Dos métodos de evaluación: criterios y teoría del programa*
2. Evaluación orientada por criterios
2.5. Consideraciones sobre la evaluación por criterios
It is difficult to locate symmetrically either cavities of both plates or cavity and core of cavity and core plates respectively by using conventional machinery. Some times even end product is not symmetrical along the parting line. This frequently happens for uneven cavities. The mould maker should have skills, patients and carefulness to do this accurately. Also it takes lots of time ifhe uses conventional methods
(a)
(b)
Figure 4.4 Non-symmetrical cavities.
4.7.2 Non alignments of guiding pin boles
Due to inaccuracies of the machine or the operator, non-alignment of guiding pinholes for two plates of the mould may occur as shown in the figure 4.5.
Since centre points for drilling are measured and marked in both plates by persons, it may leads for inaccuracies. In addition to that, the may happens due to improper clamping, inaccuracies of the machine.
4.7.3 Non uniform thickness in the finished product
Since all measurement are canied out manually either by the mould maker or by the machinist when using conventional machinery, it is difficult to keep uniform or required thickness between the cavity and core as shown in the figure 4.6. This will affect the thickness of the finished product.
Figure 4.6 Non unifonn thickness between the cavity and core.
4.7.4 Non identical cavities in multi-cavity moulds
During the manufacture of multi cavity moulds using conventional machinery, non symmetry of mating surfaces and non-identical among the cavities or cores may occur. If this happens, finished products are not identical to each other. This is entirely depends on the machine accuracy and the operator. Some times mould makers mark or engrave cavity number on each cavity to identify possible manufacturing defects in the mould.
Figure 4.7 Non identical cavities.
4.7.5 In accuracy in machining radial types of runners and gates
It is a difficult and time consuming task to machine runners and gates symmetrically and accurately. Specially radial types of runners and gates as shown in the figure 4.8 a. The design of this type of runners and gates has to be machined manually by the
machinist. Again this requires time, patients and skills. Even skilled machinists may make this type of mistakes while on machining.
( a) Radial runners and gates (b) In line runners and gates
Figure 4.8 Basic designs of runner and gate systems
Manufactures who are equipped with conventional machinery may select inline runner and gate system as shown in figure 4 . 8 b. in order to avoid radial type of designs. But in this design, plastic material does not reach to all cavities at once in the same time. Therefore filling defects in the finished product may occur.
4.7.6 Requirement of special tooling for circular complex sbapes
During mould manufacture, mould maker may come across products with shapes as shown in figure 4.9.
Mould maker need to use templates and an additional tooling in order to machine such cavities.
F or example, mould maker might use boring bars with ply cutters. Copy turning attachment use in centre lathe is another example additional tooling which helps to machine irregular shapes. Even for copy turning attachment a template is required.
Figure 4.9 An example for a shape of mould cavity
4.7.7 Waste of time
According to the discussion about problems usmg conventional machinery, it is obvious that it is taken lot of time during manufacture of injection moulding parts. In addition to that, the mould maker and the machinist should have patient to do an accurate job. Also both mould maker and machinist should have knowledge to use all types measuring gauges. In addition to these measuring gauges must be accurate or should be properly calibrated periodically. This leads to wastage of time. No body can predict when the mould will be ready for delivery. Manager or Supervisor is unable to plan the job in advance due to these problems unless he is not thorough
in
mould making process.4.7.8 Waste of material
Most times, due to inaccuracies described in above sections materials throw away and new materials will have to be used. Some times inaccurate cavities are machined and those are replaced by inserts. Author has observed several occasions, some mould
makers use welding to refill inaccurate cavities. This leads to both wastage of materials and time.
4.7.9 Dependency of highly skilled personnel
In order to eliminate all problems discussed above, mould manufacturer has to recruit skilled personnel from the manager, mould designer to machinists. Every body in the tool room must have skills, thorough knowledge of the moulding process. Management has to motivate their employees to maintain required quality of the job as well as to keep industrial harmony. This means dependency of skilled personnel has unnecessarily become a critical factor. Also manufacture has to spend more to recruit , train and keep personnel. This means usage of conventional machinery leads to wastage of time, material and dependency of personnel and mould manufacture becomes an expensive process.