The manufacture of plastic parts is a critically important area of the production sector, but there is often some confusion around the differences – and similarities – between processes.
In this article we aim to explain the parallels and dramatic differences between injection molding and compression molding, to alleviate some of the process and terminology confusion that can get in the way in development and supplier discussions.
This is presented as both an introduction to the two processes and a guide to the similarities and differences between them.
I. Types of Molding Methods
Thermoplastics are a class of synthetic materials that are formed from long chain polymerization of chemically simple monomers. Polymerization is sometimes a naturally occurring process which is simulated and accelerated at industrial scale with high pressure, heat and catalyzed reactions
- Thermoplastics lose structural integrity and then liquify when heated correctly. Their structures reform when cooled, recovering all of their mechanical properties but retaining the shape they settled to when molten – this is the basis of all thermoplastic component manufacturing processes
- Heat releases the intra chain bonds which form a crystalline or amorphous structure in the solid
- Formation of intricate and specialized shapes is a low cost process, and various molding methods exploit this property
- The great value in plastics is their ability to form resilient, elastic and low cost components of complex shapes – finding uses across all sectors of industry and product
Manufacturing methods fall into two basic categories – those which enclose the liquid polymer in a filled cavity and those which form a hollow skin within a cavity (or around a former).
This article looks at comparing two net shape, cavity molding methods that appear identical, yet differ greatly in detail.
The first method is injection molding, which closes a cavity tool and then fills it with liquified polymer. The part and tool are cooled, so the part mirrors the cavity, taking its shape until remelting.
The second method is compression molding, which fills the lower cavity of a two part mold tool with softened or molten material and then closes the tool, filling the cavity by the compression of closure. The cavity tool and component cool and the finished component is extracted.
II. What is Injection Molding?
The goal of injection molding is to turn liquified polymer into a finished component whose net shape reflects the shape of the cavity as closely as possible, and whose solid properties meet the needs of the application it is made for.
This requires the melting of plastic granules in the ‘barrel’ of a molding machine. The barrel is a heated cylinder with an archimedes screw inside, to mix the granules and distribute the heat uniformly.
Once the target temperature and viscosity of the molten polymer has been achieves, a piston then drives a charge of the melt through an injection port (named a gate) into the tool. Plastic can be distributed to multiple cavities, each with its own gate, by galleries in the tool (called runners)
This melted charge fills the tool, driving out the air at the part line of the cavity tool (the line at which it splits and opens to release the part). This seam is close fitting, but not air tight. Small leakage into the seam is the origin of ‘flash’ on a finished molding.
When the cavity is full, a cooling cycle starts, which solidifies the polymer. In this solidification, the material shrinks by up to 15%, so the barrel maintains pressure, injecting more plastic into the part to counteract this shrinkage.
Once cooling is complete, the tool is hydraulically opened and the part is ejected by ejector pins which are built into the tool for this purpose. These pins are sprung to the mold position and pushed to eject by the tool opening – leaving unobtrusive tell tale circle/rectangle marks on the finished part.
The ejected part will generally have the gate and gallery parts attached when ejected, and these must be trimmed off for completion of the component.
III. What is Compression Molding?
Compression molding is a very close analog of injection molding, though considerably simpler both in tooling and in operation. Compression molding is a manufacturing process used to create parts or components from materials such as thermosetting plastics, composites, rubbers and metal powders.
In this process, a pre-measured amount of pre heated material is placed into a heated mold cavity, which is then closed with a hydraulic press. The closure causes the material to be compressed and the pressure causes it to flow and fill the cavity precisely.
In general, the combination of heat and pressure ‘cures’ or hardens the material, causing it to develop a permanent memory of the mold cavity shape. After curing, the mold is opened, and the part is removed, often requiring trimming or finishing to remove flash, formed at the tool part line.
Compression molding is ideal for creating complex shapes with tight tolerances and a smooth finish. It is often used in the automotive, aerospace, and electrical industries to create parts such as brake pads, electrical insulators, and housing components.
IV. Compression Molding Vs Injection Molding: Which Method is Best for Your Application
The selection of compression or injection molding is primarily a selection between thermoset and thermoplastic materials. There are overriding reasons to choose thermoset polymers in many applications, but it must be recognized that the cost implications in this selection can be significant.
