what is plastic mold?

What Is Plastic Mold?

What is plastic mold?

The tooling used for plastic injection molding is called a plastic mold or plastic injection mold. The plastic mold is made from a combination of steel plates and other mold components, coupled to form an overall mold, which is then reliably assembled and installed in an injection molding machine, then applies a thermoplastic resin to the desired shape to fulfill the intended purpose.

Most plastic products are molded by plastic molds.According to the molding plastic characteristics, plastic molds are divided into thermosetting and thermoplastic mold.

By its very nature, an injection mold must satisfy a multitude of demands simultaneously when the molding process is being conducted. To form a plastic component having the shape of the mold cavity, the mold must contain the polymer melt inside the mold cavity. Heat is transferred from the hot polymer melt to the cooler mold steel, in order to give injection molded products as uniform and cheaply as possible. Lastly, the mold produces a fairly repeatable ejection of the part, which makes subsequent moldings more efficient.

The injection mold is expected to perform these three functions – contain melt, transfer heat, and expel molded parts – as well as additional requirements. Taking the example of containing polymer melt within a mold, it is imperative to have the mold resist enormous forces that may deflect or open the mold, and it must contain a feed system that directs the polymer melt from the molding machine to one or more cavity in the mold.

Additionally, these secondary functions can give rise to tertiary functions when specific mold components or features are used to accomplish them. Injection molds perform a number of functions, but they should be considered only a sample of the necessary primary and secondary functions during the design phase. Even so, a skilled designer would recognize when different functions are putting conflicting requirements on the mold design. Multiple cooling lines that are tightly spaced can provide efficient cooling by conforming to the cavity of the mold.

When it comes to removing parts, ejector pins may be required at locations where cooling lines are not desired. Designing a mold in a way that satisfies the conflicting requirements is the responsibility of the mold designer. When in doubt, novice designers tend to overdesign. The tendency to do so often leads to large, inefficient, and costly molds.

plastic mold

Based on the molding process, the plastic mold is divided into

  • injection mold,
  • blow mold,
  • casting mold,
  • pressing mold, and so on.

The injection mold comprises two main sections: the moving half and the fixed half.

The moving half is mounted on the moving platen of the injection molding equipment, and the stationary half is mounted on the stationary platen.

Over the injection molding, the moving and the stationary half are shut to develop an injection structure and a cavity structure.

Once the mold is open, the moving half and the stationary half are split up to clear away the plastic item from the plastic mold.

To decrease the mold design and developing period, the majority of the plastic molds work with standard mold bases.

The plastic mold is mainly composed of

  1. Injection system,
  2. The temperature control system,
  3. Part forming system
  4. Ejection system.

plastic mold structure

The injection system and the part forming system are in direct contact with the plastic, and they change with the different plastics and products.Those 2 systems are the most complex and most varied parts in the mold and require the highest finish and accuracy.

The part-forming process refers to the melted plastic heated by an injection molding machine is injected into the mold cavity.After cooling and solidification, it gets the molded products. Which simplifies as mold close – injection – pressure – cooling – mold open – ejection.

What is plastic mold?

The plastic injection mold is generally composed of the following parts:

  1. mold base: generally choose standard mold base from standard mold base manufacturing factories, such as LKM, DME, HUSCO…
  2. the mold insert: it is used to form plastic products; a plastic mold factory normally makes it by itself; most of the processing time is spent on producing mold inserts.
  3. Mold auxiliary parts: they include location ring, nozzle bushing, support pillar, ejector plate, guide bushing, guide pin, lifting ring, and so on.
  4. The four major systems:
  • Injection system(runner): Injection system refers to the flow path part before the melted plastic enters the cavity from the nozzle, including the main runner, the cold slug, the sub-runner, and the gate. It is directly related to the molding part quality and production efficiency.
  • ejection system: ejector pins, blade pins, and sleeves.
  • heating and cooling system: hot runner, preheating device, cooling water;
  • venting system: venting slot, parting surface venting, ejector pin venting, and insert venting;
  1. special mechanism: if the product has the undercut or sidewall hole, the plastic mold needs to design the side core pulling mechanism (also called the slide), the inclined core pulling mechanism (also called the lifters), the oil cylinder (when the side core pulling distance is long).

