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Bulk bags are highly versatile and durable packaging products that are used to store large quantities of material. Often these flexible containers are used to store and transport fine grain materials, such as powdered chemicals, sands and clays, as well as castor sugar

These powdery substances can easily sift through gaps in the fabric and seams, so sift-proof seams are required when storing them. Additionally, some extremely fine substrates may require an additional inner liner to be inserted into the bulk bag to completely cut out sifting and product loss.

Custom Bulk Bags is a manufacturer and supplier of high-quality bulk bags in South Africa. Our products range in capacity from half a cubic metre to two cubic metres. Our bulk bags can hold between 500kg and two tonnes of materials at once. In addition, they have a minimum safety factor rating of 5:1, which means that our bulk bags can hold at least five times their intended load.

Custom Bulk Bags seamstress sewing a bulk bag

How are sift-proof seams made?

The seams on bulk bags have two functions; they create a firm bond between the side panels, which gives strength to the bag, and they also prevent materials from sifting through the corners of the bags. The manner in which these seams are sewn determines whether they are sift-proof.

When choosing bulk bags, be sure to indicate whether you will require sift-proof seams or inner liners. This will minimise or completely prevent unwanted discharge from the bags. To make a sift-proof seam, a thick cord (known as filler cord) is sewn into the fabric next to the seams.

This filler cord creates a tighter bond between panels and acts as an additional barrier for powders and fine-grain substances. Bulk bags can have one, two or three sift-proof seams. This single, double or triple stitching method also makes the bags stronger, which is necessary as fine grain materials can be extremely compact and heavy as a result of their bulk density.

Are bulk bags truly environmentally-friendly

How to tell which level of sift-proofing you need

When choosing bulk bags, you will already know what type of material you’ll be storing. For coarse products, such as gravel, ore, grain, wood and rubble, no sift-proofing is required. For smaller particles, such as sand, ground materials and seeds, a single sift-proof seam is recommended.

Opt for a double seam when storing fine products and chemicals, such as castor sugar and clay. Lastly, for ultra-fine materials, such as cocoa, talc, flour and cement, a triple sift-proof seam should be used.

Custom Bulk Bags is always on-hand to provide expert guidance and consultation to ensure that you get the perfect packaging products for your needs. Be sure to speak to one of our consultants before placing an order. They will guide you through the process and provide valuable insights into the options available to you. For more information about our bulk bags, please contact us today. 


Custom Bulk Bags is a leading South African manufacturer of woven polypropylene bags for various industries, such as mining, chemicals and food. We are able to produce over 3.5-million bulk bags per year, keeping our customers in stock at all times. Our bags adhere to the highest levels of quality as a result of our stringent in-house testing and quality control programmes.

Custom Bulk Bags holds ISO 9001 certification and we currently have a number of UN-certified designs. We are a Level 2 B-BBEE manufacturer and supplier and fall under the ownership structure of Deneb Investments Limited. For more information on our products, contact Follow us on Facebook for our latest news and industry insights.