Much effort, from the thermoplastics development sector, has been focussed on the creation of new polymers and additives, to enhance thermoplastics performance.
This has lead, at the leading edge, to new injection molded options that counter some of the real weaknesses in the common thermoplastic options. This has resulted both from new materials and the creation of novel additives that enhance thermoplastics in limited but significant ways.
At the same time, the development of thermoset polymers has been slower, so the capability gap has steadily closed, in the last 30+ years – making the choice between the two basic options more difficult.
An increasing range of design problem can be solved with thermoplastics, displacing thermosets and therefore compression molding:
- Thermoplastic rubbers offer good elasticity at lower component cost. While less durable than the thermoset options, these weaknesses can be accounted for in design to a large degree
- Advanced thermoplastics improve the ultimate tensile strength of molded parts – PEEK, polyether ether ketone, for example, can approach the strength of die cast Aluminum, with greater shock resilience than a thermoset equivalent
- Where extreme chemical resilience is required, new polymer options can handle more aggressive solvents (such as fuels) – LCP (liquid crystal polymer), advanced fluoropolymers and acetal copolymers all offer good options
Compression Molding: Benefits and Applications
Compression molding is a powerful tool in the arsenal of manufacturing capabilities. It brings the designer a wide range of options that are virtually impossible to achieve, without this production method (or the related transfer molding system, plus extrusion).
Benefits of and markets for compression molding and the materials options it opens up are:
- Great electrical insulation parts, more detailed and robust than can be achieved in glass, yet resilient and much lower cost than glass
- Highly elastic, wear resistant and temperature stable rubber parts – extensively used in automotive (tyres, suspension bladders, fluid diaphragms, seals and gaskets)
- High heat tolerance rigid and rubber parts in cookware, oven components
Overall, compression molded rubber and rigid parts offer quite good levels of detail and precision, high repeatability, high strength/resilience and relatively low cost (compared with potential alternatives).
Injection Molding: Benefits and Applications
Injection molding, and particularly the steadily falling cost of both tooling and resultant parts, has made this process the most ubiquitous of all manufacturing methods. There is no sector or market that does not make heavy use of the process, and the penetration of injection molded parts into our lives, our homes and our environment continues to grow.
Injection molding unleashed the minds of designers and has allowed an explosion in both functional and cosmetic complexity, as thermoplastic components of increasingly elegant and intricate design are everywhere.
The ability to combine multiple functions into a single component has triggered product developments that even a few decades ago would have seemed almost magical.
Examples of the wide spectrum of benefits and markets for injection molded thermoplastic parts are legion;
- The socket of a prosthetic hip joint has a hemispherical internal face that has extreme polish applied to the tool, making a component that is simple in shape but remarkable in function. Molded in a fluoropolymer (PTFE, teflon), the wear resilience, low friction and high polish result in a part that can serve for 10 years plus in a high load application. The biocompatibility of the material, combined with its low surface energy, result in a well tolerated part that gets little to no deposition and triggers minimal immune reaction
- The retaining of tomato/pepper plant stems to trellis supports in commercial greenhouses is a big issue. String takes time and can strangle the plant as it grows. The solution is a range of elegant and simple injection molded clips that are fast to apply, very low cost, allow for growth and molded from a compostable material so they can go with the plant waste when the house is cleared
- Fresh food storage is an ancient challenge that remains central to healthy living today. Rigid, clear and food compatible plastic kitchen containers with flexible, integral sealed lids improve food quality and reduce waste. And there is no production method that can match injection molding for making both parts. the polished surfaces are easy to clean, the materials are hot water safe and the easy use seal systems exclude pests and air without complexity or high effort
- Automotive bumpers are designed to be essentially unaffected by an impact up to 5 mph. The level of structural distortion experienced requires them to distort by as much as 50mm and recover with only light cosmetic damage. Combine this with the demanding cosmetic standards and extreme cost controls required by car manufacturers, only injection molded ABS derivatives can combine the toughness, environmental resilience and excellent surface finish, while delivering a lower cost than the alternatives
These are a tiny sample of the astonishing range and reach of injection molding, into every aspect of technology and life. The combination of wide design flexibility, extensive material choices/properties and the lowest cost of complex net-shape, single stage to finished production has made injection molded thermoplastics the go-to choice for most components.
