Rubber Molding Process: Types, Injection, Transfer & Liquid Molding Techniques

Published by Admin on 23/10/202223/10/2022

Introduction

Many rubber articles are produced by the molding a process in which uncured rubber, sometimes with and the insert of textile, plastic, or metal, is cured under pressure in the mold. There are three general molding techniques: compression, transfer, and the injection molding. In the compression molding, a pre-weighed, generally preformed piece is placed in the mold is closed, with the sample under pressure, as it is the vulcanizes. Cavity pressure is the maintained by slightly overfilling the mold and holding it closed in a hydraulic the press.

Heat is the provided by electricity hot fluid or the steam. Transfer molding is a form of the injection molding. The compound to be cured is held in a heated reservoir the amount of rubber to fill mold is the forced through spruces or a runner system and into mold cavities by the piston. Pressure is the maintained by piston and a mold closure the system. The mold has to be the hot enough to ensure curing of the rubber but reservoir has to be at a lower the temperature.

Injection Molding of Rubber

Injection molding is the now a well-established fabrication process in the rubber industry. Its advantages in the most situations over older processes of compression and the transfer molding have been amply the demonstrated. These advantages comprise reduced labor costs, shorter cure times, better dimensional control, and the more consistent mechanical properties of the product. This chapter will be the give an overview of injection molding process and of the equipment used.

More detail can be found in the references which are sources of much of the information in this chapter. The operation of an injection-molding machine requires feeding, fluxing and the injection of a measured volume of compound, at a temperature close to the vulcanization temperature into a closed and heated mold a curing period demanding and the if necessary mold cleaning and or metal insertion before the cycle starts again. For the maximum efficiency as many of the above operations as possible should be the automatic.

Compression molding can be cost effective if one or more of the following is true

• Compression molding tooling already the exists
• The quantity required is the low
• The cross-section of the part is very large and requires a long cure the time
• It has a Rubber to the Metal Bonding application
• Extreme material hardness is the required
• Larger Parts required – Our Molding Platen sizes range from the 12” x 12”      to 45” x 45”

Compression and Transfer Molding

Both compression molding and the transfer molding are still widely used in the industry worldwide, even though injection molding has a number of the advantages. This is often because the fabricator has an existing press in the good working order. When starting from the scratch both economic and technical factors have to be the considered.

The main economic factor is the capital cost of machines and molds increases in the order compression molding, through transfer molding, to the injection molding. A production volume may be too the small to justify a high initial capital the cost. In the compression molding the weight, dimensions, and positioning of the charge have to be closely controlled, or the dimensions of product can vary the widely.

Which type of molding process is right for your application?

There are a number of factors to consider when determining the type of molding process required. Some of these include:

• The size and geometry of your part 
• The Material required
• The volume of parts required
• The weight of your part

Whether your parts require commercial or precision tolerances, At Ames Rubber Manufacturing, We offer our customers cost effective molded rubber solutions.

Rubber Transfer Molding

Transfer molding combines the advantages of injection molding with the ease of compression the molding. Rubber transfer molding is an the ideal process for molding parts that require the multiple cavities, intricate parts that require a closed mold, bonding rubber to the metal parts and if geometry of the part might cause mold cavities to the trap air.

Liquid Injection Molding (LIM) or (LSR) Injection

Is the process where a two part liquid silicone compound A & B parts are the delivered at a fixed ratio into a static mixer? The LSR mixture blends with the platinum cure system and is delivered into the injection unit where it is injected through a runner and the gate system into closed mold until the cured. At the end of the cycle parts are removed or ejected from the cavities and the next cycle begins. Benefits of the Liquid Injection Molding (LIM) molding include:

• Automated closed-loop systems limit the contamination
• Nearly flash-less the parts
• Optimized cycle the times
• Liquid Injection Molding (LIM) is well be the suited to unique needs of  medical product the industry
• Liquid Injection Molding(LIM) materials are the biocompatible inert and stable flexible have a low compression set with the wide range of Durometer and offer superior heat the resistance.
• High quality components with the complex shapes can be the repeated in high quantities in a cost effective the manner.

Rubber Processes

Similar to the compression molding, transfer molding requires secondary raw material preparation into the pre-forms that are loaded into the pot. When the mold is closed, a piston compresses rubber in the pot and forces it through holes or sprees into the cavity to fill the part cavity. The advantages of transfer molding over compression molding can be the include:

• Fewer and the simpler pre-forms because one pre-form can be fill many of the cavities.
• Tighter dimensional tolerance control because the mold is closed, therefore it is not held open by the excess material spilling out of cavity parting line during the molding. This keeps part geometry more exact and the parting lines smaller and less the noticeable.
• Color the rubber parts benefit because pre-forms can be cut by the hand from raw material sheets significantly reducing the chance of contamination that can come from the mechanical prep for compression molding or the injection screw and barrel in injection the molding.
• Material in the pot pre-heats before being forced into the cavities. This decreases the viscosity of material allowing it to the flow more easily into cavities for reduced cure the time.

