Durable plastic for cold weather toys? 3

[yayatoys]

I am seeking advice regarding what type of plastic to use for two snow toys. These toys are small hand held toys that need to be durable, not brittle, and slightly flexible. These toys which are targeted for ages 3+ would be produced with injection molding. I appriciate any help that I can get.

 

[btrueblood]

Polyethylene.

 

[yayatoys]

Thank you for your advice. Perhaps you can answer a couple more question for me. First, are there any additives that should be added to the polyethylene. Second, I know of other snow toys that are made of ABS, can you tell me what are the advantages of polyethylene over ABS and which material is more expensive. Thanks in advance.

 

[patprimmer]

Polyethylene can be had in a range of grades generally described by their density, being Low Density LDPE, Medium Density MDPE, or High Density or HDPE.

The lower the density, the more flexible, and the tougher the part will be at low temperature. For more flexibility still, Vinyl Acetate can be added to produce Ethylene Vinyl Acetate or EVA. The more the Vinyl Acetate, the softer the material right through hot melt adhesives to liquid at room temperature.

ABS is considerable more expensive than PE and more rigid than HDPE.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[btrueblood]

ABS is also (generally) less tough than PE; i.e. PE will yield, where ABS will crack.

 

[yayatoys]

Hi guys, thank you very much for your help. If I may, I have more questions to ask. One of my toys is snowball maker and I would like to know if LDPE, MDPE, or HDPE would be sturdy enough material for this toy. I also understood that these materials couldn’t be painted. Is it true? Should I use ABS with additives instead of PE? And if so, what kind of additives should I use? Thanks again.

 

[btrueblood]

No, it's pretty tough to paint PE, but it takes coloring pigments very nicely. In general, the lower molecular weight PE's will have more toughness, and lower brittleness temperature, but also are softer and have lower yield strengths at room temperature. Most of my kid's sleds that have held up are (or appear to be) a medium-density PE. The cheaper ones become brittle and crack when used in really cold temperatures (in the 10F to 15F range). As Pat mentioned, this may be because less plasticizer (EVA?) was used in their formulation, or it may also have been due to the way the part was molded (the cracks seem to follow "mold lines" or "flow lines" visible in the plastic). PE is lovely for the fact that it stays hydrophobic regardless of usage/wear, this means that snow won't stick to it and the sleds keep sliding.

Yes, you may be able to get an impact modified ABS that will be tough (not brittle) at low temperatures -- but you'll need to do your homework to ensure getting something truly durable. I believe refer shells are molded from impact-modified ABS. My freezer at home has some cracks in the shell where something got dropped...

You'll want to work closely with a molding shop and resin supplier. You will need to specify a lowest use temperature and then look at what the modulus and strength of the material choices are. Oh, and cost too.

 

[yayatoys]

After reading about PE in general and receiving your wonderful advice in particular, I feel that PE would be the best choice for my products. Can you advice me what type of PE would be the best for the snowball maker? LDPE, MDPE, or HDPE? Can you also let me know if decal can work on PE instead of painting?

Best regards, Aaron

 

[patprimmer]

I think LDPE.

The addition of a little EVA in the compound will help.

EVA is sometimes included, but the manufacturer does not advertise the fact. It is often in the so called "environmental stress crack resistant" compounds.

Just a point that is often misunderstood, Molecular weight and density have nothing to do with each other.

You can get High or low molecular weights in LDPE, MDPE and HDPE.

Generally, high molecular weight is extrusion grade and low molecular weight is easy flow injection grade. Molecular weight is inversely proportional to melt flow index.

All things being equal, higher molecular weight (lower Melt Flow Index or MFI) is tougher with no loss of other properties, but it is also harder to injection mould in thin or long parts.

Regards
pat pprimmer@acay.com.au
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.

 

[btrueblood]

Yes, decals will work on PE, if you work very carefully with your decal guy to get the "right" adhesive, and a smooth (but not too smooth) finish on the PE. 3M makes several acrylic adhesives that have good adhesion properties to LSE (low surface energy) materials like PE. And, the sled makers have found an adhesive that sticks like a $^%%$ to the sled, had to use some WD-40 to get the residue off. Grr!

I've also seen ski bases in a clear (or mostly clear) PE, with a decal or paint job beneath that shows through.

Finally, my current pair of snow skis have a PE base, with what appears to be an emblem in a contrasting color (white) PE. I think they may over-mold the emblem with the base material (black PE)? Ski bases are typically finished by grinding/sanding off the base smooth, so any flash from the overmolding would be swept away.

 

[yayatoys]

Hi guys, I heard about a type of PE that has been around for few years that can take paint(ink?) Do you know the name of this PE?
Best regards and thank you for the flow of information,
Aaron

 

[btrueblood]

No, I haven't. If you get a trade name or resin manufacturer, it'd be nice if you'd post it back!

 

[patprimmer]

The higher the vinyl acetate content, the better chance that things will stick to it.

Regards
pat pprimmer@acay.com.au
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.