How will you estimate Moulding Cycle Time

Molding cycle time

The following formula expresses the total cycle time t for injection-molding.
Formula 19 t = td + ti + tc
Whereas:
td: intermediate time
The sum of the time required to open and close the mold (referred to as the dry cycle in injection-molding), to remove the molded product from the mold, to place inserts in the mold and to apply parting agent.
ti: injection time
The sum of the time required to fill the mold cavity with molten polymer and to replenish the mold with material to avoid voids and sink marks.
tc: cooling time
The time required to coagulate the molten polymer in the cavity and to cool to a temperature and solidify within the mold so that the ejector pins will not cause deformation or strain in the molded product during part release.

Intermediate time

Recent improvements and advances in injection-molding equipment have led to the emergence of machines featuring very short dry cycle times. However, the shorter the dry cycle time, the more thought that needs to go into the material and design of the mold. The impact load increases, so the mold structure must be solid. Also, the molded product should release automatically. With shorter dry cycle times, it is important to use inserts as minimally as possible. In any event, the intermediate time can be accurately forecast based on the molding machine, product and molding materials.

Injection time

Obtain a rough estimate of the polymer filling time using the cavity volume (cm3)nd the injection rate (cm3/sec. Next, depending on the thickness and complexity of the molded product and requirements for dimensional precision, add on time for dwelling to calculate the injection time.
Note that the injection rate of a molding machine is influenced by injection speed controls, cavity wall thickness and shape, gate cross section surface area, material grade, molding conditions (polymer temperature, mold temperature, injection pressure) and more.
While these factors influence injection rate, the injection rate is usually 15-25cm3/sec per ounce in a standard inline screw injection-molding machine. Measure the injection rate after beginning molding, and gather data to assist future estimation efforts.

Cooling time

To estimate the cooling time of an ordinary molded product, use an equation related to the one-dimensional heat in a plane-parallel plate.

Based on the above equation, the relationship between cooling time tc(sec) and the central temperature of the plate at that time θ(°C) can be expressed in as follows.
Formula 20

Whereas:
ℓ: the maximum thickness (m) of the molded product
α: temperature conductivity rate (m2/S) of the polymer
θ: Temperature (°C) of the polymer in its center at tc.
See Table 6.1.
Decide the temperature at which the part can be ejected using the ejector pins, based on the shape of the molded product and the location of the ejector pins, and make that temperature tc.
(Generally, θ is used as the load deflection temperature under a low load (0.45 MPa).
θs: Mold temperature (°C)
θ0: Initial temperature (°C) of the polymer

Table 5.4: Nylon density (25°C)

Type		nylon 6	nylon 66
Name of grade		CM1017	CM3001-N
α : FTemperature conductivity rate(mm2/sec)		0.075	0.060
θ : F	Coagulation temperature°C	195	240
	Cooling temperature (°C) at center of part	185	182

For your reference, Table 6.2 shows the temperature conductivity rates and cooling temperatures for other thermoplastic resins.
Performing calculations using formula 20 provided above can be tedious. For convenience’s sake, use Figure 6.1. For molded products having a thickness greater than 3 mm, use the time required for a spot within the product located an arbitrary distance from its surface to reach θ°C as the cooling time. Doing so will provide a practical means of finding an appropriate cooling time without risking deformation upon ejection of the product from the mold. See Figure 6.2 for a graph expressing the ratio of cooling time using the center of the part to cooling time using an arbitrary distance x from the part surface.

 

Figure 6.1: Estimating cooling time

Table 6.2: Values α and θ in other resins

Material	α(mm2/sec)	θ°C
Polystyrene	0.077	87
ABS resin	0.075	98
AS resin	0.075	98
Polymethacrylate	0.065	90
Polyvinyl chloride	0.068	60
High density polyethylene	0.102	76
Polycarbonate	0.098	148

Figure 6.2: Coagulation of planar plates

Problem 1
Estimate the cycle time when simultaneously molding four CM1017 60φmm discs having a thickness of 3 mm using a three-ounce screw type injection-molding machine. Molding conditions: polymer temperature 250°C, mold temperature 65°C.

