Basic Knowledge on the Mould Clamping Force

When a injection mold is fixed in a molding machine and molten plastic is injected into the interior of the cavity from the injection nozzle, a high filling pressure acts on the inside of the cavity. Since the parting surfaces of the mold try to expand outward due to this pressure, it is necessary to clamp the mold so that it does not open instantaneously.

It is easy to imagine that flash will be generated if the parting surfaces open even very slightly. The force of keeping the mold closed tightly is called the "required mold clamping force". The unit for the required mold clamping force is N (Newtons), or kfg, of tf.

At the time of designing a new mold, it is necessary to obtain by theoretical calculations what is the optimum required mold clamping force that the injection molding machine has to have for the mold to be installed in it. For example, if a required mold clamping force of 100 tf was obtained by calculations, if this mold is installed in an injection molding machine with a 75 tf capacity, the molded product will be full of flash thereby making it impossible to carry out the molding operation. Further, if the mold is installed in a molding machine with a 300 tf capacity, even if the molding operation is possible, since usually the hourly cost of a 300 tf machine is higher than that of a 100 tf machine, the molding operation becomes high in cost.

The required mold clamping force of a mold can be calculated using the following equation.

F = p×A/1000
where, 
F: Required mold clamping force (tf),
p: pressure inside the cavity (kgf/cm2), and
A: total projection area (cm2)

Here, p will have a value in the range of 300 to 500 kgf/cm2. The value of p varies depending on the type of plastic, molded item wall thickness, cavity surface temperature, molding conditions, etc. To be more accurate, it is recommended to incorporate a pressure sensor inside the cavity, and to collect guideline data from actual measured values. Also, A is the total projection area of the cavity and the runner with respect to the parting surface. Therefore, the value of A varies depending on the number of items molded and on the placement of the runner.

Example of a Calculation

Consider calculating the required mold clamping force when four molded items are obtained using PBT plastic with 30% glass fibers added.
Let us assume that the assumptions for calculation are that the pressure inside the cavity is P = 300 kgf/cm2, the projection area of one cavity is A1 = 15.3 cm2, and the projection area of the runner is A2 = 5.5 cm2.

F = p×A/ 1000
= 300×(15.3×4+5.5)/1000
= 20.01(tf)

Therefore, an injection molding machine that has a required mold clamping force of about 20 tf is required. Giving some margin, it is considered optimum to select an injection molding machine with a 25 to 30 tf rating.

This article was composed from notes taken from posts in misumi-techcentral.com

Wear of Moulds

The parts of plastic injection mold wear out due to contact or friction between parts, and in addition, due to glass fibers contained in the plastic resin.
If the shape of the wear exceeds the tolerable range, problems occur such as the mold can no longer move correctly, the mold will be broken, or the shape of the molded product becomes deformed.

The worn out of molds is classified into normal wear and abnormal wear.

Normal wear is the worn out that is caused when parts that touch or slide against each other gradually get worn out.
Although it is technically possible to make it difficult for parts to get worn out, it is extremely difficult to prevent wear fundamentally unless the parts do not contact each other.
Normal wear is classified into initial wear and steady wear.
If the part is replaced with a new part when the steady wear reaches managed scheduled dimensions, it is possible to prevent in advance failures or problems with the molds.

On the other hand, abnormal wear is wear that is not normal wear. There are five typical classifications of abnormal wear.

1. Abrasive wear（abrasive wear）

This is the form of wear that can occur easily when there is a difference in the hardness of the materials that are rubbing against each other. The harder material bites into the softer material generating scratches, and causing wear.

2. Adhesive wear

This is the form of wear in which projecting parts of materials hit against each other causing a part to get adhered, and as a result of growth of the adhered part, it becomes a transfer particle, and eventually falls off as wear dust.

3. Fatigue wear

This is the form of wear in which metal fatigue occurs due to repletion of the application and removal of load (repetition of operation and stopping), causing wear,

4. Fretting wear

This is the form in which wear of fine pitching shape occurs on the surface of mating parts.

5. Corrosion wear

This is the form of wear that is caused when a potential difference is generated between metals in a corrosive atmosphere, thereby causing the sliding parts to disappear, resulting in damage occurring speedily due to the addition of friction wear.

This article was composed from notes taken from posts in misumi-techcentral.com

What is the Moulding Shrinkage phenomenon ?

