Principle of encapsulation, basis of formula and main points of process for TPE/plastic

Time:2019-09-27 09:41:01 / Popularity: / Source:


Let me talk about overmolding. This technique or process is nothing new. Simply saying, put a material is coated on the surface or inside of another material by injection molding. It is a process in which two materials are physically (snap, surface roll, thread) or chemically (co-bonding, miscible) joined to form an apparent single component but a double material.
There are two main means to achieve overmolding:
1.1. The first one is 2K injection molding, that is two-component molding or also called two color injection molding. It is realized on a two color injection molding machine, injecting one material into a product, and then mold is flipped and jumped to corresponding mold cavity of other plastic injection cylinder, which is used as an insert, and other material is injected thereon to form an integrated double-material product. Injection molding machine has a double barrel and a double mold cavity. Plastic injection molding process is that both products are produced at the same time, but only a cavity product will enter next cavity as an insert.
2K injection molding 
1.2. Another type is 2 step molding. A part is molded on an two color injection molding machine and placed as an insert into another mold, then second material is injected.
2K injection molding 
Vertical injection molding machine is used for 2-STEP sealing strip corner injection molding, except that first step is to extrude TPV or EPDM sealing strip and glass channel joints completed by 2-STEP injection molding.
2K injection molding 
Above two processes are collectively referred to as overmolding.
This process has been used for a long time in engineering plastics, such as PA66 GF insert, surface coated with PA66 URF gear, such as electronic charger coil skeleton package (PET GF FR + PBT GF FR ), package of a micro motor (internal PA66 or PET skeleton, coated with PBT GF).

Basic definition and principle of overmolding and encapsulation

In fact, rubber in the market segment of thermoplastic elastomer applications is a special overmolding. Special feature is that soft TPE material is used as second material component, and is coated on the surface, part or inside of various hard ordinary plastics and engineering plastics to realize effect of a single component with a variety of material effects.
Simply saying, overmolding can be plastic wrap plastic, soft plastic pack hard plastic, or soft plastic pack soft plastic, and overmolding is a habitual name. Carefully, it is shot-adhesive, that is encapsulation achieved by injection process is basically a specific overmolding process for soft plastic hard molding.

2.1. Double material parts, physical snaps and chemical bonding

As mentioned above, method of achieving overmolding (including encapsulation) can be physically snapped and chemically. The former, for example, by snap design, surface roll, surface tapping, and then coated with a second material to achieve overmolding (encapsulation). Feature of material bonding by this method is that physical joint has strong adhesion, and part outside physical joint has little adhesion.
Chemical method is a process that relies on molecular affinity between two materials and bonding force of chemical bond, two materials are bonded together to form a single component, two or more components and materials, so that there is strong adhesion between contact interfaces. Although physical snapping and chemical bonding methods are often used together in practical applications, it is clear that achieving chemical bonding between two materials is a more preferred method of greater robustness and greater design freedom.

2.2. About concept of adhesion and cohesion in encapsulation

Since it is clear that in TPE elastomer/hard plastic encapsulation application, chemical bonding of TPE and hard plastic is preferred two-material bonding method, you must understand two words: adhesion and cohesion.
It is a pity that there is no unified translation term or definition in Chinese. There are also separate translations called “sticking” and “co-bonding”. I follow internal definition of a German-owned TPE-S manufacturer.
Physical snap-type connections are no longer within scope of this article, and both of these two material bonding methods are chemical bonding.
Difference is that adhesion depends on molecular polarity attraction or hydrogen bonding force between two materials to achieve bonding between second material TPE and hard plastic; cohesion relies on one or several kinds of miscibility, polar attraction, and hydrogen bonding between two materials to achieve bonding of at least a molecular segment level at contact interface between two materials.
Theoretical level is very complicated, but from practical application, there is a very simple method of discrimination:
Adhesion effect has strong adhesion on double material interface, but as long as there is enough tearing force to tear TPE from hard plastic surface, hard plastic surface is smooth and flat, without residue of TPE material;
Bonding of cohesion effect, if TPE is strongly torn off from hard plastic surface, there is a lot of TPE residue on hard plastic surface, which means that TPE/hard plastic is actually mutually soluble on the interface, just like two metals are welded together, even if TPE itself is torn, bonding on the interface is also tight enough to cause TPE remains on hard plastic surface.
In a nutshell, adhesion is based on strong adsorption between TPE/hard plastic, and cohesion is mutual dissolution and chemical bonding of molecular segment between TPE/hard plastic. Undoubtedly, cohesion is the most ideal chemical bonding method in TPE/hard plastic encapsulation design.

