What is Rubber Molding

Introduction

The term rubber in this article is used generically to include natural rubber (India rubber), synthetic rubber or a blend of the two. Both are elastomers with synthetic rubber, often referred to as an artificial elastomer, making up about two-thirds of global rubber production. Natural rubber is sourced from Hevea tree sap while synthetic rubber is derived from crude oil. Plastics are also considered to be synthetic rubbers and are made from petroleum and natural gas. The principle difference between them is the superior elastic properties of natural rubber and their chemical structure.

Rubber’s inherent properties including excellent tensile and elongational strength, wear and tear resistance, electrical insulation, and resilience make it an ideal material to use in high volume preformed molded products for a wide range of applications.

What is Rubber Molding

Rubber molding is defined a process whereby an amount of uncured rubber or elastomer is transformed into a usable product. Rubber products are made from elastomers or uncured rubber and there are a number of different rubber molding processes.

Rubber Molding Process

Many rubber components and products are formed in a molding process during which uncured rubber is subjected to a chemical process that toughens or hardens the polymer chains in the material. The process often introduces heat to reduce curing times.

Types of Rubber Molding

There are three types of rubber molding processes:

  • Rubber compression molding – placing a predetermined amount of rubber compound directly into the mold cavity and then compressing it into the shape of the cavity by closing the two sides of the mold. Pressure is maintained while heat is added to the mold to cure the product. On completion of the process, the product is removed from the mold and excess rubber (flash) is removed.
  • Rubber injection molding – in this process the uncured rubber compound is heated to a liquid state before being injected into a mold. The product is released by opening the mold and closing it again to receive the next injection.
  • Rubber transfer molding – during this process the uncured rubber compound is placed in a part of the mold called a “pot” before being forced through an aperture into a cavity in the shape of the desired product. When cured, the mold is split releasing the final product. 

Rubber Molding Materials

There is an extensive range of natural and synthetic rubbers available for molding. The most commonly used ones are:

  • Nitrile or Buna-N – is the most popular and low-cost solution in rubber molding. Properties include resistance to water, hydraulic fluid, oil, and solvents. Other properties include strong abrasion and tear resistance with robust mechanical properties;
  • Hydrogenated Nitrile – this is a more expensive hydrogenated variation of Nitrile polymer, which increases its resistance to heat, petroleum products, and ozone almost fivefold. It is now used extensively in the automotive, marine and aircraft industries for use in seals for air conditioning units, engine parts, vehicle suspensions, and fuel systems;
  • Ethylene Propylene Diene Monomer (EPDM) – commonly used in rubber molded products for steam systems, vehicles panel seals, and braking systems due to its high resistance to brake fluid. EPDM has a wide operating temperature range for automotive applications and is easy to use in creating smooth extruded products. Its low cost makes it an attractive option;
  • Silicone – resistant to exposure to high and low temperatures, is very flexible and has good weather resistance. These properties suggest typical applications in medical devices and food processing. It is, unfortunately, a semi-organic elastomer, can be easily torn and is not an option where products are subject to high wear;
  • Fluorosilicone – combines the temperature range advantages of silicone with the resistance to oil, fuel, and solvents found in fluorocarbons. This material is now widely used in the aerospace industry due to its broad temperature range, longevity, and resistance to petroleum-based products. It has poor abrasion and friction qualities;
  • Neoprene – as a strong multi-purpose material it can be used in a large number of rubber molding solutions. It has good fire resistant and abrasion properties and is frequently used in the manufacture of mass transit and transportation equipment. In motor vehicle components, Neoprene is used for the production of constant velocity joint boot, hose covers, vibration mountings, shock absorbers, window seals and gaskets;
  • Natural Rubber – is a natural product produced from latex derived from the rubber tree which can now also be produced synthetically. It has high tensile strength with good abrasion and surface friction properties. This makes it an ideal material for the manufacture of vibration dampers, seals, o-rings, machinery and component mountings, diaphragms and bumpers. Natural rubber bonds well with metals. Due to its low tolerance to UV rays and ozone, it is not recommended for prolonged exposure to sunlight or petroleum-based products;
  • SBR (Styrene butadiene) – one of the more cost-effective polymers that can be used in rubber molding and is frequently used in the production of tires, diaphragms, seals and gaskets and the mass production of other rubber parts because of its durability. While this is an excellent material for water-based applications, care should be taken where the component is subjected to prolonged exposure to petroleum-based products and solvents;
  • Fluorocarbon – this relatively expensive rubber compound combines a wide range of chemical resistance and high-temperature properties. Common applications include hoses, o-rings, gaskets, and fuel system seals for extended component lifespan. It has a good resistance to petroleum products.  It is not recommended for use with hot water and steam exposure;
  • Butyl – has a high resistance to gas permeability. This makes it the ideal solution in the production of seals for high-pressure gas and vacuum systems. It also has good sound damping properties and is cost-effective, although marginally more expensive than traditional polymers. It should not be used if exposed to petroleum products or abrasion;
  • Urethane – is a fairly high priced, but commonly used material in applications subject to high pressure and constant shock loads due to its excellent abrasion resistance and high tensile strength. Its performance does tend to deteriorate with water absorption and loss of tensile strength at higher temperatures.

