1. Injection Molding Molds
Professor Joe Greene
CSU, CHICO

2. Introduction Background Concept is simple
Melt plastic, flow into mold and take part shape, cool, demold
Injection molding makes parts in discrete (discontinuous) process
More injection molding machines used for plastic processing than any other equipment
Almost all thermoplastic and some thermosets materials can be injection molded
Process is automated and highly repeatable parts
Injection molding parts are finished with little post molding operations
Very complex parts can be made
Machines are expensive
Molds are expensive, usually P-20 steel

3. Injection Molding Process

4. Injection Molding Operations
Fountain Effect Flow
Hot resin flow from the middle of the flow channel to the walls and cools

5. Injection Molding Operations
Cycle Time ·Injection Pressure

6. Injection Pressure Graphs

7. Injection Molding Thermal Process
Temperature History in part

8. Injection Molding Process
Fill time
How long it takes to fill part. Faster filling rate = shorter fill time
Volume of part divided by volumetric flow rate
Note: Pressure is a function of the flow rate. Faster flow rate = higher pressures, except at very slow fill which causes larger core and smaller flow channel and then higher pressures.

9. Clamping Unit Clamping Force
Clamping unit holds the molds together while the resin is injected, packed, and cooled, and ejected.
Clamping force is the rating of the injection molder, e.g., 150 tons clamping force.
Clamping force = Injection Pressure x Total Cavity Projected Area
Projected area is the area projected into a single plane, that is, the widest area of the part.
Examples
The force necessary to mold a part that has 100 in2 projected area and has 3,000 psi is 3,000 * 100 = 300,000 lbs force = 150 tons (note 1 ton = 2000 lbs)
The maximum projected surface area of a part on a 200 ton machine with a maximum injection pressure of 2,000 psi is: 400,000 lbs force / 2,000 psi = 200 in2

10. Plastics Design for Injection Molding
Part Design
The underlying principles behind part design, other than part functionality are
cooling of plastic from melt to glassy state
heat transfer from various sections
thermal shrinkage of the plastic parts
Heat transfer is best when the parts have the same thickness.
Inside portions of parts cool more slowly than the part surfaces
Center portion will shrink more than the surface

11. Cavities Number of cavities
The number of cavities depends on the available production time, product quantity required, machine shot size and plasticizing capacities, shape and size of the moldings, and mold costs.
Number of cavities
Product Quantity: If the dimensional tolerance of the part is not very critical and a large number of moldings is required.
Machine shot capacity: Number of cavities = S / W

12. Injection Molding Molds
The mold includes the shape of the part and is located between the stationary and movable platens of the injection molding machine
Key terms
sprue bushing- part of mold (cooled)
nozzle- end of injection (heated)
sprue channel- from bushing to runner
runners- feeds material from sprue to part
gate- mold area between runner and part
mold cavity- concave part of mold
mold core- convex part of mold
multi-cavity- more than one part in a cavity
ejectors- knock out pins
mold inserts- multiple cavities for same base
mold base- inserts used in same base
MUD base- Master Unit Die
draft angle- minimum angle from bottom to top of part
parting line- the split between core and cavity molds

13. Mold Timing and Terminology
Dry-cycle- Total Time required for the clamp to close and open (sum of mold opening and closing time).
Opening Time- Time it takes to open mold and usually eject part
Closing Time- Time it takes to close mold
Mold Protection- Halts molding operation if a foreign material (dirt, flash, debris) is detected in mold cavity
Mold Open- Time the mold is actually open
Mold Closed- Time the mold is closed and usually includes
Injection Time, Hold Time, Cooling Time
Ejection Time- Time is takes to eject parts and is a part of Mold Open Time

14. Injection Time Injection Time and the Machine
Faster a machine can inject the faster the injection time.
All machines are rates in the number of cm3 PS per second
Injection Time and Mold Design
Pressure drop in the runner system from nozzle to the gate in the cavity
Smaller the pressure drop, the faster the mold will fill.
Larger runners yield smaller pressure drop, but take longer to cool.
In hot runners, valve gates can provide large passages
Number of cavities
More cavities, the longer it takes to fill
Product Shape
Long flow lengths cause longer injection time
L/t Ratio
Ratio of flow length to thickness. Keep less than 200 (high Press)

