Problems in Injection Molding of Rubber

In addition to the operating problems discussed earlier, there are a number of problems that do not become apparent until the mold is opened and it and the product are examined. Some of these are briefly described below.

Shrinkage and Part Dimensions

On cooling, both the mold cavity and the molded part contract, usually by a differential amount because the metal and rubber have different coefficients of thermal contraction. Shrinkage is usually defined as the difference between the dimensions of the mold cavity and those of a molded part, when both are measuredat room temperature. It might be more exact to refer to the difference between the hot mold and the cold part but the above definition is easier to use in practice.
The amount of shrinkage has to be allowed for in mold design, but it is itself variable, being affected by batch-to-batch variance in the compound, cure time, temperature and pressure, and so there must be adequate dimensional tolerance allowed in the specifications for the part.

Adhesion

Adhesion in injection molding has two aspects: Adhesion to the mold surface, which is not wanted, and adhesion (bonding) to a metal or plastic insert in the part, which is wanted. Mold release agents are used to prevent the one, and adhesion promoters to ensure the other.
Release agents, often silicone based, are sprayed onto the mold surfaces between shots. With complex molds this can be time consuming, and it is difficult to ensure thin, even coating on all surfaces. There is also a problem of pollution from the spray carrier. One improvement on this is to deposit a layer of diamond-like carbon on the surfaces by a plasma vapor deposition system. This leaves a semi-permanent easy release surface, which does not wear readily.
Rubber-to-metal bonding agents have been used in the industry for many years, and there are many types available. Most of these depend on a two-coat system. The first coat, or primer, applied to the clean metal surface is usually halogenated polymer with heat-reactive resin, either dispersed or dissolved in organic solvents.
The metal bond is the result of van der Waals forces and is described by standard adhesion theory. Second layers, or cover cements, depend on the base polymer being used. They are usually chlorinated, brominated, or chlorosulfonated polymers, with appropriate cross-linking systems, which form cross-links with the rubber part and with the primer layer. Concern about environmental pollution due to the solvents used in the traditional systems and, in the USA, government regulations on volatile organic compounds (VOCs) and chlorine containing solvents, has led to the development of water-based adhesive systems, which are replacing solvent ones.

Backrinding

This is the term applied to the torn or gouged look that occurs at the mold parting line of compression molded parts, and at the gates of transfer and injection molds. It is caused by thermal expansion of the rubber after cross-linking, which can force the cross-linked rubber into the space at the parting line or gate, causing it to rupture. The smaller the surface area/mass ratio of the part, the worse it is (i. e., a sphere is the most severe case). The best way to minimize backrinding is to minimize the shot weight commensurate with filling the cavity, of course. Increasing scorch time can also help because it ensures that the mold is filled before curing begins, as the injection temperature can be raised.

Mold Fouling

The build-up of material in a mold, especially in corners, is a major problem. The more severe it is, the fewer the number of cycles before the surface or sharp edges of the part are affected and the mold has to be taken out of service and cleaned. The cause is usually deposition of chemicals and perhaps their subsequent oxidation or degradation. These agents may originate in the rubber, in fillers, curatives, waxes, etc. or from release agents. Thus, there are a wide variety of deposits whose severity varies from compound to compound and also depends on injection rate and mold temperature. One of the major concerns in compounding for injection molding is to minimize mold fouling. Costs of downtime and mold cleaning can be considerable.
Mold cleaning is often done by blasting with some abrasive particulate material such as glass beads, plastic, or metal beads. One effective, clean, but expensive and noisy process is blasts with solid carbon dioxide. Other techniques use liquids such as detergent solutions or hydrocarbons, often with ultrasonics. When the deposit is volatile at higher than curing temperatures, heating in a salt bath, fluidized bed, oven, or by induction can be used.

Orange Peeling

This is usually caused by the initial layer of rubber in contact with the heated mold surface having cross-linked before succeeding layers have filled the mold. The cure is usually to increase the scorch time of the compound.

Porosity

This is due to undercure and the presence of volatiles, especially water, in the compound. Higher injection and mold temperature or longer mold closed time should resolve this.

Blisters

Air entrapped in the rubber is the usual cause. This can be eliminated by a higher back pressure, slower injection rate, or effective venting of the mold.