Injection Molding Press

Plastic injection molding presses are classified or rated based on tonnage, or more specifically, the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 6000 tons. The higher the press ton rating, the larger the machine.

A machine rated for 60 tons can deliver 60 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge.

How to know the required press tonnage?

There are many factors that are taken into consideration when determining the size of the press. The size of the part, the polymer being used and something called the safety factor. The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part. Many calculations include the platen size as well as the mold and part size, however, to get an estimate of the press size your project will need, we have simplified it even further.

Reference :

Mold Design Specification

The design of the mold will be divided into 2 stages:

1) Preliminary Drawings

The preliminary design is to be submitted within 1 week after receipt of purchase order. The purpose of this design is to determine the general layout and working of the tool to a degree that will allow steel to be purchased.
This stage should include :

  • Runner system
  • Basic position of water lines
  • Clamp slots
  • Gate locations
  • Gate size
  • Support/Pillar locations
  • Overall size of the mold
  • and any conditions special to the mold.

Project engineer approval is necessary for this stage.

2) Final Design

Drawings are to be fully detailed as follows:

  • Details of final gate designs, runner style, layout and style of sprue puller must be shown.
  • Complete CAD files of the tools to be submitted. ( 2D and 3D)
  • Cores, cavities, inserts or any other part of the mold which are not standard are to be fully detailed.
  • Assembly view should be stated the approximate weight of both mold halves.
  • A material list is required.
  • Detailed drawings are required for all inserts, slides, lifters, etc., with steel hardness specified.

To summarize, in the mold design process some general requirements have to be considered:

– Tool Design
– Materials
– Mold Bases
– Mold Makers Responsibilities
– Mold Venting Requirements
– Mold Cooling Requirements
– Core Pulls/ Lifters
– Electrical Systems
– Hydraulic Systems
– Finish Requirements
– Mold Identifications
– Mold Try-Out
– Warranty
– Tool Design Checklist
– Tool Sign-off


Machine Shot Capacity: Refers to the maximum volume of thermoplastic resin which can be displaced or injected by the injection ram in a single stroke.

Mar Resistance: The resistance of glossy plastic surfaces to abrasive action. Masterbatch: A concentration of a substance (an additive, pigment, filler, etc.) in a base polymer.

Material Safety Data Sheets: Documentation regarding the toxicity or hazards associated with contact with some substances. The manufacturer of the plastic prepares these data sheets.

Mechanical Property: Properties of plastics which are classified as mechanical include abrasion resistance, creep, ductility, friction resistance, elasticity hardness, impact resistance, stiffness, and strength.

Melt Flow Rate: A measure of the molten viscosity of a polymer determined by the weight of polymer extruded through an orifice under specified conditions of pressure and temperature. Particular conditions are dependent upon the type of polymer being tested.

Melt Flow: Rate of extrusion of molten resin through a die of specified length and diameter. The conditions of the test (e.g. temperature and load) should be given. Frequently, however, the manufacturer’s data lists only the value, not the condition as well.

Melt Index: The amount of a thermoplastic resin, measured in grams, which can be forced through a specified orifice within ten minutes when subjected to a specified force. (ASTM D-1238) Continue reading “M”