While there are several methods for molding rubber in the production of industrial and precision rubber parts, there are three processes that are not only the most common but also far exceed the others in efficiency, quality and time required for production. These three methods are compression molding, injection molding, and transfer molding.
The most obvious questions for you as the consumer of rubber parts is, “which method is the most effective for my part?” This is a fair question, and we will try to address that as we compare the advantages and disadvantages to each method.
Basics of Rubber Molding Methods
Let’s begin with an explanation of the basics of the molding process. All rubber molding processes start with the base polymer, i.e., Nitrile, Fluorocarbon, Ethylene Propylene, Silicone, et al. which determines the fundamental properties of the elastomer compound. Additional ingredients (plasticizers, curing agents, carbon black, et al.) are then added during the batch process to give the base polymer the desired physical properties such as compression set, elasticity, tensile strength and so on.
Following this batch process, the uncured raw material is then placed in a mold, exposed to heat, activating a chemical reaction that cures the elastomer within the mold into its final shape. The main difference between molded rubber and most molded plastics is the waste created. For most plastics, this waste material can be ground up and reused in future molding processes. For molded rubber, once cured, the physical properties of the compound change permanently and therefore the material cannot be reheated and remolded–any extra material (waste) becomes scrap.
The fundamental factors to be considered when determining the best method for molding a part are the complexity of the part, the material to be used, the required cure time and finally, the expense incurred from the waste material left behind. All of these factors contribute the molded part’s rate of production and its cost of production. Ultimately these factors impact your concerns of the availability of the part, and its final price to you.
Here’s a quick summary of the three most common rubber molding methods.
Rubber Compression Molding
The process of compression molding rubber involves creating “pre-forms” in the basic shape of the end product from the rubber compound or mixed raw material. The “pre-forms” provide an excess amount of material which is placed in the mold cavity, to ensure a complete cavity fill. When the mold is closed, both heat and pressure are applied to the “pre-form” filling the cavity; any excess pre-form material spills out into overflow grooves. The rubber is mostly cured when removed from the mold and continues the curing process in a post-mold operation.
Advantages of Rubber Compression Molding:
- Effective for simple shapes
- Cost effective tooling
- More pieces per heat cycle through higher cavity counts
- Effective in medium to small batch runs
- Tools exist for many standard, metric and non-standard size o-rings.
- Suitable for large parts requiring longer cure times.
Disadvantages of Rubber Compression Molding:
- Longer production times due to
- Longer curing times – requires a post-mold curing process
- Requires post-mold operations such as “deflashing”
- Not suited for complex parts due to limited flow of the material to fill voids and air pockets created by the complex cavity.
Rubber Injection Molding
For injection molding, the rubber compound is heated to a low viscous and flowing state. It is then injected into the mold and travels through a system of runners and sprues before entering the mold cavities through one or more gates. Once filled the rubber is heated within the mold to complete its cure cycle.
Advantages of Injection Molding of Rubber:
- Shorter Production Times
- “pre-forms” not required, reducing prep time and labor costs.
- Curing process begins as the rubber is forced into the mold.
- May have flashless tooling to reduce post curing operations.
- Minimal material waste.
- Effective for very high-volume parts.
Disadvantages of Injection Molding of Rubber:
- The tooling can be expensive compared to compression or transfer molds.
- Small runs may be costly due to the setup times.
- Some part design restrictions such as wall thicknesses.
- Material restricted to high flowing/low viscous compounds.
- Tool modification often difficult.
Similar to compression molding, transfer molding requires the preparation of the raw material into “pre-forms.” Unlike the compression molding process, these “pre-forms” are prepared in a sheet configuration (transfer pad) which positions the “pre-forms” in a “pot” located between the top plate and a plunger. When the mold is closed, the material is compressed by a hydraulic plunger and transferred through sprues into the cavity below where the curing process completes. Last, the plunger is raised, the remaining transfer pad material “waste” is extracted, and the finished molded rubber part is removed from the mold.
Advantages of Rubber Transfer Molding:
- Higher cavity counts
- Requires fewer and simpler pre-forms than compression molding.
- Provides part design flexibility. i.e., sharper edges.
- Provides near flashless tooling.
- Shorter cycle times
- An effective process for over-molding components.
Disadvantages of Rubber Transfer Molding:
- Tooling may be complex and expensive.
- Results in a higher volume of waste material.
- Intricate tooling requires more maintenance (periods of downtime)
Additional Factors to Consider When Choosing a Rubber Molding Process
Deciding between the three rubber molding processes comes down to several variables including the complexity of the part (design), the production cost, the performance requirements of the part, the production quantity, and the environmental conditions (temperature, pH, etc.) for the application. Each molding process has advantages and disadvantages associated with these variables which ultimately affect, the quality, value, and profitability of your finished product. Finding a cost-effective process that does not hinder a high-quality and profitable product is key.
- Injection molding allows for more intricate designs and provides a faster process, but certain materials are not viscous enough to support the injection process, and the initial tooling can be costly.
- Choosing the transfer molding process involves more waste, but has a faster cycle time than compression molding and has the added benefit of colored pre-forms that can be cut by hand from raw material sheets.
- One of the oldest known methods of molding rubber, compression molding is a very cost-effective molding process solution if the molding tool needed is already in existence, the quantity required is minimal, and there is time for a longer cure process. It is also ideal for larger molded parts.
While this can be an overwhelming process, there is good news. You do not have to become a rubber expert, and in fact, in our opinion, you should not. For the majority of our customer’s, their rubber spend is only a fraction of their total overall spend so it doesn’t make sense for the to become an expert in rubber. However, getting it right is still incredibly important.
Through our network of vetted suppliers, Rothkopf & Associates has access to all three of these processes; processes that allow us to find the most cost-effective and highest quality part of your assembly or finished product. We can compete your part for you across our network of domestic, and low cost country manufacturers.
At Rothkopf, we understand that your reputation is only as good as our performance. Contact us to have the right part made, from the right material, with the right process, right on time.