Injection process for the production of pet and RPET articles and articles obtained thereby
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Solution Overview
Problem
Current injection molding processes for PET articles face challenges in achieving a balance between mechanical and thermal resistance, cycle time, and surface finish, particularly due to the poor thermal characteristics of amorphous PET and the fragility associated with high crystallinity, which limits the production of thick or thin articles with good surface quality.
Innovation Solution
Incorporating a nucleating agent into the PET composition and cooling the mold to a temperature of 30°C or lower, with the nucleating agent concentration between 0.10 wt % and 0.25 wt %, allows for improved crystallinity, mechanical properties, and reduced cycle time, enabling the production of articles with good thermal stability and rigidity without fragility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the mold is cooled to increase crystallinity and thermal resistance, then thermal stability is improved, but cycle time increases
Solution Approach 1:
The mold is pre-cooled to a temperature of 15°C to 30°C before injection, creating favorable thermal conditions in advance. This preliminary cooling action enables faster crystallization during injection, allowing the article to be extracted sooner while maintaining good thermal resistance, thus resolving the contradiction between thermal stability and cycle time
Solution Approach 2:
The invention changes the mold temperature parameter from conventional high temperatures (150°C) to low temperatures (15°C to 30°C). This parameter change fundamentally alters the crystallization kinetics, enabling rapid crystallization during the injection process itself, thereby reducing cycle time while achieving the desired thermal stability
2Stability of the object's composition
If the article is left in the hot mold for crystallization, then crystallinity is achieved, but cycle time increases
Solution Approach 1:
The mold is pre-cooled to create favorable thermal conditions before injection, enabling crystallization to occur rapidly during the injection process itself rather than requiring extended holding time in a hot mold. This preliminary cooling action allows crystallinity to be achieved while significantly reducing cycle time
Solution Approach 2:
Instead of using a hot mold to promote crystallization (conventional approach), the invention inverts the approach by using a cold mold (15°C to 30°C). This inversion enables rapid heat transfer and controlled crystallization during injection, achieving the desired crystallinity without extending cycle time
3Productivity
If the article is extracted immediately from the mold, then cycle time is reduced, but crystallinity is insufficient
Solution Approach 1:
The mold is pre-cooled to 15°C to 30°C before injection, creating optimal thermal conditions in advance. This preliminary action enables rapid crystallization to occur during the injection process itself, allowing the article to be extracted immediately after injection with both adequate crystallinity and reduced cycle time
Solution Approach 2:
The invention rushes through the crystallization process by utilizing the temperature differential between the cold mold and molten PET. This rapid crystallization occurs during the injection process itself, eliminating the need for extended holding time and enabling immediate extraction while maintaining sufficient crystallinity
4Volume of moving object
If the molten PET is injected into a thin mold, then the article can be molded, but the mold does not fill completely
Solution Approach 1:
The invention changes the mold temperature parameter to a low range (15°C to 30°C), which creates a strong temperature gradient during injection. This parameter change enhances the driving force for molten PET flow, enabling complete filling of thin-walled molds while maintaining good surface finish and dimensional precision
5Volume of moving object
If the article thickness is increased, then more material can be molded, but surface finish deteriorates due to shrinkage defects
Solution Approach 1:
The invention changes the mold temperature to a low range (15°C to 30°C), which creates controlled thermal conditions during injection. This parameter change enables uniform cooling and crystallization throughout the article thickness, minimizing differential shrinkage and preventing surface defects even in thick-walled articles, thereby maintaining good surface finish
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process results in PET articles with enhanced mechanical and thermal properties, allowing for a wide range of shapes and significantly reducing cycle time, thereby increasing production efficiency and enabling the molding of both thick and thin articles with improved surface finish.
Implementation Method 1
adding to PET a nucleating agent... to obtain PET articles having improved characteristics... a crystallinity level that allows a good thermal stability
Implementation Method 2
the mold is cooled at a temperature equal to or lower than 30°C... by using water and the temperature of water is kept at a temperature equal to or lower than 30°C
Data Source
Figure 1~2
AI summary
The present invention is directed to a process for the production of PET articles, which process comprises the following steps: adding a nucleating agent to PET; homogenizing PET and the nucleating agent; injecting the homogenized mixture into a mold; wherein the mold is cooled at a temperature lower than 40°C. The invention is further directed to a PET injection molded article comprising from 0.03 wt % to 1.0 wt % of a nucleating agent and having a degree of crystallinity as measured by DSC comprised between 15 % and 30 %. The inventors have found that by injecting PET comprising a nucleating agent in a mold cooled down to a temperature lower than 40°C, it is possible to obtain a large variety of PET articles which were not obtainable by injection molding before. Furthermore, by dosing a proper amount of nucleating agent, it is possible to obtain an excellent compromise between mechanical and thermal properties of the article.