Intro
In fact, in the field of thermoplastic elastomers, overmolding is a special type of overmolding. So what is overmolding? Simply put, overmolding can be plastic-on-plastic, soft-rubber-on-plastic, or soft-rubber-on-soft-rubber.
Two main ways to achieve overmolding:
- Two-component molding, also known as two-shot injection molding, is performed on a two-shot injection molding machine. One material is injected into the product, then flipped over and transferred to the corresponding mold cavity of the other injection barrel. This part serves as an insert, and another material is injected over it to form an integrated two-material product. The injection molding machine uses two barrels and two mold cavities. During the injection molding process, two materials are produced simultaneously, but the product in one cavity is transferred to the next cavity as an insert.
- Another type is two-step injection molding, where a part of one material is molded on one injection molding machine, the part is placed as an insert in another mold, and then the second material is injected.
The methods for achieving overmolding
The methods for achieving overmolding (including overmolding) can be physical snap-on methods and chemical methods.
The former, for example, relies on a snap-fit design, surface knurling, and threading, followed by a second material coating to achieve overmolding (rubber coating). This method alone is characterized by strong adhesion at the physical connection, but little adhesion beyond the physical connection.
The chemical method relies on the molecular affinity and chemical bond strength between the two materials to bond the two materials together to form a single component, two or even multiple components.
Although physical snap-fit and chemical bonding methods are often used together in practical applications, it is obvious that chemical bonding between two materials is the preferred method that is more reliable and has greater design freedom.
This strong chemical bonding includes the mutual solubility, permeation, penetration, and molecular entanglement of molecules or molecular segments.
The key to achieving molecular segment bonding
The key to achieving molecular segment bonding of TPE/hard plastic is to do the following three things:
- The polarity of TPE material is similar to that of hard plastic (otherwise they cannot dissolve, penetrate or penetrate each other in the melt state);
- The surface tension of TPE material is lower than that of hard plastic (otherwise, the TPE melt will not be able to spread on the surface of the hard plastic insert);
As for surface tension, it is easy to understand if you think about the fact that mercury shrinks into a ball on the surface of glass, while water droplets can spread out on the surface of glass.
- When the TPE melt flows along the hard plastic surface in the mold cavity, the cooling process releases heat, which can quickly and effectively melt the hard plastic surface to form a thin layer that can be interpenetrated.
But the actual situation is that TPE encapsulation technology is not very mature, and there are not many companies in China that can do it.
The first reason why the coating cannot be completed is definitely the TPE material formula. This is also the customer’s first reaction. Then they change manufacturers to find test materials, wasting a lot of time and energy. However, if it is an experienced salesperson and modification manufacturer, when receiving customer needs, they will definitely understand the customer’s situation in depth before making recommendations. Therefore, generally speaking, unless it is really because of immature technology, there should be no problem with the coating of the material. Because if the coating cannot be completed, it not only wastes the customer’s time but also the salesperson’s time and sample fees.
Therefore, we often need to deeply analyze the reasons why the coating cannot be covered from the perspective of the injection molding process.
Technical reference
From a technical point of view, the following points are worth your reference.
1,Two-color injection molding (2K molding) is better than two-step injection molding (2-step molding).
Reason: Simply put, the hard plastic is still hot when it is injected in the first step and immediately enters the second cavity of the 2K injection molding machine. There is no moisture absorption during the intermediate transfer process. It is easy for the high-temperature TPE melt to ablate an ultra-thin layer on the surface, and there is no influence of water vapor adsorption on the surface.
2, Within the processing allowable range of TPE material, choose a higher barrel (melt) temperature as much as possible, otherwise TPE will not easily have enough heat to burn the surface of the hard plastic material.
3, In the second step of encapsulation, the mold cavity of the hard plastic insert should be kept at a high mold temperature as much as possible. The higher the hard plastic temperature, the slower the TPE melt cools, giving it enough heat and time to ablate the hard plastic and form an ultra-thin, mutually soluble layer.
4, In the second step of encapsulation, the TPE melt is ejected at the highest possible speed without generating TPE flash.
1) The injection speed is high, the TPE spreads on the hard plastic surface for a short time, and there is a longer time to ablate the hard plastic surface during the molding cycle;
2) High injection speed, friction between TPE and hard plastic surface causes frictional heat, and the TPE melt temperature cools down slowly, taking longer to contact and ablate the hard plastic surface;
3) The injection speed is high, and the melt viscosity of most TPEs (except TPU, which is more sensitive to temperature) undergoes shear thinning, and the surface tension decreases, which is conducive to spreading on the hard plastic surface;
5, avoid moisture absorption or contamination on the hard plastic surface, especially during two-step injection molding, where the hard plastic undergoes a stop-and-go process. Polar hard plastics such as PC, PBT, PET, and POM, especially highly polar hard plastics such as nylon 6 and nylon 66, absorb moisture on the surface, forming hydrogen bonds with adsorbed water molecules. This prevents TPE from effectively forming intermolecular bonds with the hard plastic surface molecules, naturally reducing the bonding strength of the encapsulation.
The surface of hard plastic parts is contaminated, such as unclean gloves of workshop workers or cotton fibers of gloves adhering to the surface of polar hard plastic parts, which naturally affects the encapsulation of TPE melt; TPE that coats polar hard plastic must also be a polar formula, and if moisture-proofing is not paid attention to, the cohesion effect of the encapsulation will also be affected.
6, Pay attention to the problem of washing the barrel in the early stage of trial operation
1. PVC encapsulation was used before
2. Previously, it was a machine for producing hard plastic