2.3. Interface chemical reaction principle of cohesion in encapsulation process

As discussed above, in order to achieve special overmolding effect of encapsulation, it is most desirable to form a molecular segment level bond between TPE soft/hard plastic materials. This strong chemical bonding involves mutual dissolution, penetration, and molecular entanglement of molecular or molecular segments.
It sounds like a weld of two metals, isn't it? That's right, original idea of cohesion process comes from metal soldering process! One metal is used as a substrate, and the other metal is applied to surface in the form of a high-temperature melt. An ultra-thin layer having a thickness of several molecular layers is formed on the interface between two metals by high temperature. Two metal molecules are in a liquid or molten state at high-temperature ultra-thin contact interface layer, penetrate each other and penetrate into interface of each other through free diffusion movement of liquid molecules, and after cooling, form a thin layer of mutual solubility, penetration and permeation interface.
According to theoretical model of cohesion of above metal-like welding mechanism, bonding of molecular segment of TPE/hard plastic is realized, and key is to do following three points:
2.3.1. TPE material is similar in polarity to hard plastic (otherwise it cannot dissolve, penetrate in melt state);
2.3.2. Surface tension of TPE material is less than surface tension of hard plastic (otherwise TPE melt will not spread on the surface of hard plastic insert);
2.3.3. When TPE melt flows along hard plastic surface in mold cavity, cooling process releases heat, which can quickly and effectively melt hard plastic surface layer to form a thin layer that can penetrate each other;
Formulation design of rubber-coated grade TPE, molding process of rubber-clad molding, and even some seemingly irregular and difficult-to-coat problems in rubber-coated rubber. After knowing three basic points of above basic theoretical model, it is not difficult to grasp and understand.

2.4. Technical points and difficulties in realizing true chemical bonding properties of TPE/hard plastic bonding

Since TPE/hard plastic overmolding is a molecule of more mature overmolding process, it seems that mechanism of TPE/hard plastic encapsulation is not very complicated. Why do major TPE manufacturers still package TPE? In particular, especially nylon TPE encapsulation can be used as a major technical selling point (if you master this technology), or as a technical challenge (for those who have not yet mastered technology)? Is it just a character problem?
To achieve true chemical bonding properties of TPE/hard plastic bonding, technical points are also difficult:
2.4.1. First of all, it must be ensured that TPE has sufficient temperature resistance. It will not degrade in the barrel of injection molding machine when it is shot at high temperature, and it must ensure that temperature of TPE is much higher than melting/softening temperature of hard plastic, so that thin layer of hard plastic surface is melted.
-> Two requirements are combined together, in fact, TPE is required to have a high temperature resistance;
-> The higher melting point/softening point temperature of hard plastic, the greater technical difficulty;
2.4.2. When high temperature TPE melt flows at high speed along surface of hard plastic part in mold cavity, heat released during cooling process is sufficient to melt surface of hard plastic substrate;
-> The higher melt temperature during TPE injection, the better;
-> The higher melting/softening temperature of hard plastic, or the higher crystallinity, the larger melting enthalpy of crystal, the more difficult joining;
2.4.3. Even if TPE has sufficient temperature resistance, injected melt has a sufficiently high temperature and cooling enthalpy, which is sufficient to melt thin layer of hard plastic surface. It also requires that polarity of TPE and engineering plastic are similar in polarity to reach melt molecule. Thermodynamic compatibility of segmental stages is diffused and penetrates into interface of other party during melt contact time of short-term TPE/hard plastic substrate. Otherwise, once TPE melt cools, only polar adsorption force between molecules is formed, and bonding force of TPE/hard plastic is adhesion type;
2.4.4. Surface tension of TPE melt must be as small as possible to surface tension of hard plastic substrate. Otherwise, TPE melt cannot be spread out as soon as possible, melting of thin layer of hard plastic surface, and melt miscibility of two components. Then TPE is cooled in mold cavity, and only bonding force of adhesion type can be formed;

A variety of general plastics / engineering plastics / high temperature resistant special plastics and TPE plastic coated difficult point analysis

3.1. General plastics such as PP, PE, ABS

-> Melting point / softening point temperature is not high, material polarity is not strong,
-> SEPE/TPV/TPO/POE type TPEs are all non-polar,
-> Using above non-polar TPE injection temperature to pull high point, melt will melt a thin layer on the surface of these plastics, and then two sides dissolve, penetrate.
-> Conversely, if a polar TPE is used, such as TPU, TPEE, TPE-A, etc., TPE melt temperature is sufficient to melt surface of above-mentioned hard plastic material, but surface layers of two materials have poor compatibility and cannot achieve strong bonding because it violates requirement of "similar polarity"!

