Hi Everyone:

I have inherited a mold that produces a tamper evident cap almost identical to those used in plastic milk/water jugs.  The cap has a ring that sits below the cap body that must break away from the cap at 9 evenly divided points along the circumference when the cap is initially unscrewed from the bottle (the ring stays on the bottle neck finish).  The problem we are having is that it requires excessive torque to break the cap from the ring.  We have samples in Exxon HDPE HMA 016.  I am trying to find out which physical properties to concentrate in order to make an educated resin selection for our next few sample runs.  Would it be the tensile strength, i.e. a lower tensile strength will allow the cap to break away from the ring?  Thanks in advance for your help.

I'm not a plastics engineer by far, but I would think shear strength would play a huge role in this one.  Are you able to change the design any, e.g., makes the tabs thinner so they break more easily?

Dan - Owner
http://www.Hi-TecDesigns.com

Tensile and elongation are the two properties you want to reduce.

Introducing notches to the design helps.

If it did work, then stopped working, someone probably polished the mould and removed some sharp edges at the intended break point.

A little talc in the resin or  lower molecular weight will probably help.

Regards

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Thanks everyone.  Making the connectors/tabs/gates(?) thinner really isn't an option right now.  I'm hoping to make good parts by trying alternate resins and, to a limited extent, playing with the process.  Someone else here mentioned trying a lower molecular weight (sp. gr.).  I think we'll head in that direction first.  I'm of the belief that modifying a tool should always be your last resort.  Thanks again.

When selecting a material, you will want to make sure it complies with the product it comes in contact with. You say it's like the caps on a milk bottle - Are you making these caps for a food product? make sure the material selection is FDA grade material. Are the tabs exactly like the other caps in thickness and width? Has this mold ever made acceptable parts for your customer? Why did you inherit the tool? Price, or was it because of poor quality issues?


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