Plastic Parts Design

The nature of design analysis obviously depends on having product-performance requirements.

Even though many potential factors can influence a design analysis, each application fortunately usually involves only a few factors. Here we explain some of the factors:

Geometrical Shape:

There are different techniques that have been used for over a century to increase the modulus of elasticity of plastics. Orientation or the use of fillers and/or reinforcements such as RPs can modify the plastic. There is also the popular and extensively used approach of using geometrical design shapes that make the best use of materials to improve stiffness even though it has a low modulus. Structural shapes that are applicable to all materials include shells, sandwich structures, and folded plate structures. These widely used shapes employed include other shapes such as dimple sheet surfaces. They improve the flexural stiffness in one or more directions.

In the discussion of uniform wall thickness, the ribbing was one of the suggested remedies. Ribs are also used to increase load-bearing requirements when calculations indicate wall thicknesses are above recommended values. They are provided for spacing purposes, for supporting components, etc. The first step in designing a rib is to determine dimensionally limitations followed by establishing what shape the rib is to have in order to realize a product with good strength and satisfactory the appearance that can be produced economically.

If performance calculations indicate wall thicknesses well above those recommended
for a particular material, one of the solutions to the problem is to find equivalent cross-sectional properties by ribbing. Heavy walls can be responsible for the reduction in properties due to poor heat conductivity during fabrication, thus creating temperature gradients throughout the cross-section, and thereby causing residual stresses. Cycle times are usually longer, thus adding another potential cause for stresses when using too short a cycle time. Also, close tolerance dimensions are more difficult to maintain, the material is wasted, quality is degraded, and material and processing costs are increased.


Example of using ribs and changing the weight part [1]

Reference: Plastic Design Handbook, DOMINICK V. ROSATO , DONALD V. ROSATO,  MARLENE G. ROSATO

Plastic Processing Data

Processing means setting up different parameters to produce a part which has all dimensions and tolerances per drawing and also meets the quality requirements. Process needs to be optimized to give us a part with minimum cycle time and best quality. First thing to start will be material properties (Physical, Mechanical and Thermal) which are provided by material supplier. These data can be used as guidelines for setting parameters on injection machine.

Material Properties

  • Material 
  • Melt Temperature (  or ° F)
  • Mold Shrinkage  (%)
  • Specific Gravity  (unitless)
  • Coefficient of Thermal Expansion, (in/in)/°F
  • Drying Temp (  or ° F)
  • Water Absorption % (24 hr@ 73 °F)
  • Heat Deflection Temp @264 psi
  • Specific Heat btu/lb/°F
  • Injection Pressure on Material (PSI)
  • Back Pressure on Material (PSI)
  • Melt Flow Index (g/10 min)

Process Setup Sheet

  • Press tonnage (tons)
  • Barrel Zones Temperature (°F)
  • Screw RPM
  • Gates Temperature  (℃  or °F)
  • Nozzle Temperature (°F)
  • Mold Zones Temperature (°F)
  • Clamp Force (ton)
  • Filling Time (sec)
  • Hold Pressure 
  • Cooling Time (sec) 
  • Injection Pressure (PSI)
  • Cushion (in)
  • Injection Speed (in/sec or mm/sec)
  • Ejector speed (in/sec or mm/sec)

Clamping Concepts

The injection molding machine clamp is used to close the mold, hold it closed during the injection and curing of the plastic material, and open the mold for the removal of the formed part. There are 3 different types of clamp design:

  1. Straight hydraulic clamp
  2. Linkage or toggle clamp
  3. Hydromechanical clamp

1- Straight hydraulic clamp

This design uses hydraulic fluid and pressure to open and close the clamp and to develop the force required to hold the mold closed during the injection of plastic. The basic concept is to direct hydraulic fluid to the booster tube to move the clamp ram forward. Oil fills the main area by flowing from the tank through the prefill to the main area. as the ram moves forward, a slight vacuum is developed in the main area, pulling fluid from the tank into this chamber. Once the clamp is closed, the prefill valve is closed, trapping the oil in the main cylinder area. High-pressure fluid is put into this area, compressing in this area. The maximum pressure is controlled by a pressure control valves, which closely controls the clamp tonnage ( the max. hydraulic pressure times the area it pushes against.)