In the injection moulding of thermoplastic plastics, it is possible to obtain a molded product with the desired dimensions using the mold shrinkage phenomenon. Mold shrinkage is the phenomenon where the volume of the molten plastic filled inside the cavity of a mold is shrinking at the time as being cooled and solidifying.

The extent of this shrinkage is called the "molding shrinkage factor", and if this molding shrinkage factor is known accurately both scientifically and by experience, by preparing the mold making the dimensions of the cavity a little larger by the amount of shrinkage, it is possible to form the molded item by so that it has the intended dimensions.

The value of the molding shrinkage factor is generally a number in the range of about 2/1000 to 20/1000 (about 0.2 to 2%).

If the molding shrinkage factor is expressed by the symbol α (alpha), it can be defined by the following equation 1.

α=(L0−L)／L0 .........(Eq.1)
Where, L0: the cavity dimensions (mm) L: Dimensions (in mm) of the molded product at room temperature (usually 20ºC).

Further the molding shrinkage factor is affected by the following factors.

1. Type of molding material

The range of the basic shrinkage factor is determined by the type of plastic material being used. However, there will be fine differences depending on the material manufacturer and the grade of the material.

2. Cavity surface temperature

The molding shrinkage factor varies depending on the cavity surface temperature during injection molding. In general, the shrinkage factor tends to be large when the temperature is high.

3. Maintained pressure × pressure maintenance time

The molding shrinkage factor varies depending on the magnitude of the pressure maintained after plastic injection and the time of maintaining that pressure. In general, there is trend in the shrinkage factor becoming smaller when the maintained pressure is high and the pressure maintenance time is long.

4. Wall thickness of the molded item

The shrinkage factor also varies depending on the wall thickness of the molded item. There is a trend in the shrinkage becoming larger as the wall thickness becomes larger.

5. Gate shape

The shrinkage factor varies depending on the gate shape and the gate size. In general, there is a trend in the shrinkage becoming smaller as the cross-sectional area of the gate becomes larger. There is also a trend in the shrinkage becoming smaller in the case of a side gate rather than in the case of a pinpoint gate or a submarine gate.

6. Presence or absence of additive materials to the molding material

It is very common that there is a large difference in the shrinkage factor between natural materials and materials having glass fibers. There is a trend in the shrinkage factor being smaller in the case of materials with glass fibers. In actuality, the molding shrinkage factor for mold design is determined by comprehensively investigating the above conditions.

This article was composed from notes taken from posts in misumi-techcentral.com

Common Steels Used in Injection Mould Making

When it comes to injection mold making, choosing the right tool steel can make a huge difference. Making an incorrect choice can cause disasters that fly-in-the-face of many hard hours of work.

Making a poor tool steel choice for your injection mold can mean a cracked core or cavity, causing it to wear out long before it is expected it to.  To help avoid this problem ask yourself these questions before making your tool steel choice:

• How many parts is the mold expected to produce?
• Surface finish of the molded part?
• Are there any shut offs that could wear?
• What cycle time is expected?
• Are there any long cores that there is no way of getting cooling into?
• Will there be thin steel areas venerable to cracking?

When in the process of considering which steel to choose for your injection molding process, there are basically two types to choose from although always there will be your exceptions to the rule.  

·    Steel choices for injection mold making include hardened steel and pre hardened steel.     

The commonly used hardened tool steels will contain S-7, H-13, 420 Stainless Steel, while the pre hardened tool steels are comprised of P-20 and mod pre-hardened stainless steel.

Some specialty type steels are Maraging 300 for toughness and PAS 940 for heat transfer.

Choosing The Right Materials is Critical

As with the tool steels - choosing the right materials for other aspects of your injection mold making processes also need to be considered. Mold material selection is well-known to have a dramatic impact on outcomes.

While having proper materials inevitably improves the design, build and repair processes for specialty injection mold making and timely product delivery, it often also saves both time and money. This is what keeps injection mold making able to remain competitive.

When it comes to tracking, maintenance costs, tooling - and when considering wear resistances, part geometry, cooling and part stability - even cycle times - all of these considerations become essential.

Mold makers and tooling specialists alike agree on the huge impact choosing the right materials can make. Each engineer has their own experience of the risks, factors to consider and scope of results obtained when doing their own evaluations of the success of their injection mold making processes.

