Silicone Overmolding Design Guide

Silicone Overmolding Design Guide

Silicone Overmolding Guide

Silicone overmolding is also known as rubber overmolding, liquid silicone overmolding, lsr overmolding, insert overmolding, multi-shot injection molding, as a process that apply silicone rubber part - overmold onto the second part - substrate by compression molding, transfer molding, plastic injection overmolding or liquid silicone rubber injection molding(LSR overmolding ).

By silicone overmolding, silicone rubber part will form a bonding with the substrate.

Silicone overmolding is widely used for overmolding silicone, overmolding rubber, overmolding glass, overmolding plastic(PP, PC, ABS ), overmolding metal, silicone overmolding electronics, medical silicone overmolding, etc.

Such as TPU/TPE overmolding, cable overmolding, fabric overmolding, overmolded connector, pcb overmolding, overmolded gasket, overmolded o ring, etc.

Silicone overmolding products we made are as follows: silicone spatula(silicone head+PP handle ), baby toothbrush(silicone+plastic ), respirator(PVC mask+silicone gasket ), auto shockproof mount(rubber+metal ), screwdriver(rubber+metal ), and so on.

Silicone Overmolding Main Ways:

Silicone Overmolding Way	Application	Strength	Weakness
Compression Molding	Mainly used for when the substrate is metal material/plastic(PBT) that is resistant to high molding temperature	Economical	Mold life cycle is affected by semi-automatic operation; After many shots, the burrs and parting line will increase; The tightness of positioning pin installation will affect the product quality, if it is too tight or too loose, the product will be trapped or misplaced
Transfer Molding	Mainly used for when the metal substrate part is so small/light that easily run if silicone rubber material vulcanized unevenly	By transfer molding, the silicone rubber material(overmold ) in the material pot can be vulcanized quickly and evenly. So that the substrate - metal part can be overmolded well.	Waste raw material
Plastic Injection Overmolding	Widely used for plastic overmolding and big/thick product overmolding	1. The wall thickness distribution of parison can be accurately controlled by using injection mold, so that the wall thickness of the obtained container is relatively uniform; 2. the obtained product does not need to be processed for e times, has no seams, and saves the finishing time of the product; 3. All parisons made by injection can be blown into the blow mold for molding, and no waste is produced during the processing, thus reducing the waste of molding materials; 4. Products with smooth surface can be obtained without scratches, and the surface characters and patterns are clear; 5. Small batch production can be carried out economically; 6. Wide application range for plastic varieties; 7. High degree of automation, multi-mode production and high production efficiency.	1. Two sets of molds for injection molding and blow molding and a hollow core mold with valve must be used for product molding, and the mold for injection parison should be able to withstand high pressure, and the cost of molding equipment is high; 2. The injection parison has a high temperature, which needs a long cooling and setting time after inflation in the injection mold, which prolongs the whole molding cycle of the product and affects the production efficiency; 3. The internal stress of injection molding parison is large, and it is easy to be cooled unevenly in the process of changing molds. When producing products with complex shapes and large sizes, stress cracking is easy to occur, so the shape and size of the products are limited, which is only suitable for producing small products used in cosmetics, daily necessities and medicine packaging, food packaging and other fields; 4. The operating skills of operators are required to be higher.
LSR Overmolding	Widely used for overmolding for silicone rubber part, plastic substrate part,  not resistant to high molding temperature or metal substrate part, or silicone material safe grade is highly required	1. Short curing time 2. Small burrs & parting line 3. Sealing tightly at the junction of silicone and substrate 4. Automatic mechanical operation with high precision, tight tolerance in size control	Higher cost

Silicone Overmolding Design Guide - Case Analysis

Silicone overmolding case 1:

Our customer wants to manufacture this overmolding mount - silicone overmolding onto metal part, the below picture shows our engineer’s mold design layout.

If only use this metal substrate and silicone rubber, have you observe any problems?

The most important problem is that the edge sealing(at the junction of silicone and substrate ) will be not tight, besides, the burrs will be large.

So, how to solve this problem?

The keyword is cap!

See below picture:

Our engineer suggests applying metal protective sleeve/cap.

Metal protective caps are added at both ends of the metal substrate part, and the metal protective caps matched with the metal substrate parts are prepared in advance, and then the metal substrate part is sleeved in the metal protective caps and put into the cavity of the mold together, and then the silicone rubber is put into the cavity for silicone overmolding.

Because it takes time and pressure to vulcanize silicone rubber, it flows into the mold cavity after a little vulcanization, not instantaneous vulcanization.