Answer: t = 18 seconds, arrived at by referencing Figure 6.1 and Table 6.1.



Problem 2
Using the same molding equipment as in Problem 1, estimate the cycle time required to make the same product using a CM3001-N material. Molding conditions: polymer temperature 280°C, mold temperature 70°C.

Answer t = 23 seconds, arrived at by referencing the above problem and Figure 6.1.


Problem 3
Estimate the cooling time for a 60 mmφ disc having a thickness of 6 mm. Because the thickness is greater than 3 mm, use as a cooling time the time required for a point 1.5 mm from the product surface to reach β. Also, calculate the temperature at the center of the product. The material is CM1017. Molding conditions are: polymer temperature 250°C, mold temperature 70°C.

Answer: tc = 13.4 seconds, arrived at by referencing Figures 6.1 and 6.2.

The temperature at the center at that time would be 237°C; 33.6 seconds would be required for the center to reach 185°C.

Options for Plastics Prototype and Production Parts

PLASTIC PARTS

With Stereolithography (SLA), Selective Laser Sintering (SLS) and Fused Deposition Modelling (FDM) there is considerable choice when prototyping plastic parts. However, with all the hype around 3D Printing techniques it is easy to overlook the more established option of CNC Machining. This remains a very cost effective solution, especially for larger parts and functional materials.

Let's look at the options.

Rapid Prototyping (RP) is ideally suited to handling high component complexity. It provides a fast and economic means of validating such designs before tooling release. 

Despite the growth of RP, CNC machining still plays a significant part in plastic prototyping and low volume production and should not be overlooked as a viable option. Material choices are wider and when combined with our global sourcing, which includes the Far East, the results we can give you are often extremely economically attractive.

Vacuum casting is the first step into tooling; we can offer through this process good detail combined with a range of polyurethanes designed to emulate the mainstream production of plastic parts. Depending on part and materials chased, tool life is typically 15 - 25 off.

Bridge tooling is an extremely viable option when you need to produce plastic parts from production intent material whilst avoiding investment in expensive production tooling. Part quantities can range from low 10's to 1000's off, and can use loose inserts to avoid automation.

Injection moulding still dominates the production of the majority of plastic parts, and whether combined with bridge tooling or full production tooling, offers a consistent high quality source of parts. Usually considered to involve long lead times and high costs, this is not always the case and is really a case of matching the requirements to the type of tooling produced. - this is something we can help you with.

Depending upon the combination of processes and material selected, varying levels of evaluation from basic visuals through form and fit, to full functional verification can be performed.

This article was composed from notes taken from posts in plunkettassociates.co.uk

MMT's December Digital Edition

MMT's December Digital Edition Is Available

This month’s features cover how to keep hot runner system costs down, why thread milling is often faster and more accurate than tapping in tough mold applications, and how the right laser welding formula can lead to efficient mold repair and increased tool life.

Department coverage includes: From the Editor on 2016 Leadtime Leader Award nominations, EAB Insight on survival tools, Your Business on new regulatory impacts, a Profile of Choice Tool & Mold, a mold component Case Study, Pellet 2 Part on thinking in plastic first, a Product Focus on inspection & measurement, our monthly MoldMaking Business Index, medical and automotive/transportation end market reports, a surface treatment tip of the month and a special year in review.

Download it here.

Selection Criteria for Plastics Materials

The selection of plastics material for a specific application is always a challenging task. After careful consideration, the possibilities may be lessened to few and the final selection is then determined by testing. A complete and in detail understanding of material properties, behaviour, flow properties has to be considered. It requires comprehensive knowledge of the part design, process limitations, advantages and disadvantages, success and failure effects with a collective practical significance of design and manufacturing process. Wrong material selection leads to product failure.

In a study of over more than 5000 plastics product failures at Smithers Rapra Technology, the product failures have been classified on the basis of primary failure mode as shown in Figure 1.0. A further breakdown of plastics product failure due to human causes is given in Figure 2.0 of which 45% are due to material mis-selection and poor specification.