Evaluations and materials results can differ from machine to machine and from process to process. Some major manufacturers will swear by the evaluation of their part geometry.

They also consider the cycle time impact and the nuances of part stability from cooling processes and materials selections. Mold material impact is always reinforced with these outcomes and they also have a big impact when it comes to cooling and water channels.

Other Considerations for Tool Steel When Making Critical Choices

Pre-hardened steels are used for the low production tools.  Many times the mold plate is P-20 steel and the molding can be cut solid into the plates.  Areas of the plate could be inserted with hard steel if needed for shut offs or wear surfaces.

Hard stainless steel tooling would be used to minimize corrosion, either from cooling channels or corrosive materials such as PVC.  Stainless steel will crack quicker than other hardened steels and the thermal conductivity is not good.  Stainless steel will not hold a sharp edge.  Stainless steel will be used for high quality surface finish needed to produce lenses and clear parts.

H-13 and S-7 steels are tough materials.  These materials hold up well to wear and constant pressures of injection and the mold closing.  Special care must be taken for corrosion.  Water channels will rust in time.

PAS940 is used for transferring heat.  The material is not very hard so plating is sometimes used to add surface hardness.

Maraging 300 is used for thin steel areas and for strength and toughness.

This article was composed from notes taken from posts in crescentind.com

Fool proof in Mould Design

Literally, the meaning of "fool proof" has the nuance of "prevention of fooling".
In concrete terms, this refers to a construction or shape that prevents wrong assembly or disassembly by humans making an inadvertent mistake.

Taking the example of a mold, consider that there are two core pins A and B that are extremely alike. The position of incorporating in the main core is decided.
If, the mold was disassembled and maintenance work was done on it, there is naturally the possibility that the assembling positions A and B are reversed while assembling again the mold. It is not possible to eliminate completely inadvertent mistakes or misunderstanding even in the case of highly experienced persons.
In view of this, if the flange parts of the core pins are cut in different shapes and the assembly holes of the main core are also machined to those shapes, since it will only be possible to assemble the core pins A and B only at the correct positions irrespective of who does the assembling, there can be no wrong assembly.
These kinds of measures are called "fool proof" measures.

When there are parts that are likely to be assembled wrongly, or when there is a sequence for assembling, it is necessary to make fool proof designs so that there is no inadvertent mistake by the operator while assembling.
If a mistake is made in assembling a large sized mold or a mold for export, carrying out that work again can take a long time. In addition, by clamping a wrongly assembled mold can break the mold which can lead to a huge loss of money, etc.

For judging whether or not fool proof measures are required, it is very important to give training to mold designers not only about simple cost reduction considerations but also about making reviews regarding fool proof measures.

This article was composed from notes taken from posts in misumi-techcentral.com

Merits or Demerits of Standardization for Mould Design and Fabrication

In the world of design and fabrication of molds for plastic injection molding, it can be said that standardization spread relatively faster than other fields of machine design and fabrication.
Misumi has had a role to play in this matter , and we would like to discuss here again the merits and demerits of standardization of design and fabrication by making quantitative comparisons.

Merits of Mold Design and Fabrication

1. The design time can be made short

Because of this, the design delivery time becomes short, and even the design cost gets reduced.

2. It is possible to shorten the machining time, the finishing and adjustment times

Because of this, the component preparation time gets shortened, and even the production cost gets reduced.

3. It is not necessary to increase the number of machines and equipment

Since it is possible to depend on purchasing, it is not necessary to increase the number of machines and equipment within the company. Therefore, the fixed expenses (cost of depreciation) need not be increased.

4. The interchangeability during maintenance gets enhanced

By using standard components, since it is possible to unify the specifications of replacement parts, the interchangeability becomes excellent.

5. Maintenance becomes easy in the case of exports to overseas destinations

At the time of maintaining molds exported to overseas destinations, if the same standard component can be procured locally, it becomes possible to carry out maintenance speedily.

6. The cost of order management can be reduced.

The additional work load of ordering and associated accounting is eliminated, and as a result it is possible to reduce the cost.

Demerits of Mold Design and Fabrication

1. It is likely that it becomes difficult to pass on the basic knowledge or mold design to the freshly recruited designers.

2. The basic component production technology within the company may be lost.

In order to counterbalance the demerits of standardization, there are some companies that carry out design seminars within the company or internal production within the company of parts for repairs.
Considering the advantages and disadvantages and the losses and gains, it can be said that further progress in the standardization of design and fabrication of molds is definitely advantageous.

