Source: Link Testing Instruments Co.,Ltd.
In the final packaging of ready-to-eat meat products, transparent heat-shrink film is often used for secondary wrapping to achieve a tight fit, enhance appearance, prevent scattering, and allow for additional brand printing. However, if the film's shrinkage characteristics do not match the product shape and the heat tunnel parameters of the production line, it can lead to problems such as packaging wrinkles, film breakage, localized overheating damaging the product surface, or incomplete shrinkage affecting aesthetics. A large meat processing company encountered challenges with unstable finished product packaging yield when introducing a fully automatic heat-shrink packaging line for its newly launched square-sliced ham.
I. Specific Problem: Yield Fluctuation in the Packaging of Square-Shaped Ham Sausages on the New Production Line
The company's new production line is designed to arrange six square-sectioned ham sausages into a rectangular array, then automatically wrap them with film and send them through a heat-shrink tunnel. During initial trial production, the yield of packaged products fluctuated at only around 85%. The defects mainly fell into three categories: 1) noticeable longitudinal or diagonal wrinkles on the packaging surface; 2) excessive film shrinkage at the corners of the ham sausages, leading to localized thinning, whitening, or even tearing; 3) incomplete shrinkage at the bottom of some packages, resulting in loose film. The equipment supplier has repeatedly adjusted the hot air temperature, airflow speed, and conveyor belt speed, but with limited improvement, and improving one problem often exacerbates another. The packaging engineer suspected that the root cause might be that the shrinkage characteristics of the selected three-layer co-extruded polyolefin shrink film were unsuitable for the geometric structure of the square-shaped product, but lacked data to prove this and guide the selection of new film materials.
II. Film Material Performance Diagnosis and Selection Using the LTRSY-03 Heat Shrink Tester
The company's technical center used the LTRSY-03 heat shrink tester to conduct a systematic quantitative analysis of the shrinkage performance of the current film and various candidate film materials. Heating in a liquid medium simulated a uniform heating environment, eliminating the interference of uneven hot air on the production line.
Test method: Samples were cut from the current film roll (designated as M-Current) in both the transverse direction (TD, perpendicular to the roll direction) and the machine direction (MD). In the LTRSY-03, a series of temperature points (110°C, 120°C, 130°C, 140°C) were set. The samples were immersed in silicone oil for 10 seconds and then rapidly cooled. The dimensions before and after heating were precisely measured, and the shrinkage rates in both the MD and TD directions, as well as their ratio (TD/MD shrinkage ratio), were calculated. Key Data Findings:
The M-Current film exhibits a shrinkage rate of 55% in the TD direction at 130°C (close to the actual effective temperature of the production line), while the shrinkage rate in the MD direction is only 18%, resulting in a TD/MD shrinkage ratio as high as 3.06.
Further observation revealed that the film's shrinkage becomes very intense and rapid after the temperature exceeds 120°C.
Problem Correlation Analysis: The extremely high transverse shrinkage rate and asymmetrical shrinkage ratio are the direct causes of the packaging problems. When wrapping square products, the excessive transverse shrinkage force causes the film to experience extreme stress at the corners, leading to whitening or tearing; insufficient longitudinal shrinkage force prevents effective tightening of the bottom, and the intense shrinkage process is extremely sensitive to temperature fluctuations, easily resulting in random wrinkles.
Test Method: Three shrink films nominally suitable for "heavy and irregularly shaped products" were purchased (designated as M-A, M-B, and M-C). The same multi-temperature point shrinkage rate test was performed on the LTRSY-03.
Data Findings:
M-A film: At 130°C, the TD shrinkage rate is 45%, the MD shrinkage rate is 25%, and the TD/MD shrinkage ratio is 1.8. The shrinkage process is relatively smooth.
M-B film: At 130°C, the TD shrinkage rate is 48%, the MD shrinkage rate is 30%, and the TD/MD shrinkage ratio is 1.6. Its shrinkage initiation temperature is slightly lower, but the change is smooth in the 125-135°C range.
M-C film: The performance is similar to the currently used film, with significant asymmetry (TD/MD ratio > 2.5).
Theoretical Preference: From the perspective of shrinkage characteristics, the M-B film has a more balanced longitudinal and transverse shrinkage ratio, and the shrinkage process is stable. Theoretically, it can better adapt to the geometric shape of square products and apply shrinkage force uniformly. 3. Validation Testing Simulating Actual Constraint Conditions
Innovative Testing: To more realistically simulate the constraints on the film in actual packaging, the laboratory designed a simple fixture. The film sample was pre-wrapped around a square wooden block (simulating a product array) and secured at both ends. The entire assembly was then immersed in a silicone oil bath in the LTRSY-03 and heated at 130°C.
Observation and Verification:
When using M-Current film, the film at the corners of the wooden block was noticeably thinned, showing stress-induced whitening patterns, consistent with observations on the production line.
When using M-B film, the film adhered evenly to all sides of the wooden block, with good coverage at the corners and no significant stress concentration. After cooling, the film remained flat on the surface of the wooden block.
III. Data-Driven Process Reset and Standard Establishment
Based on the clear data provided by the LTRSY-03, the company collaborated with the equipment manufacturer to implement systematic improvements:
Replacement of Core Film Material: The company decisively replaced the existing M-Current film with M-B film. The procurement department used the key performance data from the LTRSY-03 test report (TD: 48±3%, MD: 30±3% at 130°C) as the new technical standard for procurement.
Optimization of Production Line Parameters: Due to the more moderate shrinkage characteristics of the M-B film, equipment engineers were able to lower and stabilize the temperature setting of the heat shrink tunnel from the previously fluctuating high-temperature range to a narrower, more efficient range (e.g., 125-128°C). The air speed was also correspondingly reduced, lowering energy consumption and the risk of product surface overheating.
Establishment of Film Material Incoming Inspection Specifications: The quality department incorporated the LTRSY-03 shrinkage rate test into the mandatory inspection items for each batch of shrink film. It was stipulated that the TD and MD shrinkage rates must meet the new standard range when tested in a 130°C silicone oil medium, and the TD/MD shrinkage ratio must be between 1.4 and 1.8, thus controlling material performance consistency from the source. IV. Implementation Results and Extended Value
After replacing the film material and adjusting the process, the first-pass yield of packaging on the new production line steadily increased to over 98.5%, and the three main types of defects were virtually eliminated. Furthermore, due to the wider process window, the production line became more tolerant of minor temperature fluctuations and operated more stably.
The key takeaway from this case study is that for processes like heat shrink packaging, which are highly dependent on the interaction between materials and equipment, simply adjusting equipment parameters is often a superficial solution. The value of the LTRSY-03 heat shrink testing instrument lies in its ability to independently and accurately quantify the core thermodynamic properties (shrinkage rate, direction difference, and temperature sensitivity) of the film, which is a "reactive" material.
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