DFM | Mould design: part 2
Figure 2: Flow analysis shows that adding a chamfer on the rib eliminates the gas trap with no need to vent
dimensions of the vents. We can create a small gap on the shut off area of the tool that is large enough to allow the gas to escape but not large enough to allow the part to fl ash. ABS/PC blends will generally allow good venting but no fl ash using a gap of 0.0012mm, whereas PA would require a smaller gap of 0.0008mm to avoid fl ashing problems. This is usually the reason why venting is considered at the design stage but not executed on the tool completely - the tool maker starts to open the venting grooves only after the fi rst trials when venting issues will have been seen.
Keeping the vents clear Cleaning of the venting grooves is essential to keep consistent part quality during production as the venting grooves and channels tend to get contaminated and close down after a period of operation. A maintenance schedule for cleaning appropriate to the material being used must be determined to prevent this (where glass fi bre reinforced materials are involved the interval of cleaning stops will need to be shorter). Whatever the cleaning frequency, it is very benefi cial if venting features can be accessed for cleaning without disassem- bling the whole tool or even while it is sitting on the moulding machine. The tool designer should always design a tool so that it can be cleaned easily and in a short time to minimize production interruptions. Ejectors can be considered to be self-cleaning as they move after each shot. Even complex steel features can move a little while the mould tool is opening or can
Figure 3: Incomplete
fi lling of this part (left) was
overcome using mould cavity evacuation technology from Frimo
follow the ejection stroke completely. During a recent mould optimisation project running a 50% glass reinforced PA it was found that by putting all the venting pins onto the ejector package it was possible to leverage the self-cleaning aspect to extend cleaning interruptions to 100,000 shots rather than the previous 30,000.
Allow the gases to vent This very important point is regularly missed. In some moulds all venting requirements may be taken care of by careful implementation of staggered venting inserts, pin-venting, venting grooves on insert splits and the like, but no consideration given to exhausting the gases, which will just compress somewhere within the tool structure. If the gases cannot escape freely to the outside atmosphere of the tool the venting performance will be impacted dramatically. Some moulders have achieved good results on
problematic parts using vacuum pumps to support cavity venting. Connecting a vacuum pump to exhaust drillings allows air to be removed from the cavity prior to injection of the polymer. The only additional element required is to install seals around the component geometry on the main parting line and on the ejector box to prevent air being sucked back into the cavity. Figure 3 shows a component which could not be
fi lled due to incorrect calculation of the maximum fl ow path during development. However, by evacuating the mould tool prior to injection it was possible to achieve a complete fi ll. The additional cost of the seals and vacuum pump components saved having to construct a complete new hot half with three drops to replace the original single drop version. It should be noted, however, that vacuum techniques should not be considered as a fi rst option – the priority should always be placed on proper DFM work in the fi rst place.
Cooling considerations Cooling is generally seen as “the processing step” where most money can be saved and, with it accounting for 90-95% of a typical moulding cycle, it is the most attractive area to target to reduce cycle time. Shorter cycle times can make a big impact on production cost for a number of reasons: Fewer mould tools or cavities need to be manufac- tured to meet the anticipated production volumes;
Smaller moulds can be used, allowing smaller and less costly moulding machines to be employed;
Fewer moulds and cavities means labour cost can be reduced; For the same reasons, the amount of energy
required to product each part can be reduced. A great deal can be done on the tool side to improve
48 INJECTION WORLD | April 2013
www.injectionworld.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56