This article was composed from notes taken from posts in misumi-techcentral.com

Mean Time Between Failures of Molds (MTBF)

In plastic injection molding, when molding operations are being carried out in mass manufacturing, initial failures or random failures can occur at certain probabilities.

The average time after a mold has failed until the next failure occurs is called the MTBF. Therefore, the unit of MTBF used is time (hr, min, etc.) or the number of shots.

When the value of MTBF is large, it can be evaluated that the mold is one in which failure is difficult to occur.
On the other hand, if the value of MTBF is small, it can be said that the mold is likely to fail easily, and it is difficult to make a plan of production of stable molding operations.

Let us consider MTBF in a more realistic example. When several repeated molds are manufactured using the same design drawings, although initial failures and random failures are more likely to occur in the first mold, if technical improvements are made in response to such failures, it will be difficult for such failures to occur in the second and subsequent molds. Therefore, the MTBF of second and subsequent molds will be a large number compared to that of the first mold.

When considering the "production cost + maintenance and management cost" of a mold from it is initially produced until it is disposed off, if the cost of a mold is evaluated based only on the fact that the initial cost (initial production cost) of the mold is cheap, in the end it may become an expensive purchase.
MTBF has a lot of importance as an index for evaluating the maintenance and management cost.

On the other hand, as an index for evaluating the ease of repair of molds, there is what is called the Mean Time To Repair (MTTR).
In case a mold fails, the MTTR of that mold has a small value if it is possible to repair it quickly.
As an example, if spare parts are always in stock, and also if the structure is such that it is possible to replace the spare parts from the parting surface even without removing the mold plate of the cavity, then the MTTR will become a small value.
MTTR, also, has a lot of importance as an index for evaluating the maintenance and management cost.

This article was composed from notes taken from posts in misumi-techcentral.com

Moulds for Plastic Magnets

Plastic magnets are molding materials produced by mixing magnetic powders in a plastic resin or an elastomer. Using injection molding, it is possible to manufacture magnetic molded products with a high degree of freedom in the shape.
Compared to sintered magnets, it is possible to produce products that are lightweight and have shapes with thin walls. However, the magnetic force is larger in sintered magnets.

The following types of magnetic powders are used.

(1) Ferrite type
- Barium ferrites
- Strontium ferrites

(2) Rare earths type - Samarium - cobalt
- Samarium - iron - nitrogen
- Neodymium - iron - Boron

Further polyamides (nylon) such as PA6, PA66, PA12, etc. are used as binders.
In some cases, PPS (polyphenylene sulfide) and PVC (polyvinyl chloride) are also used as binders.

In the case of plastic magnets, in some cases the devices for generating magnetic fields are incorporated in molds for giving polarities to the molded product inside the mold.
There are many methods for this such as having a structure in which permanent magnets are embedded inside the cavity, strong magnetic fields are generated using coils, or receiving the supply of magnetic fields from the molding machine, etc.
Therefore, for all components of molds, it is necessary to separate the use of magnetic materials (steel, nickel, etc.) and non-magnetic materials (stainless steel, copper alloys, etc.).
In addition, since the mold gets worn strongly due to magnetic powders, it is necessary to take measures to enhance the resistance to wear.
Since the molded products taken out stick to each other due to magnetic force, care should be taken in the handling of molded products after they are taken out (making suitable trays, etc.).

This article was composed from notes taken from posts in misumi-techcentral.com

Mould for Food Containers

A number of plastics are used for food containers. PET bottles, food cups, and food packaging are mostly made of plastics.
Quality control of food containers is necessary to ensure that they are hygienic and that they do not have any flash that can hurt fingers or lips so that people can feel safe about consuming food from the containers.
In addition, even quality defects such as holes or cracks are not allowed because bacteria can penetrate through them and cause the food to rot.

Some typical food containers are the following.

- Margarine container: PP
- Ice cream container: HIPS, HDPE
- Lactobacillus beverage (fermented milk drink) container: HIPS
- Pudding container: PS, PP
- Snacks container: HIPS, PP, HDPE
- Soft drinks bottle: PET

Depending on the usage of the food item, the container will have to have heat resistance (for foods that are heated and consumed), low temperature resistance (for foods consumed after refrigerating or freezing), or gas barrier ability (for food items that are to be stored so that they do not come into contact with oxygen).

