Industry · Plastics and injection moulding
Injection moulding: when the process varies without anyone touching it
In injection moulding the dimensions of a part are affected by temperature, pressure, material and the condition of the mould, and a mould with several cavities can give parts that vary differently. Much of the quality is decided before the part is measured.
How the process works
Plastic granulate is dried if needed, melted in the barrel and injected under pressure into a mould. After injection a holding pressure keeps the material in place while it cools, and then the part is ejected. A cycle can take from a few seconds to several minutes depending on part and material.
Many moulds have several cavities, so several parts are made in each cycle. Each cavity can have its own small differences.
What affects the result
- Melt and mould temperature
- Injection speed, holding pressure and holding time
- Cooling time
- Material batch and moisture content
- The condition of the mould and which cavity the part comes from
Typical quality issues
- Dimensions affected by shrinkage and warpage
- Weight
- Surface defects such as short shots, flash and sink marks
- Approval of first parts after start-up and changeover
The process step by step
Where the questions arise
The same quality questions, placed where they arise in the process: from the material to the measurement.
Where in the process do the quality questions arise?
Illustration01
Material
Batch, moisture content, drying
"Is the material dry enough, and which batch went into which parts?"
02
Injection and holding pressure
Melt temperature, speed, holding pressure and time
"Is the process where it was when the parts were good?"
03
Cooling
Mould temperature, cooling time
"Does the cooling cause shrinkage or warpage in the dimensions?"
04
Mould and cavities
Mould condition, cavity
"Do the cavities differ?"
05
First parts
Start-up, changeover
"Are the first parts approved before production starts?"
06
Measurement
Dimensions, weight, surface defects such as short shots, flash and sink marks
"Does the dimension match the process values from the same cycle?"
Much of the quality is decided before the part is measured. Each step has its own variables, and the cavity question is special to multi-cavity moulds: the same cycle can give parts that vary differently.
Requirements and customers
Moulded parts are often delivered to customers with their own requirements for dimensions, capability and documentation, for example in automotive, medical devices and packaging. The requirements differ between customers and industries and should always be read in the customer's own documents.
Where the information loses its context
Where in the chain is the context lost?
Illustration- 01
Customer requirementCustomer
Who: Customer, quality
Where: The customer's drawing and requirement documents
- 02
Planning
Who: Engineering
Where: Drawing, tolerances, control plan
- 03
ProcessSupplier
Who: Engineering, production
Where: Process settings, instructions
Context lost · The material batch is not linked to the parts it was used for.
- 04
Machine
Who: Operator
Where: The machine control system (PLC)
Context lost · The measurement is done in the measuring room, while the process settings are in the machine.
- 05
Measurement
Who: Operator, measuring room
Where: Gauge, measuring machine (CMM)
- 06
Quality
Who: Quality
Where: Quality system, spreadsheets
- 07
Decision
Who: Operator, quality, production
Where: Conversations, meetings, email
- 08
Action
Who: Production, engineering
Where: Order, maintenance, instruction
Context lost · Capability evidence for the customer is compiled by hand.
- 09
EvidenceCustomer
Who: Quality
Where: Reports and certificates
Situations
Common in the industry
Situation
Values creep towards the limit
Every part passes, but the measured values drift in one direction as the tool wears.
Read about the situationSituation
Changeover to the next part
Tools, programs and settings are changed. When is the first part approved?
Read about the situationSituation
Incoming material does not meet requirements
A batch deviates at incoming inspection, or causes problems only in production.
Read about the situationSituation
Two operators, same part, different values
The same part gives different results depending on who measures or with what.
Read about the situationSituation
The customer wants to see the requirement was met
A complaint or an audit. Can you show what was made and how it was inspected?
Read about the situation
Where Rektron is today
What we can show
| What you want to follow | What is available today |
|---|---|
| Differences between cavities | The cavity number can be recorded per measured value, and in Rektron SQC the values can be selected per cavity and compared. |
| Temperature and other process values, against the dimension | Process values sent by the machine, for example temperature, can be recorded as characteristics in Eyesight and compared with dimensions on the same part (serial number). |
| Data straight from the machine | Eyesight can collect data from production machines, PLCs and sensors. |
| Capability the customer requires | Capability studies, with indices such as Pp and Ppk, are available in Rektron SQC. |
Does the dimension follow the mould temperature, part by part?
Illustration · Example dataWhen the process value and the dimension belong to the same part, through the serial number, you can see whether they move together. A correlation shows where it is worth looking. It does not prove what causes the deviation.
Step by step
- Each point is one part. The mould temperature comes from the machine and the dimension from the measurement, and they are linked by the serial number of the part.
- The dimension should be 25.00 ± 0.10. Every part in the example is within tolerance.
- The higher the temperature, the larger the dimension. The parts from the hottest cycles, above about 55 °C, are close to the upper tolerance limit.
- The correlation shows where it is worth investigating. It does not prove that the temperature is the cause.
Do you work with injection moulding?
Tell us about your process, your machines and your requirements, and we will show you what it can look like for you.