I Need Two Properties For A Piece Of Plastic

[Guest Runiuc]

Hi!

 

Im not an expert on plastic materials or injection moulding. My issue is I need to design a small plastic component where to form a plug on the one side of this. The rest of the component should work as a grip.

 

The purpose is to plug a hole with this and bend the grip part of the component so it removes and left is the plug in the hole.

 

So, I need the area between the plug itself and the gripp part of the component to be brittle while the plug should be as flexible as possible.

 

Can you control this in the injection moulding process in some way, I think of cooling at different intervalls or something? Is there anymaterial where you can achieve these properties?

 

Thanks!

 

/Chris

[Cyberdemon 17]

From the way you describe it, it sounds like you're talking about over molding.

 

Over molding is taking two separate materials (a hard substrate like ABS or PC) and a soft rubber/elastomer.

 

 

You can see this used all over in products from the iconic "Good Grips" type products to toothbrushes, etc.

 

Theirs no way to get 1 single material to behave like you describe, so it involves a two step process, and there are certain design considerations required (you need a shut off type feature to prevent the second material from leaking out against the first part which causes flash)

[Guest Runiuc]

I need it to be simple, with only one material.

 

I want Delrin 500 P or equivalent but in the area of where the component should break I want it to be as brittle as possible.

 

Maybe its wrong to say I want it to be brittle. I just want it to break at the right location between the plug and the grip part due to the material properties, without doing some change in the design like a weakened waist.

 

The method you explain Cyberdemon, is the tooling very expensive?

 

/Chris

[Cyberdemon 17]

Do you have a sketch of the part design?

 

I can't think of any materials that would functionally allow you to make it more brittle. You can design parts to break away by designing a functionally weak section that can be snapped with one material, but that depends on the part design and it sounds like you don't want to do that.

 

Is the tooling very expensive? No. Is it more expensive then a single shot? Yes. The process is used on toothbrushes, even the pen I'm using right now has an overmolded ergo grip.

[Guest Runiuc]

[Cyberdemon 17]

 

Ah I see now...so you want the peg to break off in the object?

 

Why don't you just want to build in a break away feature at that joint?

 

Does the plug have to seal something or just fill some kind of hole? IE if the plug was hard is it actually going to do the job?

 

Could you mold the whole part in a soft material and then just provide a slit to cut it with a scissor?

[Guest Runiuc]

Yes, but the surface of the peg should be plain :S

 

Let me arrange with some better pictures and I will try to explain more in detail

[Cyberdemon 17]

Yeah it's hard for me to clearly understand the beginning and end result. If you have some more pictures or even a quick napkin sketch it might help me understand a little better.

 

You can also give a molder a call and see if they have some feedback for you.

[Guest Runiuc]

Yes, I will have a meeting with a moulder next week.

 

See attached pic:

 

 

Here is a schematic picture of the components in use. As you see the "problem" is the small size of the parts. How do you get a strong snap fit of this? :S

 

The male part should snap in to the female part at left. When its in position I need to break the handle from the peg. The snap fit should hold for forces F1 and F2. Since its very small dims, its hard to design a snap fit that holds. The force F1 will cause the snap fit package to bend, I should have marked it just at the edge instead.

 

Lets say money is not an issue here What about making the peg and handle in two different materials, where the connection between the materials isnt "perfect" so is breaks perfectly but is joined together with enough force so you can push the peg in to position without its breaks to early.

 

Is there any good sites on internet with information about different snap fits?

 

I think of annular snap fit (I think its called) with an edge inside the female and with the male designed with hooks. What about tolerances with these sized parts?

[KQD 0]

I'm still not too sure exactly what you need the parts to do. Do you need the flat part to rotate relative to the round part? Can the parts be rigidly joined? You want the flat part pin to break off at the flat section - at what force?

 

But the biggest problem is the tiny dimensions! You are really going to struggle to get an effective snap fit with those size of parts. This is VERY specialist moulding stuff (but you are in the right country for that!).

 

If the parts can be rigidly joined why not design as a single part with a break point where needed, or possibly use a plastic assembly system like ultrasonic welding or staking to join the two parts. But without know the exact application and requirement it is impossible to give useful advice.

[vahid 2]

can you dip the part in some really cold stuff before putting it in the hole and breaking it. something like liquid nitrogen, but maybe not that cold. maybe about -50 to -60 degrees celcius.

but this again depends on the volume of production. how many parts per week/month/year?

[Guest brayn]

All you need to do is mould a notch into the part. The notch is a small sharp cornered cut-out at the point of maximum bending or the point at which you want separation. The notch will cause a crack to propagate through the part and hey presto you have a part moulded in one material with a designed in failure mode.

However, if you do intend to use Delrin, it is very important that you try and keep the wall thickness constant, with any ribbing being roughly 0.75 of the main thickness. Also, from the injection point thru to the end of the part (flow), the thickness should not increase at any point. If it does, In all likelihood you will mould in voids into the part. These are a great way of making your part fail uncontrollably, and often unexpectedly.

Also, don't forget radii, and as they increase inscribed thickness, they should be kept small.... Sharp edges/corners are in general to be avoided regardless of the material used, unless of course you want your part to break.....

 

If any of this is too technical (or if I am described it a bit hamfisted), check plastics.dupont.com for some tips on what I'm on about....