Since the sales of food containers is influenced greatly by the design (treatment for beautiful appearance) of the molded item, three dimensional curved shapes and designs are given importance. Therefore, three dimensional solid data becomes important in the design of the molded item and in the mold design.
In addition, since there is the risk of flash being generated at the parting surface of the mold, it is necessary to give sufficient considerations to the parting surface.
Similarly, even the position and method of providing gates are also important.

It is necessary that the cavity and the core are made of a steel material having very good corrosion resistance. Also, since it is desirable that the mold is not coated with grease or oil for promoting sliding, the recommended mold structure is one which can operate safely without any lubrication.
Electrical motor driven type injection molding machine is preferable, and it is still better that the molding and machining is done in a clean room.
Valve gates and hot runners can be used. If large volume production is expected to grow, the effect of eliminating scrap and high cycle speeds are extremely advantageous.

This article was composed from notes taken from posts in misumi-techcentral.com

Moulds for Medical Parts

Plastics are used a lot in medical parts. Injection molding is used widely as the method of their manufacture.
Therefore, it can also be said that a substantial part of the medical parts are being manufactured using molds.

Some typical medical parts made of plastic are the following.

1. Cylinders of syringes (injection tube): PP, PE
2. Pistons of syringes: PP
3. Pipette tips: PP
4. Catheters: PVC, PC
5. Blood collection test tubes: PC
6. Petri dishes for culture growth: PS

The plastic materials used for medical parts, depending on the application, can only be used if they satisfy the quality standards of laws related to pharmaceuticals and drugs or of regulations of related governmental ministries or organizations. The use of these materials is limited to plastics that can withstand sterilization processing by irradiation with ultraviolet rays or gamma rays, and to plastics that have passed the clinical approval tests such as blood coagulation reactions or allergy reactions, etc., on human body.
In addition, since these devices are basically of a disposable type, the materials should also be those that do not destroy the environment when disposed off by burning.

It is recommended that molds are manufactured using materials that are resistant to rusting. Therefore, structures preventing corrosion of the cavity and core are used widely such as using stainless steel or ion plating coatings, hard chromium electroplating, etc.
Further, valve gate molds are also used widely for molded products that are manufactured in very large quantities.

Since molded products become quality defects if burrs are present on the periphery of the molded product, precision products are used for the positioning guides on the movable side and fixed side, and also various techniques are adopted for preventing abnormal wear of core pins of various structures.

In order to stabilize the quality of molded products, since it is necessary to design delicately even the mechanisms for the temperature control of cavities and cores, it is necessary to pay attention to the structures of the cooling circuits and heat pipes. The cooling structure on the interior of the core becomes extremely important in the case of the tubes of syringes, etc.

In terms of quality control, removing any dirt or soot adhered to the surfaces of molds is also a very important point. Setting up of gas vents and forced suction structures are also being used.

This article was composed from notes taken from posts in misumi-techcentral.com

Design Concepts of Mould Design

The mould design is an intellectual work of realizing the idea of the mold so that the molded product is within the desired specifications and the cost is within the requested limit, and the items to be investigated can be classified into the following factors.

Functional design

- Maintaining the dimensions of the molded product

- Investigations of the runner and gate structure

- Investigations of the ejection method

Shape design

- Three dimensional shape design

- External appearance quality investigations (glossy surface, graining, etching, etc.)

- Investigation of shape transfer, etc.

Strength design

- Cavity and core deformation, breakage

- Deformation of the mold base, etc.

Thermal design

- Investigation of the thermal shrinkage with respect to the mold

- Investigation of the cooling capacity of the molded product

- Investigation of the temperature controlling means

Safety design

- Investigation of the hook bolt strength

- Investigation of the mold opening prevention structure, etc.

Maintenance design

- Investigation of the maintenance parts structure

- Investigation of spare parts

- Investigation of ease of disassembling and assembling

- Investigation of repeatability

Cost design

- Investigation of the mold cost

- Investigation of the molded product cost

- Investigation of using standard parts, etc.

Mold design is not merely creating a space for producing the shape of the molded product, but it is very important to carry out various studies from the above viewpoints.

This article was composed from notes taken from posts in misumi-techcentral.com

Key points of Maintenance of Moulds

When molds for plastic injection molding are used for molding in mass production, the performance of molds decreases gradually due to wear, rust, etc. Maintenance (maintenance management) is necessary in order to repair such decreases in performance.