Classification of injection mold:

  1. 2 plates mold: another name of the sprue gate mold, also known as the single parting line type plastic injection mold, its characteristic is the simple mold structure, but the sprue is a part of the injection molded part, and it needs to be manually removed later, and plates mold structure is widely used for various kinds of mold.
  2. 3 plates mold: the pinpoint gate mold and the double parting line type injection mold. Its gate characteristic is pinpoint, and the gate cross-section is small. The injection molded part appearance is good, and because it does not need the manual removal of the gate later, it is advantageous for the automation production. But three plates mold structure is more complex, and the cost is higher. It is generally applicable to small and medium plastic parts. And plastic molding material is with good fluidity.


The most significant difference between the three plate mold and the two plate mold is that the three plate die has an extra runner plate (automatic remove runner).

2 plates mould

The main plastic mold system

There are four key concepts to consider when designing a mold, and the next few lines provide information on how to design a mold.


To design a good runner, its geometry, size, and layout also should be correct, in addition to its cooling ability, ejectability, and minimization of regrinds. It is best to fill all cavities at once using a balanced runner system to minimize cycle time and ensure the greatest possible dimensional integrity of the molded product.

The long and thin runners or any runner shaped like a half-moon or a half-circle need to be filled at higher pressures to prevent the mold from cooling down prematurely and causing incomplete parts. The length of a long and thick runner leads to an increase in regrinding, which, in turn, reduces the efficiency of the molding process.

In cases where the runners’ intersections should have ejector pins positioned to eject with sufficient force, ejecting the runner should still be possible. On the ejector portion of the mold, it is preferable to have runners installed to be forced out with the ejector.

  • Main Runner

This is a portion of the mold that attaches the nozzle of the injection molding press to the sprue. The top of the sprue is concave for touching with the nozzle.

The one end diameter of the sprue needs to be a little bigger than the nozzle size (0.8 mm) to prevent excessive streaming and avoid the two from being clogged due to wrong positioning.

The size of the one end is determined by the item’s dimensions, generally 4-8 millimeters. The size of the runner ought to be increased inwards at an angle of 3° to 5° to aid the ejection of runners.

In addition to the correct runner geometry, size, and layout, good runners must also be cool quickly, ejectable, and have very little regrind. For filling all cavities simultaneously, a balanced runner system is necessary, which minimizes cycle time and allows the molded product to remain dimensionally intact.

Long and thin runners or half-moon runners require higher injection pressures to ensure that parts aren’t rendered incomplete if the mold cools too quickly. Long and thick runners result in more regrind, which decreases the effectiveness of the operation. Ejector pins should be in place to expel the runners at the convergence of the cold runners.

Runners should be installed at the core half of the mold so that the ejector should be able to push these out of the mold.

  • Sub-runner

This is basically a small channel joining the primary runner and each cavity for the multi-cavity plastic mold building. As a way to come up with the melted resin occupies the cavity in equivalent velocity, the layout of the runners for the mold needs to be symmetrical and equidistantly spread.

The form and dimension of the runner impact the stream of the plastic melt, the discharge of the item, and the mold building. In many cases, the trapezoidal or semi-circular cross-sections are employed for runner design, and they are machined on ejection half of the mold for the ejector pin to push out.

The exterior of the runner has to be finished to minimize the stream resistance to produce a quicker filling speed. The dimensions of the runner vary according to the sort of plastic material, the dimensions, and the thickness of the item.

For most thermoplastics, the runner’s cross-sectional circumference is not over 8 millimeters, Maximum 10-12 millimeters, Minimum 2-3 millimeters. The cross-sectional region needs to be created as small as possible to decrease the resin misuse and shorten the cooling period.

3 plates mould

3 plates mould

  • Cold Slug

It is deemed a prolonged runner situated at the far tip of the main runner to capture the cold resin among the 2 cycles, therefore avoiding the possible blocking of the main runner or the gate.

Should the cold resin blend directly into the cavity, the interior stress will probably stem from the injection-molded item.