To open the clamp, hydraulic fluid is directed to the pullback side of the cylinder while the prefill valve is open, with fluid from the main cylinder being returned to the tank. One of the major advantages of the straight hydraulic clamp is its very precise control of the clamp tonnage.

2- Linkage or Toggle Clamp

This concept uses the mechanical advantage of a linkage to develop the force required to hold the mold closed during the plastic injection portion of the cycle. Normally the linkage design is done in such a way that slowdowns are built in. The advantage of a toggle clamp is that less hydraulic fluid is required to open and close the clamp. A disadvantage is that the clamp tonnage is not precisely known.

A small hydraulic cylinder travels at a constant speed with the slowdown for mold close built into the linkage. The mechanical advantage of the linkage is extremely high so the relatively small closing cylinder can develop high tonnage.

3- Hydromechanical Clamp

This design uses a mechanical means for high speed close and open. A short stroke cylinder is used to develop tonnage identical to the straight hydraulic design. This concept is said to offer the advantage of toggle clamps for high-speed close and open, and the advantage of a straight hydraulic for precise control of the clamp tonnage. The hydromechanical design normally has a high-speed clamp close and open device which is usually a hydraulic cylinder or actuator. The closing and opening mode occurs with relatively low force. Once the clamp is closed, a blocking action takes place allowing a large -diameter hydraulic cylinder to build tonnage similar to the straight hydraulic design.

When the clamp is to be opened, the blocking member is removed, and the clamp opens rapidly. The blocking action is normally a mechanical device,  and the tonnage action is done by hydraulic; hence the name hydromechanical.

Comparison of Clamp Designs

Over the years many arguments have been presented showing each clamp design concept to be superior to the others. In reality, each concept has merit.

The straight hydraulic design has proved over the years to have long-term reliability, excellent control of low-pressure mold protection, and exact control of tonnage, and it will not allow the clamp t be overstressed due to high injection forces.

The toggle clamp has extremely fast closing and opening actions and is typically lower in cost than the straight hydraulic, but this energy is small compared to the total energy usage of the machine. With good lubrication, the toggle bushings and pins last well, but they still must be reworked after several years of service.

The toggle design will also develop higher than lockup tonnage if the clamp is overpowered by the injection end, or due to temperature buildup in the mold. The hydromechanical tends to have the advantages of the straight hydraulic, whereas the toggle is more complex because of the block action required. The debate over the three clamp concepts will continue for many years.



Standard For Using Interchangeable Molds

The society of plastics industries has published standards in regard to mounting molds in the injection molding machine. The obvious advantage of these standards is that they easily allow molds to be designed to run on more than one brand of machine. These standards are listed here for descriptive purposes.

1- Platen Bolting Pattern

These standards specify the location and size of tapped holes in the stationary and moving platen for the attachment of the mold. They cover 3 sizes of machines:

  • Up to 750-ton clamping capacity
  • Overy 750-to 1600-ton clamping capacity
  • Over 1600- to 4100-ton clamping capacity

2- Knockout Pin Locations

Knockout pins are used to push the molded part of the mold during the portion of the machine cycle called ejection. Since the location of these pins is critical to the mold design, the society of plastics industries has written a standard for locating the knockout pins. The location and size of the knockout pin holes are classified according to machine sizes identical to those used for mold mounting holes standards.

3- Machine Nozzle and Die Locating Ring

The die locating ring is used to align the mold to the machine platen. This alignment is needed for the knockout pins and for the injection unit to line up with the mold. The machine nozzle provides the mating necessary between the injection unit and the mold.