Industrial or linear robots
There’s a lot in favour of automated injection moulding: increased performance – greater flexibility – constant process stability – higher output of parts without loss of quality – integration of several functions in one operation. But which type of robot is the better choice – a linear robot or an industrial robot?
When planning a fully automated injection moulding cell, the choice of robot – linear or industrial – is of prime importance. But which type of robot is more suitable for the application? – The following comparison throws light on their differences and respective advantages and disadvantages.
Linear robots operate in a threedimensional Cartesian space (hence also the term “Cartesian robot”) with defined coordinates. These robots have three independent main linear axes (X,Y, Z), which are in most cases servo-driven, the Y axis being additionally equipped with up to three pneumatically or electrically driven swivelling or rotating axes (A, B, C).
Basically, linear robots are able to carry heavy loads over very long distances within a large working space, perform insert-placing and partsremoval operations quickly and with a high degree of positioning accuracy, permit automation at relatively low cost – three servo axes are generally sufficient for simple pick-and-place applications, can be integrated within the safety housing of the injection moulding machine (e.g. ER-HLi), can be operated (programmed) comparatively easily and, in the case of Engel ERC linear robots, integrated into the machine control system.
Industrial robots, also known as articulated robots, can be equipped with up to six servo-driven axes, all of them rotational, and have a spherically defined working space.
Industrial robots can work in variable directions and are used both for universal and for even the most complicated applications, are capable of transmitting high forces and can even demould undercut parts, can be installed separately in order to reduce the overall height of the machine. This is advantageous, for example, in cases where a laminar flow box is located above the clamping unit (clean room application), are ideal for use with peripheral equipment and permit the integration of highly demanding tasks (e.g. assembling of parts) into the operating cycle of the machine.
Industrial robots have one particular disadvantage in that their complexity demands a lot of expertise and experience from the programmer.
One might easily assume from these brief descriptions that difficult tasks are best performed by the industrial robot. Is that in fact the case? Not necessarily, for much depends on the actual application.
Here are two examples:
A production cell for a multicomponent screwdriver consists of two two-component injection moulding machines linked by means of an ERC 63/2-C linear robot. The tiebarless machines are joined together not only mechanically – by a gantry – but also via a software package linking the two control systems. Both injection moulding machines are equipped with an 8+8- cavity Combimelt mould. In the first machine, the screwdriver blades are placed in the cavities of station 1 and encapsulated with a handle core of PA. The parts are then transferred to station 2, where the handle cores are overmoulded with PP. The linear robot then transfers the semi-finished parts to the second machine, where the handle layers 3 and 4 are added.
This production cell consists of two Engel injection moulding machines equipped for unattended operation. The parts are welding fittings for pipe systems. In the first stage, metal cores are placed in the cavities and encapsulated with the core material for the heating coils.
The parts are then transferred to the second machine for completion. Industrial robots have been selected for this application because many tasks have to be performed – sprue removal, fitting of the heating coils, functional tests, packaging of the finished parts – in addition to the actual placement, transfer and removal of the parts during the moulding cycles.
When it comes to automating an injection moulding cell, a combination of both types of robot is often the best solution. The linear robot takes care of the insert-placing and parts-removal operations and then hands them over to the industrial robot for downstream finishing and/ or assembly. In one respect, however, the linear robot is far superior to any industrial robot.
As the industrial robot must always move all six of its axes, even when moving its gripper head along just one axis, it can never achieve the speed of a linear robot. It is for this reason that for fast cycling applications – for the production of packaging containers, for example – only linear robots are used.
When designing and equipping injection moulding cells, Engel’s automation specialists always follow the same basic rule: the choice of robot must be of maximum benefit for the actual application.