Co‐extrusion involves in extruding two or more materials through a single die. The materials for each of the layers (active pharmaceutical ingredient, polymer, plasticizer and other additives) are premixed or separately fed into an extruder. In each heated extruder barrel the material is softened, mixed and finally extruded through the die, where the different melt streams are combined into the final co‐extrudate. The co‐extrudate is then shaped, cooled and further processed. Throughout the entire process several important process parameters need to be controlled.
All equipment of a co‐extrusion line need to fit together and the extruders have to be positioned in a way that they can easily be connected at the die. Therefore the overall design of the co‐extrusion line, fit to the dimensions of the production facility, is important.
Variables that need to be controlled throughout the entire process are feed rates of the ingredients, barrel and die temperatures, motor speed and specific parameters for the downstream equipment, e.g. extrudate diameter control by a laser gauge. Parameters that need to be monitored are actual feed rate and temperatures, screw speed, torque and die pressure.
This co-extrusion technology gives full display to the characteristics of different plastic materials and improves the performance of custom co-extruded profiles with special requirements. such as moisture resistance, oxidation resistance, therm-forming and thermal bonding ability, good mechanical properties, etc., thus improving the quality of plastic extrusion profiles and reducing the cost of co-extruded plastic profiles. Therefore, co-extruded profile technology is increasingly used in the production of profile parts in many industries.
From material selection and co-extrusion modification technology, mold design technology and process parameter design are the key technologies that can affect the hard and soft co-extrusion processing.
Process parameter design: It is relatively harsh on the extrusion processing temperature for soft and hard co-extruded profiles production. Especially the extrusion die temperature. If the extrusion die temperature is too low, the plasticization is insufficient, it will result in the poor strength and wrinkled surface of the finished flexible and rigid profiles.
if the extrusion die temperature is too high, the melt viscosity of soft material is low (PVC material will also decompose and foam phenomenon), the sticky mouth die is more serious, and the tear strength of soft and hard co-extruded blanks is obviously reduced.
Therefore, soft and hard co-extruded profiles must be carefully studied and adjusted to the appropriate die process temperature. In addition, during the shaping stage, attention should be paid to the size of the water curtain in the soft cavity of the shaping die and the relative position of the vacuum port, the cooling temperature, the vacuum degree, the shaping length, the speed of traction and other process parameters, so as to obtain the suitable shaping parameters and ensure the stability of the product.
PVC is very temperature-sensitive and easily degrades at high temperatures in an autocatalytic reaction, generating hydrochloric acid. For high-volume rigid PVC production, the PVC resin is purchased in PVC powder, which is premixed with the necessary stabilizers, plasticizers, colorants, and fillers in batch mixers. The PVC is subsequently fluxed for a prescribed time and temperature to adhere to the thermal stabilizers and plasticizers to the PVC particles.
The colorants and fillers are added sequentially to the batch to produce a thermally stable formulation that is extruded into a custom profile. Since PVC is an amorphous resin, it does not have a defined melting point, and it continuously softens as the temperature is increased until the viscosity is low enough to flow and be processed by the extruder. The softening and flowing are called plasticization. Conical twin screws take the formulation and gently heat it and massage it until it is hot enough to flow in the extruder. In some instances, the melting process is called fluxing, because PVC does not go through a well-defined melting point; instead, it continuously softens at higher temperatures until all the ingredients are fluxed and mixed together. As a result, downstream feeding is not normally done, as all the ingredients are preblended prior to addition to the extruder.