Hydraulic Presses - Molding Processes

What molding process is right for you?

Depending on the part, material, volume, part size, and part shape requirements, a particular molding process can be optimized for cost and part quality. Some of the most commonly-used molding processes are described here. Contact French engineers for help with determining what process is best for your application.

Compression Molding

This is the most widely used production method for molded rubber products. It is ideal for low to medium volumes and can be used for a largest variety of part sizes and materials, including high cost materials, and applications that demand extreme hardness. It is a very useful molding process for forming bulky parts, gaskets, seals and O-rings. It is also a very efficient, low waste method that offers the simplest process, lowest investment, and greatest flexibility. Compression molding generally results in lower amounts of scrap. It does not consume excess rubber in the runner of an injection mold, or in the pot of a transfer mold.

The most commonly viewed drawbacks to compression molding are longer cycle times and costly labor costs. However, both of these can be addressed to equal or surpass the injection molding process. Cycle times for compression molded parts using preheated preforms can be less than for injection molded parts. Automated preform, loading/unloading, and post handling equipment can be integrated with a compression press to nearly equal the labor cost of injection.

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Traditional Processing Applications:

1. High Cost Materials – reduced waste can provide lower overall manufacturing cost. Compression molding produces the lowest amounts of scrap material. It does not require additional rubber to fill a runner cavity of an injection mold, or the transfer pot of a transfer mold.
2. High Durometer Materials - Medium hardness materials (60A-90A) work the best in compression molding. Compression molding is easily capable of molding material of even higher durometers. Higher cavity pressure can be difficult to obtain with other molding processes without degradation of material properties.
3. Filled Materials -The compression molding process is well suited for molding materials that are combined with stiffening fillers. Fiber orientations are not altered of destroyed as commonly observed in injection molding.
4. Virtually all thermoset materials, from bulky material compounds to very high durometer materials.
5. Mostly every molding industry from industrial to aerospace.

Typical Products:
Largest ranges of products from O-rings, gaskets, diaphragms, seals, bearings, bushings, golf balls, shoes soles, to highly valued critical components used in the medical, fuel cells, printed circuit board industry.

Common Materials:
SMC
BMC
SBR
Polyurethane
Polyethylene
Neoprene

High Viscosity Materials:
Epoxy Resins
Polyamide
Melamines
Polyesters
Phenolics
Nylons
Polycarbonites
Acrylics
Filled Materials

High Cost Materials:
Fluoroelastomers
Fluorosilicone
Perfluoroeleastomers
Common names: Viton, Kalrez, Aflas, PTFE, PEEK

Other Compression Press Processing Applications

Compression presses are used in virtually every molding application from various thermoset to thermoplastics, including laminates and composites. There are many others processes that can use compression presses in a stand only condition, or in combination with auxiliary material dispensing systems. These include:

Cast Molding

Urethane material can be cast using a compression process. The polymers and curative is premixed and poured into a mold. The mold is overfilled with raw material so that when the mold plates are fit together and placed under pressure, the excess material helps to carry out the air. After parts reach a "green strength", they are demolded and moved to a secondary post cure oven.

Typical Materials:
Polyurethane
Filled Polyurethanes
Epoxy Resins

Compaction Molding

Plastic molding powder, mixed with such materials or fillers as wood flour and cellulose to strengthen or give other added qualities to the finished product, is put directly into the open mold cavity. The mold is then closed, pressing down on the plastic and causing it to flow throughout the mold. The pressing and compaction rate can be critical to the process. Entrapped gases can create voids, defect and poor material properties that result in high scrap rates.

Typical Materials:
Phenolics
Polyamides
Melamines
PTFE

Reaction Injection Molding

Reaction injection molding (RIM) is a relatively new processing technique that has rapidly taken its place alongside more traditional methods. Unlike liquid casting, the two liquid components, polyols and isocyanates, are mixed in a chamber at relatively low temperatures (75° - 140° F) before being injected into a closed mold. An exothermic reaction occurs, and consequently RIM requires far less energy usage than any other injection molding system.

Typical Materials:
Rigid Structural Foam
Low-modulus Polyurethane
High-modulus Polyurethane
Epoxy Resins

Reinforced RIM (R-RIM) consists of the addition of such materials as chopped or milled glass fiber to the polyurethane to enhance stiffness and to increase modulus, thus expanding the range of applications.

Typical Materials:
Polyurethane
Glass-Filled Polyurethane
Epoxy Resins

Structural Reinforced RIM (SRIM)

Low-viscosity polyurethane RIM systems can be injected into a mold through glass mats or preformed glass fiber mats to produce very stiff, high-strength structural RIM (SRIM) parts. After the glass-filled preform is loaded into the open mold, the mold is closed and the polyurethane resin is injected where it permeates and surrounds the preform to form the part.

SRIM is used more commonly used in demanding structural applications. This process promotes non-uniforms cross-sections, and allows designers to add strength where required. The overall outcome is a single component with the optimum weight for the physical requirements.

Typical Materials:
Polyurethane
Glass-Filled Polyurethane
Epoxy Resins

Resin Transfer Molding (RTM) is a low pressure (typically under 100 psi), closed molding process, which offers a dimensionally accurate, and high quality surface finish composite molding, using liquid thermoset polymers reinforced with various forms of fiber reinforcements.

Typical Materials:
Epoxy
Vinyl Ester
Methyl Methacrylate
Polyester
Phenolic
Filled Materials such as with Glass, Arimid, Carbon and Synthetic fibers

Transfer Molding

In concept, transfer molding is a simplified version of injection molding. It is provide many of the benefits of both injection and compression. It allows the molding of intricate parts while providing highly accurate dimensional control for low to medium production volume requirements. The cycle times are generally longer than injection molding, however they can be very costs competitive.
Transfer molding can also reduce the cure time by heating the material before it reaches the mold. The material can be and is forced into a closed mold by means of a hydraulically operated plunger, or by using the compressive force of the press in combination with a tooling with an internal transfer pot.
Transfer molding was developed to facilitate the molding of intricate products with small deep holes or numerous metal inserts. It is ideal for insert molding because the tool is closed prior to the material being transfer, which limits the amount of shift with the insert parts.

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Injection Molding

Typically known as the fastest, most accurate, most repeatable method for rubber goods production. Numerous specialized cycles such as injection/transfer, injection/compression, preform, vacuum purge and volumetric bump cycles provide many molding and process options. These various tools can help solve many molding problems, and optimize the cycle time. However, injection molding requires much more processing knowledge and strict control over the machinery and material parameters. One serious problem with injection molding of thermosetting materials is that, under heat, many materials will first soften, and then harden to an infusible state. Great care to limit to the amount of time and heat put into the thermoset material. Otherwise, anything from minor material degradation, part non-uniformity, to complete freezing up the injection unit can result. Another concern is that the equipment is generally not suited for short production runs. Tooling and material changeover can be very timely, and costly.

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