Figure; 1.0 Material/Phenomenological causes of failure

Figure; 2.0 Human causes of plastic product failure

It’s crucial for designers and engineers to understand the basic nature of plastics. Poor material selection results into product failures which are very much frequent in the plastics design and engineering due to lack of awareness and understanding of plastics properties.

The most important step in selecting a plastics material from the broad range of available materials (i.e., acrylic, polycarbonate, UHMW, Delrin, nylon, etc.) is to cautiously classify the requirements of the application, the various properties required and the environment in which the material will perform.

There are certain considerations like physical and mechanical properties, thermal and chemical properties, wearing and bearing properties and some standards, which should be used to define the application as completely as possible before selecting a perfect and particular plastics or an entire family of plastics. The more accurately the application is defined, the better the chance of selecting the best material for the exact requirement.

Physical & Mechanical Considerations

• Overall part dimensions (length, width, thickness)
• Load the plastic part carry
• Duration and times the plastic will carry higher loads
• Maximum stress on the part
• Type of stress (tensile, flexural)
• Dimensional shape retention
• Projected life of the part or design

Thermal Considerations

• Temperatures the plastic part see and its duration
• Maximum temperature the material must sustain
• Minimum temperature the material will sustain
• Will the material have to withstand impact at the low temperature
• What kind of dimensional stability is required
• Is thermal expansion and contraction an issue

Chemical Considerations

• Exposure to chemicals
• Duration the plastic might be submerged in water
• Exposure to steam
• Plastic material painted and/or glued? If so, what kind of paint and/or adhesive will be used
• Plastic material exposed to chemical or solvent vapors? If so, which If so, which ones?
• Exposure to other materials that can outgas or leach detrimental materials,
such as plasticizers or petroleum-based chemicals?

Bearing and Wear Considerations

• Will the material be used as a bearing?
• Will it need to resist wear?
• Will the material be expected to perform as a bearing? If so, under what condition?
• What wear or abrasion condition will the material see? If so, under what condition?Materials filled
with friction reducers (such as PTFE, molybdenum disulfide, or graphite) generally exhibit less wear
in rubbing applications.

Standards

• Regulatory requirements
• Is UL94 Flame retardant rating required and at what level (5VA | 5VB | V-0 | V-1 | V-2 | HB)
• Materials color and/or appearance?
• Material be used outdoors
• UV Resistance needed

In order to pre-empt product failure it is strongly advised to make an independent material selection course. Even the selection of the right kind material can be left to the materials supplier which is an alternative, and the advice given is generally of excellent quality but it will certainly be limited to the grades available in their own product range.

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

Techniques of Mould Maintenance

After molding is carried out, it is necessary to carry out maintenance of the plastic injection molds such as disassembling and cleaning, etc. (maintenance and inspection work).
The "soot" constituents generated from the molding material, and the condensed gas deposits, etc. accumulate on the surface of the cavity or core and on the parting surface.
Although these constituents are still in the liquid state when the mold is still at a high temperature, but when the mold cools down they solidify, moisture content in air gets adhered because of these, eventually causing the generation of rust and corrosion of the mold.
When this state is reached, the surface quality of the molded item decreases, there will be fluctuations in the dimensional accuracy, insufficient air escape, which causes the generation of short shots.

The frequency of maintenance varies widely depending on the status of quality management of the molded item, and on the size of the mold, etc.
In the case of molds with short periods, it is necessary to carry out mold maintenance once in a few days, and about once in two months in the case of mold with long periods.

A common method of maintenance is to disassemble the mold, cleaning each of the part by ultrasonic cleaning, or by rinsing in an organic solvent, and to remove the rusted part by polishing and then electroplating.
In the case of parts having a lot of rust, the parts are replaced, or the inserting part is corrected.

It is necessary to modify tools, jigs, and equipment in order to carry out maintenance efficiently.
Cleaning brush, disassembling tools, wooden tools, cloth jigs, buffing tools and sand paper, polishing cream, lapping material, special chop sticks, bamboo spatula, bamboo comb, air tools, crane supplementary tools, Z light, magnifying glass, etc., need to be modified.

