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.
Best suited for:
Low to medium production volumes
Medium to large sized parts
Thick cross-sectional parts.
Low to high durometer materials including very high hardness - Ideal
durometers 60A-90A
More expensive rubber formations, and other high cost materials
Molders who require quick tooling changeover.
Advantages over other methods:
Lowest investment for tooling and machinery
Shortest mold setup times make a perfect match for short production runs
Internal stress is minimized, producing less warping
Ability to process very stiff, high durometer materials
Generates less waste than other production methods
Ability to process thin to large thick parts
Greatest flexibility in molding various part sizes and materials
Less shrinkage of material leads to greater accuracy of parts
More cavities per mold are possible as lower molding pressure is required
Disadvantages:
Requires a preform (a pre-measured slug of material)
Can produce a higher rate of dimensional inconsistency
Generally produces the largest parting line
Flash removal requires a secondary operation
Typically the most labor intensive, but can be automated to nearly equal
injection.
Traditional Processing Applications:
- 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.
- 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.
- 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.
- Virtually all thermoset materials, from bulky material compounds to very high durometer materials.
- 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.
Best suited for:
Mid range dimensional tolerances
Low to medium volume production
Small to medium sized parts
Delicately shaped parts
Low to medium durometer materials
Insert molding
Colored and translucent compounds
Advantages over other methods:
Shorter production cycles than compression molding
Ability to maintain closer dimensional tolerances than compression molding
Excellent uniformity from mold cavity to mold cavity
Rapid mold setups
Typically less flash than compression because the cavity plates are closed
For multi-cavity tools, labor cost is lower than compression since only a
single pre-form is necessary.
Processing Applications:
Low to medium durometer materials
Low to medium production volumes – flexibility with part sizes
Small delicate parts
Disadvantages:
Higher Investment Cost than compression (press, tooling and
auxiliary equipment)
Requires a preform (a pre-measured slug of material)
Not well suited for filled materials
Generates some amount of material degradation
Cannot process high durometer materials
Tool maintenance costs are typically higher than compression
Wear of cavities is less than injection, but gates and runners erode
Labor content typically higher than injection
Flash Pad or Pot Pad is excess scrap material
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.
Best suited for:
Close dimensional tolerances
Large production volumes – (particularly suited for products similar to
grommets, O-rings, and seals - small to medium sized parts)
Low to medium-high durometer materials
Insert molding
Small delicate parts<
Advantages over other methods:
Shorter production cycles mean lower unit cost
Ability to maintain closest dimensional tolerances
Computer control allows the best repeatability from cycle to cycle and
production run to production run
Specialized cycle availability provides solutions to tricky production problems
Lower production cost can be obtained with large production volumes
Disadvantages:
Highest Investment Cost (press, tooling and auxiliary equipment)
Most complex processing method
Typically the highest excess material (scrap)
Not well suited for filled materials
Generates the greatest amount of material degradation
Lowest cavity pressure that cannot process high durometer materials
Longest set-up time / costly for short production runs
Requires closed loop processing feedback
Specialized process and cycles parameter
Requires higher skill set for operators and processing staff
Tooling Maintenance due to erosion of gates or runners
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