Mold Making

The single most important aspect of creating quality rubber molded products is the design, fabrication, installation, and operation of the mold.  The mold forms a part of a more complex machine, depending on injection, compression or transfer method employed, that allows a mold to continuously reload material and create finished components. The mold making process involves:

  • Design – managing this process in a business that makes high volumes of single or multiple types of rubber components can be simplified using 3D (3 dimensional) modeling software. The design of the mold must be compatible with the actual molding machinery that it will be fitted to. A 3D designer creates a digital model of the mold base, core, and cavity to match the component specification;
  • Materials – in a mass manufacturing process the mold must be able to replicate the components accurately with minimal material spillage and at a high frequency. The size, weight and curing temperature of the component will determine the size and material of the mold. Molds for rubber molding can be made from plaster or other rubbers but primarily from various engineering grade metal alloys in the case of reusable molds;
  • Manufacture – this process involves the casting of the components and the use of high precision equipment that includes milling, drilling, grinding and polishing machines to create the contours of mold components;
  • Testing – molds components are tested to ensure that they meet all aspects of the design specifications in terms of surface finish, dimensional stability, durability, and mechanical properties;

Advantages of Rubber Molding

  • The physical attributes of various rubber compounds including strength, flexibility, stress and wear resistance, recyclability, and easy moldability make them idea for mass-produced components with a wide range of applications;
  • Rubber molding is generally a low energy usage processes, and most rubbers, synthetic and natural are easily recyclable. The carbon footprint of any rubber product is therefore low, and the lightweight of molded rubber components in aircraft, motor vehicles and other methods of transportation further reduce energy consumption;
  • Production of rubber molded components is cost effective due to the simplicity of the processes involved. Minimal materials are lost during production and scrap is easily recycled. The components can be made at very high production rates with perfect accuracy and too small tolerances;
  • Molding machines can produce a wide range of components simply by changing individual molds leading to high machine efficiency rates. Most processes lend themselves to complete automation leading to low operational costs.

Challenges with Rubber Molding

Although the mechanical aspects of molding rubber products are relatively straight forward, there are some challenges in the process:

  • Shrinkage – due to the difference in expansion coefficients between the mold and the component there will always be a degree of shrinkage of the cured component. This must be compensated for in the designed size of the mold.
  • Mold flash – when the mold is split after producing the component there will be a parting line (flash) cause by a slight seepage of the rubber material between the two surfaces. The extent of mold flash is controlled through reduced tolerances between the surfaces of the mold and to preheat the rubber material.
  • Porosity – blisters on the molded component surface are caused by trapped gasses while porosity is the result of uneven or incomplete curing during the molding process.
  • Flow marks – flow marks or blemishes on the component surface may be as a result of a fault on the mold surface or a fault in the flow process or flow direction.
  • Surface delamination – contaminated materials may cause layers that appear to be coatings to appear on the surface of components. This could be as a result of foreign materials and excessive release agents.
  • Short shot – refers to an insufficient supply of rubber compound into the mold that creates an incomplete or deficient component.
  • Warping – due to non-linear cooling the component twists or warps on release from the mold.
  • Burn marks – caused by heat degradation of the cured component surfaces as a result of excessive heat or too high injection speeds.
  • Set-up efficiency – refers to the time taken to rejig the production molds between different product lines on the same molding machine. 