It is recommended to pay attention to the following points as the key points in the maintenance of molds for plastic injection molding.

1. Scratches in the pinch off surface of the core pin.
2. Wear in the pinch off surface of the core pin.
3. Fusing of the pinch off surface of the core pin.
4. Depression in the butting surface of the core pin.
5. Scratches and corrosion in the mirror finished parts.
6. Peeling off of electroplating.
7. Wear and deformation of the submarine gate hole.
8. Wear and deformation of the pin point gate hole.
9. Depression in the periphery of the parting surface.
10. Wear of the ejector pin hole.
11. Biting of the ejector sleeve hole.
12. Soot collected in the air vent parts.
13. Biting of the sliding surface of the slide core.
14. Weakening of the coil springs.
15. Warping and deformation f the cavity frame.
16. Cracks in the corner parts of the cavity.
17. Biting of the ejector guide pin and bush.
18. Wear in the nozzle touching surface of the sprue bush.
19. Adhesion of scale and rust in the cooling water hole.
20. Water leakage in the cooling water hole.

Why does a Mould Break ?

Although no unnecessary force is applied to a mold when it is being assembled, when it is actually installed in an injection molding machine and the molding operation being conducted, it is subject to various external forces unlike those experienced during assembly.

For example, the mold clamping force when a mold is being clamped can be from several tons to several hundreds of tons, even several thousands of tons. It is necessary that the mold has enough strength to withstand that compression stress.

In addition, in order to completely fill the mold with molten plastic via the sprue and via the runner, it is necessary to apply pressure to the plastic and make it flow inside the mold. The reason for this is that since molten plastic is a fluid having viscosity, a sufficient pushing force is necessary to make it flow into the mold. The force of the pressure can be 1000 to 2000 kgf/cm2 near the sprue inlet, and even inside the cavity the force of the pressure is 200 to 600 kgf/cm2.

In addition, since the force of the pressure acts for a very short time which is normally not even 1 second, considerable shock is applied to the core pin and the walls of the cavity, and in some cases, this may cause buckling of the thin and long pin.

In this way, if we sequentially look at the process by which the parts of a mold break, we can find the corresponding causes. In order to make sure that a mold does not break, at the time of designing a mold, it is very important to make clear the basic environment of use (injection pressure, mold structure, acting stresses, etc.) in terms of numerical values, and to verify in advance the actual operation of the mold. This is because fatal damages can occur that cannot be covered by fine adjustments after the mold has been prepared if the mold preparation is done without carrying out the strength calculations of the basic structure and while defects are allowed to be present in the structure.

In addition, even when machining the parts of a mold or at the time of assembling and adjustment, it is very important to give considerations to machining after understanding the shapes of the parts, the surface quality, the accuracy of mating, etc. In the case of machining, although the minimum possible responsibilities can be said to have been carried out as long as the work has satisfied the dimensions, accuracy, and tolerances specified in the drawings, in order to make a more superior mold, it is desirable to understand the functions of all the parts of the mold, so as to advance one step further.

In order to prepare molds that do not break, it is very important that there is a balance between the basic concepts and the considerations in machining and assembly.

This article was composed from notes taken from posts in misumi-techcentral.com

Basic Mould construction

The Figure below shows Three-Plate Mould base type with closed position, basically 3 plate type and 2 plate type has some main plate.

1. Fixed Clamping Plate or Top Plate: The function of top plate is to holds the
fixed side of the mould to attached at the fixed platen of the injection machine at this plate will attach locating ring, eye bolt, and sprue bush.

2. Runner Stripper Plate: The runner stripper plate only used in 3 plate moulds type, the function is to cut resin from nozzle in top of sprue bush, and pull the
runner by runner locking pin.

3. Fixed Mold Plate or Cavity plate: The function of cavity plate is to hold
cavity side of product, leader pin, support pin, Puller bolts, and Angular pin when slider attached.

4. Movable Cavity Plate or Cavity plate: It used to attach core side of product, return pin, leader bush and slider core if needed.

5. Back up Plate or Support plate: The function of support plate is to support
cavity plate, attach the hole for return pin's spring, and cooling channel when
in cavity plate can not make it.

