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