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Metal insert molding and over-molding

Metal insert molding refers to a molding process where resins is molded over a metal insert prepared in advance in the mold. The molten material binds curing inserts and produces an integrated product.
Since metal insert molding may be prone to heterogeneous shrinkage, marginal tests should be done in advance to check the shape and dimensional accuracy of critical areas. During injection molding metal inserts are subject to easy deformation and displacement, one should take full account of the mold structure and shape which is easy to maintain the metal insert. If the metal insert shape can’t be changed, testing is indispensable. Predictable issues of restricting mold construction such as gate location and molding cycle should be resolved or improved in advance. One should confirm the need for preheating or drying metal inserts in order to ensure product quality and shape stability.
To avoid metal inserts and fine piece of molding to accumulate in the mold cavity, if necessary, one can install gas blowing device.  Insert molding rate, the cost of production and molding conditions are determined by various factors and technical tricks such as the precision of metal insert, insert shapes, whether the mold is conducive to insert molding and shapes of molded parts. An automatic insert molding system is critically determined by an effective combination of  injection machine, molds, automation equipment and how to make the system function in a short time.
Since 2004 Hengtai has been making auto and industrial sensor housings for a U.S. company. These sensor housings require the use of different material metal inserts with different plating requirement on the surface of metal inserts (including gold, nickel and tin plating). 
In the over-molding process, the covering material is injected into the above, below, around or within of the base material to form a complete unit. This process can be completed through multiple injection or insert molding.
Commonly used covering material is thermoplastic elastomer. During the over-molding stage, the slide or insert must be moved to another mold cavity or another injection molding machine.
In the process design of over-molding, non-return device, the gate, the vent and the mold surface texture are key factors.
Non-return device between the base and covering materials is critical for adhesion and the gradual thinning of the injected covering material or flash should be avoided. If the covering material is too thin, it will cause adhesion not strong, stripping or crimping. A good non-return device design should clearly separate the covering material and substrate.
The gate design is equally important to the success of the over-molding. The ratio of runner length and wall thickness is a key factor affecting the bonding effect. According to experience, the ratio should not exceed 150:1, and should keep at 80:1 in the development of new process design. In order to shorten the runner, the gate should be placed at the maximum wall thickness.
When using TPE, one should pay attention to the gate size. Materials such as TPE should be gated with large diameter in order to accommodate the higher viscosity and prevent excessive  shear force and resulting material degradation. Materials such as SEBS require higher shear rate to obtain the best flow rate. A better approach is to use small diameter of gate at the initial stage and adjust the gate size after first articles.
As with the gate, venting is also an important factor to affect bonding.  How to control the air residual is a major problem. If not properly managed, bonding may be not strong and filling flash may occur. The depth of the vent is critical to prevent flash. According to the viscosity of different covering material, the depth of the vent should be 0.0005-0.001 inches.
For the design of some parts, decorative surface texture can be used to help ejection. Most TPE materials are easy to adhere to mold surface because TPES material has metal affinity or  vacuum is formed between mold surface and the material.  Because many materials have not yet formed a stable chemical bonding at the time of ejection, if adhesive with the mold surface, they will affect the adhesion.  This means that after processing parts should be handled with care. If necessary, adhesion test should be done after 24 hours after stable chemical bonding is formed.  When the mold surface is parallel to the parting line, adhesion may occur if pull isn’t enough. Also the coating of mold surface helps parts to eject.

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