Rubber Molding Machinery

Rubber molding involves three different processes, compression, injection, transfer and specific machinery for each type:

Rubber compression molding machinery

Compression molding is the most common process which involves applying pressure to a preset amount of molding material before and during the curing process. This process is most suitable for low to medium volume production runs not requiring tight tolerances and smooth finishes. Compression molding is recommended for the manufacture of diaphragms, valve parts, gaskets, dampers, O-rings, seals, rubber wear parts, etc.

Rubber injection molding machinery

The fully hydraulic mold clamping machine is currently the rubber injection molding machine of choice for many manufacturers. The injection molding process is especially suitable for the mass production of high-quality precision components. It involves injecting fixed volumes of a rubber material under pressure into the closed mold before curing and releasing the component.

Rubber transfer molding machinery

I the case of rubber transfer molding, a rubber material flows into the open mold until it closes. This may lead to gas formation and shrinkage of the component that cannot be rectified during the curing process. Rubber transfer molding machines are used for precise molding of rubber, bonding rubber to metal parts, encapsulating electronic parts, medical products and inserting electrical connectors, etc. Transfer molding machines are relatively inexpensive and have a simple manufacturing process.

Typical Rubber Molding Products and Applications

 

 

PRODUCT TYPICAL APPLICATIONS
Rubber bellows Cooling water systems, condensers, gas lines, water pipelines and piping, pumps, turbines, boilers, automotive suspension systems, and mechanical vibration dampers
Rubber grommets Automotive, home appliances, electrical circuits, computers, furniture, shoes, clothing, and tools
Rubber Diaphragms Pumps, air and gas compressors, actuators, flow control valves, measuring gauges and controls, turbochargers, switches, and accumulators
Rubber expansion joints Roads, railways and bridges, industrial and domestic piping, seals, housing, appliances, ducted air and air-conditioning systems, pumps, compressors, and blowers, piping systems, and fire-fighting systems
EMI shielding Computers, data cabling, sensitive instruments, wiring harnesses, mobile phones, radios, and television appliances
Hoses And Ducting Air ducting in buildings, equipment and vehicles, water hoses, fuel lines, protection of cables, pipes and wires, underwater diving, welding, and agricultural irrigation
Vibration isolators Cooling systems, chassis to body interface on vehicles and equipment, isolation of sensitive equipment, suspension systems, engine, transmission, steering, and exhaust mountings, pipe work, electrical enclosures, mounting of generators, cooling systems, and compressors in buildings
O-rings and gaskets Hydraulic cylinders, internal combustion engines, syringes, pumps, filtration units, valves, dust protection, liquid dispensing, compressors, refrigeration, heaters, and condensers
Rubber plugs, caps and stoppers Powder coating, anodizing and plating, spray paint masking, component protection, securing and protecting sharp edges, concealing screws, holes, fixtures, bolts, and nuts, blanking of unused apertures and holes, mechanical stops and closing tube or pipe ends
Electrical insulators gaskets and shields Insulation of electrical components, insulator pads, isolator boards, heater circuits, circuit boards and, semiconductors
Custom Molded Rubber Bumpers & Feet Product protection, securing and protecting sharp edges, protection of surfaces and floors, reducing or increasing friction, support components, fenders in wheel arches of vehicles, and reduce impacts between boats and solid objects

Summary

Natural rubber and synthetic rubber compounds have unique chemical and physical properties that lend themselves to mass production of high performance, molded components that are cost effective and have a very wide range of applications. The process of creating these products is usually through rubber compression molding, rubber injection molding or rubber transfer molding. The design and fabrication of mold making machinery is highly specialised and each mold making process requires a completely unique design and each type of rubber mold is suited to a specific use case.

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