Preventing warpage in a flat plate

[cityjack]

Morning all,

I have a plate plate that will be molded out of PC/ABS(Cycoloy). This part is approximately 10 " long x 5.5" wide and .10" thk. This is a cover to a battery box that has 8 even spaced countersunk holes around the perimeter of the piece. The are two pieces of text that are raised .050 above the surface at each end of the piece as well. The part is presently being gated at the top surface directly in the middle of the part. I am sure I need to put some ribbing on the underside of the part to help with stiffness and to help prevent warping. I am just unsure of the thickness, height, spacing and orientation of the ribs with regards to the gating and flow. Any help would be greatly appreciated. I am going to try to attach a pic of the part as well.

Sid

 

[Pud]

PC/ABS is an amorphous material with a very small shrinkage (thermal rather than crystallisation). You should be able to mould a flat plate ok. If it's not stiff enough, ribs could be added later.
Ideally, it should have a fan gate at one end.
As an observation, if the c/s holes are for fasteners, it is bad practice to use c/s in plastics due to the high pressure contact points on the c/sink angled faces (creep). Use c/bores with pan head screws if possible.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[cityjack]

Thank you for the reply sir Pud,

I also saw where the shrinkage rate for PC/ABS was low. Curious to know why you opt for a fan gate at one end rather than the center surface tab gate that the molder indicated he would use? Just trying to learn. I agree with the C'sink holes not being optimal but really had no choice or room for c'bores. What do you mean by "creep"? The plastic moving or being displaced over time?

Thank you again.

Sid

 

[APDplastmold]

I suggest designing the thermal conditioning (cooling) of both sides of the cavity properly. The distance between the edges of the cooling channels and the molding surface (EDGE-TO-PART (E2P)) should be >= EDGE-TO-EDGE (E2E). E.g. 0.8"(better: 0.45") E2P with 0.7(better: 0.4") E2E for steel; Alum and Be-Cu are more forgiving: e.g. 1.3" E2P with 1.2" E2E. The Wetted Surface should be apx. 50% of molded part surface in steel. The Packing/Holding conditions (pressure and time) will be very important in warpage control. Center gating will suffice; however, you have to make sure no colder material is coming from the machine nozzle causing splay around the gate.

 

[Paulkuklych]

Just to add to the points above, make sure your runner size is a full circle as opposed to a half circle or trapezoid so that you minimize stresses (and thus warpage) in the part.

As far as the part design is concerned, flat parts in the food packaging industry use a feature called an "angular panel" which acts like a rib but is not a rib. This helps to keep the part flat, reduces rejects due to hesitation effects or venting issues and keeps a low nest height for stacking. For more information go to:

http://www.improve-your-injection-molding.com/plastic-parts-design.html

 

[cityjack]

Thank you for that link sir. It just became a bookmark. Unfortunately, I do not the luxury of added height by adding teh "angular panel" effect or to the walls. My cover is basically a flat plate with molded in text on it. I think I will just remove any length and width wise running ribs all together. I have the molder coming over this afternoon to discuss the present part design. I will also discuss the c'sunk holes that are a concern. Funny thing is, Mr. Pud brought up the fact that c'sink holes are not a good idea in plastic parts. I kind of suspected that, but I also inherited this part design and all hardware had been picked out and purchased before I got handed the project. From another engineering forum, I also got the comment that "those c'sink holes make me nervous". Voicing their concerns that with the inherent hoop stress of a c'sink hole and weak knit lines on the back side of these holes at the extremities or furthest from the gate, these parts will be very prone to cracking. That is if the torque applied to the fasteners going through these holes is not strictly monitored. I do not see that monitoring happening or the air torque tools being calibrated frequently enough. The OTHER funny thing is when the present molder, the same one who is visiting me today, visited a month ago and discussed part design and gave us their first stab at a mold flow for all parts and FEA results, not one time did either mold engineer mention the c'sink holes. I will bring it to them today though.

I have another question regarding some additional features I'd like to add to the present parts. We have some wiring of various gauges in and about the plastic parts. Can anyone give some examples or point me in the right direction of possible wire retention features. One that comes to mind is something I have seen inside of close/tight space tools. Drills, weedeaters, and things like that. It is basically a small track or channel that is sized for the appropriate gauge wire running through it. Inside the track on either or both walls that form the track ar little "nubs" or bumps that protrude from these side channel walls in effect making an interference fit in that area for the wire. The channel routes and the the nubs hold. Any additional help would be most welcome. I do not want to buy little aftermarket press in clips, or tie wraps or anything else that I have to purchase. I am already buying the mold, so in essence these features are free.

Thanks guys.

Sid