6. Spacer Block: The spacer blocks are mounted between the movable clamping plate (bottom plate) and the movable cavity plate to give space and allow the ejector plate to move when ejecting the part and the required length of spacer block depend on ejector stroke that needed to eject product.

7. Ejector retainer plate: The function of the ejector retainer plate is to hold
the ejector, Z pin, shoulder bolts,and give space to ejector leader pin and support pillar.

8. Ejector Plate: The function of ejector plate is to pushes the ejector pins and
return pins, connected with ejector rods.

9. Movable Clamping Plate or Bottom plate: The function of bottom plate is
to holds the movable side of the mold like spacer block, support plate, cavity
plate and ejector mechanism to the movable platen of the injection machine.

Basic Mould Construction

Strength theory of Mould Design

While moulds are precision machines for molding plastic, it is necessary to acquire appropriate strength in order to maintain its functions. In addition, since a mold becomes a heavy load when carrying out the operations of installing it or removing it, sufficient strength has to be acquired so that accidents causing injury to humans is not caused due to sudden problems such as the mold getting deformed or damaged due to its own weight while it is being carried.

In order to do this, it is not sufficient to ensure the strength based merely on experience or intuition, but it is important to carry out mold design using strength calculations based on the mechanics of materials.

In the design of plastic injection mold, it is necessary to utilize the data of the following experiments based on the mechanics of materials.

■ Static strength tests

• Tension test

• Bending test

• Shear test

• Compression test

• Buckling test

• Torsion test

■ Dynamic strength tests

• Impact test

• Charpy impact test
• Izod impact test

• Fatigue test

■ Industrial tests

• Creep test
• Wear test, etc.

Empirical formulae that have been technically established should be used for the calculations related to the strength of materials, and it is necessary to use a safety margin factor regarding the results of calculations.

Further, although it goes without saying, the results of calculations should strictly follow the "ethics of engineers", and it is necessary to take decisions based on good intents.

Since the prevention of mold damage accidents and acquisition of safety depend upon the judgment by the designing engineer, one should always be strict with oneself so that too much weight is not given to designs for cost reduction thereby blunting one's own most important decisions. This is something a mold design engineer should always remember.

This article was composed from notes taken from posts in misumi-techcentral.com

Fail Safe in Moulds or Injection Moulding Machines

Fail Safe is a scheme for acquiring the safety of operators even when a system or an element constituting the system fails, by fixing the state to a predetermined state on the safe side and limiting the effect of the failure, so that no labor disasters occur as a consequence of that failure. (From "Guidelines for achieving fail safe operation in the control mechanisms of metal working machines, etc.", Vol. 464 dated July 28, 1998.

In order to achieve fundamental safety of machines such as molds and injection molding machines, after first accepting the two facts of "1. Machines can fail" and "2. Operators can make mistakes", it is necessary during the stages of design, manufacturing, and modifications of machines in order to build a structure that ensures the safety of operators even if these occur by chance. For this purpose, although a "safety confirming system" is adopted, if the "safety confirming system" itself fails, since the safety of the operators is not acquired, and it is possible that a labor disaster occurs, it is necessary to have characteristics that even when the "safety confirming system" fails, always the equipment is on the safe side (stopping the machine in a state in which it does not cause labor disasters).

In order to achieve fail safe, it is necessary to carry out the design, manufacturing, and modifications of molds, injection molding machines, automatic machines, etc., while following the "fundamental rules for achieving fail safe" listed below.

	1）	As a rule, the following are the control mechanisms that are the target of implementing fail safe. a. Restart preventing circuit. b. Interlock circuits for guarding c. Circuits for sudden stop d. Circuits for emergency stop e. Circuits for preventing exceeding the limits f. Circuits for monitoring operations g. Circuits for hold stop monitoring h. Circuits for speed monitoring i. Hold to run circuits
	2）	As a rule, the control mechanism should be designed to have "asymmetrical error characteristics". Asymmetrical error characteristics are the characteristics by which even when a system or a component constituting the system fails, the frequency of failures on the safe side is far greater than the frequency of failures on the danger side, or the characteristics of failing only on the safe side.
	3）	When adopting electronic control equipment such programmable controllers for the control mechanism, use only those having "asymmetrical error characteristics".
	4）	Danger or fault conditions are not mistakenly reported as safe by making safety information correspond to a high energy state, danger signals and fault signals correspond to a low energy state.
	5）	Safety information should be conveyed in an information conveying mode (unate information conveying) so that, unless safety information is input to the system, an operation permitting signal is not issued by mistake.
	6）	In order to acquire resistance to the maximum environmental noise that can be predicted, the safety information should be made to possess sufficient energy.

