The Top Ten Considerations For Switching To Polypropylene:
Polypropylene Is An Incredibly Economical Food Packaging Solution That Is Rated As A Highly Recyclable Material.
1. Polypropylene has higher expansion and shrink rates.
Polypropylene has a typical shrink rate of up to .015 per inch. This is higher than what you would traditionally experience with HIPS (High Impact Polystyrene) or PET (Polyethylene terephthalate). This makes it necessary for you to review your product and mold design to ensure that the different expansion and shrink rates are accounted for. As in any of these processes, a qualified BMG expert can assist you in this evaluation and get you the data on acceptable ranges of tolerances.
2. Know your material filler and how much to use.
Talc is the common filler for Polypropylene material, but it is important to know how much talc has been added to your material. Talc filler at 10% or less generally does not incur excessive wear on your tooling or extrusion equipment. However, a filler rate of greater than 10% can cause premature wear and be an issue. Replacing worn components in tools and extruders can be costly in the long run, but simple material monitoring can reduce exposure to this.
3. Increase blow form.
The molecular orientation of Polypropylene material during the thermoforming process can vary. Polypropylene may require more stretching and orientation within the tool, necessitating higher psi for finer product detail. The minimum blow form required is 100 psi.
4. Increase thermoformer oven stops.
Polypropylene has a lower coefficient of thermal expansion than polystyrene, which means that it may not expand as much during the heating phase. Using additional oven stops in the thermoforming machine extends the plastic material’s heating or curing time. Recommended oven stops are 5 stops for post trim and 4 stops for Form/Cut/Stack applications.
5. Roll-fed thermoformers require material preheating.
As stated above, Polypropylene’s molecular orientation can vary. It is easy to have distortion in the heating, causing hot spots, uneven thickness across the sheet, and even material degradation. If the material is not fully cured or heated, the expansion of the molecules will affect your finished product. If your thermoforming machine’s oven is too short to heat your roll of material thoroughly and evenly, a preheater can extend the heating process of the sheet.
6. Superior material heating from inline extrusion.
Inline extrusion is the suggested or preferred method of introducing Polypropylene material into the thermoformer. This is when the extruder melts the plastic pellets into sheets, and the heated material is fed directly into the thermoforming machine’s infeed. The extruder has heated the sheet to the ideal temperature, and the molecules are fully expanded. This is an expensive method and is dependent on product volume and mix.
7. Decrease trim mold die clearances.
The expansion and shrink rates of heated Polypropylene affect your trim tools. Match metal (post-trim) applications require a die clearance from .005” to .0005”. The most common clearance used for dies is .0025”.
8. Heat trim knives.
Form/Cut/Stack trim molds use steel rule or forged knives. Since Polypropylene is more flexible and durable, trimming products is more complicated. Heating the knives provides a superior trim.
9. Increase mold cooling.
As discussed above, polypropylene requires more heating to cure the material properly. Because the material is coming into the mold hotter, the mold requires additional cooling to form and cool the product. Molds should be designed to remove 25% to 50% more BTUs per hour than a traditional HIPS tool.
10. Review product design.
As previously mentioned, polypropylene is a different material to heat and cool. It is more flexible, and durable, and has different expansion and shrink rates than similar materials. The trim tool die clearance, blow form psi, and mold cooling require review. Be confident that you have thoroughly reviewed your tool to form the best product possible.