I. Material Formula Factors
The proportion and characteristics of material components are the basis for determining the shrinkage rate. Different components will directly affect the thermal shrinkage response of the film.
Ratio of resin raw materials: POF shrink film is co-extruded from an outer layer (homopolymer polypropylene PP) and an intermediate layer (linear low-density polyethylene LLDPE). The ratio of the two will significantly affect the shrinkage rate. For example, the higher the proportion of the intermediate layer LLDPE, the more flexible the film, but the shrinkage rate may be slightly lower; the higher the proportion of the outer layer PP, the more sensitive the film's thermal shrinkage response at high temperatures, and the shrinkage rate is usually higher.
Amount of plasticizers and modifiers: Adding plasticizers can improve the flexibility of the film, but excessive amounts will reduce the intermolecular forces, resulting in a lower shrinkage rate; while adding specific modifiers (such as nucleating agents) can optimize the crystal structure, making the film more prone to shrinkage during heating and thereby increasing the shrinkage rate.
Molecular weight distribution of raw materials: A narrower molecular weight distribution of raw materials leads to more regular molecular arrangement, resulting in more uniform molecular chain contraction during heating and a more stable shrinkage rate; if the distribution is too wide, some molecular chains may contract at different rates due to differences in heat resistance, causing fluctuations in the shrinkage rate.
II. Production Process Factors
The processing parameters during the production process directly "set" the shrinkage potential of the film, with the key being the control of the stretching and cooling stages.
Bidirectional stretching process parameters: POF film needs to undergo longitudinal and transverse stretching. The stretching ratio and stretching temperature are the core factors. The higher the stretching ratio (typically 3-5 times longitudinally and 5-8 times transversely), the greater the "shrinkage stress" stored in the film, and the higher the shrinkage rate during subsequent heating; if the stretching temperature is too high, the molecular chains will relax prematurely, resulting in a lower shrinkage rate.
Cooling and setting speed: The film after stretching needs to be rapidly cooled and set to "lock" the stretched state of the molecular chains. The faster the cooling speed, the less time the molecular chains have to return to their original position, and the more stress is stored, resulting in a higher shrinkage rate; if the cooling is slow, the molecular chains will easily relax naturally, and the shrinkage potential will decrease.
Uniformity of film thickness: If the film thickness is uneven during production (such as local areas being too thick or too thin), the molecular chains in the thinner areas will contract faster during heating, while those in the thicker areas will contract slower, leading to inconsistent overall shrinkage rates and even causing wrinkles.
III. Heating Conditions Factors
The heating method and parameters used in actual application are the direct factors that trigger shrinkage and determine the final shrinkage rate.
Heating temperature: Temperature is the core driving force for shrinkage. Within the range from the critical shrinkage temperature (typically 70°C) to the maximum tolerance temperature (120°C), the higher the temperature, the more intense the molecular chain movement, and the higher the shrinkage rate; however, if the temperature exceeds 120°C, the film may soften and deform due to excessive heating, and thus cannot shrink normally.
Heating time: At an appropriate temperature, the longer the heating time, the more time the molecular chains have to contract, and the shrinkage rate will gradually approach the maximum value; if the heating time is too short, the molecular chains will not fully contract, and the shrinkage rate will be lower; if the time is too long, it may cause the film to age and become brittle.
Heating method: Different heating methods (such as hot air circulation, infrared heating, steam heating) have different heat transfer efficiencies. Hot air circulation heating is uniform, allowing the film to shrink uniformly as a whole, resulting in a stable shrinkage rate; infrared heating is prone to local overheating, which may cause local shrinkage rates to be too high, leading to wrinkles or cracks.