15. Sprue Guidelines
The sprue must not freeze before any other cross section. This is necessary to permit sufficient transmission of holding pressure.
The sprue must de-mold easily and reliably.
Dco  tmax mm
Ds  Dn mm
  1º - 2º
tan  = Dco - Ds / 2L

16. Injection Molding Runners
Distribution system for the resin from the sprue to the cavities.
Runners must be large enough to fill the cavities even with the resin cooling along the runner walls.
Materials with high viscosities (low MFR) runners should be large enough to prevent gate freeze-off, e.g., PC has a large viscosity and should have a larger runner than nylon which has a low viscosity.
Runners should be designed to minimize shear, e.g., acetal can decompose if subjected to excessive shear.
Length of runner should be minimized
When secondary runners are used as in multi-cavity molds, the flow should be streamlined (Figure 21.7). Streamlining minimizes shear.
When the same part is made in a multi-cavity design the runners should be balanced and fill at the same time with similar pressures. (Figure 21.8)
When cavities are producing different parts, balanced runner design is more complicated and flow analysis should be used.

17. Runner System Several types of runners single part runner
multiple part runner
symmetrical runner
non-symetrical runner
runner-less designs with hot manifolds

18. Runner System Runner size considerations
Although properly sizing a runner to a given part and mold design has a tremendous pay-off, it is often overlooked since the basic principles are not widely understood.
Pros and cons of large runners
While large runners facilitate the flow of material at relatively low pressure requirements, they
require a longer cooling time, more material consumption and scrap, and more clamping force.
Pros and cons of small runners
Designing the smallest adequate runner system will maximize efficiency in both raw material use and energy consumption in molding. At the same time, however, runner size reduction is constrained by the molding machine's injection pressure capability.

19. Runner System
Runner Balancing is an essential for a balanced filling pattern with a reasonable pressure drop.
Payoffs of good runner design
A runner system that has been designed correctly will:
Achieve the optimal number of cavities
Deliver melt to the cavities
Balance filling of multiple cavities
Balance filling of multi-gate cavities
Minimize scrap
Eject easily
Maximize efficiency in energy consumption
Control the filling/packing/cycle time.

20. Runner Guidelines Common runners Full-round runner Trapezoidal runner
Modified trapezoidal runner (a combination of round and trapezoidal runner)
Half-round runner
Rectangular runner

21. Runners
Shape
Round channels give best flow characteristics but are difficult to machine.
Semicircular channels are less expensive but should have these rules:
depth of channel is at least two-thirds the size of the width
sides are tapered 2º to 5º.

22. Hot Runner System
The ideal injection molding system delivers molded parts of uniform density, and free from all runners, flash, and gate stubs.
To achieve this, a hot runner system, in contrast to a cold runner system, is employed. The material in the hot runners is maintained in a molten state and is not ejected with the molded part. Hot runner systems are also referred to as hot-manifold systems, or runnerless molding.
FIGURE 1. Hot runner system types: (a) the insulated hot runner, (b) the internally heated hot-runner system, and (c) the externally heated hot-runner system

23. Gates
Gate is the end of the runner and the entry path into the cavity.
Shape of gate strongly affects the ease with which the part is removed from the runner system.
Gate is the most restricted point in injection path and first to freeze-off
Designs
Edge gate: small rectangular opening at end of runner channel. Low cost gate.
Submarine gate: starts from the edge of the runner and goes into cavity edge at an angle. Gate is sheared off at part ejection. Creates high shear.
Tab gate: formed by connecting the runner directly into the cavity with no reduction in runner cross section. Used for large parts.
Fan gate: made by reducing only the thickness and not the diameter of the runner channel as it goes into the cavity. Used for parts of intermediates size and when reinforcements can’t flow though edge gate.
Ring gate: Used to make hollow cylinder parts. Covers the entire top of the cylindrical part so the resin flow is downward into the walls of the part. Used to minimize weld lines.

