Stack mold / Family mold

[obsigmatec]

We have a 64 cavity stack mold. It is a family mold so 32 cavities in the first stack make part A and 32 cavities in the second stack make part B. Parts A and B are the two sides of an electrical switch housing. Therefore parts A and B have very similar geometries. The differences are primarily in the interior of the parts where there are additional features on part B for holding electrical connectors. The tool utilizes a hot runner system. The material is polystyrene. The problem - of course - is that Part A and Part B fill out differently and production was constantly fighting the tool to get it to run properly. After a long period of sorting out defects related to this problem, a decision was made to run each side of the tool independently because less scrap was generated. This of course has doubled the amount of time required to produce the parts and significantly increased the cost so now the pressure has come back to engineering to get both sides of the tool running simultaneously again. I've previously had some experience with cold runner family molds and with putting flow inhibitors in some of the runners to balance out the flow. This met with a limited degree of success but now is not possible due to the hot runner system. What other options exist for balancing out a family mold?

 

[plastx]

If you have heat control for each cavity you can try to slowly lower the hot runner tip heat for the cavities that pack out first. Also the tip design might allow for less clearance for these cavities. Both of these methods will require trial and error and monitoring to ensure that variation in the process is kept to a minimum.

Mike

 

[patprimmer]

Tip size.

Tip temperature.

In my opinion, family moulds are the work of the devil.

I can understand them for low volume, with a single cavity for each part, but once multiple cavities for each part are required, two moulds should be made and run in smaller machines to keep costs down. In the example above if two stack moulds were made with 2 X 16 cavity of part a and 2 X 16 cavity of part b, I expect very high yields could be produced and although 2 small machines costs more than one big one, good mouldings produce more income than does scrap.

The quicker you produce scrap, the more you lose.

True cycle times are number of parts that the customer accepts per hour, not the number of parts made per hour.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

I've been induced to buy a number of family molds over the years. I have _never_ seen them make both/all parts at once successfully.



Mike Halloran
Pembroke Pines, FL, USA

 

[ToddCook]

As you can see from the previous posts family molds are a poor way to get productivity. The reason, implied though not stated directly, is that although there are many parts per cycle, getting a predictably good part puts you at the mercy of a mold designer whose principal aim is to make the mold last and open/close reliably. Getting the part to work well requires a special skill set. Family molds can work, but the best examples of this require that the parts be the same injection volume, both parts be optimized for gate location, and the runner system be optimized for overall performance--CAE software can help with this, but the complete answer comes from experimentation as some effects are not captured by simulations. Good luck.

 

[obsigmatec]

Thanks for all the valuable input. It will be useful as we try to make this mold work.

 

[dhuffy]

It is true family molds causing filling problems. But if they are designed properly they can be run successful. We have helped several companies design/run family molds successfully. The problem you mention is most likely due to the design of the hot runner system in that it does not account for shear imbalances created in the sprue bar. My guess is that the parting line closest to the nozzle fills first. This is very common, though it may reverse with some materials and process setting (but very few instances are like that).

To learn more about the root cause, I suggest visiting

http://www.meltflipper.com/p&s/mf/hot/hot.html.

Figure 1 shows a stack mold and the development of shear biased flow to parting line 1.

Who manufactured the hot runner system?

dhuffy

 

[SteveAf]

Well, as for the surface runner molds that you look at in the future, use a Melt Flipper. This will solve the problems of balancing a poorly laid out surface runner molds. Never try to gauge primary to secondary runner and increase or decrease diameters to accomodate a filling imbalance.

The stack mold that you have described has two problems.

1). It uses a common manifold system to fill both parts "A" and "B".

2). Its a family mold.

If it is a valve gated mold then I suggest looking into weather or not the valve gates can be driven from seperate energizing sources. This will allow you to sequence the valve gates to the desired filling rates. i.e.. make one part "A" then the other "B" allow for cooling, open and repeat.

If this is a hot drop mold, meaning no valve gate. Then I suggest you look at the cooling and venting scheme of the mold. Make adjustments to one part "A" or "B" to create the fill desired.

 

[obsigmatec]

It is a Husky 500 series hot runner system. Valve gates are not used.