May 1, 2023

What is Overmolding?

over molding parts

Overmolding is also called 2 times injection molding in China. the final part is consist of 2 independent parts in different material, color, finish, Compared with the third-party material bonding, overmolding process makes the process faster and more cost-effective, it provides better bonding strength and better look, so it has been widely employed in plastic part design.

Overmolding type:

Over molding are categorized into 3 types: insert molding, double shot molding, co-injection molding.

1. Insert molding:

The most commonly used to mold soft elastomers to rigid plastics, insert molding is the most economical way to do overmolding. In the insert design, the hard segment has been inserted into tools in advance, and then eject the soft materials in. Elastomers are adhesion bond, which can be mechanical locks, chemical and other methods. In order to achieve chemical adhesion, the elastomer must be sufficiently heat to melt the surface of the hard segment to achieve chemical bonding.

Elastomers are adhesion bond, which can be mechanical locks, chemical and other methods. In order to achieve chemical adhesion, the elastomer must be sufficiently heat to melt the surface of the hard segment to achieve chemical bonding.

2. Two-shot molding:

This over molding process requires two injection molding machines. Hard segment is injected first, and then led by the elastomeric mold to the injection molding machine, finished the process of hard segment into the elastomer injection. Heat substrate is usually semi-solid, gel state. These molds can be designed and shaped projecting laterally of the hard segment on the substrate. This can achieve a stable mechanical bond.

3. Co-injection molding:

In co-injection molding, while the hard substrate and the elastomer is injected into the same mold, compatibility between materials is essential, it must be carefully controlled.

Co-Injection is very expensive and difficult to control; it is used least comparing with other 2 method. However, because the hard and soft substrates elastomers in a fully molten state, it provides the best bonding between soft and hard parts, the molded parts quality is also the best.

Over molding material

1. Hard material

The first step of injection material is called the base material or hard material, the hard material can be ABS, PA6/PA66-GF, PP, PC, PC+ABS, etc. the plastic parts in hard material are made in the first time injection molding.

2. Soft material

The second step injection material is called covering material, or soft material. The commonly used covering material is elastic resin, TPU, TPR, TPE, soft PVC, in the second time injection process, soft material will be injected to cover on the hard plastic ready in first time injection.

3. Over molding material choosing

The choice of overmolding resin material depends on a variety of factors, depending on substrate properties on the one hand and application performance on the other. Specifically, there are the following points:

  • Chemical corrosion resistance.
  • Flame retardancy (conforming to the requirements of ecological and environmental protection). The eco-friendly label is a sign that a product meets environmental and social standards.
  • Abrasion resistance (so as not to sink or fall off).
  • Shore hardness (in line with soft feeling or other requirements).
  • Impact resistance (in line with structural requirements).
  • Melting point (meets application temperature requirements, will not soften or deform).
  • Adhesion method (a mechanical interlocking effect is formed when the two materials do not match, and a chemical adhesion is formed when the two materials match).

In general, matched materials should have similar chemical properties or contain matched composite components. When the substrate and cover material do not match, there is usually only a mechanical interlock, not a chemical bond.

Soft material can be molded with different hard material; however, hard material over molding on hard material is subject to strict restrictions. it can only be used for partial area; it cannot be used for large-area or closed-loop designs.

If hard material can also over molding on another hard material, but here are the restrictions:

  • There must be a molding temperature difference (melting point) between the two materials, at least 30 degrees; the molding temperature of the base material should be higher than that of the covering material;
  • The encapsulation (color separation) area of the hard rubber covering material should be as small as possible to avoid the risk of cracking caused by the shrinkage of the larger material itself and shrinkage;
  • The shrinkage rate of the hard rubber covering material should be small, and the shrinkage rate is usually 0.4~0.6% when using non-crystalline or semi-crystalline plastic materials.

Over molding design principles

  • There must be rubber grooves on the parting surface of the substrate.
  • The width of the base glue groove is 1.2 or 1.8mm. The sealant is flat, choose 1.2mm, and choose 1.8mm where the sealant process is complicated.
  • The corners or slender parts of the rubberized area need to have rubber holes with steps; there are rubber grooves or interval rubber grooves on the substrate around the rubberized area, and the depth of the groove is 1.0mm and the width is 1.0mm. Make as many nail holes as possible (20~30mm spacing) and try to be as close to the edge or corner as possible. The edge distance between the nail holes and the ribs on the frame should be greater than or equal to 1mm to prevent the lagging material from overflowing into the gaps of the ribs, resulting in glue leakage or draping.
  • When the substrate is beveled, the bevel should not exceed 45 degrees. The sealing surface should be at least 1.2MM wide and try to be parallel or perpendicular to the parting surface. If it is an inclined surface, the slope should be as small as possible to reduce the difficulty of mold matching. The slope should not exceed 45 degrees.
  • For holes or slots deeper than 5mm, there should be a buckle design at the end of the lagging to prevent the lagging from sticking to the model head.
  • Similar to the 2 small dots in structure “B”, technical requirements: must be greater than 2*2mm.
    The width of the peripheral wireframe should be at least 1.5mm, and the root should be 2mm (trapezoidal).
  • Lagging wall thickness:
    Recommended value: 1.5mm. The thinnest part of the part shall not be less than 1.0mm. The flow length of TPE (including gate length) / product thickness ratio is less than 150:1. If the ratio is too large, there will be glue shortage and insufficient glue; Product design can be designed according to 80:1.
  • The sealing gap between 2 features must be more than 1.0mm. Otherwise, there will be sealing problems and burrs.

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