Benefits of Dimensional Testing in Plastic Injection Molding

Dimensional Inspection Equipment:

  1. Hand Tools – Consist of a simple inspection equipment to measure dimension.
  2. Optical comparators – This equipment use optic (light) projections on a photographic screen to measure dimensions
  3. Structured light scanners – Used with components where manual measurement is not possible
  4. Laser scanners – Laser based non-contact inspection method
  5. Vision System – Applicable where high accuracy and precision measurements are necessary. These systems are capable of reliable and quick measurements.
  6. Coordinate measuring machine – Advanced measurement technology, which senses discrete points on components to measure its dimensions. The following sections detail the working of a CMM machine and its benefits.

What is CMM

While manufacturing a component, dimension and geometry inspection is an important step. Using conventional methods and measuring instruments such as micrometers and depth gauges can give rise to open errors, which can potentially jeopardize the production process. 

Whereas, a CMM machine on the other hand uses well automated and pre-defined inspection methods, resulting in accurate measurements. Therefore, studies have revealed that using a CMM (Coordinate Measuring Machine) shall minimize the errors largely.

CMM employs coordinate technology to measure the dimension of a manufactured part using a probe to sense discrete points on the components’ external surface. Probe position is controlled by a computer or manually by an operator. It gathers the measurement value with reference to the pre-set three-dimensional Cartesian coordinate system. Probes are even capable of measuring inaccessible locations on the parts’ surface.

CMM Machine Components

  1. Mechanical probe or stylus: An important component of the CMM machine to measure dimension. The probe is resistant to temperature to give accurate and precise dimensional value. Ruby and zirconia are commonly used probe materials.
  2. Main Body or Table: The table is a robust and stable structure to support the CMM machine. Granite is often considered a suitable material for the CMM Table due to its resistance to temperature and other forces.
  3. Fixtures: Fixtures are used to mount and hold a component rigidly during the measurement. Rigid fixtures are therefore necessary to eliminate dimensional errors in measurement.
  4. Software: The probe movement with respect to a defined reference is analyzed by the software, which provides accurate measurement data. This software needs to be calibrated and updated in order to display accurate measurements. Polyworks and PC-DMIS are generally accepted as industry standard for CMM Measurements.

Types of coordinate measuring machine

Though advancements have been made to enhance the functioning of a CMM using various technologies, four CMM machine types are globally accepted for their reliability and measurement accuracy:

  1. Bridge Type CMM: The most stable type of all the four CMM types. Based on an application, Bridge type can either be a fixed and portable unit. Portable bridge types can provide inspection close to the machine.
  2. Gantry Type CMM: Used for the inspection of large parts. This type of CMM eliminates the need of lifting the parts onto a table as floor serves as the datum for the machine. Cost of this type of CMM is more when compared with other CMMs.
  3. Cantilever type CMM: As the name indicates, this type is fixed at one end overhang from another end, making it less rigid than other types with two-point support. It is limited to use only for small parts and allows access from all three sides.
  4. Horizontal Arm CMM: This type fixes their probes horizontally rather than vertically, as with other types. It is used for long and slender parts, which is out of range in machines. Generally, it is less accurate than other CMMs.

Benefits of CMM Dimensional Testing 

CMM,with its unique technology can save valuable time and cost for plastic injection molding companies. With the complexity in part design, precise and accurate measurement becomes essential. Utilizing CMM for injection molders will bring good results for a company with remarkable authentication.

  1. Saves time and cost – Using CMM for the measurement of component inspection reduces production costs and saves time in longer period.
  2. Guaranteed QA- Several benefits such as CAD comparison and reverse engineering with reliable results assist to achieve Quality Assurance requirements.
  3. Versatile – CMM machines measures all complex components and it is compatible for different sizes of components. A variety of probes and techniques can be integrated into CMM as per application.
  4. Less Labor – As CMM is a computer-controlled machine, it reduces manual involvement, minimizing human error.
  5. Accurate Results – Single reference point of components is w.r.t. to origin leads to reduction in measurement error.
  6. Flexibility – One fixture is sufficient for the mounting of various sizes and shapes.
  7. Hardware Life – Proper maintenance of CMM machine ensures longer life of components.
  8. Easily upgradeable – Only updating software system whole machine is upgraded.
  9. Cost-Effective – Though the initial investment is higher compared with conventional measuring techniques, ROI (return on investment) is lower for CMM. CMM benefits the owners through lesser labor cost, immediate error detection, and increased efficiency.

Reasons to incorporate dimensional inspection testing in Plastic Injection Molding

  1. Risk Management – Validate product dimensions before initiating the manufacturing process shall help in reducing liability and overall risk.
  2. Effective Decision – Making key decisions during the development process before it is late to make any changes.
  3. Determining process capability – Capability study of parts saves time and money. This is necessary when parts are subjected to harsh environments such as force, pressure, shear etc.
  4. Regulatory Compliance – Validating using a CMM ensures that your company’s parts are compliant with ISO and other International Standards.

Preventive Maintenance (PM) for CMM

PM is the essential for any machine, including CMM. If a proper maintenance schedule not planned and executed then machine life, overall performance reduces drastically. Therefore, PM should be done for all major components such as motor, bearings, controllers, and probe system.

Benefits of doing PM for CMM:

  1. Optimal Performance
  2. Long Component Life
  3. Diminished Downtime
  4. Reduces Repairing Cost


CMM machine measures physical and geometry property of manufactured components and needs few components such as body, fixture, probe, and software. Various CMM machines can be selected based on suitable applications with different probe types. CMM is the best machine for measuring manufactured components for precise and accurate results with all mentioned benefits.

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