24. Gate System
Several types of gates
rectangular simple gate
fan gate

25. Gate Design Gate Design Overview Single vs. multiple gates
Single gate is usually desirable because multiple gates have weld lines
Gate dimension
The gate thickness is usually two-thirds the part thickness.
The gate thickness controls packing time
Chose a larger gate if you're aiming for appearance, low residual stress, and better dimensional stability.
Gate location
Position the gate away from load-bearing areas.
Position the gate away from the thin section areas, or regions of sudden thickness change to avoid hesitation and sink marks

26. Gate Design Gate Design Overview Gate Types Manually trimmed
Requires an operator to separate parts from runners during a secondary operation
Types include sprue, tab, edge, overlap, fan, disk, ring, film, diaphragm, spider
Automatically trimmed gates
Automatically trimmed gates incorporate features in the tool to break or shear the gate
Should be used to
Avoid gate removal as a secondary operation
Maintain consistent cycle times for all shots
Minimize gate scars
Types include Pin, Submarine, hot-runner, and valve

27. Gate Design Design Rules Gate location Gate Length
Should be at the thickest area of the part, preferably at a spot where the function and appearance of the part are not impaired
Should be central so that flow lengths are equal to each extremity of the part
Gate symmetrically to avoid warpage
Vent properly to prevent air traps
Enlarge the gate to avoid jetting
Position weld and meld lines carefully
Gate Length
Gate length should be as short as possible to reduce an excessive pressure drop across the gate. Ranges from 1 to 1.5 mm (0.04 to 0.06 inches)
The gate thickness is normally 50 to 80 percent of the gated wall section thickness. Pin and submarine gates range from mm (0.01”- 0.08”)
The freeze-off time at the gate is the max effective cavity packing time.
Fiber-filled materials require larger gates to minimize breakage of the fibers

28. Boosting structural integrity with ribs
Structural integrity: the goal of every design
The major component of designing for structural integrity, in many cases, is to design the structure to be stiff enough to withstand expected loads. Increasing the thickness to achieve this is self-defeating, since it will:
Increase part weight and cost proportional to the increase in thickness.
Increase molding cycle time required to cool the larger mass of material.
Increase the probability of sink marks.
Well-designed ribs can overcome these disadvantages with only a marginal increase in part weight.
Typical uses for ribs
Covers, cabinets and body components with long, wide surfaces that must have good appearance with low weight.
Rollers and guides for paper handling, where the surface must be cylindrical.
Gear bodies, where the design calls for wide bearing surfaces on the center shaft and on the gear teeth.
Frames and supports.

29. Ribs Design Design Rules
Keep part thickness as thin and uniform as possible.
This will shorten the cycle time, improve dimensional stability, and eliminate surface defects. .
If greater stiffness is required, reduce the spacing between ribs, which enables you to add more ribs.
Rib geometry
Rib thickness, height, and draft angle are related: excessive thickness will produce sinks on the opposite surface whereas small thickness and too great a draft will thin the rib tip too much for acceptable filling.
Ribs should be tapered (drafted) at one degree per side.
Less draft can be used, to one-half degree per side, if the steel that forms the sides of the rib is carefully polished.
The draft will increase the rib thickness from the tip to the root, by about mm per centimeter of rib height, for each degree of draft angle.
The maximum recommended rib thickness, at the root, is 0.8 times the thickness of the base to which it is attached.
The typical root thickness ranges from 0.5 to 0.8 times the base thickness.

30. Recommended Design Parameters.
See Figure 1 for recommended design parameters.

31. Ribs Design Rules Location of ribs, bosses, and gussets
Ribs aligned in the direction of the mold opening are the least expensive design option to tool.
As illustrated in Figure 1, a boss should not be placed next to a parallel wall; instead, offset the boss and use gussets to strengthen it.
Gussets can be used to support bosses that are away from the walls. The same design rules that apply for ribs also apply for gussets.
Alternative configurations
As shown in Below, ribs can take the shape of corrugations.
The advantage is that the wall thickness will be uniform and the draft angle can be placed on the opposite side of the mold, thereby avoiding the problem of the thinning rib tip.
Honeycomb ribbing attached to a flat surface provides excellent resistance to bending in all directions.
A hexagonal array of interconnected ribs will be more effective than a square array,with the same volume of material in the ribs.

