What is a rubber compound?

It is a homogenous mixture of ingredients and additives used in a particular Rubber Product manufacture such that the properties of most of the ingredients are unchanged in the final product whereby providing a defined set of mechanical properties. It is converted into a dough like mixture which is called "compound" then milled into sheets of desired thickness. Rubber may then be extruded or molded before being cured.

What are some of the additives that are combined with rubber during compounding?

Rubber is first compounded with additives like sulfur, carbon black and accelerators. Lubricants are one necessary additive for compound production. That improved compound flow and release. Kennwu expertize in its rubber technology development that can formulate a compound that suit your applications and needs.

What is a Compounding?

It is the process of slecting and combining polymers and additive to obatin physical and chemical properties for a finised rubber product that suite its application.

Compression Molding

Compression molding is a process that involves taking a rubber compound or mixed raw material and creating "pre-forms" in the basic shape of the end product. The pre-forms provide a surplus of material to be placed in the cavity, thus ensuring a total cavity fill. Once in place, the mold is then closed, applying both heat and pressure to the pre-form and allowing it to fill the cavity. When the cavity is filled, excess pre-form material spills out into overflow grooves. Following this step the rubber is then demolded, usually by hand, leaving us with the molded rubber product.



Compression molding is often chosen for medium hardness compounds in low volume production or in applications requiring particularly expensive materials. This process helps to minimize the amount of overflow, or flash created during the rubber molding process.

In creating compression molded rubber products, the pre-forms can be difficult to insert into more complex mold designs. Furthermore, the compression molding process does not lend itself well to the material flow requirement of harder rubber compounds.

Benefits of Compression Molding

• Cost effective tooling


• Maximized cavity count


• Economical process for medium precision

Transfer Molding

As with compression molding, transfer molding requires secondary raw material preparation into pre-forms. It differs, however, in the placement of these pre-forms into a "pot" located between the top plate and a plunger. When the pre-forms are placed into the pot, the mold is then closed and the material is compressed by the plunger and transferred through sprues into the cavity below.

Some possible advantages of transfer molding over compression molding can include:

1. High Cavity Count - In many cases, transfer molded rubber products require fewer and simpler pre-forms. One pre-form can fill hundreds of cavities

2. Cost effective tooling

3. Tighter control of dimensional tolerance - the mold is not held open by excess material spilling out of the cavity parting line; all the excess hold the plunger open from the pot

4. Economical process for medium to high precision components

5. Color molded rubber parts benefit - pre-forms can be cut by hand from raw material, thus reducing the chance of contamination that can result from the mechanical prep of compression molding or the injection screw and barrel in injection molding.

6. Capable of producing overmolded components

The primary disadvantage of transfer molding is found in the increase of waste or excess flash. This can be seen in the "flash pad" or the rubber left in the pot after the transfer is cured which can either be recycled or discarded.

Injection Molding

Based off of a process intended for the molding of plastics, injection molding of rubber. Rubber injection molding successfully alters the plastics process by heating the rubber and placing it under significantly more pressure per square inch of cavity surface in molding. This is different from the plastic injection molding process where the materials are cooled under less pressure. Through various innovations, injection molding has become one of the most efficient ways to create molded rubber products in many cases.

The process of injection and injection-transfer molding starts with efficient material preparation. To begin, the material is mixed in bulk and then stripped immediately into continuous strips. These strips are then fed into a screw which in turn, fills a barrel with the appropriate predetermined amount of rubber material.

Advantages of injection molding:

1. The complete elimination of pre-forms

• The production and need for pre-forms is a labor intensive step that can potentially affect the finished product through variability in pre-form weight and shape.

2. Elimination of operator placement of pre-forms.

• Since pre-forms are eliminated, the need for operators to place the pre-forms in a cavity (compression molding) or pot (transfer molding) is removed.

3. Injection screw pre-heats material before forcing it into cavities

• This process decreases the viscosity of the material, allowing it to flow more easily into the cavities.
• This pre-heating provides the potential for decreased cure times through

1. More rapid cavity filling due to lower viscosity

2. Material already being in the curing process through the heat added during screw charging and shear created during injection

4. Reduced cycle time

5. Flashless tooling

6. Economical process for high volumes of medium to high precision components

7. Minimal material waste

Rubber Deflashing

Removal of the waste edge, or flash, from a molded rubber productcan be accomplished in a number of ways. Depending on the material, part size, tolerance and quantity deflashing methods commonly include:

• Manual tear trimming



• Cryogenic processing



• Tumbling



• Precision grinding

Extruded rubber products differ from molded rubber products based on how they’re made. Extruded parts are forced through a die of the required cross section under pressure of an extruder. These parts are often made with soft, unvulcanized rubber compounds that are fed into the extruder. The resulting extrusion leaves the rubber in a soft, pliable state. In this case, the finished extruded products will typically need to be vulcanized, or hardened, before they are rendered usable.

Extrusion Die for Extruded Rubber

An extrusion die is a precise and specific tool made by cutting an opening through a blank of steel. The shape of the opening will match the finished rubber cross section desired for an extruded part. Once in place, the rubber material will be forced through this die via the pressure that builds up from the revolving screw of the extruder.

Many rubber compounds tend to swell when passing through the extrusion die, causing them to experience an increase in dimensions. Thus, each die is made according to each particular part and material to ensure that all tolerances are met for the finished extruded rubber part.

Extrusion Advantages

There are many advantages to the rubber extrusion process, including:

• Variety of shapes available


• Efficient and economical


• Flexible and lightweight products


• Minimal waste

Extruded Rubber Products

Kennwu can also cut custom rubber extrusions in a range of lengths, shapes, and rubber compounds to meet various temperature, weather, and technical specifications. Some of them, such as EPDM or Neoprene, can be extruded like dense rubber or sponge rubber. Common extruded rubber products include:

• Connectors


• Tubing


• Mandrel formed hoses


• Door, cabinet and window seals


• Vibration reducers


• Gaskets


• Spliced gaskets


• Window seals for mass transit


• Pressure switch square tubing

How to (Co-Molding) Bond Rubber to Metal

Bonding rubber to metal is a means by which rubber is mechanically bonded to a metal insert during the molding process.

Rubber to Metal Bonding Preparation

To begin the bonding process, the metal inserts are first prepped for production using a degreasing system to rid the parts of any contaminants before the adhesive is applied. Next, a bonding agent is activated adhesive is applied to the metal inserts using a technique similar to spray painting. Once the metals are prepared, they are ready for production.

Rubber to Metal Bonding Process

Once the metal inserts are prepared, they are then physically placed, one at a time, into each cavity of the mold. For inserts on the top of a part, special magnets / jigs are incorporated into the mold to hold them in place while the mold is being loaded. For inserts being encapsulated into the rubber, special chaplet pins are incorporated into the mold to suspend the insert in the mold and allow the rubber to flow around the metal.
Once the metal inserts are in place, the normal rubber molding process commences. After the mold is closed, and the molding begins, the adhesive on the metals is activated, allowing the metal to bond to the rubber.

Rubber to Metal Bonded Parts

Typical applications for Rubber Bonded metal include any part requiring the flexibility of rubber and the stability of a metal. Applications range from bonding Natural Rubber, EPDM or even Silicone to metals like steel and aluminum. Components ranging in size from small mounts for motors, brakepads to large locomotive suspension parts are just a sampling of rubber to metal bonded parts produced using this process.