3.2. Amorphous engineering plastic PC, PC/ABS; PPO

-> Softening point temperature is higher than that of general-purpose plastics, requiring TPE to have a high melt temperature;
-> Material is weak to medium polarity;
-> Hard plastic is amorphous, TPE does not require a lot of heat to compensate for melting enthalpy in crystallization zone;
-> Therefore, it is not difficult to encapsulate rubber, TPE of SEPE/TPV/TPO, polarity of formula is easily adjusted to meet requirements, and TPU/TPEE is no problem, regardless of hardness requirements of TPE.

3.3. Semi-crystalline engineering plastics, polyester (PBT, PET)

-> Material polarity, similar to above PC, PC / ABS;
-> Material is semi-crystalline. TPE melt melts on the surface of polyester hard plastic to form an ultra-thin surface layer, which requires more heat, so TPE temperature should be higher, so rubberization difficulty is slightly higher than PC, PC. /ABS;
-> PBT melting point 220℃, PET melting point 260℃, TPE melt temperature requirements are still high, SBS-based TPE may not be suitable, PET encapsulation is more difficult than PBT;
-> TPE-S/TPV formula adjustment to meet polarity requirements, temperature requirements can be easily achieved; TPU / TPEE to achieve encapsulation naturally.

3.4. Semi-crystalline engineering plastic POM

-> Whether it is copolymerized POM or homopolymerized POM, melting point is not high (170℃ ~ 180℃), hardness of hard plastic melting point is not too high for temperature of TPE melt, and temperature resistance of TPE is not great;
-> Crystallinity is very high, heat required for TPE melt to melt POM surface is large, which has higher temperature resistance requirements for TPE;
-> Molecular polarity is not very strong, TPE-S/TPV type is easier to adjust formulation to achieve this requirement in terms of mutual compatibility;
-> POM is a self-lubricating material with a low surface tension, which means that even if TPE is similar in polarity and melt temperature is high enough to melt POM surface, in actual coating process, it is difficult for TPE melt to spread quickly and effectively on POM hard plastic surface layer!
-> So TPE-S/TPV wants to cover POM, it is very difficult! At present, we have only seen TF6MAA series of plastic bags covering POM (100P, M90).

3.5. Semi-crystalline engineering plastic nylon (PA6, PA66)

2K injection molding 
-> PA6 melting point 220℃, PA66 melting point 260℃, need for TPE melt temperature is higher, TPE temperature resistance requirements are higher;
-> Polarity is higher than all common plastics, PC and PPO, PBT, PET, POM, so it is more difficult to adjust formula of TPE-S/TPV soft rubber; TPU/TPEE polarity is sufficient, but temperature resistance, fluidity and hardness range in processing will be insufficient, which will be discussed later;
-> PA6, PA66 are semi-crystalline and have high melting enthalpy. It also requires higher TPE melt temperature to melt hard plastic surface more effectively and also improve temperature resistance of TPE.
-> PA6, PA66's strong polarity leads to easy water absorption and moisture absorption, and has higher requirements for control ability of injection molding process;
-> Therefore, TPE-S / TPV coated nylon is the most difficult area in current encapsulation formula and process!

3.6. Special high temperature resistant plastic

-> Hard plastics are high temperature plastics, such as PPS, LCP, PPA, PA46, etc., and polarities are similar to PBT, PET, PA6, PA66, etc. in above engineering plastics;
-> Ultra-high melting point (270℃ ~ 310℃) is a challenge for TPE-S, TPV, TPU, TPEE, latter TPE melt is at least 30℃ above high temperature hard plastic melting point to achieve melting and mixing of contact interface, which is very difficult for these TPEs;
-> It is easier to adjust polarity of formula and corresponding high temperature plastics to achieve adhesion effect.
-> Really coated TPE, LCP and other cohesion-coated TPE, are high temperature resistant TPE such as silicone/TPE-A based, and patented!
Above is difficulties and easy point analysis of encapsulation based on basic characteristics of general-purpose plastics, five engineering plastics, and special high-temperature plastics. Different formulation types, additive usage, and process conditions of each plastic have not been considered, and basically only a broad overall analysis.

Analysis of performance characteristics and difficult points of substrate used in rubber-coated TPE

Above is analysis based on characteristics of various general plastics, engineering plastics, and high temperature resistant special plastics. Obviously, to achieve ideal encapsulation effect of TPE/hard plastic, characteristics of various TPE substrates must also be taken into account, which inevitably involves formulation adjustment requirements of various TPE substrate characteristics.

