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Rubber Compression and Injection Molding Process

The basic rubber molding processes we utilize are compression molding and injection molding.

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 Diagram

Compression Molding Diagram

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

Compression molding can be a cost effective solution in situations where:

  1. The tooling already exists
  2. The cross-section of the part is very large and requires a long cure time

Applications of compression molding range from simple rubber grommets to complex air intake hoses, Hebei Shida Seal Group can offer a variety of other molded rubber products through compression molding.

Injection Molding:

This process is the most efficient way to mold rubber in most cases. Injection and injection-transfer molding start with more efficient material preparation. The material is mixed, typically in 500-pound batches, and then stripped immediately after being mixed, into continuous strips measuring approximately 1.25″ wide and 0.375″ thick. This strip is fed into a screw on the injection molding machines, which charges a barrel as needed with a pre-defined amount of material. When the mold is closed, the material in the barrel is injected into the mold cavities and cured.

Injection Molding Diagram

Injection Molding Diagram

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. Capable of producing overmolded components
  8. Minimal material waste

About us:

Hebei Shida Seal Group is an ISO/TS 16949 accredited manufacturer of molded rubber products and extruded rubber products from Hebei, China. We are tier-2 supplier to Hyundai and Kia. Look no future than Hebei Shida Seal Group if you need molded rubber products of high quality!

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Rubber Molding Processes

Rubber Molding Processes

Processing rubber to produce final products for end use really depends on what the shape and product is. The main processes for shaping rubber are Extrusion, Compression Molding, Transfer Molding, and Injection Molding.

1. Rubber Extrusion

Extrusion is a continuous manufacturing process in which rubber is squeezed through a die and then vulcanized (industry terminology for cured). The die gives the extrudate its shape. The required pressure is produced via a conveying screw, in which the material is mixed, compressed and extruded through the die form to produce the final shape.

Extrusion Process Steps

  • The extrusion process begins with the unvulcanized (uncured) rubber compound being fed into the extruder.
  • Next, the flutes of the revolving screw will carry the rubber forward into the die, with an increase in pressure and temperature occurring as the material gets closer to the die itself.
  • Once it reaches the die, the built up pressure forces the material through the openings, where it will consequently swell in various degrees based on the material compound and hardness.
  • Because rubber has this tendency to swelling, many extruded parts require compensation on the cross sections of the die.
  • During the vulcanization (curing period), the extruded rubber will swell or shrink in both its cross section and its length depending on the type of rubber compound used.

2. Rubber Compression Molding

Compression molding is a process that involves taking a rubber compound or mixed raw material and creating “preforms” in the basic shape of the end product. The preforms 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 preform and allowing it to fill the cavity. When the cavity is filled, excess preform 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 Process Steps

  • Uncured rubber is preformed into a control weight, shape and specification.
  • Rubber preform is then placed into the mold cavity.
  • Rubber mold is then closed compressing the rubber to fill the molds cavities.
  • The mold remains closed under pressure and held at a temperature to allow the rubber to reach optimal cure.
  • Compressed rubber parts are then removed from the mold and the process is ready to begin again.

3. Rubber Injection Molding

Injection molding of rubber is based upon a process intended for the molding of plastics. 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 particularly with more complex shapes.

The operation of an injection molding machine requires: feeding, mixing, and injection of a measured volume of compound, at a temperature close to the vulcanization temperature, into a closed and heated mold; a curing period; demolding; and, if necessary, mold cleaning and/or metal insertion (Overmolding process), before the cycle starts again. For maximum efficiency, as many of these elements as possible should be automated.

Injection Molding Process Steps

  • The uncured rubber is fed into the machine in the form of a continuous strip.
  • The uncured rubber is worked and warmed by the rotation of the screw in a temperature controlled barrel.
  • As the rubber stock accumulates in the front of the screw, the screw is forced backwards. When the screw has moved back to a specific point, this indicates the correct amount of rubber (shot amount) is ready for injection.
  • With the mold held closed under hydraulic pressure, the screw is pushed forward. This forces the rubber into the mold (this is the injection process).
  • While the rubber cures (vulcanizes) in the heated mold, the screw starts rotating and filling with the new rubber (the next shot).
  • The mold then opens and the part can be removed. The machine is ready to make the next shot as soon as the mold closes again.

About Us:
Hebei Shida Seal Group is located in Qinghe, Hebei, China, and is a Tier I supplier of molded rubber products to the automotive industry, and has attained TS 16949 designation. Main molded rubber products are rubber grommets, wire harness rubber parts, air intake hoses, rubber boots, rubber seals, rubber components, rubber diaphragms, rubber bumpers and stoppers using latest molding technology.