Even the work bench needs to be modified so that the work becomes easy. Modifications are to be done about the work bench height to improve ease of carrying out the work, walking space, crane position, air gun piping, etc.
It is also useful to use a table or white board for spreading the drawings, a digital camera, and a video camera.

In order to increase the efficiency of maintenance, it is necessary to give considerations from the point of designing the mold.
Holes are provided so as to facilitate disassembly of the mold, screws for hook bolts are provided, and supplementary screw holes are provided for assembling.
The cores should have the frame block construction so that they do not come apart, and it is also effective to use a construction of fixing using keys.
Very often the placement numbers of core pins are engraved, and tag cuts are made in order to fix the directionality.

Mould Design Considerations - Hot Runner Application.

Plates Requirement

Manifold and Back Plate Material 
• High strength material must be used for the plates.
• Minimum plate material is 1.2311/1.2312, 30HRC, 800MPa Yield Strength.

Manifold and Back Plate Thickness
• The minimum recommended back plate thickness is 40mm which should increase as plate sizes increase.
• The minimum manifold plate material required below the nozzle head is 30mm. The material below the nozzle head
should increase as the manifold pocket size increases to ensure system rigidity.

Clamping
• Sufficient clamping between the back plate and manifold plate is required to prevent the plates from being forced apart through thermal expansion of the hot runner system and to resist injection pressure.
• A minimum of 2 bolts per drop positioned as close to the drop positions as possible.
High tensile grade bolts (12.9) must be used and sized according the system size.

Minimum Plate Size
• Sufficient material is required around the manifold pocket (Dimension A) to ensure the rigidity of the system.
• 75mm of material around the pocket is required to give sufficient space for bolts, wire slots, guide pillars. This also maintains the system strength.

Central Supports
• To ensure rigidity of large systems - pillars may be required through the manifold to provide additional
bolting close to the sprue bush.
• These integral supports reduce the risk of back plate bowing over large cavity pockets, or the hot runner system forcing the plates apart due to thermal expansion.

Venting
• Vent slots must be machined into the manifold plate to ensure any moisture from condensation is released from the
manifold pocket and nozzle detail.
• 1mm x 16mm slots are recommended as depicted in the diagram below.

Deformation of Mould Components

After the molds are used actually for injection molding for some time, the mold components that use carbon steels and alloy tool steels and that have been heat-treated may generate warps, deformations, or their dimensions may have increased slightly.
The phenomenon of changes occurring in the dimensions after the passage of time in this manner is called "aging".

The main cause of changes with time occurring in carbon steels of alloy tool steels is known to be the expansion in volume caused when the austenite structures remaining during quenching changes to martensite structures.

The process of quenching is that of changing the austenite structure to the martensite structure by suddenly cooling from the quenching temperature (about 800°C, but this varies depending on the type of steel).
While the sudden cooling is done using water, oil, or a salt bath, although the conversion to martensite is promoted when the temperature is less than 0C, if the temperature is not that low, a small quantity of austenite structure remains within the martensite structure. This remaining part is called "remained austenite".
It has been known that this remained austenite gradually changes to martensite structure with the passage of time as mentioned above, and at that time the volume also expands. As a consequence, if the quantity of remained austenite is large, it can be assumed that the trend is that of a large change with time.

In order to reduce the remained austenite, it is effective to carry out deep cooling treatment (subzero treatment) by creating a low temperature environment of about -80°C using a coolant such as Freon (the user of Freon cannot be recommended due to environmental protection), and to carry out sudden cooling in that environment.
If liquid oxygen or liquid nitrogen is used, although it is possible to cool down to -180°C to -190°C, since the cost becomes high this is only done under special cases.

Therefore, in the case of components that should not undergo changes with time, it is recommended to carry out subzero treatment.
However, although the remained austenite would have become small in a subzero treated steel, at the same time, since even the hardness would have increased and the internal strain would have become large, appropriate annealing processing will be required. (There are recommended values for the conditions of annealing depending on the type of steel.)

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

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