP and PET, a special plasticizing system can be adapted to avoid raw material degradation.
IML labeling system: Configure a fully automatic IML labeling mechanism, including an integrated device for label feeding, picking, positioning and bonding, with a positioning accuracy of ≤±0.1mm, and the labeling speed is synchronized with the injection cycle, supporting mass production of multi-cavity molds (usually 4-8 cavities).
Auxiliary equipment: Equipped with a dehumidifying dryer (for PET raw materials, drying temperature 80-90℃, drying time 2-3 hours, ensuring moisture content ≤100ppm), a mold temperature controller (controlling the mold temperature at 30-50℃ to ensure the cup forming accuracy and label bonding effect), and an automatic part-taking manipulator to realize fully automatic production and reduce manual intervention.

IV. IML Thin-Wall Production Mold Structure and IML Optimization

The mold is the key to determining the forming quality and labeling effect of IML thin-wall yogurt/milk tea cups. A precision IML thin-wall cup mold needs to meet the following optimization requirements:

Cavity and core design: Adopt high-precision processing technology to ensure the cavity surface is smooth (Ra≤0.2μm), avoiding scratches and defects on the cup surface; the fit gap between the core and the cavity is controlled at 0.01-0.02mm to prevent flash and ensure the cup mouth precision.
Gate and exhaust system: Adopt pinpoint gate or hot runner gate to reduce gate marks and ensure the cup body appearance is clean; add annular exhaust grooves on the cavity parting surface and label bonding area, with exhaust groove width 0.01-0.02mm and depth 0.1-0.2mm, effectively discharging the gas in the melt and avoiding gas trapping and bubble defects.
Label positioning and bonding mechanism: Set positioning pins and adsorption devices in the mold to ensure precise label positioning and avoid deviation; optimize the mold cooling water channel, set an independent cooling circuit in the label bonding area, control the temperature stably, ensure that the label and melt are quickly bonded and firmly combined without wrinkles or bubbles.

V. IML Thin-Wall Production Raw Material Selection and Pre-treatment

The performance of raw materials directly affects the forming quality, sealing performance and safety of IML thin-wall yogurt/milk tea cups. It is necessary to strictly follow food-grade standards and focus on the following points:

Main raw material selection: Priority is given to food-grade PP (polypropylene) or PET (polyethylene terephthalate). The melt flow index of PP raw material is controlled at 5-10g/10min, which has good fluidity, heat resistance and toughness, suitable for thin-wall molding; the melt flow index of PET raw material is controlled at 3-6g/10min, with high transparency and good hardness, suitable for high-end milk tea cups and yogurt cups.
IML label raw material: Select food-grade IML labels with good compatibility with the main raw materials. The label surface needs to be specially treated (such as corona treatment) to improve the bonding force with the melt and avoid label falling off; the label thickness is controlled at 0.03-0.05mm, with good flatness, no wrinkles or damage.
Raw material pre-treatment: Before molding, PP raw materials need to be dried at 60-70℃ for 1-2 hours, and PET raw materials need to be dried at 80-90℃ for 2-3 hours to ensure that the raw material moisture content meets the requirements (PP≤200ppm, PET≤100ppm), avoiding silver streaks and bubbles on the cup body caused by water vapor, which affect the sealing performance and appearance.

VI. IML Thin-Wall Production Quality Control and Testing Standards

To ensure the product consistency and safety of IML thin-wall yogurt/milk tea cup production, it is necessary to establish a full-process quality control system, focusing on the following testing items and standards:

Appearance inspection: Online inspection of the cup surface without scratches, defects or bubbles, the label is flat, no deviation, no wrinkles, no falling off, and the label pattern is clear and not blurred; offline sampling inspection, the appearance qualification rate is ≥99.5%.
Dimensional accuracy inspection: Use non-contact optical inspection equipment to sample and inspect the inner diameter, outer diameter, height, wall thickness and flatness of the cup mouth, with the error controlled within ±0.02mm; the roundness of the cup mouth is ≤0.03mm to ensure perfect fit with the cup lid.
Sealing inspection: Use the pressure difference method to conduct air tightness test on the cup body. Under the pressure of 50-80kPa, keep the pressure for 30 seconds without air leakage or liquid leakage; the sampling inspection qualification rate is 100% to avoid food leakage and deterioration.
Food safety inspection: Regularly sample and inspect the heavy metals, peculiar smell, migration and other indicators of the cup body and labels, which comply with the food-grade packaging standards of GB 4806.1-2016 and GB 4806.7-2016 to ensure food safety.
Production process control: Through SPC statistical process control, real-time monitor the injection process parameters, labeling accuracy and equipment operation status, timely find and correct process drift, and ensure the stability of the production process.