Technical Papers Plastics
Brittleness at ambient temperatures makes thermoforming of PLA different

Brittleness at ambient temperatures makes thermoforming of PLA different

Polylactic acid (PLA) represents a new group of thermoplastics for packaging applications obtained from renewable resources. PLA can be produced from renewable resources such as wheat, sugar beet or agricultural by-products containing natural plant sugars. In its production, first dextrose is fermented to form lactic acid, which is subsequently converted into cyclic dimers (lactides) by condensation and esterification. The dimers are converted to polylactide by catalytic ring opening polymerisation. PLA�s properties make it suitable for various thermoforming applications, particularly cups for cold beverages and trays for fruit and vegetables. No major change in equipment or tooling is required when thermoformers switch to PLA. However, some important considerations need to be addressed before making a successful switch from oriented PS and PET.
All safety precautions normally followed in the handling and processing of thermoplastic sheet should be followed, as per NatureWorks® for its PLA. As with most thermoplastics, thermal processing and the variability of those conditions may result in minor decomposition.
PLA sheet should be stored in an environment designed to minimize moisture uptake, and in a cool place at temperatures below 40°C. It is not necessary to dry PLA sheet prior to use to obtain haze free parts. At temperatures above 40°C, the sheet is susceptible to blocking and would resist unwinding. Above 40°C and above 50% RH, the sheet is susceptible to molecular weight breakdown and loss of physical strength.
PLA thermoformed parts should be shipped and stored in an environment that minimizes exposure to heat, moisture, and humidity; including maximum temperatures below 40°C. . Clear, amorphous PLA thermoformed parts at temperatures above 40°C are susceptible to distortion. At temperatures above 40°C and above 50% RH, PLA thermoformed parts are susceptible to molecular weight breakdown and loss of physical strength. PLA is highly polar and can retain a charged surface, if untreated. A charged surface can attract dust; but the charge can be mitigated by the use of local electrostatic eliminator bars. Protection of sheet goods with packaging material is also recommended to control dust collection on the sheet.
PLA sheet is relatively brittle at room temperature. The elongation to break under tensile stress is between 4 and 8%. Good tension control during web handling is critical as sudden increases in tension during any portion of the unwind process may result in web breaks. Power driven nips at the unwind station are recommended. In addition, unwind stations and skeleton rewind stations should have web paths that minimize tight radius paths of the web. Minimum skeleton rewind radius should be 10 inches (25 cm) to ensure smooth travel of the web and minimum breakage. The toughness of PLA increases with orientation and therefore thermoformed articles are less brittle than PLA sheet, particularly in the regions that have been highly stretched during the forming operations. Experimentally, the elongation to break under tensile stress has been seen to increase from 4-8% in sheet to about 40% in the sidewall of a drinking cup. Flange or lip areas that receive less orientation tend to be more brittle than the rest of the thermoformed part. If the sheet must be slit to size prior to thermoforming, then a rotary shear knife is required for trimming. Edge preheaters are necessary to prevent the sheet from cracking at the pins and to minimize rail chips. The edge preheaters will be set (temperature and proximity to the sheet) to warm the sheet to near 200°F (190°C). Contact heat edge preheaters would typically be set to 212°F (100°C). Non-contact heat edge preheaters would typically be set warmer than would be used with similar thickness sheet made of polystyrene or PET, and may approach the thermoforming oven set points.
PLA is frequently thermoformed using forming ovens, molds and trim tools designed for PET or polystyrene (more specifically in the classifications HIPS or OPS). It is critical to note that PP shrinks much more than PLA so that molds and trim tools designed for PP are less optimally used with PLA. Chain rail systems designed to stretch warm sheet to compensate for sagging PP are not necessary or desirable systems for PLA, and may cause PLA sheet to be pulled out of the pin chains. In post trim operations, comparable shrinkage is observed between PLA and PET, so PET trim tools adapt well to PLA service. In trim in place operations comparable shrinkage is observed between PLA, PET and polystyrene. Trim in place molds, matched metal die punches and less optimally heated steel rule die punches (120°C or 250°F) are recommended for trimming PLA thermoformed articles. An ambient temperature steel rule die punch would not be recommended. For plug assist thermoforming, plugs manufactured from Syntactic foam are typically used. The plug can be coated with a slip coating to prevent sticking in some deep draw applications. Plug shape has had more impact on part quality than has the material of construction. Plug shape is more dependent upon the particular part being molded so no general recommendation can be made regarding PLA. PLA has a lower softening temperature than PET or PS. Typically oven settings are about 55°C (100°F) or more lower than PS, and about 40°C (75°F) or more lower than PET oven settings. The sheet should be about 90 to 110°C (190-230°F) entering the mold. Aluminum molds are recommended for thermoforming PLA. In addition to being the traditional choice for production thermoforming molds, aluminum is a good metallurgical choice for PLA service (in contrast to carbon steel, which would be more susceptible to corrosion).
PLA has a thermal conductivity that is lower than Polystyrene and PET. The Tg (deformation temperature) of PLA is also lower than both polymers. In addition, the density of PLA is greater than Polystyrene. All of these factors indicate that the cooling time in the mold will be greater for PLA than either PS or PET. In many thin wall parts, this increase in cooling time is negligible compared to the overall cycle time so that forming rates equivalent to PS and PET have been achieved. However, some thick wall parts will require additional cooling time, which will adversely affect the overall cycle time.
PLA regrind is not compatible with regrind from any other sheet product. It is necessary to both wipe and water wash clean the grinding equipment and transfer lines or to have dedicated systems for PLA. PLA must be about 40°C or less to grind efficiently. Some grinding systems require additional cooling to efficiently grind PLA. Various PLA blends will behave differently during thermoforming and may require completely different process parameters. Overall, the cycle times for PLA are quite consistent with those of PET and OPS
  Back to Articles

Previous Article

Next Article

{{comment.Name}} made a post.




There are no comments to display. Be the first one to comment!


Name Required.


Email Id Required.

Email Id Not Valid.


Mobile Required.

Email ID and Mobile Number are kept private and will not be shown publicly.

Message Required.

Click to Change image  Refresh Captcha



Moulds for lotion pump

Moulds for lotion pump