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Mixing materials into a blend for an injection moulding project is sometimes required in order to provide the final product with specific properties or to enable it be to used for critical applications in medicine, aviation or the automotive sectors.

There are several ways of introducing materials into the resin blend, with the most cost effective being by hand. There are, however, issues in applying this method, such as ensuring accuracy of dosing, which could render the entire batch unusable, notwithstanding a complete waste of materials. This being the case what other methods can we consider when we require a blend to be created? Pre-mixed materials are readily available and, although this can be an expensive option, it does mean that you are certain of the exact blend and the final quality of the finished product, assuming the supplier of the material is also a high-quality manufacturer.


Dosing is the act of introducing one material into another in the correct quantities required to pass quality control standards. This can be as simple as a pre-regulated flow of material from a hopper into the resin, which is then fed into the mould along with the original resin and can be used to add different materials into the finished product, as well as different colours.


Blending is the act of introducing an additive into the resin material, regulating exactly how much material should go into the finished product and also how this should occur. Examples of blending are virgin material, regrind and different colours which are all precisely blended together in a controlled and repeatable procedure for extended production runs.

Feeders, which allow the correct dose of material or colour to be added into the resin, are available in two versions:

Volumetric Feeder: This relies on free-flowing materials and requires calibration to achieve any level of accuracy. This approach is cheaper both in terms of initial investment and operation, but is not very accurate, which can increase the cost of production if errors re made during the process.

GravimetricnFeeder: Apply a “loss in weight” strategy in order to monitor and accurately dispense the correct dosing level by measuring the weight of the material in the hopper supplying the additive.


Blenders are accurate, multi-component dispensing and mixing machines which allow the exact amount of material to be combined prior to being introduced into the moulding process and also come in two different versions:

Volumetric Blenders: These are typically regarded as adequate if the proportions material you are hoping to add are not critical to the final product. They rely on the timed, free-flow dispensing of materials, which if compromised in any way can have a very detrimental effect on accuracy, meaning that each component of the final blend must be calibrated and accurate in order for the blender to know the exact volume left in the hopper. An operator is required to adjust the flow rates as required and there still remains the possibility of using more virgin or additive than is necessary, which will cause issues with quality control and manufacturing cost per piece.

Gravimetric Blenders: This type of blender is the most expensive option, but is very quick and accurate, therefore allowing the initial cost to be recouped quickly. With these machines you can be assured that the proportions of ingredients remain accurate and, once calibrated with the correct recipe, most gravimetric blenders will run without intervention, automatically calibrating to ensure accurate proportions and mixing of the elements of the blend, with certain manufacturers have advanced control systems that will allow precise documentation of the blending process for the purposes of quality control. All metered doses of additive are checked and introduced by weight and if there are any variations in flow or other parameters, then this is taken into account in the amount dispensed throughout the production run.