There are options when one needs to introduce materials into a blend for plastics processing requirements.
The first option is to work by hand. While it may seem like a cheap option, particularly when considering how inaccurate this method is. In the long run, it could turn out to be the most expensive option. It is definitely open to human error and expensive additives are invariably added in higher quantities than necessary. It is also possible that not enough of one element may be added, leading to poor product quality and ultimately customer dissatisfaction. So, although this may be a cheap initial investment, long term this could results excessive use of high-priced additives, high reject rate and overall poor quality.
Another alternative is to purchase pre-mixed material. This is an expensive option and does mean that the blending of a recipe is in the control of the supplier. Changing the blend on short notice can be a challenge.
Blending offers control not only of the additive going into the processing machine, but all of the necessary components for the recipe. For example, virgin material, regrind and color are all precisely blended together in a controlled and repeatable procedure.
Dosers come in two versions: volumetric and gravimetric.
A volumetric option features free flowing materials and it does need to be calibrated to achieve any level of accuracy. Generally, although cheaper in terms of initial investment, these dosers are not considered to be as accurate as the gravimetric option.
The gravimetric doers typically operate in a “loss in weight’ mode to ensure they can accurately monitor and keep correcting the desired dosing level. They do this by measuring the weight of the hopper supplying the additive.
Over the past five years, there has been an increase in the demand for gravimetric dosing. However, this trend hasn’t come at the expense of demand for gravimetric blending.
Gravimetric dosing feeders are more economical than blenders, when it is an either/or choice. Volumetric feeders, once popular, today are eclipsed by gravimetric blenders. This takes control away from the operators. The volumetric option requires the operator to do the math to figure out blend ratios, which can be less accurate.
Dosing also gives plastics processors the option to introduce a selected material element, such as color, into the flow of material going into the machine.
Over the last decade, shops have moved from press-side blenders to throat blenders. However, more recently shops have trended back to press-side blenders for safety and maintenance concerns.
Gravimetric feeders also offer more precision compared to volumetric options. The cost of the options is comparable, and in some instances volumetric mixers can be more expensive.
Operators must be careful not to put additional weight on the gravimetric feeder. That can lead to issues. Shops can have a direct system with a flexible hose in the middle to avoid this issue.
Whether blending or dosing, selecting the option that is right for a shop will ensure production moves forward.