The Cold slug features a diameter around 8.5-10.5 mm and 6.5 mm deep. To be able to help ejection, the base is usually grabbed by a puller. The tip of the puller needs to be created to be a zig-zag catch or a depressed slot to ensure that the cold slug might be easily removed in the course of ejection.runner & cold slug

2.Temperature Control

To meet up the mold temperature demands of the injection molding procedure, a temperature control technology is necessary to regulate the heat level of the mold.

For injection molds to inject thermoplastics, the cooling system is usually built to cool the mold. The most popular way of mold cooling would be to drill cooling water lines inside the mold and work with moving cool liquid to take out the mold’s heat.

Along with heating the mold, hot liquid or vapor should be considered within the water lines, and a heating bar might be mounted within and around the mold.

Mold cooling is an essential metric for determining the quality of the product given to the customer in terms of dimensional integrity, physical properties, surface finish, shrinkage, and the strength of weld lines.

Irregular cooling inside a long cavity will result in poor warpage control. Cooling all core pins is imperative, especially if the ratio of length to the diameter of the core exceeds four. Hot core pins result in surface imperfections and prolong the molding process.

The heat transfer efficiency of a water layer on the pins is much higher than that of the air layer. Ejector pins for flexible resins need cooling before ejection.

The ability to control the temperature of the sprue puller pin area reduces the time it takes to cycle the mold and the number of interruptions it causes during the ejection process.

For effective temperature control, the fluid flow must be high-volume and turbulent.

Prohibiting corrosion within the water lines is achieved by using stainless steel mold plates; other ways to prevent corrosion include plating the cooling channel or adding rust inhibitors into the water. The mold plates must be thick enough to accommodate the cooling channels of proper size.


This is a not deep groove cut in the mold to release the air within the cavity or the soft plastic material introduced.

Should the melted resin is shot into the cavity, this kind of trapped air in the cavity needs to be released out of the mold via the grooves at the end of the streaming front in time.

Or else, the item will likely have voids inside(especially for transparent resin injection molding), vulnerable weld lines, um-completed shot -. Perhaps the build-up of air would produce excessive-high temperatures because of the high pressure to make the item charred.


plastic mold vent slot

The venting grooves might be situated both at the end of the melt stream and on the P/L of the mold. The last-mentioned place is a shallow slot that is a depth of 0.03-0.2mm and a width of 2-6.5mm cut on the cavity side of the plastic mold.

The vent grooves won’t escape much-smelted resin throughout the shot since the melted resin will cool down around this location and congeal the grooves. The placement of the vent grooves ought not to be confronting the operator to avoid an unexpected splash shot of melted resin. The gap between the ejector pins and the ejector hole, between the ejector bar stripper plate and the core insert, may be a method to release the air.

plastic mold venting slots size:

plasticslot deepth /mmplasticslot deepth /mm
PE0. 02PA(GF)0. 03 〜0. 04
PP0. 02PA0. 02
PS0. 02PC(GF)0.05〜0. 07
ABS0. 03PC0. 04
SAN0. 03PBT(GF)0. 03〜0. 04
ASA0. 03PBT0. 02
POM0. 02PMMA0. 04


Ejector pins, sleeves, stripper rings, or stripper plates should work without obstruction to realize consistent ejection.

A guided ejector system allows for precision alignment of the core and pins and will also bear the loads so that the pins do not wear out and go out of alignment. An early return system is another safety feature that should be included.

The early Return system drives all ejector pins into seated positions before the mold closes, ensuring no accidental contact with the ejector that has not fully retracted. Every mold incorporating ejector pins or sleeves under any slides should have a protector pin.

To prevent the ejection pins from colliding with the slides, this lock secures it in the retracted position. Plastic mold should use special ejection systems for parts that consist of flexible, thin-walled, deep sections that are difficult to eject.