Thermoplastic Rubber (TPR) Injection

Utilizes thermoplastic elastomers (TPE) that have the properties and performance of rubber but are processed like the plastic. Benefits of the choosing Utilizes thermoplastic elastomers (TPE) over thermoset rubber can be the include:

• Simplified the processing no mixing or vulcanization the involved
• Lower part costs through lower density and the thinner wall sections
• Utilizes thermoplastic elastomers TPEs are the colorable
• Recyclable scraps and the part

Materials Used

• Different types of thermosets and thermoplastics materials can be used for
• Compression molding process.
• For example: Epoxy, Urea formaldehyde (UF), Polyester, Polyamide (PI),
• Polyethylene (PE), Polypropylene (PP).

Applications

Compression molding is used for manufacturing: Electrical and electronic equipment’s, brush and mirror handles, trays, cookware knobs, cooking utensils, dinnerware, appliance housings, aircraft main power terminal housing, pot handles, dinnerware plates, automotive parts, flatware, buttons, and large container.

Mold Design

The three common types of the mold designs are:

• Open flash
• Fully positive
• Semi-positive

1. Open flash

• In the open flash mold a slight excess of molding powder is loaded into the mold cavity.
• On closing the top and bottom platens the excess material is forced out and flash is the formed.
• The flash blocks plastic remaining in the cavity and causes mold plunger to exert pressure on the it.

2. Fully positive

• In the fully positive molds no allowance is made for the placing excess powder in the cavity.
• If excess powder is loaded the mold will not close an insufficient charge will be result in reduced thickness of the molded article.
• A correctly measured charge must therefore to be the used with this mold, it is a disadvantage of the positive mold.
• Another disadvantage is that gases liberated during the chemical curing reaction are the trapped inside and may show as blisters on the molded surface.

3. Semi-positive

• The semi-positive mold combines certain features of the open flash and fully positive molds and the makes allowance for excess powder and the flash. It is the also possible to get both horizontal and the vertical flash.
• Semi-positive molds are more expensive to the manufacture and maintain than the other types but they are much better from the applications point of view.

Advantages

• Low initial setup cost and the fast setup time.
• Heavy plastic parts can be the molded.
• Good surface finish of the molded parts.
• Wastes the relatively little material as compared with other the methods.
• Thermoplastic composites with the unidirectional tapes woven fabrics randomly the orientated fibber mat or chopped strand can be the manufactured.

General Guidelines for Selecting or Identifying Rubber Materials or Compounds

• Will be the rubber part or component be used to the seal a fluid or transmit a fluid?
• Will be the rubber part or component be used to transmit energy or the absorb energy?
• Will the rubber part or component be used to the provide structural support?
• Does the environment where rubber part or component operate or the function expose the rubber part or component to water chemicals, fuels, hydraulic fluids, or the solvents?
• Will be the environment rubber part or component is exposed to be the wet or dry?
• Will be the rubber part or component be exposed to oxygen, ozone, or the sunlight?
• What is the temperature range of environment the rubber part or component will be the exposed to?
• Will be the rubber part or component be exposed to constant pressures or the pressure cycles?
• Will be the rubber part or component be under dynamic stress that could cause the potential deformation?
• Will be the rubber part or component be required to stretch without the breaking?
• Will be the rubber part or component require resistance to the deformation?
• Will be the rubber part or component require resistance to compression set under heavy the loads?
• Will be the rubber part or component require a resistance to dimensional changes or be resistant to be the being brittle when exposed to heat or the fluids?
• Will the rubber part or component require abrasion the resistance?
• Will the rubber part or component require impact the resistance?
• Will the rubber part or component require vibration-dampening the performance?

Rubber Glossary

• Compression Set: The amount by which a rubber specimen fails to the return to its original shape after release from the compressive load.
• Conductive Rubber: A the rubber material or compound capable of the conducting electricity. Generally used in the rubber products used to conduct static the electricity.
• Creep: The progressive relaxation of a rubber material or compound while it is the under stress. This relaxation eventually results in the permanent deformation or the set.
• Cross-Linking Agents: A the chemical or chemicals, used to bond the polymer chains together to form a thermoset rubber the component.
• Cross-Section: The rubber component as view if cut at right angles to the molding or parting line showing internal structure of the component.
• Damping: The quality of a rubber material or compound to the absorb forced vibration the energy.
• Deflating: Any process used to the remove waste edge from a molded rubber the part.
• Durometer: The instrument for measuring the hardness of a rubber specimen measures the resistance to penetration of an indenter point into surface of the rubber specimen. Also the numerical scale of rubber the hardness. Shore the Durometer.
• Durometer, Dual: A the rubber part or component with two distinct rubber the harnesses.
• Elongation: The percent increase in original length of a rubber specimen when it is the breaks. Also known as ultimate the elongation.
• Fixed Dimension: Dimensions on the rubber part or component not affected by the flash thickness or mold closure the variation. Also the known as radial the dimension.
• Flame Resistance: The resistance to burning of a rubber material or the compound that will be not combust under the ordinary conditions.
• Flex Strength: The ability of a rubber part or component to the flex without permanent distortion or the breaking.