Overmolding and Insert Molding

We have been in the plastic injection molding industry for decades, we have done a lot of overmolding insert molding projects, these projects are used in a wide range of industries, in the process we have accumulated a lot of experience about overmolding insert molding.

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We supply one-stop overmolding insert molding service from product design, mold manufacturing to injection molding and product assembly.

Our dedicated engineers will provide a response within 8 hours, ensuring the process goes on smoothly.

Comprehensive analysis product manufacturability and optimization recommendations for your design.

Use CAE technology to simulate the real Injection molding process before Injection mold making.

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Few or A dozen samples should be produced for assembly/functional testing before mass production.

After the trial insert molding phase, we start batch molding, which uses powerful machining to manufacture parts at fast rates to save on time and costs.

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Table of content

Insert molding is a comparable process however instead utilizes a preformed part– often metal– that is filled right into a mold where it is then overmolded with a thermoplastic resin to develop a last part. When the run is full, parts are boxed as well as shipped shortly thereafter.

There are two ways to place the insert in the mold during insert molding.
1) Manual placement; this can significantly increase the injection molding cycle time and is suitable for small batch production or products with complex structures.

2) Automated placement; this is a better choice for high volume production, which helps to reduce manual errors, improve efficiency, shorten the molding cycle and increase the reliability of the product.

With overmolding, the production of the substratum parts is a conventional shot molding procedure involving an aluminum mold without heating or air conditioning lines running through it. Cycle times are a bit much longer, which allows our molders to keep an eye on fill pressure, aesthetic issues, and also the basic top quality of the components. When the complete run of substrate parts are built, overmold tooling is then constructed to journalism. The substrate parts are put by hand into mold and mildew where each component is overmolded with either a thermoplastic or fluid silicone rubber product.

1) The combination of the easy formability and elasticity of plastic and the rigidity, strength and heat resistance of metal can make complex and delicate metal-plastic integrated products solidly; reduce the size and weight of the product;

2) Using the combination of plastic insulation and metal conductivity, the formed products can meet the basic functions of electrical products; they can also meet some special requirements, such as magnetic permeability, wear resistance and fastening;

3) Adding metal inserts to plastics can improve the strength of plastic parts;

4) overmolding increases the flexibility of product design and provides new possibilities for the combination of plastic and metal and other parts;

5) Avoid secondary processes such as hot melting, welding and riveting, and have the opportunity to shorten assembly time and cost;

6) Inserts are not limited to metal, but also include cloth, paper, wire, plastic, glass, wood, coils, electrical parts and plastic parts;

7) For rigid molded products and bending elastic molded products on the rubber gasket plate, after the integrated products are made by injection molding on the substrate, the complicated operation of arranging the sealing ring can be omitted, which makes the automatic combination of the subsequent processes easier;

8) Because of the combination of molten plastic and metal inserts, compared with the press-in molding method, the metal insert gap can be designed to be narrower, and the product reliability is higher; it is easier to pass vibration and other tests;

9) Select appropriate plastics and molding conditions, that is, for products that are easily damaged (such as glass, coils, electrical parts, etc.), they can also be sealed and fixed through plastics;

10) Select the appropriate mold configuration, the inserts can also be completely enclosed in plastic.

11) The combination of vertical injection molding machine, manipulator, insert product alignment device, etc., most of the insert molding projects can realize automatic production.

1) The placement of the inserts often complicates the mold structure, prolongs the injection molding cycle, increases the manufacturing cost, and increases the difficulty of automated production;

2) The thermal expansion coefficient of the insert and the plastic is inconsistent, which is easy to cause internal stress in the final product, resulting in product rupture(see below picture); this is particularly obvious in the injection molding of the nut insert;

3) Or because the thermal expansion coefficient of the insert and the plastic is inconsistent, causing the product to deform(see below picture);

4) Inserts (especially nut inserts) often require preheating or drying to reduce internal stress;

5) The insert must be well fixed in the mold, otherwise it is easy to shift or deform under the impact of the molten material;

6) Once the overmolding produces defective products, such as poor injection molding, missing inserts, and poor location, the entire product will be scrapped, and the cost will be huge;

7) overmolding is not conducive to product recycling and scrapping.

When designing overmolding insert molding, the process characteristics of overmolding insert molding must be fully considered, and the DFM design must be carried out

4) In order to facilitate placement and positioning in the mold, the overhanging part of the insert (that is, the part placed in the mold) should be designed to be cylindrical, because it is easiest for the mold to process round holes; (see picture Design of the overhanging part of the insert)

5) The overmolding insert should have structures such as sealing bosses to prevent overflow during injection molding; (see picture Insert boss design)

13) In the connector, the firmness of the terminal insert in the plastic is a priority for every product design engineer. The following methods are commonly used, which can greatly enhance the firmness of the terminal in the plastic;

-Punching holes in plastic middle terminals;
-Plastic middle terminal special-shaped;
-Terminal bending in plastic;
-The terminal in the plastic increases the roughness and bumps;
-Increase the plastic wall thickness.