V. Compression Molding vs Injection Molding: Common Traits and Characteristics
There are a range of close parallels between injection and compression molding – it is often practical to make the same part by both methods, so the selection of process will be driven by a variety of design and application factors;
Color selection in most thermoplastics is essentially unlimited – colors can be masterbatched (custom made by the granules supplier) or locally colored at the molder to deliver any color. This applies equally to thermoplastic elastomers/rubbers and rigid materials.
Some coloring agents can have mild adverse effects on some material properties, so it is usual to use ‘natural’ materials (i.e. uncolored) for engineering components.
Color selection in rigid thermoset plastics is less flexible, though they are generally very accepting of paint finishes. The natural color for many rigid thermosets is light to dark brown, black or sometimes shades of yellow.
For thermoset natural rubbers, few color options exist. The ‘natural’ color for the raw material natural rubber is made from (latex) is white, but processing of the latex alters this to dark brown or black. This material is commonly compounded with Carbon black, making it very deep black in color.
The natural color for most synthetic thermoset rubbers is dark to very very dark brown, almost black. The exception to this is silicone rubbers, which are mostly clear.
Synthetic rubbers such as nitrile are generally compounded with Carbon black to improve their properties, so they are black in color. Silicone rubbers can be compounded with a wide range of coloring agents, allowing selection of color to be as wide as it is for thermoplastics.
Low Labor Costs
Both the compression molding and injection molding processes are focussed on reduced labor costs, in comparison with any alternative manufacturing technique for similar parts.
Compression molding is generally used for moderate volume products and components, and often includes a manual loading (raw material) and unloading (finished part) stage.
In addition, the low cost nature of the tooling often results in high flash, often requiring extensive manual trimming. For higher volume manufacture, the loading and unloading can be automated and the trim can be performed in a relatively low cost press tool.
Some manual effort is required to ensure that the tool is clean and clear of detritus before the next cycle is run.
Injection molding is best suited to medium to high volume production scales and has often no manual unloading stage, for small parts. The small part unloading process can rely on dropping into a chute/box after ejection, slight when cosmetic damage to the still warm parts is of low concern.
Chutes reduce drop damage and allow extra cooling time before the part drops into a bin with previous shots.
Where parts are larger, or cosmetic standards high, a manual (and often robotic) unloading stage is required. However, for higher value parts this is a first quality check point, allowing the operator to flag molding issues before large numbers of parts are made. Machine settings may need adjustment, or contaminants be removed from the machine.
Visual checking of the tool is a routine stage, but where production quality is stable the frequency of such checks/cleaning may be reduced.
Reduced Part Costs
Whether compression or injection molding is used, manufacturers have a tight focus on cost minimization. Where cost sensitivity is a primary driver AND volumes are large, it is generally considered better to use injection molding.
The loss of performance in some areas of component properties can either be managed in design and thermoplastic materials choices, or a thermoset/compression process must be used.
Part cost reduction for compression molded parts can be achieved to a limited degree:
- downgrading material class or using a higher filler content – thermosets are relatively high cost materials, so an additional 10% filler can influence part cost measurably
- More cavities can speed up production, especially if efficient loading methodologies are used – but be aware that scale up will not reduce labor per part and can increase material wastage
The part cost reduction ability in injection molded parts is very large:
- Use of multi cavity mold tools increases tooling costs but drives down component cost, so a decision must be made as the the ‘sweet spot’ for cavity count
- Use of automation will reduce labor content, but be aware that a tool/product that is not fully stabilized can generate a lot of unmonitored scrap before the issue is noted
- Use of ‘hot runners’ reduces overall plastics use in multi cavity tools, as the runners are not cooled and ejected but kept liquid. The cost effect of hot runners is significant in small parts only
VI. Differences Between Compression Molding and Injection Molding
The similarities in the process don’t survive any close scrutiny of the detailed stages. Shaping a charge of polymer using pressure is a loose enough description to encompass every moulding and forming process used on polymers!
Compression molding is a process used for thermoset polymers – and a range of other materials such as brake linings, cake decorations and toasted sandwiches!
