A number of things come to my mind of using materials in injection mold, because some colors (especially phthalocyanins) could cause nucleation in both HDPE and PP, this could lead to stresses which are showing up in the colder environment. Just thinking out of the box. Have you thought of using any of the super tough grades of Nylon6 or Nylon 6/6? An easy way to select your grade would be just to look at their datasheets for Impact resistance at -30 deg C and -40 deg C. I know from experience that these grades of Nylon are used for a lot of automotive applications for large parts and are used in varied range of environments. The added advantage is that Nylon being crystalline will give good chemical resistance as compared to the amorphous materials suggested earlier, better mold filling properties. The only disadvantage as compared to the polyolefins would be the increase weight and cost (considering current Nylon prices and higher Specific gravity).
Impact additives add strength but also add cost, and may reduce stiffness (rigidity) which will be important during stacking. The injection mold is built so you aren't going to change the dimensions. The key then is cycle time -- related to available machine hours, your accounting system, financing and things like that. There is little virtue in running faster if you're not selling out the machine time.
There are plenty of lower-MFI grades that are stronger, but they will mold slower, even at higher melt temps and injection mold temps. What is the real cost of running more slowly? If the products are cracking in service, the current cycle time is not a goal but a wish. HDPEs for pipe, milk bottles, bags and the well-known (in USA) post office tote boxes are made from fractional-melt grades, but you may not have to go that low. If you do use these, beware creating flow-line weaknesses which will encourage cracks in some places and not others, based on mold design.
BTW keeping it very cold doesn't make it brittle -- some materials are just brittle at that temp and some are not, and internal stresses make a difference, too. The FlintHills material noted above is interesting and worth follow up. Maybe you can blend it or even your current resin with recycled milk bottles to lower the cost, but you need very good-mixing and melt-screening equipment.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
