What Is An Overmold?

After the read, you will learn about:

What is an over-mold

The difference between plastic over mold and two-color mold

What are the problems that easy to occur in the molding process

With the continuous improvement of living standards, the requirements for material and aesthetics are also continuously improved. More and more manufacturers choose the multi-color concept of encapsulation when designing products.

For the plastic mold industry, the market demand for overmolded molds will increase, and the improvement of product grades is an urgent need for social development.

The mold company should always follow the footsteps of the market and create exquisite workmanship and high-quality rubber-coated two-color molds for customers.

What is an over-mold?

The over-mold is actually a kind of two-shot injection mold. The product generally includes the main body (hard rubber) part and the special outsourcing (soft rubber) part.

In-mold manufacturing, the hard rubber body is manufactured first, and the main hard rubber mold design is as normal.

The procedure is outsourcing (soft rubber) mold design should take the hard rubber finished product as a reference, no need to shrink, the back mold part is basically the same as the hard rubber main body, the front mold is different in that it has a rubberized shape, which is produced during production.

Hard rubber body, then put the hard rubber body into the outsourcing mold and then enter the outsourcing soft rubber injection production.

Overmolding is a process where a single part is created using two or more different materials in combination. Typically the first material, sometimes referred to as the substrate, is partially or fully covered by subsequent materials (overmold materials) during the manufacturing process.

How to make an over mold?

The practice of overmolding

Many products in our lives use plastic encapsulation molds, such as toothbrushes, handles, mice, buttons, car buttons, etc., composed of two or more materials, among which hard plastic and soft plastic are rare, soft plastic Hard rubber-coated products are mostly used for hand-contact products, and hard rubber-coated hard rubbers are mostly used for high-precision products.

  1. Two-step injection molding

The semi-products produced by ordinary injection molding machines are processed and then manually put into another set of molds for injection. Finally, get the desired product. In terms of equipment requirements, ordinary injection molding machines can meet, so it is also a common practice in factories. Also called set beer mold.

1) In the second injection molding, the mold needs to design the clamping position and the avoiding position of the product for the second time, so that the product can be placed in the designated position faster and more accurately, and avoiding compression molding.

2) When designing the mold, the product only shrinks the first mold product, because the product has been retracted to the actual size when it is taken out and put into the second mold, so the second mold does not shrink.

The dimensional tolerance can only be measured accurately for the first mold, and the size of the second mold is usually a soft rubber appearance product surface, and the dimensional tolerance requirements will not be very high.

  1. Two-color injection molding machine

The two sets of molds opened are installed on the two-color machine at the same time, or the soft and hard plastics are set on one set of molds, and the two materials are injected simultaneously in the same mold.

Generally, the two-color machine has at least two barrels. After the injection is completed, the index plate of the back mold rotates 180 degrees, and the second injection is performed at the same time to obtain the desired product.

(Two-color molds can be integrated into two-color molds and split two-color molds. They are all installed on two-color machines. The principles are the same, but there are differences in mold processing.)

Split two-color mold

Two sets of molds need to be opened, and the mold base size of the two sets of molds is the same. The rear mold part is the same, and the front mold part is different. During installation, it is necessary to ensure that the two sets of molds are at the same height and cannot be offset.

Integral two-color mold

Combine the split two-color mold on a set of molds. The pouring system and ejection system are set up on the same set of molds, but they need to work independently. That is to say, the back mold part is the same, and the front mold part is different.

It has two locating rings and an ejector washer. The advantage of the integral two-color mold is that only one set of molds is required, and the processing cost is lower than that of the split two-color mold, and the installation of the mold will also reduce the installation of one set of molds.

1) Different from two-step injection molding, there is no need to take out the product when the primary molding is converted to secondary molding.   

Therefore, the two sets of molds are scaled and shrunk during the design, and the dimensional tolerance is still based on the size of the first mold. Tolerance shall prevail.

2) The number of two-color molds in the factory is limited. When scheduling production, it is necessary to determine whether the machine meets the molding requirements of the product. The measurement of the molding machine, the size of the turntable, and the tonnage of the two-color machine shall prevail.

3) The product structure of the two-color mold is relatively simple, and the structure of the inclined top and the slider is designed as little as possible to ensure the safe use and stability of the mold during the operation of the two-color machine.