                                                                         ▲Terminal insert punching, special-shaped, bending

If you want to get the English version for above overmolding insert molding design guide, please contact PTS, a professional insert molding factory.

The nut insert is one of the most common inserts. The nut insert and the corresponding plastic pillars must be designed correctly, otherwise there will be quality problems such as insufficient pull-out force and pillar cracks.

There are three ways to embed the nut into the plastic part:

1) Hot melt nut
Hot-melt embedding is the most common and common way of embedding. Generally, the nails are embedded with hot-melt machines and manual electric irons;

2) Insert injection nut
Injection molding embedding generally has strict requirements on the aperture of the nut strut, and the aperture is controlled within 0.05mm, because the product is fixed with ModingPin and placed in the injection molding mold, and the nut aperture should be controlled by the size of the PIN pin of the injection molding machine;

3) Ultrasonic nut
Ultrasonic embedding is a kind of ultrasonic vibration, which causes the friction between the nut and the surface of the workpiece and the internal molecules to increase the temperature transmitted to the interface. When the temperature reaches the softening temperature of the workpiece itself, the nut is embedded in the plastic part. In the middle, when the vibration stops, the workpiece is cooled and shaped under a certain pressure at the same time.

The three methods have their own advantages and disadvantages. This article discusses the insert injection nut.

–The d dimension is the nut base, also known as the guide positioning part, which is matched with the plastic part C before being embedded, so the d part of the bottom end of the nut is smaller than the diameter C of the inner diameter of the pillar of the plastic part to facilitate positioning;
–The D dimension is the outer diameter of the nut, which matches the inner diameter C of the plastic pillar. Generally, the plastic inner diameter of the mobile phone nut is about 0.25-0.3mm smaller than the outer diameter of the nut;
–The L dimension is the length of the nut, which is matched with the Y dimension of the pillar hole depth of the plastic part. Generally, the depth of the plastic hole is 0.5-1.0mm larger than the height (length) of the nut for glue storage;
–W is the wall thickness of the plastic hole. Generally, the thickness of the pillar hole of the plastic part is 0.8-1.0mm or more. The larger the size of the nut, the greater the thickness of the meat. (contact us to get all details parameter.)

1) The correct size of the pillar hole of the plastic part is very important. If the pillar hole of the plastic part is too large, the nut will not be able to eat the glue after being embedded in the plastic, which will cause insufficient torsional tension; if the pillar hole of the plastic part is too small, the phenomenon of glue overflow or rupture will occur;

2) If there is no problem with the size of the nut and the size of the plastic, and there are abnormal phenomena, it is usually considered to improve through some optimization designs;

Due to the shallow depth of the pillars of the plastic parts, if you choose a nut with double twill, under the proportion of each size of the nut, especially on the embossing, the difference in the embossing section is short. If it is too small, it will cause insufficient torsional tension. Therefore, it is generally recommended that the depth of the pillar hole of the plastic part be set to more than 2.5mm, and the length of the nut is generally recommended to be more than 2.0mm.

Improvement plan: Similar to this situation, if it is inconvenient to change the mold, it is recommended to change the nut flower shape to monoslope BS1, as shown in the figure on the right, in this way, the embossing step difference will be increased, and the plastic of the embossed rubber surface will increase. , thereby increasing the torsional tension.

3) Glue overflow or rupture in the pillar holes of plastic parts;

Due to the choice of the style nut on the left, after the plastic is embedded, A1 and A2 expand, but B shrinks sharply, making it difficult to squeeze the plastic, and the upper and lower ends of the nut are prone to overflowing glue, which seriously affects the appearance of the product;

Improvement plan: The “C” part is used as the guide for positioning, so that the nut can be securely placed in the plastic hole, which improves the efficiency and yield rate. Due to the addition of the C-end guide part, after A2 squeezes out a certain amount of rubber, it is also reserved. At the same time, because the A1 and A2 parts are twill with a 90° angle of 45°, they can form a high-strength node, resist the torsional force on the circumference, and greatly increase the torsional tension.

7.1.1 First of all, in the product design, the product structure’s feasibility analysis must be carried out, mainly for the strength of the metal parts, metal parts and plastic parts Positioning, metal parts molding positioning hole design and other aspects of the overall comprehensive analysis.

