Diagnosis and Suggestions on Causes of Poor Adhesion of TPE Shot Adhesive Coated Plastics
Source：德创化工Time：2020-06-16Hits：2330
Injection molding is a common processing method for TPE materials. However, during the test of new products, many processing plants have consulted the author about the problem of weak encapsulation. The author briefly analyzes the reasons and provides the following suggestions:
Theoretical Analysis: In fact, TPE should cover plastic. Three factors are considered: first, the compatibility between TPE and plastic (or their similar polarity); The second is the temperature when TPE is formed. The third is the flow length ratio when TPE is coated.
Detailed interpretation
1. Compatibility between 1.TPE and Plastics
TPE and coated plastic should have close polarity (good compatibility). Polymer materials believe that liquids with close solubility parameters SP can be better mutually dissolved. It is understood that polymer materials with close solubility parameters SP can be better mutually infiltrated to form an interfacial mutual melting layer when melting and softening at high temperature.
This has become the theoretical basis for the formulation design of TPE coating materials. When selecting coating materials, TPE brand with close polarity to the coated plastic should naturally be selected, so that good coating adhesion is possible. The theoretical analysis of coating adhesion between different plastic varieties and different TPE materials is as follows:
2. Forming temperature
Only when sufficient temperature is reached can interpenetration between TPE and coated materials be realized, thus providing good coating effect. Different plastic varieties adopt different molding temperatures for TPE (the following table shows the common plastic and TPE overmolding temperatures summarized by the author). In general. TPE overmolding temperature is slightly lower than the melting point or molding temperature of hard plastic parts by 5-10 degrees.
3. flow length ratio L/T
The flow length ratio L/T probably means that the flow length/product thickness ratio of ——TPE is less than 150:1 (refer to Baidu Encyclopedia entries created by the author). For example, if the thickness of TPE coating is 2mm, the maximum flow length of TPE melt cannot exceed 300 mm. It is not difficult to understand that if the flow length ratio is too large, the TPE melt may not be able to reach the required size of coating due to flow resistance, temperature drop in the mold cavity and other reasons, resulting in coating dissatisfaction.
Specific analysis
There are many kinds of soft rubber TPE used for encapsulation. This paper mainly discusses the SEBS modified TPE coating. PP, ABS, PC and PA are the most common hard plastics. TPE, PP, ABS and PC are relatively simple to encapsulate, while nylon is much more complicated to encapsulate.
1.TPE coated PP,ABS,PC
Under normal circumstances, there is no situation that cannot be covered. If there is a situation that cannot be covered, there may be two factors:
1.1TPE materials are not selected correctly, and there are components in the blending system that differ greatly in polarity from PP, ABS and PC.
1.2 The overmolding temperature is low. The temperature is low, TPE is not fully plasticized, let alone coated and bonded.
2.TPE-coated nylon
Because the melting points of different nylons are quite different, and many nylon materials have been modified by adding fiber and flame retardant, nylon has strong water absorption. The following issues should be paid attention to in TPE formulation and overmolding process
1.TPE blending system
1.1 improvement of polarity
The polarity of TPE blending system should be adjusted to make it have adhesion with PA.
1.2 matching of melting points of TPE and PA nylon
TPE molding temperature should be slightly lower than nylon injection molding temperature by 5~10 degrees, and cannot differ too much. Therefore, for nylon materials with different melting points, the choice of TPE blending system is different.
2. Design and process of molding die for rubber-coated nylon
2.1 Design of Product Glue Entry Point
Because nylon overmolding requires a higher molding temperature, the flow length ratio should be as small as possible when designing the runner of the mold machine. Excessive flow length ratio will make the temperature of TPE melt drop obviously when filling and flowing in the mold cavity, thus affecting the bonding effect. According to the product structure, when designing the mold,
Designing one or two more glue feeding points can shorten the flow length ratio l/t.
2.2 Preheating and Baking of Nylon Parts
This is to eliminate the moisture that may be contained in nylon parts, which is beneficial to encapsulation and appearance. At the same time, increasing the surface temperature of nylon parts will be more conducive to coating and bonding.
2.3 overmolding temperature
The melting point of nylon parts and the first-stage injection molding temperature T1 are clarified, and the two-stage injection molding TPE overmolding temperature T2 which is slightly lower than the injection molding temperature T1 of nylon parts is adopted. Generally, T1 is 5-10 degrees higher than T2.
