The right choice of blowing agent combined with the application and processing is crucial to successfully producing good products. Blowing agents (or foaming agents) have been known for a long time and mainly reduce product weight and material cost per product. Commonly used in injection moulding and extrusion applications, such as cast film and profile extrusion, blown film, extrusion blow moulding and rotational moulding. There are different types of blowing agents, but when should you apply them? And how?
Why use a blowing agent?
Blowing agents are used to change specific properties, such as:
- density reduction (weight and cost savings)
- reduction of shrinkage and warpage
- sink mark reduction
- improved flow properties
- creation of optical effects (f.e. wood structure, pearlescent, matting )
- improving the machining properties of the finished product
- increased stiffness in larger products
- improvement of the insulation properties
In general, blowing agents are used for thick-walled products from mainly polyethylene and polypropylene (wall thickness more than 10 mm) to reduce weight, whether or not combined with a shorter cycle time.
Less well known is that blowing agents can also be used in engineering plastics such as POM, PC/ABS, SAN, ASA, PET, PBT, elastomers (TPE, TPU, TPV, SEBS/SBS), PMMA and various polyamides.
For these plastics, this can be done both unfilled and filled.
Various foaming processes
Structure Foam Moulding. A well-known process, with the aim of maximum weight reduction, greater stiffness, reasonable control of warping, collapse and shrinkage. Because of the visible traces of the blowing agent at the surface, you have to paint the products most of the time. However, it is possible to reduce specific gravity by 35%. Wall thicknesses up to 40 mm
During filling, the die is kept under pressure until it is almost filled. By only then lowering the pressure, the blowing agent will work later compared to TSG. In this case, weight reduction is achieved while maintaining surface quality. The products formed through this process have a smooth surface and therefore do not need to be painted.
Multi-component injection moulding
In this process, blowing agents in the core of the product ensure higher rigidity, a tighter product and better flow. Weight reduction is usually not the primary goal, but 15-25% weight saving is achievable.
Compact injection moulding
Here, blowing agents are mainly used to prevent collapse and warping and reduce cycle time with very little or no surface defects.
Low pressure and ‘direct gassing’ physical foaming processes
Little applied in Europe, but is very popular in the United States. As a result, it is possible to manufacture very large products with relatively low weight and high rigidity. In combination with a gas (nitrogen or air), blowing agents are used as a ‘nucleation agent’ for the products’ most delicate possible cell structure. Think of pallets, large crates and slides.
In addition to these injection moulding processes, blowing agents are used for:
- extrusion of profile and tube, plate and rod, and wire and cable, but also, for example, for structural wallpaper
- extrusion blowing of bottles and technical parts rotational moulding
Also used in injection moulding, but chemical foaming grades are much easier to use, lower cost, and as effective as an alternative to physical foaming processes like MuCell while keeping better mechanical and surface properties.
Which blowing agent?
If you select the correct type of blowing agent, it is necessary to consider the final product, the production process and, of course, the kind of plastic. Thus, there is a choice of numerous types of blowing agents, which in principle can divide into two main types:
Exothermic blowing agents
- Based on azodicarbonamide and other hydrazine derivatives, compounded with different carriers.
- Very high gas yield (approx. 220 ml/g)
- Uniform gas distribution provided that in the right concentration
- Low corrosion for moulds
- A gas formation can adapt to the plastic
- No inclination to ‘blooming.’
- Not approved for food contact!
Endothermic blowing agents
- Based on bicarbonates and citric acid-like compounds.
- Relatively low gas yield (approx. 130 ml/g)
- Large, long-term production series may require moulds with a high chromium content, as some contain citric acid.
- The gas formation can adapt to the plastic
- In some types of ‘blooming’ tendency
- Excellent cell structure
- Shorter cycle times
- They produce CO2 and a minimal amount of water
- Approved for food contact
Depending on the reason for using a blowing agent, there are several possibilities:
- To prevent a slump, the addition of 1.0 – 3.0 wt % of blowing agent masterbatch with a low concentration of active ingredients for optimal distribution of the gas is recommended.
In TSG, gas-pressure or multi-component injection moulding, preferably 1.0 – 3.0 wt % of a highly concentrated blowing agent masterbatch, is added.
- For an optimal result, the blowing agent and the polymer must be matched in all cases. Important factors are the type of polymer (melting temperature, melting index, etc.), residence time in the cylinder, shape/volume of the product and the intended effect. In selecting the right type, the supplier can be beneficial.
For injection moulding with a blowing agent, below are some basic guidelines:
- Set up the machine as usual
- Reduce the holding pressure to 0 bar
- Increase the back pressure
- Use the maximum injection rate, but inject a lower volume than the cavity volume.
- Leave the machine’s noise against the mould during the entire process and preferably use a closeable nose or hot runner. This is necessary to prevent loss of melt and gas.
- Ensure proper die venting, lower clamping force is possible.
- Ensure sufficient mould cooling
- If this does not result in a stable process and good products, consult the supplier about the correct parameters and the blowing agent grade.