Elements of a Plastic Mold

Designers and people interested in creating a mold should realize that molds are basically made up of a number of different elements from which to choose a design that is appropriate for use.
The following elements are essential to every injection mold:
1. Determination of the cavity space(s) with corresponding cores ( molds can have up to 144 cavities for preform mold).
2. A conduit for conveying (hot) plastics from the machine nozzle to the mold cavity.
You can choose from 
  • Cold runners (two or three plate)
  • Hot runners (of various types)
3. Ventilation:
Natural venting or vacuum venting are both options. 
4  A cooling system designed to allow the molded product to be ejected from the cavity
5. Various options are provided for ejecting the molded article:
These include
  • Hand removal
  • Sleeves and pins for ejectors
  • Stripper 
  • Ejection by air
  • Force ejection
  • Various methods of in-mold product removal methods
  • Automated removal methods 
6. Attaching the mold to the machine:
Several methods are available
  • Molds are limited to one machine
  • Mold can be used on more than one machine
  • Easy mold change options 
7. Aligning the cavities and cores:
The following methods may be used:
  • No alignment feature
  • Brass bushings and pins (2, 3, or 4)
  • Leader pins and bushings between ejector pin plates
  • Taper locker between individual cavities and cores
8. A number of (mold) plates would be required here to support and back the above elements.
 Although each of these features can increase the cost of a mold (often quite significantly), they can also improve the productivity of a mold and lower the cost of a product. If you are looking to choose the most suitable (and most economical) mold for a particular application, these factors may not all be necessary.

Plastic Mold Gates:

The gate of plastic mold refers to a short flow path between the runner and the cavity, which is the entrance of the resin into the cavity.

It is a channel connecting the runner and the cavity.

  • The cross-sectional area of the gate can be equal to the runner, but it is usually reduced. So it is the smallest part of the entire runner system.
  • The design of the gate is related to the size, shape, mold structure, injection conditions, and properties of the plastic parts.
  • The role of the gate :
  1. It could control the flow rate:
  2. The early solidification of the melt in this part can prevent backward flow:
  3. The molten material passing through is subjected to strong shearing to increase the temperature; thereby, it will lower the viscosity to increase the fluidity:
  4. It could facilitate the separation of products and the runner system.
  • plastic mold gates

    plastic mold gates

  • The design of the gate shape, size, and location depend on the plastic, the size, and the structure of the article.
  • The shape of the cross-section of the gate is rectangular or circular, and the cross-sectional area should be small, and the length should be short.
  • Gate location should generally be selected where the product is thickest without affecting the appearance.
  • The shape, quantity, size, and location of the gate will greatly influence the quality of the plastic parts.
  • So gate selection is one of the key points in plastic mold design.
  1. Direct Gate

  • direct gate

    direct gate

  • Pros:
  • 1) low-pressure loss;                      
  • 2)the processing is simple.
  • Cons:
  • 1) the stress near the gate is large, and the product stress is uneven and easy to deform.
  • 2) it is necessary to remove the gate with extra works manually. Also, it will leave obvious gate marks on the product surface.
  • Application:
  • 1) it is suitable for large and deep barrel-shaped parts. For plain parts, it is prone to warpage due to shrinkage and stress.
  • 2) for parts that are not allowed to have gate marks on the surface, the gate can be set into the inner surface of the part, which is an inverted mold.
  1. side gate

  • side gate

    side gate

  • Pros:
  • 1) simple shape and easy machining.
  • 2) it is easier to remove the gate.
  • Cons:
  • 1) gate can not separate from the product automatically.
  • 2) the plastic part will leave the gate marks on the plastic part, obviously
  • Application:
  • Suitable for all kinds of parts, but not for long barrel profile parts.
  1. pinpoint gate

  • pin point gate

    pin point gate

  • Pros:
  • 1)gate position could set on the most surface;
  • 2) the gate can be separated from the part automatically.
  • 3) the gate is small, and the gate marks the glue is small.
  • 4) the stress near the gate location is small, and the injection molded parts are not easy to warp.
  • Cons:
  • 1) injection pressure is large, and it is not suitable to use plastic material with poor fluidity.
  • 2) generally use 3 plate mold structure, the mold structure is complex, and the cost is high.
  • Application:
  • Because several gate points can be designed, it is often used for shell parts with a larger surface.
  1. fan gate

  • fan gate

    fan gate

  • Pros:
  • 1) when the plastic flows through the gate, melt plastic distributes more evenly in the transverse direction and reduces the inner stress.
  • 2) prevent air entry into the cavity and avoid defects, such as silver streaks and bubbles.
  • Cons:
  • 1) the part can not be separated from the gate automatically.
  • 2) the remaining gate material will leave on the plastic part and be trimmed manually.
  • Application:
  • Commonly used to produce thin sheet and wide parts, and poor fluidity material, such as PC, PMMA, etc.
  1. submarine gate (set on ejector pins or ribs)