4.1. TPE-S, TPV for non-polar substrates:

-> Packing non-polar or weakly-polarized hard plastic materials is no exception, no need to repeat them;
-> Difficulty is that it is difficult to pack polar materials and high temperature resistant plastics.
-> Hardness range is very wide, not TPU, TPEE, TPE-A and even most other TPE substrates are comparable;
-> Feel good, not sticky, not slippery, so all like it very much!
Therefore, key points of adjustment formula for encapsulated TPE-S/ TPV and polar hard plastic materials are:
-> TPE-S, improve temperature resistance, use SEBS less than SBS (temperature difference);
-> TPE-S, high-polarity substrates for substrates (supplier such as Kraton, Kuraray has their own unique cheats), or MAH grafted substrates, which is blended with a polar substrate such as TPU + solubilizer to change polarity. Otherwise encapsulation may not be good, but it is prone to migration or precipitation of added polar substrate and solubilizer!
-> TPV and polar hard plastic material encapsulation, TPV is actually a dynamically crosslinked EPDM rubber fine particles distributed in PP matrix, in process of melt mixing of TPV coated hard plastic material, fine particles of EPDM rubber which have been crosslinked cannot penetrate with polar hard plastic materials, so key point of formula design is to increase polarity of PP matrix phase as much as possible (such as Chemtura Polybond series grafting PP), and It is possible to increase content of graft modified PP in formulation and reduce content of fillers and plasticizers (such as paraffin oil) in formulation. Simply saying, there are more resins in formulation, and more grafted modified PP in dispersed phase of resin!

4.2. TPEE, ETPV for medium polarity substrates

-> TPEE is a polyester elastomer, ETPV is an ACM (AEM) / TPEE rubber / plastic blended elastomer produced by dynamic vulcanization technology (technical similar to EPDM / PP substrate TPV);
-> Due to TPEE main substrate or continuous phase of ETPV, they are all TPEE polyester elastomers. Cover PCs, PBT, PET with similar polarities and polyester functional groups are not difficult.
-> Nylon 6, nylon 66, polarity is still different, resulting in poor compatibility, unless substrate is improved in polarity;
-> TPEE, ETPV has a very good temperature resistance, allowing a very high melt temperature during injection molding, so that high temperature melt melts an ultra-thin layer on hard plastic surface to achieve cohesion;
Problems and deficiencies are:
-> TPEE hardness is very high due to chemical structure, at least from Shore A 90 / Shore D 35, corresponding to pursuit of soft hand-applied applications, can not achieve desired results;
-> ETPV actually uses a dynamic vulcanization process to disperse ACM (AEM) rubber particles into TPEE continuous phase substrate to achieve purpose of reducing hardness and maintain original TPEE high temperature resistance, fatigue resistance, chemical resistance and other advantages. Hardness can only achieve the lowest Shore A 60, can not be compared with low hardness and super softness of TPE-S, TPV;
-> The most critical point is that TPEE or ETPV materials are expensive! ! !

4.3. TPE-U (TPU), TPE-A (nylon elastomer), PEBA (long carbon chain copolymer nylon elastomer)

-> Undoubtedly, these highly polar TPEs are very similar in polarity to strong polar hard plastics such as PA6 and PA66, which facilitates mutual dissolution in rubber;
-> These short-term temperature resistance of TPE is very high, and it is easy to use high melt temperature in injection molding, injected into surface of strong polar nylon hard plastic to form a molten thin layer, then two materials form a cohesion grade encapsulation effect through penetration between molecular segments;
Problems and deficiencies are:
-> Hardness of TPU is not easy to achieve soft feel from Shore A 50, and it is sticky due to polarity of material;
-> Melt viscosity of TPU is sensitive to temperature. Sensitive temperature range is from 180℃ to 200℃. Above this temperature range, TPU melt will suddenly become too fluid, causing flashing after injection molding onto a hard plastic surface;
-> TPE-A and PEBA theoretically have good compatibility with strong polar PA6 and PA66, temperature resistance in injection molding is also high enough, but hardness is generally high starting from Shore A90, and unit price of material is also a constraint factor;
All of above results in the realization of cohesion effect in ideal encapsulation with TPE-S/TPV, which is the most soft, dry feel, and the most economical direction, so it is also difficult for Shenma TPE-S/TPV coated nylon, but this is also background reason why TPE manufacturers have been working hard to develop this technology.
Of course, these discussions are still based on characteristics of various TPE substrates to explore difficulties or requirements of formulation, without considering molecular structure design differences of various substrates, additives in TPE formulation components, etc. Even so, it is a very wide topic. Hard plastic formulation factors and process factors in processing have not been included.

