The key to designing the hem height of a square-bottom bag is balancing the bottom support area with the bag's effective volume. The hem height directly determines the length of the square bottom structure (the size of the support surface) and the starting height of the bag from the bottom (the starting point of the volume). The hem is formed by folding the bag's bottom edge inward. A higher hem increases the length of the support surface after the fold, widening the contact area between the bag's bottom and the storage surface, making it less prone to tipping when stacked. However, an excessively high hem compresses the effective vertical height from the bag's bottom to the bag's opening, reducing the space available for contents. Conversely, a hem that is too low reduces the length of the bottom support surface, reducing the contact area and making it more likely to tilt or collapse during stacking due to insufficient support. However, the bag's effective height increases accordingly, theoretically allowing it to accommodate more contents. Therefore, the primary design principle is to find the optimal hem height between "sufficiently stable support surface" and "sufficiently practical volume," ensuring that neither compromising core requirements.
The hem height needs to be tailored to the characteristics of different contents to achieve a targeted balance between capacity and stacking. If a square bottom bag is used to hold fluffy items (such as snacks or home textile fillings), these items have low density and large volume, requiring a higher capacity. Furthermore, their low weight puts less pressure on the bottom when stacked. In this case, the hem height can be appropriately reduced, preserving more effective height to increase volume. Furthermore, the lower hem provides a support surface that can withstand the weight of the fluffy items without compromising stacking stability. If a square bottom bag is used to hold dense, heavy items (such as rice or hardware parts), these items have high density and weight, placing greater pressure on the bottom when stacked, requiring even greater support stability. In this case, the hem height should be appropriately increased to distribute the weight by increasing the bottom support surface, preventing deformation or tipping during stacking. Furthermore, since dense items are relatively small, even if the effective height of the bag is slightly compressed, the target capacity can still be met, preventing capacity deficits caused by a slightly higher hem.
Making the hem height proportional to the overall bag dimensions is a key detail in ensuring a balanced capacity and stacking performance. The overall shape of a square bottom bag is composed of the bottom side length, bag height, and bag width, and the hem height should be in harmonious proportion to these three dimensions. For example, if the bag is wide and tall, if the hem height is too low, the bottom support surface will be much shorter than the bag width. This can cause the bag's center of gravity to shift when stacked due to its "wide at the top and narrow at the bottom," leading to tipping. In this case, the hem height should be appropriately increased to bring the bottom support surface's length closer to the bag's width, creating a stable "equal width at the top and bottom" shape while ensuring sufficient bag height and capacity. If the bag is narrow and short, if the hem height is too high, the bottom support surface's length will exceed the bag's width, creating a "wide at the bottom and narrow at the top" shape. This not only wastes material but also compresses the bag's effective height, resulting in a significant reduction in capacity. In this case, the hem height should be reduced to align the bottom support surface with the bag width, while maximizing capacity within the limited space of the bag's height.
The hem's folding process and structural strength can optimize the balance between capacity and stacking at the same hem height. Even with the same hem height, different folding methods can affect bottom support strength and volume utilization. Double-folding (folding the fabric or paper twice at the hem) significantly improves bottom stiffness and enhances support stability. This allows stacking requirements to be met without increasing the hem height, thereby preserving more effective bag height for storage. Single-folding, however, offers weaker bottom stiffness. To ensure stable stacking, the hem height must be increased to increase the support surface and compensate for the lack of strength, but this will also reduce bag capacity. Furthermore, reinforcing the hem with indentations or heat sealing (for plastic materials) can reduce bottom deformation, ensure more even load distribution during stacking, and prevent partial collapse of the bottom from squeezing the bag interior. This maximizes bag volume while ensuring stable stacking.
The actual needs of warehouse stacking will further refine the design logic for hem height. If square bottom bags are stacked in multiple layers (e.g., five or more in a warehouse), the bottom layer must bear the full weight of the layers above. The hem height must be high enough to distribute the weight across the bottom support surface, preventing deformation and tilting of the stack. Furthermore, the hem height must be uniform, ensuring the bottom support surfaces of each layer are perfectly aligned. This allows the center of gravity to be vertical when stacked, minimizing the risk of tipping. In this case, ensuring safe stacking is a priority, even if capacity is compromised. If a single layer is stored flat or stacked in a low number of layers (such as supermarket shelves), the pressure on the bottom support is minimal, and the hem height can be appropriately reduced, prioritizing increased capacity to meet the required contents. Furthermore, a lower hem height allows for a lighter bag, making it easier to display and handle, without sacrificing capacity due to stacking requirements.
The stiffness and toughness of the square bottom bag's material also influence the design of a balanced hem height. Square-bottom bags made of high-stiffness materials (such as thick kraft paper or composite plastic film) maintain good support at the bottom even with a slightly lower hem height, making them less susceptible to deformation due to stacking pressure. Therefore, a lower hem height can preserve more capacity. However, square-bottom bags made of low-stiffness materials (such as thin polyethylene or thin paper) are prone to bottom collapse and require a higher hem height to increase the support surface. This may also require a multi-layer folding process to enhance stiffness and prevent deformation and capacity compression during stacking. Furthermore, square-bottom bags made of highly resilient materials are less susceptible to tearing at the hem due to folding or stacking stress, allowing for greater flexibility in hem height design without being overly conservative, ensuring stable support while effectively allocating volume.
Dynamic testing and optimization of hem height are critical to achieving a precise balance between capacity and stacking performance. After determining the initial hem height, the effect is verified through actual loading tests (filling the bags with the target contents and measuring whether the actual capacity meets the requirements) and stacking tests (stacking them according to storage requirements to observe whether they tilt or the bottom is deformed). If the capacity is insufficient but the stacking is stable, the hem height can be appropriately lowered to expand the effective volume of the bag body and retest the stacking stability. If the stacking is unstable but the capacity is sufficient, the hem height can be slightly increased to enhance the bottom support, while also checking whether the capacity still meets the requirements. By repeatedly adjusting the hem height, the optimal value of "meeting the capacity requirements and stacking stability" is found, while also taking into account material costs (excessively high hems increase material consumption). Ultimately, a hem height design that meets usage requirements and is economical and practical is achieved.