  • submarine gate

    submarine gate

  • Pros:
  • 1) the choice of gate position is more flexible;
  • 2) the gate can be separated from the plastic part automatically.
  • 3) both the two plate die and the three plate die can be applied.
  • Cons:
  • 1) the cloudy area is hard to remove.
  • 2) it should cut off the redundant gate relics artificially;
  • 3) the pressure loss from the gate to the cavity is large.
  • Application:
  • It is suitable for plastic parts with an external appearance with no gate marks allowed.
  1. horn gate (also called banana gate)

  • banana gate

    banana gate

  • Pros:
  • 1) the gate can separate from the plastic part without post-processing of the gate
  • 2) the gate location is inside the plastic parts and will not affect the appearance of the part.
  • Cons:
  • 1) the appearance of the plastic part surface may find fingerprinting marks.
  • 2) the machining process is complex;
  • 3) the unreasonable design would easily lead to break the gate and block the gate channel.
  • Application:
  • Commonly used for ABS, HIPS, not used for POM, PBT, and other crystalline materials, also unsuitable for PC, PMMA, and other rigid materials, design should prevent the arc gate from breaking and blocking the gate.
  • Sliders and lifters
  • Any structure that hinders the mold opening or ejection is referred to as an undercut. The way to handle the undercut on the mold includes sliders, lifters, core pulling, gear rotating, etc.
  • The most commonly used is the sliders and the lifters.
  • slider
  • The slider could be set on the static or movement mold plate, and sliders on the movement plate the most widely used.
  • The slider generally consists of a slide body, locking block(heel), Gibbs, cam pin (angle pin), wear-resistant block, spring, and so on.
  • injection molding
  • lifter
  • The lifter generally is used to handle the undercut in the interior of the plastic part. At the same time, the slider generally treats with the external undercut of the plastic parts, but the lifter’s structure is simpler than the slider structure.
  • The lifter has the function of releasing the internal undercut in the plastic parts, and it can also play the role of ejection.
  • So the design of the lifter on the mold generally reduces the arrangement of the ejector pins.
  • The lifter comprises an inclined top body, guide bushing, lifter seat, and wear-resistant block.
  • plastic mold design process:
  • plastic mold design process
  • Troubleshooting of Plastic Mold Design
  • To summarize the causes and cures of faults in injection molded components, the Table below should be consulted. 
  • injection molding troubleshooting
  • Fault
  • Generalized cause
  • Possible cures
  • Short moulding Sinks and voids
    Weld and flow marks
    Poor surface finish
    Cavity not filling properly
  • Increase injection pressure
    Increase injection rate
    Increase cylinder and mould temperature
    Increase feed
    Improve mould venting
    Increase size of gate,sprue, and runner
  • Bum marks
  • Lack of mould venting
  • Improve mould venting
  • Mechanical sticking
  • Over-pressurization of cavity
  • Reduce injection pressure
  • Silvering and splash marks,Decomposition
  • Overheating of material
  • Reduce cylinder temperature
    Reduce effects of mechanical
    Increase cooling time
  • Distortion
  • Ejecting too hot
  • Increase cooling time
    Eject over larger area
  • Jetting
  • Linear velocity of melt too high
  • Reduce velocity or break up
    stream of melt
  • Lamination, crazing
  • Frozen-in stresses
  • Ensure rapid filling at lowest
    possible pressure

5 Responses to what is plastic mold?

  • Zachary Tomlinson

    Thanks for helping me learn more about plastic molds! My dad has been into collecting plastic models and I got curious as to how they’re being made. I find it interesting that the injection system and part forming system can modify the appearance of the plastic. I should share this with my dad so he’d understand how they’re made as well.

    • Steven

      I am so glade that my website could be helpful

  • Steven

    I will,thanks for your information.

  • Penelope Smith

    This is some really good information about plastic molding. I liked that you showed that diagram. That seems like it would help you understand molding.

    • Steven

      thanks for your information