32. Ejection System
Ejection system is driven by power system separate from the one that opens and closes the mold.
The ejector plate, to which the ejector pins are attached, advances after the mold has been opened.
Ejector pins move through the support plate and the B plate and into the back of the cavities to push parts out of cavities.
Stripper plate mold is used when ejector pins are not used to push part out of cavity by moving against a feature in part.
Bases can allow for complicated features
Slides or lifters when have undercut sections in part which have to move out of the way before mold can open.
Threaded features.
Machine hydraulics drives slides and lifters.
Cooling is achieved through channels in cavities. Important

33. Injection Molding Molds
Mold Bases (Figure 21.4)
Built to have one parting line for one mold opening
Built to have more than one mold opening- stacked molds
Used when relatively small parts are made with large machine
Parts are created on two mold plates
Like having two or more molds running simultaneously
Mold bases mounted to platens by mold clamps or die bolts
Locating ring assists in aligning the sprue bushing with the injection machine
Bases hold two plates
Front Cavity (A plate)
Rear Cavity (B plate)
Resin flows through sprue cavity and reaches a pin that is inserted into B plate when the mold is closed.
Pin is mounted on ejector plate and ensures runners stay on B side
Several types of sprue end types as shown in Figure 21.5

34. Lifters and Slides UNDERCUTS
(ref:
Undercuts are those portions of the part that can not be pulled in the line of draw. If an undercut were to be machined into a mold without a mechanism to relieve it, the part would be destroyed upon mold opening or ejection.
Slides are usually used for relieving external undercuts or to allow for zero degree draft on part exteriors.
Their motion can be driven by one of several mechanisms. Any slide mechanism must also employ a locking device to hold the slide against the ravages of molding pressures in excess of 10,000 psi
Undercuts can usually be divided into two categories, internal and external.
SLIDES are used to pull external undercuts and
LIFTERS to pull internal undercuts.

35. Peckerslide
Referred to in prudish circles as a cam pin or angle pin, this is the most common and versatile of all slide mechanisms.
It is driven by the opening of the press. The action of the angled pin withdrawing from the angled hole drives the slide back.
Springs hold the slide in the retracted position. Upon press closing the pecker pin returns to drive the waiting slide back almost to molding position.
Finally then the lock seats against the back of the slide, driving it to its home position and clamping it there for the duration of the next shot.

36. Lifters
Lifters are used primarily for relieving internal undercuts or for zero draft internal faces.
Their motion is driven by the molding press pushing on the ejector plates. A simple side action lifter withdraws at 90 degrees to the line of draw, simultaneous with ejection.
The angle of the lifter passing through the core frequently confuses the novice into thinking that it pulls the feature at an angle.
Relative to the core the lifter indeed moves at an angle.
Relative to the part however a lifter moves directly sideways because the part is being ejected forward at the same rate as the lifter.

37. Mold Materials
P-20 Pre-hard steel. Rc The most common tool steel used, it is a favorite among mold makers for its durability and machining qualities.
H-13 Air hard steel. Rc Great for high production cavities and cores, slide bodies, lifters, and gibs.
S-7 Air hard steel. Rc Used for many of the same applications as H-13 however somewhat less stable and more prone to cracking. Great for long wearing gibs and guides.
M-2 High speed steel. Rc used where tough rigidity is required such as tall thin core pins or blades.
420 SS stainless steel. Rc Best for achieving high polish finishes. Also for corrosive polymers such as PVC.
Beryllium Copper. Primarily used for inserting small areas in tools that may be prone to over heating. The thermal characteristics of beryllium copper make it the best choice for wicking heat out these areas.
7075 T-6 Aluminum. Generally used for short run or prototype tooling, however the hardness, wear resistance and ability to take a good polish has caused many a short run mold to outlast its projected life.
Lamina. "Lamina" is a name brand material that is used for wear plates. This material is a bronze over steel laminate that imparts long life to moving parts with a minimum of lubrication. In addition to wear plates it is commonly used for large locking surfaces.