Principle and process points of encapsulation to achieve ideal cohesion effect:

5.1. Three main points of realizing ideal encapsulation:

After reviewing again, from theoretical model of encapsulation process, to achieve cohesion encapsulation effect, basic points are actually only three sentences:
5.1.1. Polarity matching, two materials can dissolve and penetrate molecular segments in melt state;
5.1.2. TPE melt temperature is high enough to ablate hard plastic surface;
5.1.3. Surface tension of TPE is lower than that of hard plastic surface so as to spread rapidly on hard plastic surface, thereby achieving ablation and material penetration;
Above three basic points have actually been decided: TPE and hard plastic formulations match and meet each other's requirements, which is basis for achieving the ideal cohesion effect.
From perspective of commercial operation, the best way is of course TPE and hard plastic are produced by same manufacturer. They are very clear about their TPE and hard plastic formulas, design formula in strict accordance with three main points of theoretical model of above-mentioned rubber coating requirements to avoid factors or materials that are not conducive to cohesion in formulation design.
However, because of professional division of labor in TPE and hard plastic formula design, TPE and hard plastic material manufacturers each have their own main business direction.
In most cases, it is not guaranteed that same manufacturer will be able to provide matching TPE plus hard plastic grades (recipes) at the same time. Therefore, second best method is to test cohesion effect of mainstream hard plastics according to application direction of TPE formula developed by themselves, and carry out screening of hard plastic grades. Then selected hard plastic grades and their own TPE grades are recommended to customer as a rubber coating solution.

5.2. Process points for achieving ideal cohesion effect of encapsulation

No matter how it is done, if we have selected appropriate, matching TPE and hard plastic grades (recipes) enough to meet three main points of theoretical model mentioned above, so can you guarantee ideal cohesion encapsulation effect?
Answer is: Of course NOT yet !
Because even if TPE matches hard plastic grade, process still has a big impact!
In general, optimize encapsulation to achieve cohesion effect, main points of process are summarized as follows, and a simple explanation based on three main points of encapsulation principle:
5.2.1. Two color injection molding (2K injection molding) is superior to 2-step molding.
-> Simply say, the first step of hard plastic is hot when it is injection molded. It immediately enters second cavity of 2K injection molding machine. Intermediate transfer process does not absorb moisture, and it is easy to be ablated by high temperature TPE melt on the surface of ultra-thin layer, and there is no effect of moisture being adsorbed on the surface;
5.2.2. Use a higher barrel (melt) temperature as much as possible within allowable range of TPE material processing;
-> Otherwise, how easy is TPE to heat surface of hard plastic material?
5.2.3. In second step of encapsulation, mold cavity with hard plastic as insert to adopt a high mold temperature as much as possible;
-> Hard plastic temperature is high, TPE high temperature melt is slow to cool, there is enough heat, enough time to ablate hard plastic to form a super soluble thin layer;
5.2.4. In second step of encapsulation, TPE melt is injected at a high rate as much as possible without generating a TPE flash;
-> High injection speed, TPE has a short spreading time on hard plastic surface, and it has a longer time to ablate hard plastic surface during molding cycle;
-> High injection speed, friction between TPE and hard plastic surface leads to frictional heat generation, TPE melt temperature cooling is also slowed down, and it is used for contact and ablation of hard plastic surface for a longer time;
-> High injection speed, most TPE (except TPU, TPU melt viscosity is more sensitive to temperature) melt viscosity is shear thinning and surface tension is reduced, which is good for spreading on hard plastic surface;
5.2.5. To avoid moisture absorption or staining of hard plastic surface, especially in the case of two-step injection molding, it is more likely to occur during process of stopping and transferring hard plastic.
-> Polar hard plastics such as PC, PBT, PET, POM, especially strong polar hard plastics such as nylon 6, nylon 66, surface moisture absorption leads to formation of hydrogen bonds with adsorbed water molecules, TPE can not effectively form an intermolecular bond with hard plastic surface molecules, and natural encapsulation bonding force is reduced;
-> Surface of hard plastic part is dirty, such as workshop worker's gloves are not clean or glove cotton fiber adheres to surface of polar hard plastic part, which naturally affects encapsulation of TPE melt;
-> TPE coated with polar hard plastic is definitely a polar formula, which does not pay attention to moisture, and also affects cohesion effect of rubber;
In general, on basis of understanding principle model of encapsulation, it is easy to design a reasonable TPE and hard plastic matching grade (recipe), select appropriate process parameters in process and avoid some undue operational errors to achieve an optimized cohesion effect of TPE and hard plastic.

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