Increasing Plastic Rigidity

[slaiman]

Hi All,

I'm new into this forum and I have just started my own plastic manufacturing process.

I'm producing a plastic tray which is 60cm(L)x30(W)x3cm(H), with all things constant, how can I increase the rigidity of the plastic tray. I think this is more of a design issue.

Do I increase the ribs ? I noticed that my plastic tray flexes very much as opposed to another tray done by a different manufacturer with the same dimension, He has some ribs on his tray but besides the ribs and the material, what other additions can I add to my tray to make it rigid from a design point of view.

[Will Call]

Start by adding ribs, they will add a lot of rigigity if properly sized and placed.

What material are you using? Does it have a filler?

[DwightDixon]

Burn one of your competitor's trays. If it has filler or fiber reinforcing they will remain after the fire. Weigh the remains. That is the percentage of filler or re-enforcer. Ribs should be only 1/2 the thickness of the overall tray and should be radius-ed into the abutting wall. You still may get a little shrink mark where they intersect. The ribs should be in the direction of flow to fill properly. Optimal placing is dependent on use and may dictate that the gate may have to be moved to properly fill the tool. No rib should have blind corner fill zones and may require knock out pins in those areas to provide vents allowing complete fill without dieseling.

[Louis]

Molded tray?

[slaiman]

Hello everyone,
Thank you for the insight, the tray is constructed using recycled PP as this is what the customer wants and it is a molded tray.

I'll inform my injection engineer to check and see if we can insert some ribs onto the tray, i'll try to get some drawings of the tray so any further insights into where the ribs can be placed will be excellent

Again thank you all for the quick updates, I appreciate the suggestions very much

[DwightDixon]

I'm sorry, I thought you were an OEM and had material and functional responsibility. A processor needs work with the OEM to understand their performance vs cost trade-offs. Anything you do to improve the "performance" of a 100% recycled PP tray is likely to cost you more money, either in upfront mold revision cost or increased material cost for the long run. Your only potential product improvement, at no long term costs, would be to down gauge the basic tray and increase the amount of the material used in the ribs, therefore reduce the cycle time.

Another potential for the OEM of a high volume application, would be switching to recycled PS using a thin vacuum formed or pressure formed tray with lots of ribs in a fast cycle automated machine fed by an extrusion line.

[Len]

Hello everyone,
Thank you for the insight, the tray is constructed using recycled PP as this is what the customer wants and it is a molded tray.

I'll inform my injection engineer to check and see if we can insert some ribs onto the tray, i'll try to get some drawings of the tray so any further insights into where the ribs can be placed will be excellent

Again thank you all for the quick updates, I appreciate the suggestions very much

Why can't you add a mineral filler to your recycled PP material and thereby increase stiffness or modulus? Talc or delaminated clay could be used at 20 - 40% by weight adding stiffness. If you don't have material compounding capability, then ask your sullpier to make a filled version of your recycled PP resin. Or shop around, find a recycler who can supply the "value added" material.
Depending on what you're currently paying for the recycled PP and a modist up caharge for the compounding, you could net a savings because these fillers could well be less expensive than the base PP resin. Density would be in the range of 1.32 g/cc for a 40% clay filled PP and procesability could be tailored closely to the neat polymer if the formulator knew his stuff.

Just thinking out loud,

Len

[slaiman]

Dear Len /Dwight,

I was given the design of the tray by the customer and ordered no further changes to be made to the original design...when the first run was done, I found the tray to be flimsy and it flexes because of the length..I then went back to my customer and told them the shortfalls..again they reiterated that the original design & weight cannot be changed and so if adding ribs would cause the tray to be strengthened but at a cost of say 5% of the total weight then it is acceptable but nothing more.

Now Len has suggested that adding mineral fillers would strengthen the properties of PP Recycle, I may want to go in that direction as additional cost in producing the tray will be beared upon by me.....I'll consult with my injection engineer as that is an option, I would now want to check with my PP recycle supplier to see if the material that he provides contains enough fillers or what not.

Now I have some doubts....

1.If I were to use mineral fillers, will that in any way disrupt the properties of PP recycle since the ratio of PP recycle has decreased because of the additional mineral fillers

2. Will adding mineral fillers affect my cycle time, the cycle time currently is 45sec

3. Will the addition of fillers affect the durability of the tray ? PP recycle without fillers & PP recycle with fillers, will they last approximately the same lifetime ? any degradation in terms of durability as I would not want to compromise on that either since this is my first project with the customer.

Thank you again guys

[DwightDixon]

Adding filler or reinforcing filler will not change the useful life of the product, but will significantly increase the weight of the product as the specific gravity of the filler is much higher than the 0.90 of the recycled PP. The cycle time will be about the same, since you can eject at a higher temperature but the part size will be much larger that the original part since the filler reduces the shrinkage of the material significantly. Therefore, if there are tolerances on this part you will need to modify the tool to account for it. As I suggested before, I would work with the customer to determine the critical parameters and functions of this part and reduce the thickness of the basic tray and Increase the amount of ribs to obtain a more rigid tray (if that is a benefit) and a shorter cycle time at the same part weight. I also wish to remind you that you are selling parts (cubic centimeters of product) and so the cost per pound of the material is not directly related to the cost of the part. If the weight of the tray effects the function that is a different matter and if that is so you can not add filler without completely redesigning the tray.

[Yoda]

Adding geometry is the best way to stiffen your tray. Ribs don't have to be very thick to make a difference.

[POLYnnovation]

Hi There,

Some time since I visited forum, so apologies for the delayed addition.

Lots of sensible comments already made on filler content, S.G., cycle time, etc.

I guess you mean you want to increase torsional rigidity.That being the case, ribs running diagonally will be FAR more efficient in stiffening the tray than running end-to-end or cross-wise. See the image of the seed tray I designed in the 1970's. Much lighter than its predecessor, but much stiffer.

Even better use of material is to run a thick bead around the periphery at the top (rather than a simple flange, with its own rib network, as you see in the photos) and bottom (around the edge), then use gas or water assist to hollow it out (see Cinpres site, for instance). Needs tooling changes, a moulder that's licensed and the actual harware to do it. You'll get either one or two so-called "torsion boxes" by doing this and by far the best stiffness-to-weight ratio. It's a design trick" used often for automotive door pockets.

Thickening the section (even without ribs), then using enough blowing agent to take the density down by a percentage greater than the thickness gain will also give HUGE stiffness increases (basic structural engineering - section modulus, moment of inertia, etc.). Otherwise, Mucell, or similar technology to create foamed structure. Cosmetics will be very different (not necessarily inferior, depending on expectations), as swirl patterns will be evident.

In conclusion, increasing the material's inherent stiffness (modulus) will increase the part's stiffnes, but only pro rata. Diagonal ribbing, hollow beads or foaming the material and thickening the section (or a combination of all three approaches) uses macro- or micro-shaping to increase stiffness exponentially.

Regards,

John McLoughlin POLYnnovation