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proper bumping technique for molding rubber

proper bumping technique for molding rubber

(OP)
I've seen automated bump cycles but I always thought they were not as effective as they should have been and required too many bumps.
Should you spend more time pushing, less time on the upstroke, slower closing speed, vent design issues and/or bleed hole size, cavity pressure issues, etc. ?
Thanks
gumby69

RE: proper bumping technique for molding rubber

All of those are important Gumby but all I ever used to do was release the pressure on the mould 3 or 4 times. Did the trick for me.thumbsup2

RE: proper bumping technique for molding rubber

As with most rubber technology matters.it depends..on many things, The formulation for example... many ingredients can volatolise during the process and the gas needs venting,some cure systems produce water on curing and this needs venting, complex shapes or thick mouldings will need different venting ( bumping techniques) and it needs to be done at the correct point in the cycle. a good starting point if you have one is to look at a rheometer trace for the formulation and cure temperature you are using, from this you can infer at what stage the the rubber is at any point in time. The bumping should occur before the onset of cure but at just about the point when mould filling completes. If you do it too soon you may introduce air, too late and you get moulding flaws

it is a bit of inteligent trial and error

Johnnymat

RE: proper bumping technique for molding rubber

(OP)
Graham, I agree most parts require a minimal amount of bumps.
These parts are very thick and in some cases dual duro so the process itself increase the possibility of trapped air.
Better preforming would of course be helpful so that you start out with less air, but these are very large parts as well as thick.
Johnnymat thanks for your comments yes i think bumping too early may pull air back into the tool.
Lower temps buy you more time to form the part when plausible.
thanks guys.
Gumby69

RE: proper bumping technique for molding rubber

On thick section rubber moulded parts, you need time for the gases ( given off during heating/curing) to build as the rubber takes heat from the hot mould. Don`t forget rubber is a good insulator so even if you pre-warm the rubber, the mould temperature will still take time to penetrate the whole section of the rubber slug as it is only in contact with maybe 3 hot mould faces maximum when you first load it into the mould.
Bump too early and you won`t have any effect other than allowing air into the mould. Bump too late and the part will have begun to cure so will distort as the mould closes again. Check the rheometer cure trace and bump as late as you can without causing part distortion and only have a small bump opening gap of up to 0.5mm max.
This way the volatile gases in the rubber will have started increasing the pressure in the cavity so when the mould opens slightly, the gases that are under pressure will rush out to the lowest pressure area, which is outside of the mould cavity not inside it. The small gap will increase the velocity of the gases out of the mould, like putting your finger partly over the end of a hose pipe makes the water velocty incease, so don`t have your bump timing set for too long. 2 or 3 seconds should be easily enough.
Bumps are still needed even if your press has vacuum, as the vacuum only tends to intialy remove air from the gaps around the uncompressed slug and the mould cavity contours. Once the slug is compressed and formed by the mould being fully closed, the vacuum is effectively sealed off from the inner part of the moulding by the moulding itself. Hope this is of some use.

RE: proper bumping technique for molding rubber

(OP)
I agree you want to get out evolved gases, but you have to consider the scorch delay which is likely to be more of a factor with a part that is inches thick.
Plasticizers are low and selected for fairly high BP so should not be a factor.
The cure does not create moisture like in FKM compounds using dihydroxy cures, but moisture pickup in storage can be an issue at times.
Desiccants can be helpful also.
I guess I need to know what volatiles are you concerned with and how long are you thinking before bumping in general terms?
Thanks.
gumby69

RE: proper bumping technique for molding rubber

(OP)
abiento maybe I should have been asking what sort of temperature throughout the part were you talking about in general?
Thanks.
gumby69

RE: proper bumping technique for molding rubber

Whenever we have moulded thicker sections, we go as low as we can circa 135-140 Deg C actual mould temperature but it does depend on the material you are moulding and when the cure starts to kick in.
We always go for low temperature and long cure time (like baking a big cake) although again depending upon material choice, it may need a post cure.If it does, the post cure needs to be ramped up slowly because if you have not fully cured the part there may still be some volatiles in there that will expand and split open your moulding if you try to heta it too quickly. We have had this on sections as thin as 15mm using FKM elastomers whre it looked like someone had cut slices in the parts with a knife.
One thing you are more at risk of with thick sections is backrind where the outer surfaces of the moulding have cured but not the inner and as the centre heats up and expands it extrudes rubber out through the mould split lines leaving torn or rough product edges.
We get around this by either dropping the trim adge by 0.05mm right at the product corner but still supported by the outer mould land to prevent full closure to allow the rubber to expand out of the 0.05mm gap or mould a sacrificial lip on the part that is machined off afterwards along with the backrind. This also allows you to increase mould temperature and reduce cure time.
Depending upon your press type you can also program the press to mould at for example, full pressure for the first 20% of the cure then back off to a much lower holding pressure for the remaining cure time to help reduce or get rid of backrind. If you are using manual presses you can do this by just backing off the valve and watching the pressure until it has dropped to the lower pressure. The actual pressure you need is dependant on product and material so just experiment but it would be worth cutting open a "good" part to make sure you have no porosity in the middle due to the pressure being too low. Depending on the design/type of manual valve you have, watch out if you try to control the lower presure by putting the valve lever in the neutral position once you have the desired pressure figure. We had one valve almost glowing red and smoking when we left it in the neutral position for about 30 minutes as the hydraulic oil was being forced through a very small orifice when in neutral as it was fed back to the resevoir tank and picking up huge amounts of friction/heat.

RE: proper bumping technique for molding rubber

(OP)
abiento I would be a bit nervous about reducing pressure too early with a peroxide cure especially since significant volatiles are created from the peroxide during the curing process recognizing that less volatiles are created later, but sufficient crosslinking must be established before to prevent vacuole formation. Some of that is true with any cure but especially with peroxide.
With dihydroxy cures (especially) the problem is also because water is generated by the cure mechanism so desiccants are almost mandatory for thick section parts.
Yes a step post cure is strongly recommended by most of the FKM suppliers to minimize the chance of internal fissures in combination with adequate levels of desiccants to scavenge the water before it can form steam.
A step post cure is especially desirable for a process that requires post cure temps that are notably higher than final mold temps to minimize the rate of volatile coming off to manageable levels. Much of that is trial and error from past experience.
Should be less of an issue where post cure temps are comparable to internal molding temps.
Little additional volatiles are created and level of cross link density should be adequate you would expect.
gumby69

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