A fail safe design philosophy is required during the design of molds that are opened and closed at a high speed during high cycle rates or of molds of large sizes.

This article was composed from notes taken from posts in misumi-techcentral.com

Consideration for Sourcing Injection Mould Tooling for Plastics Components.

When it comes to injection mold building, strong engineering capabilities, technologies and journeymen tool makers provides options and solutions to customers to help mitigate the most difficult aspects of part and mold design.    

It is critically important to work closely with your injection mold builder from the beginning of your project to maintain risk while helping keep the mold build on its timeline as well as on budget.

A quality injection mold that optimizes cycle times, reduces mold downtime will quickly pay for itself during production runs.

Technologies, Equipment and Personnel Achieve High Quality Molds

Tooling engineers utilize technologies to develop 2D and 3D computer generated files of injection molds and mold fill simulation software to create mold designs with proper actions, runners, gates, venting and cooling to achieve reliable and consistent plastic components.  From these 2D and 3D injection mold models detailed tool paths are created for specialized CNC machining. 

CNC high speed machining centers and EDM's are using this software and other advanced technologies to help journeyman tool makers manufacture injection molds with enhanced accuracy, efficiency and precision. This provides an almost unlimited geometry which has become good news for the production of complex plastic components.

An injection mold building tool maker's job  requires advanced training to calculate the feeds and speeds required to make precise cuts with drills, end mills and other sophisticated CNC cutting tools - a job that requires a 4 to 5 year apprenticeship program, and significant on the job training.

Injection Mold Building and Tooling Classifications

The Society of Plastics Industry is responsible for establishing the customs and practices of the injection mold makers industry. This society classifies injection molds to help eliminate confusion and create uniformity between mold types in the mold quoting system which increases customer satisfaction. These classifications are Class 101, 102 and 103.

Class 101 – is built for high production runs of over 1 million or more cycles. This is the highest priced mold and is made with the highest quality materials. Mold base should be a minimum hardness of 280BHN and molding surfaces (cavities and cores) must be hardened to a minimum of 48 RC. 

Steels moving against one another should be dissimilar and have a hardness differential of at least 4 Rockwell. Temperature control provisions are to be in cavities, cores and slides wherever possible.

Class 102 – is built for medium to high production runs - not to exceed 1 million cycles. These molds are good for abrasive materials and/or parts requiring cost tolerances. This is a fairly high quality mold as well as being relatively high priced. 

This mold base should also be a minimum hardness of 280BHN, and molding surfaces (cavities and cores) will be hardened to a minimum of 48 RC. Temperature control previsions should be in cavities, cores and slides wherever possible. 

Class 103 – is built for medium to low production runs of fewer than 500,000 cycles. The mold base should be a minimum hardness of 165 BHN and cavity and cores of 280BHN or higher.

Different Types of Injection Molds

The selection of the type of injection mold needed depends completely on the customer's requirements and product specifications. 

MUD units: these are standard frameworks for tools, allowing for custom machined inserts for specific components.

Unscrewing Molds: these are sometimes used when the plastic parts have details like threads or ridges that cannot be easily injected using the standard knockout methods. These parts are carefully unscrewed from the mold to avoid thread damage.  For this to be cost effective - these complex molds need to move at high speeds and clear previously molded parts efficiently in order to begin the next cycle. These types of molds are good for products like medical syringes, vials, caps and connectors.

Action molds: these molds have some sort of mechanical action incorporated in the design to enable molding of complex parts and detailed geometry like a hole, slot, undercut or thread that is not perpendicular to the parting line of the mold.

Hot runner molds: are typically more expensive to manufacture but allow savings by reducing material waste, cycle time and labor costs.  These types of molds can produce more complex components while providing a variety of gating options that improve part quality.

Three plate molds: these molds have a runner plate in between a moving half and a fixed half.  These molds have two parting lines and are utilized for their gating location flexibility.

Family molds: family molds are used when all the products being manufactured are from the same material. These help to eliminate color matching problems because the products are manufactured at the same time. It is important with these molds to have proper flow balance, runner shutoffs and additional cooling circuit control.

This article was composed from notes taken from posts in crescentind.com