In polymer molding terms, forms of compression molding are used to manufacture from:
- Nitrile, EPDM and other rubbers
- Silicone rubber, in its catalyzed, two part forms
- Bakelite (phenol formaldehyde)
- Cyanate esters
- Diallyl phthalate
A modest range of additives are used as modifiers, in addition to the base materials.
Injection molding is used on a huge range of polymers, modified variously with additives that provide enhancement of some properties:
- Polyethylenes (low, medium, high density)
- Styrenes (polystyrene, high impact polystyrene, ABS)
- Polyesters (aliphatics of various densities, aliphatic-aromatics [polyethylene terephthalate-PET, polybutylene terephthalate-PBT], unsaturated polyesters [thermoplastic elastomer – TPE])
- Nylons (N6, N66, N11)
- Acrylics (polymethyl methacrylate)
- Acetal (polyoxymethylene homopolymer and copolymer, Delrin)
There are too many more to list all. Additionally, a huge range of additives and modifiers are used to tune properties.
Compression molding uses technically simple cavity tooling, which is mounted into a vertical axis hydraulic press (a vertical molding machine) and opened. A pre-heated and softened charge of unpolymerized (or partially polymerized) material is placed into the lower cavity half of the tool and the hot tool is then closed.
This forces the charge of material into the shape of the cavity, squeezing excess through waste vents to make certain of not making a short shot (part filled cavity). Sufficient pressure is applied to fully close the tool, then heat and time are allowed for cure to be sufficient for dimensional stability.
Once the molding is stabilized by reaching a sufficient level of polymerization, the tool is opened and the part is removed, usually by hand. It is then trimmed and often passed to a further bake stage, to ensure complete polymerization.
Injection molding requires considerably more complex tooling, but still utilizes a basically 2 piece tool that splits to open a cavity.
The cavity tool is fitted to a moulding machine (usually a horizontal axis, but small machines can be vertical). A hydraulic ram pushes the two tool parts together with high force and then molten plastic is injected through a small hole (the gate) into the cavity.
The cavity surface is quickly cooled, often with a flow of water, and the tool opens and in opening, pins are driven towards the cavity to push the part out.
Most mold tools push out the gate feeder with the part, but some tool types use a side gate which can be placed to shear off the gate waste as the tool opens. If not self trimming, parts are hand or press too trimmed and passed to assembly or the customer.
Production capacity for compression molded parts is relatively slow. A typical tool plate with 20 to 30 small parts molded in one operation might have 5-10 minutes cycle time, to allow sufficient heat/time for catalyzation to be advanced enough for the part to retain its shape when pulled out of the tool, handled and trimmed.
Some silicone rubber molding processes can be faster catalyzed, requiring only 2-4 minutes of cycle time, but some large parts can take longer
Typical cycle times vary depending on the material, the heating/cooling methods and operator efficiency, but they are generally minutes per operation (resulting in multiple small or smaller numbers of larger parts).
Injection molding is a much faster process, and parts with shot weights measured in one or more kilogram of weight will generally have only 2-5 minutes cycle time, where small parts in the 10-100 gram range may have cycle times as low as 1-2 minutes, sometimes less.
So compression molding productivity is low, compared with injection molding – but given the necessity of thermoset parts for some applications, the actual alternatives to compression molding are VERY slow.
Compression molded parts are by their nature moderately precise. The moulding cavities are generally CNC cut and highly accurate. However, the tool closure can allow some vertical error to occur, as can the flexure of the tool.
This results in greater or lesser degrees of flashing, and for many applications the increase in part height is a tolerance issue that will already be factored into the design.
In addition, some distortion can occur during fast processing, as the operator handles partially cured parts. This is bad practice and can easily be avoided.
Finally there can be some thermal effects in post molding curing, causing both shrinkage and distortion. The distortion is avoided by proper support in the cure stage and the contraction SHOULD be planned for at the tooling stage, so oversize parts are molded and shrunk to size.
This shrinkage can, however, distort parts where wall thicknesses vary widely and this can be hard to manage and is best obviated at the design stage, with retaining features that hold the shape in place.
Injection molded parts are considered to be of higher precision than compression molded parts. They are made under much higher pressure and the cooling/solidification and shrinkage processes are entirely managed in the tooling. On that basis, local features can be managed to give repeatable tolerances on +/-0.1 mm and even better if required.
