一,The technical principle is that there are two paths for dyeing fibre and treating the surface.
There are two basic ways to change the colour of moulded pulp packaging: fibre level dyeing and surface treatment. The first method puts dyes or pigments directly into pulp fibres to make the colour even from the inside out. The second method makes a colour layer on the surface of the product by printing, coating, and other methods. Each option has its pros and cons, so you should choose the one that works best for your situation.
1. Fibre grade dyeing: the dye goes all the way through the raw materials and into the finished products
To dye fibre grade, the dyes and fibres must be mixed well during the pulp preparation stage, and the dye molecules must be embedded in the fibres through procedures like pulping and grinding. For instance, Dongguan Billion Packaging Technology Co., Ltd. dyes sugarcane bagasse pulp with natural plant dyes like tea polyphenols and anthocyanins. This can make colours that are good for the environment, like red, dark green, and light green, and it can also match brand colours exactly using PANTONE colour cards. This technology is good for situations where colour uniformity is very important, like packaging for high-end electronics. However, care should be taken to make sure that the dyes and fibres work well together. Some acidic dyes can make fibres weaker, so additives like fixing agents should be used to make them stronger.
2. Surface treatment: cheap and quick way to change the colour
Printing, coating, hot stamping, and other techniques are all part of surface treatment. This is best for places where colour richness is very important. For instance, screen printing can print patterns directly onto the surface of pulp moulds using thick ink and scraper pressure, creating a multi-color gradient effect. Transfer printing, on the other hand, uses silicone rubber heads to transfer images and text onto concave and convex surfaces, which solves the problem of traditional printing on uneven packaging. The design team for a creative tea packaging case used paper pulp moulded with natural dye colouring to make a teapot shape. They then added two layers of cotton bags and printed the surface with screen printing to repeat the texture of the tea garden terrain. This is both eco-friendly and artistic.
二, Process flow: full chain control from sorting raw materials to checking the finished product
Colour customisation has to happen at every stage of making moulded pulp packaging. This includes five main steps: sorting the raw materials, preparing the pulp, adding the additives, drying the mould, and treating it afterward.
1. Sorting raw materials: laying the groundwork for colour
The dyeing result is directly affected by the fibre qualities of the different raw materials. For instance, the pulp made from recycled corrugated cardboard boxes tends to turn yellow after dyeing because it has a lot of impurities. On the other hand, the paper edge pulp made from recycled paper from paper mills is closer to milk white (natural white), which makes it a better base for dyeing. A certain company was able to make high-purity white packaging by sorting and recycling white waste paper (such straw pulp colour paper and wood pulp natural colour paper) and mixing it with raw pulp. This met the purity standards for cosmetic packaging.
2. Making the pulp and adding the additives: controlling the colour stability exactly
The time of adding the dye, the pH value, and the ratio of additives are all important in the dyeing process. For instance, sizing agents like rosin size and paraffin latex should be added 3 to 5 minutes before the beating process is over to keep the colours from changing because of the reaction between the dyes and the sizing agents. Aluminium sulphate, on the other hand, should be strictly controlled at a dose of 1.5% to 3% of the dry paper volume. Too much of the product can cause it to crystallise on the surface, which can damage the printing effect. A food packaging company discovered through research that a pH level of 4.5 to 5.0 in a mildly acidic atmosphere is the best for binding colours to fibres. This increases the color's staying power by 40%.
3. Moulding drying and post-processing: Keep the colour details safe
Controlling the slurry concentration (0.6% to 2%) and moulding time is important during the moulding stage of the vacuum suction moulding method to avoid uneven fibre deposition and colour spots. The tunnel drying equipment needs to keep the temperature between 60 and 80 degrees Celsius during the drying cycle to keep the dye from breaking down. UV coating can make surfaces shinier and protect colour in the post-processing stage. For instance, the packaging for a certain mobile phone employs water-based polyurethane (WPU) coating, which has a contact angle of 152 °, makes the surface superhydrophobic, and keeps colours for 30% longer.
三, Cost Control: Finding the right balance between the minimum order quantity and the complexity of the process
The minimum order quantity, dying procedure, and equipment investment are the primary things that determine the cost of colour customisation. Dark colours (like black) need a lot of orders to get the right colour. The minimum order quantity is usually 5000 pieces or more because dyes are expensive and the processes are complicated. However, light colours like beige and light green can be made with just a few colour changes, and the minimum order quantity can be as low as 2000 pieces. Fibre grade dyeing also needs more dyes and additives, which costs 15% to 25% more than natural colour packing. Surface treatment (like printing) is charged by area, and multi-color printing costs 2 to 3 times more than monochrome printing.
四,Industry Case: Real-Life Testing of Electronic Products and Food Packaging
1. Electronic product packaging: customising both the colour and the function
The Huawei Mate 60 series phone tray is made of a blend of sugarcane bagasse pulp and bamboo pulp. It gets its dark grey colour through fibre grade dyeing. It also passes a 1.2-meter drop test thanks to its honeycomb construction design. In this case, the dyeing process is closely linked to structural engineering. This not only helps people remember the brand, but it also makes sure that the package works well.
2. Food packaging: the use of natural colouring and ideas for protecting the environment
The container for organic soybean milk powder is shaped like the growth cycle of soybeans. It has light green, dark green, and light yellow pulp, and the colours change naturally using pigments like chlorophyll and carotene. This design works well with the product's features and uses colour psychology to portray the idea of health, which helps the brand stand out.
3. Luxury packaging: precise colour control for high-end customisation
A certain worldwide jewellery company employs sugarcane bagasse moulding packaging and transfer printing technology to get 0.1mm fine printing of the logo. The colour variation is kept to Δ E<2, which is the range that the human eye can't see. This case shows that moulded pulp packaging may also compete in the high-end market.