Therefore, if there is no protective cap, the first vulcanized silicone may flow to the place where the metal parts should not be overmolded with silicone, then causing the appearance of the overmolded part will be irregular, not sealing well and burrs will be not small.

Adding two metal protective caps effectively protect the metal substrate from contamination.

 

Silicone overmolding case 2:

When applying silicone overmolding onto metal screw, how to prevent silicone rubber from overflowing into the screw parts that do not need to be overmolded?

This picture shows our customer’s original design:

This picture shows our engineer’s design:

As above pictures show, adding a step(height: 0.5mm ) to the metal screw, can prevent the metal screw from rotating and let the metal screw be firmly fixed in the mold cavity.

Besides fix the screw in the mold cavity well, this step design can also prevent silicone rubber from overflowing into the screw parts. 

Silicone overmolding case 3:

Silicone overmolding onto plastic buckle, how to accomplish?

As the same as the first silicone overmolding case, our engineer designs a cap for the plastic buckle, not metal cap, but silicone cap.

You must be puzzled in this case, why our engineer applies silicone cap rather than metal, won't the silicone cap and silicone watch wrist stick together?

Please see below pictures, we will give you detailed explanation:

As the above picture shows, when mold plates closed, the green buckle separate the silicone material and yellow silicone cap, thus they cannot touch each other, there is no need to worry about the adhesion of silicone in two places.

The most important reason why choose silicone cap rather than metal cap is that silicone cap is flexible and elastic so that can provide better positioning to plastic buckle and itself positioned well in mold cavity.

 

From theses silicone overmolding cases, we know the material of substrate plays an important role in affecting silicone overmolding.

Different substrates need different silicone overmolding design.

For example, different substrate with different temperature requirement, will influence choosing overmolding method, compression molding, plastic injection overmolding, transfer molding, or liquid silicone rubber injection molding(LSR overmolding ).

As the silicone overmolding case 3 shows, the plastic buckle is made of PA+GF, so we apply compression molding to overmold the silicone watch wrist to the buckle.

Because adding GF(glass fiber ) can increase the hardness of buckle, prevent the buckle from deforming & shrinking, and make the buckle can be resistant to high temperature.

 

(Note: Generally, adding 8%~10% GF to plastic. 

If adding too much GF will lead to floating fiber, the plastic will looks white with fog feeling and leave flow mark on the surface of plastic part. 

Because too much glass fiber will increase the plastic rigidity, then the fluidity of the material is deteriorated. )

 

If the substrate is PC, we must use lsr overmolding.

Because work temperature of PC is lower than 130℃, if applying compression/transfer molding, the PC will soften, then the overmolding cannot seal well with bigger burrs.

Most silicone overmolding onto plastic applies liquid silicone rubber injection molding(LSR overmolding ), which is suitable for low work temperature material.

When silicone overmolding applies lsr overmolding, we will add platinum vulcanizing agent/ platinum catalyst(a kind of good low temperature vulcanizing agent ).

 

Characteristics of platinum vulcanizing agent:

Temperature: low temperature;

Color: transparent;

No new low molecules after vulcanization;

Odorless;

Rapid vulcanization speed and low molding temperature;

Excellent physical properties after formation;

One time vulcanization ,without secondary vulcanization.

Application of platinum vulcanizing agent:

Mainly used for low temperature vulcanization and silicone with odorless or thick products.

Such as: molded silicone nipple, cake pan, ashtray, sealing ring, silicone bag, PC mobile phone case or thick silicone product.

Besides silicone overmolding onto common plastic, food grade/ medical grade plastic can also overmolding onto the same grade silicone, such as PPSU, which can be used for maternal and child supplies.

 

Silicone overmolding can not only overmold on plastic, but also on metal, glass, cable, pcb, etc.

Generally, for metal and glass substrate, overmolding agent/primer is necessary.

 

Silicone Overmolding Onto Metal/Glass Process Guide:

1. Surface treatment - before applying the primer, it is necessary to carefully clean the surface of metal parts or degreased by chemical cleaning agent and make them dry;

2. Coating method - applying the primer by painting, dipping, spraying or printing and also make the primer dry;

3. Silicone overmolding - put silicone rubber material into mold cavity with metal/glass substrate together and molding;

4. Silicone overmolding finished.

(Note: different substrate need different primer. For glass substrate, LSR silicone overmolding glass silicone bonding primer is suitable. )

 

Silicone Overmolding Attentions:

1. Substrate material & overmolding way

2. Primer/treating agent

3. Detail design for sealing, burrs, parting line