Overall precision is good in well designed, well tooled and well molded parts, so accuracies of +/-0.3 mm on features of 300mm length are commonly achieved.
Tool closure in injection mold tools is much more certain, with closure pressures measured in tonnes. This ensures that the cavity precisely reflects the intended dimensions, when closed. The tool must resist very high pressures from the injection process, without allowing flash at the part line.
Finally, shrinkage is a major consideration in many thermoplastics, but it is also a well understood factor that is allowed for in the tool. Parts are ‘fed’ molten plastic as they cool, to maintain a faithful reflection of the cavity at ejection. Cavities are also cut oversize, to allow for the shrinkage of the part as it cools from ejection temperatures.
These factors compound to allow well designed, well tooled and carefully molded parts to be very precise.
Complexity of Models
Compression molded parts are generally quite simple. All detail is generally in the ‘line of draw, so undercuts are avoided in the design stage.
An exception to this is that compression molded rubber parts CAN have modest undercuts in thin sections. The thin section ensures good cure before the part is pulled from the tool and the fact that the rubber is able to flex allows the parts to pull out from under some tool features.
Undercuts in rigid materials require parts of the tool to move sideways before part removal, and this is very challenging in simple compression mold tools.
Small features can mold unreliably and an easy flow path must be maintained – so high aspect ratio features (long and narrow) are a bad choice, unless these features can be arranged a) on the part line and b) with a waste channel that feeds from them.
Shallow surface features of high complexity are generally achievable, but overall it is best to keep the parts of simple form and a single body.
An advantage of compression molding that can allow more complexity is that the sensitivity to wall thickness is low, so a mix of thick and thinner sections is easily molded, as long as the material has a clear flow path that fills the cavity.
Injection molded parts offer the designer considerably greater shape complexity freedom. A complex mix of very small and larger features can be made reliably moldable – powerful flow analysis tools are available to assess cavity fill and weld line expectations. Weld lines are the areas where flow parted by features rejoins to make a whole part.
In particular, the surface finishes of the cavity parts will be very faithfully reflected in the finished component. This allows designers to use subtle texture changes to serve purpose in the part, and speak to the user about precision and quality.
Overall, injection molded parts offer the designer considerable freedom from process constraints, other than the limitations on undercuts (because of their tooling cost rather than feasibility) and the need to maintain careful control of wall thicknesses, as sudden transitions can lead to visible cosmetic shrinkage, referred to as sinking.
Lead Time Length
Transfer mold tooling is technically simple, which results in short lead times and lower costs. It is common for simple, two plate transfer mold tools to be manufactured in days and tested immediately. If the tool design was executed well, this often results in transfer to production immediately.
Injection mold tools are considerably more intricate and entail complex mechanical operation that requires exacting precision.
- Tool design, particularly for multicavity tooling and hot runner systems, is a standalone skillset in the design spectrum and requires both skill and time
- Given the very high pressures used in injection molding, the tools must be built to be exceedingly stiff, to resist distortion
- Designed for million cycle level endurance, moving surfaces are carried on high quality bearings which demand high precision
- Internal moving features (to allow undercuts) require either entrained mechanical drive, hydraulic or electrical power. These aspects add considerable complexity in design and manufacture
- It is usual for tools to be run when mechanically completed but before surface textures have been added, to allow any additional machining
- It is common for flow issues and fine detail adjustments to be addressed after the first tool trial, which process can be time consuming
These factors conspire to make lead times for injection molded parts of production quality to rarely be less than 25 working days, and often as much as 60 working days for more complex or awkward components.
Shape of Final Product
Compression molded parts are generally; of relatively simple shape, to allow easy material flow at low pressures; tolerant of large variations in wall thickness, as material shrinkage is often low; lacking undercuts, to allow tooling to be simple; of relatively thick sections and low feature count; of moderate precision (+/-0.3mm typical on small features).
They can be surface textured, but this cannot generally be as fine as some processes allow, as the pressures are low and the molding process viscosity is relatively high.
Injection molding allows; unlimited complexity of features; undercuts of varied size and complexity; high aspect ratio features (long and narrow); extremely fine surface finishes; fine detail, of the order of 0.1mm faithfully reproduced; high precision (+/-0.1mm easily achieved on small features).