Metal parts structure control, general overmolding insert molding metal parts thickness control in 0.3-0.4mm (the following picture showed). 0.2mm is too thin and easy to be warped, and it is difficult to control the accuracy of its metal parts and injection molding products after the overmolding insert molding.

7.1.2 Metal parts structure in the normal case, four sides should have ribs position structure, this structure is not easy to deformation, but also convenient for its positioning with the plastic products.

7.1.3 The positioning of plastic products and metal parts can be positioned with plastic products through the buckle position and small holes on the side. It is not easy to separate from the product.

7.1.4 The fitting between metal and plastic parts should keep the kiss-off structure. Prevent the generation of burrs and metal or plastic parts scratches. Try to design the metal and plastic parts mating with the kiss-off structure; avoid the shut-off structure.

7.1.5 Metal products to avoid the overall flat structure, (the following figure) this structure is not strong enough, easy to lead to product deformation and other problems during overmolding insert molding.

7.1.6 The positioning structure must be stable to avoid misaligning the cavity and core mold due to unstable positioning structure and crushing the mould. The normal situation needs at least three holes positioning – 3 corners positioning Or 4-hole positioning.

7.1.7 The thickness of plastic products is generally controlled above 0.7mm; if the thickness is too thin, it will easily lead to the product can not be filled well. Then need, a relatively large injection pressure; will cause the product deformation due to excessive injection pressure.

7.1.8 The thickness of plastic products must be uniform. Avoid uneven shrinkage due to uneven thickness, causing product deformation.

7.2.1 The overmolding gates should be evenly distributed and arranged symmetrically as much as possible to make the filling parallel and the shrinkage balanced; Avoid the product’s shrinkage caused by the gate’s uneven location.

7.2.2 When you put the product shrinkage for overmolding, you must separate the metal parts and plastic parts and put the shrinkage of the plastic part alone. And the vertical and horizontal direction shrinkage is not the same.
The length direction shrinkage is larger, The width direction is smaller. (The overall product shrinkage is small)

7.2.3 After the shrinkage, the metal and plastic parts have obvious breakage and step. In this case, when designing the mould, make sure to make up the step position. The metal piece is not moving. Make up the plastic position and flatten the plane of the metal sheet.

7.2.4 Suppose the product’s structure allows the metal and plastic parts to shrink simultaneously. Metal parts processing must be made by processing the product after putting shrinkage. It is possible not to replace the steps that appear after the release shrinkage.

7.2.5 Positioning pin with 3-5 degrees on one side, easy to insert into the cavity side, and the pin in the core side need to leave part of the straight body position about 2mm, convenient to fix the metal parts deviation.

7.2.6 For overmolding Metal parts area, non-plastic sealing position, it is recommended to have 0.5mm clearance space or more, mainly to prevent the deformation of metal parts, and the mould can not be fitted well with metal parts.

7.2.7 The overmolding mold fitting surface must fit well, Preventing the product have warping and deformation. The cavity and core side close together can also correct the slight deformation of the metal parts.

7.3.1 For this type of product, high-speed and low-pressure injection is suitable. Because the product’s structure is thin, it is difficult to fill due to high injection pressure, and the overmold also needs to have a good venting system.

7.3.2 The holding pressure needs to be large enough, and the mould temperature needs to be controlled at a proper temperature to prevent the internal stress of the injection moulded parts from cooling down. Insufficient holding pressure or mold temperature will lead to product shrinkage, thus causing product deformation.

7.3.3 Choose a plastic material with less shrinkage and better flowability. This can prevent the product from deformation caused by too much shrinkage.

7.3.4 PC +10%GF has been gradually promoted in the industry; when appropriate, you can try this new process because the addition of corrugated fibre can properly prevent the deformation of the product due to shrinkage.
However, due to the poor fluidity of the wave fibre, the need for higher moulding temperature, and the instability of the wave fibre and high requirements, This makes the process difficult.

7.4.1 The overmolding mold machining accuracy is much higher than that the general mold. Especially the positioning is very important. Accuracy must be well controlled. Otherwise, it will lead to metal parts and plastic offset.

7.4.2 The overmolding mold machining process needs to be re-understood because the overmolding insert molding. The parting surface is easy to appear burrs, so the processing technology needs to be the perfect processing method. Especially, the kiss-off and shut-off surfaces are easy to appear burr, The overmolding manufacturer need to strictly control.

7.4.3 With overmolding Runner and product appearance as much as possible to have better polish. Because PC or PC+GF10%, Nylon+GF20%. Flowability will be poor. Therefore, we need better polish on the runner and product to reduce the cutting rate and cutting stress, so that the plastisol can fill the cavity quickly.