4) The two-color mold produced by the two-color machine is used as a reference. The guide pin cannot be like the guide pin of an ordinary mold. The reference angle is offset by 2.00mm to the center position.   

Otherwise, the guide pin and guide bushing will be in phase when the two-color machine is running Cannot close the mold.

The difference between plastic over mold and two-color mold

  • Two-color mold

Two plastic materials are injected on the same injection molding machine, and the mold is molded twice, but the product is only ejected once. Generally, this kind of molding process is also called two-component injection molding, which is usually completed by a set of molds and requires a special two-color injection molding machine.

(2) Over mold (secondary molding)

The two plastic materials are not necessarily injected on the same injection molding machine, but are molded in two parts; after the product is taken out from one set of molds, it is put into another set of molds Perform the second injection molding.

Therefore, this kind of molding process is usually completed by two sets of molds, and no special two-color injection molding machine is required. The mold structure is the same as that of the single-color injection mold, and it is mainly controlled by adjusting the flow rate of the injection parameters the binding point.

The encapsulation mold is mainly soft rubber encapsulated hard rubber. The soft rubber is usually made of artificial rubber, TPU, TPR, and other rubber materials, and the hard rubber can be ABS, PC, PP, etc.

What are the problems that easy to occur in the molding process?

  1. Size deviation

When injecting hard rubber first and then soft rubber, it is necessary to understand that the hard rubber is shrunk in the mold design, and the soft rubber does not shrink. The size deviation has a great relationship with the mold design and part processing;

Secondly, the soft rubber part is usually they are all appearance requirements. The size requirements are not very strict. It is easier to happen that after the first injection is completed, the second injection molding does not meet the requirements in time.

At this time, it is necessary to debug the first mold product. The required molding parameters and conversion time can be correctly obtained.

  1. Product injection

The overmolding die is the most unwilling thing to see in the factory, and it usually happens when using two-step injection molding. When the semi-product of the first injection molding is placed in the second molding mold, the product is not placed in the designated position or the product position is shifted, which is a common cause of compression molding.

Therefore, when the production efficiency and yield are low, the only way to strengthen the production awareness of employees, or switch to two-color molding production.

  1. No package

In order to make the two materials adhere tightly, it is best to use special two-color mold materials, and the smooth surface roughness of the mold is also one of the factors. When debugging the product, avoid the use of mold release agents at other times except to prevent the product from sticking to mold and stuffing when debugging.

Using the two-step injection molding method, the first batch of semi-finished products is not immediately transferred to the secondary molding process, and when the secondary molding is retained for a period of time after being transferred to the warehouse, the surface needs to be cleaned of dust and 80℃-100 The pre-heat treatment at ℃ can have a better encapsulation bonding effect.

  1. Lack of glue in secondary molding

It is necessary to ensure that there is no shortage of glue in the product in one-time molding, so as to ensure that the yield rate of secondary molding is improved. However, the soft rubber of TPE material is usually used for encapsulation in the secondary molding.

Due to the poor fluidity of the material, the finished product may have loose packages and a lack of edges. It is necessary to re-adjust the molding parameters to maximize the injection time and molding cycle.

  1. One-time forming deformation

The barrel temperature of the secondary molding and the selection of materials must be above the melting temperature of the surface of the primary molding product, and the temperature difference is about 20°C.

Secondly, try to reduce the holding pressure parameters or not use holding pressure during secondary molding, otherwise, it is easy to cause the shape of the joint surface to be misaligned or the first half-product warping deformation.

  1. Long molding cycle time

This is a common problem in overmolding. The main reason for the slowness is that the overall two-color mold is more efficient than split two-color molds and greater than two-step injection molding in the practice of overmolding.

  1. Stick the front mold

The problem of sticking to the front mold when injecting TPE soft rubber in the second molding process is easy to occur. The draft angle should be larger when the mold is designed, and the surface should be etched.

  1. The font after encapsulation is not clear

When we are molding over molds with fonts, the fonts or the logo is usually unclear, mainly because the height of the fonts is too low, which brings great troubles to the process of debugging, and the standard preload value is higher. 0.02~0.05mm of the theoretical value. The font of the formed product will be clear.