Injection molded parts, much like compression molded parts, have all sides formed by the cavity tool, so it can be difficult to distinguish the process that was used, at a glance.
VII. Compression Molding Vs Injection Molding: An Overview of Each Molding Process
Though the two processes share many similarities in a general sense, the detailed differences are profound.
These differences relate to all aspects – material selections possible, format of molding machine, complexity of part, molding method, finishing processes and more.
Compression Molding: Principle of Process
Compression molding is a relatively straightforward process that really is only two stages.
Once the tooling plates are mounted into the usually vertical molding machine and opened up:
- Both sides of the tool are heated electrically
- A prepared material charge is manually placed into each cavity
- The tool closes and squeezes the slightly oversize charge into the cavity, with excess flowing out into waste galleries
- The heat and pressure are maintained until curing is sufficient for removal from the tool
- The tool is opened and parts are manually stripped
- Parts are trimmed to remove the waste galleries and clean up the injection point
- Some materials then require post molding cure time, so parts are put into shape supports and further baked, if necessary
Injection Molding: Principle of Process
Injection molding is the optimal method for manufacture of larger volume products with high precision and/or cosmetic requirements.
All features are formed from the molding cavity, but parts can be of essentially unlimited complexity and fine detail. The molding stages are:
- The polymer granules are loaded into a hopper where they are prewarmed to condition and dry them
- Beads are fed into a ram where they are fully melted and then pushed into the cavity using an hydraulic ram.
- Plastic is then fed into the cavity either directly, or through tooled galleries that distribute it to multiple cavities
- Polymer floods the cavity and conforms to its shape
- As the tool/fill cools, the material shrinks by up to 15%
- Liquid polymer is pushed into the tool during cooling, to counter the shrinkage
- After cooling, the part is ejected as the tool opens, by integrated ejector pins
- Gallery and gate parts are then trimmed, to reveal a finished part
VIII. Should you Choose Injection Molding or Compression Molding
The choice between the two processes is generally straightforward.
If you require the properties of a thermoset polymer material, i.e.:
- greater stiffness; higher electrical breakdown voltage; greater temperature stability; flame resilience; or thick section tolerance
Or you require a part that is made of a true rubber, offering:
- high elasticity and extension tolerance, strong elastic shape memory under long term distortion (compressive or tensile), high temperature resistance, high electrical breakdown voltage and high UTS (ultimate tensile strength)
Compression molding (or one of the derivative and related approaches) in either a rigid thermoset polymer or a natural/synthetic true rubber is appropriate.
If you require the properties of a thermoplastic polymer:
- Low cost; fast processing; excellent cosmetics; good chemical resilience properties and more
Then injection molding is a likely option.
Or if, as a trade off for cost and cosmetic qualities, you can tolerate the more limited resilience properties, compared with various other, more expensive material choices:
- Moderate to low strength – at best comparable with mid grade Aluminum parts
- Poor to moderate thermal resilience – generally much poorer than thermosets and metals
- Moderate to low abrasion resistance – most thermoplastics are quite soft
Then injection molding is likely the right choice.
IX. Choose Kemal for Your Injection Molding Needs
Kemal positions itself at the leading edge of the technologies in which we operate. We offer huge experience and knowledge and we can readily demonstrate both our abilities and the quality of our outcomes.
- Molding Manufacturing
- Mold Design
- Mold Manufacturing
- CNC machining
- CNC machining service
- CNC milling
- CNC turning
- Injection molding
- Injection molding service
- Plastic injection molding
- Clean room injection molding
- Insert molding
- 3D printing
- Online 3D printing service
- Metal parts
- Die casting
- Metal stamping
- R&D and manufacturing solutions
- Rapid Prototyping
- Low-Volume Manufacturing
- Surfaces Finishing
Kemal has a skilled team, and up to date equipment. Our spectrum of services is extensive and we are the best partner for your manufacturing needs, offering adherence to quality standards and excellent project management and communications.
To understand our capabilities in injection molding, transfer molding and many related areas of service, please contact us to allow us to introduce our team and capabilities. Allow us to show how we can make your transfer to production smoother and easier.